ArticlePDF Available

Abstract and Figures

Abstract. The athermalized panchromatic imaging system (APIS) was the low-resolution refractive camera proposed by the Laboratorio de Instrumentación Espacial as a CubeSat payload. APIS flew on-board OPTOS CubeSat designed and developed by INTA using the methodology of European Cooperation for Space Standardization and space qualification tests. APIS had two main objectives: to analyze the performance degradation of commercial off-theshelf (COTS) components due to space radiation and to verify in-flight functionality of the passive athermalization system.We summarize the design, manufacturing, and assembly integration and verification phases of the instrument, as well as the analysis of the radiation tests. Additional studies are included, such as thermal behavior, tolerances and sensitivity analysis, signal-tonoise ratio, and ghost images, as well as their implications during the design process. Three main goals were achieved during the mission lifetime: (1) the viability of a small refractive Earth observation camera on-board a CubeSat, (2) the validation for low Earth orbits of a passive athermalization system, and (3) the use of COTS elements, such as commercial glasses and detectors based on complementary metal–oxide–semiconductor technology, on a 2-year Earth observation mission. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.JRS.13.032502]
Content may be subject to copyright.
APIS: the miniaturized Earth
observation camera on-board
OPTOS CubeSat
Daniel Garranzo
Armonía Núñez
Hugo Laguna
Tomás Belenguer
Eduardo de Miguel
María Cebollero
Sergio Ibarmia
César Martínez
Daniel Garranzo, Armonía Núñez, Hugo Laguna, Tomás Belenguer, Eduardo de Miguel, María Cebollero,
Sergio Ibarmia, César Martínez, APIS: the miniaturized Earth observation camera on-board OPTOS
CubeSat,J. Appl. Remote Sens. 13(3), 032502 (2019), doi: 10.1117/1.JRS.13.032502.
APIS: the miniaturized Earth observation camera
on-board OPTOS CubeSat
Daniel Garranzo,a,*Armonía Núñez,aHugo Laguna,aTomás Belenguer,a
Eduardo de Miguel,aMaría Cebollero,bSergio Ibarmia,a
and César Martíneza
aInstituto Nacional de Técnica Aeroespacial, Torrejón de Ardoz, Spain
bIngeniería de Sistemas para la Defensa de España, S.A (ISDEFE), Madrid, Spain
Abstract. The athermalized panchromatic imaging system (APIS) was the low-resolution
refractive camera proposed by the Laboratorio de Instrumentación Espacial as a CubeSat
payload. APIS flew on-board OPTOS CubeSat designed and developed by INTA using the
methodology of European Cooperation for Space Standardization and space qualification tests.
APIS had two main objectives: to analyze the performance degradation of commercial off-the-
shelf (COTS) components due to space radiation and to verify in-flight functionality of the pas-
sive athermalization system. We summarize the design, manufacturing, and assembly integration
and verification phases of the instrument, as well as the analysis of the radiation tests. Additional
studies are included, such as thermal behavior, tolerances and sensitivity analysis, signal-to-
noise ratio, and ghost images, as well as their implications during the design process. Three
main goals were achieved during the mission lifetime: (1) the viability of a small refractive
Earth observation camera on-board a CubeSat, (2) the validation for low Earth orbits of a passive
athermalization system, and (3) the use of COTS elements, such as commercial glasses and
detectors based on complementary metaloxidesemiconductor technology, on a 2-year Earth
observation mission. ©The Authors. Published by SPIE under a Creative Commons Attribution 4.0
Unported License. Distribution or reproduction of this work in whole or in part requires full attribution
of the original publication, including its DOI. [DOI: 10.1117/1.JRS.13.032502]
Keywords: camera; athermalization; CubeSat; Earth observation; radiation; commercial off-the-
shelf.
Paper 180861SS received Oct. 26, 2018; accepted for publication Feb. 21, 2019; published on-
line May 10, 2019.
1 Introduction
The CubeSat standard was developed in 1999.1This development opened a low-cost opportunity
to design, manufacture, and test small satellites for low Earth orbits (LEO). Therefore, CubeSats
allow testing new spacecraft technologies, scientific experiments, miniaturized optical instru-
ments, etc., whose cost would not be justifiable in a larger satellite. Following this idea, the
OPTOS nanosatellite was intended to qualify the platform and payloads in orbit.
The OPTOS CubeSat was launched on November 21, 2013 from the Yasny Cosmodrome
(Russia) as a secondary payload on a Dnepr-1 launch vehicle. International Space Company
Kosmotras operated the launch.
OPTOS included four payloads: (1) athermalized panchromatic imager system (APIS),
(2) fiber Bragg gratings for optical sensing (FIBOS) to measure temperature, (3) giant magneto-
resistance system (GMR) to measure the magnetic fluxes produced by Earths magnetic field,
and (4) OPTOS dose monitoring (ODM) to measure the radiation environment in space.
However, 10 technologies were tested for their qualification in space. For example, a distributed
on-board data handling subsystem based on field programmable gate arrays and complex pro-
grammable logic devices, an optical wireless communication system (OBCom) with the imple-
mentation of a reduced controller area network (CAN) protocol and an internal structure based
on composite materials,29among others.
*Address all correspondence to Daniel Garranzo, E-mail: garranzogid@inta.es
Journal of Applied Remote Sensing 032502-1 JulSep 2019 Vol. 13(3)
The nanosatellite external structure corresponded to the triple CubeSat format (3U) in size
and mass, i.e., 10 cm ×10 cm ×34.5 cm, and 3.8 kg. Internally, a composite structure was
selected to support all the elements and to allow easy integration and tests. The spacecraft struc-
ture featured an external aluminum casing provided by Pumpkin Inc. of San Francisco,
California, and an internal carbon fiber structure, designed and manufactured by INTA.1012
OPTOS CubeSat was a technological challenge regarding the size and cost reduction. These
constraints forced certain design decisions. For example, the development of a deployment sys-
tem for the solar panels to double the nanosatellite power as it is shown in Fig. 1(a), and wireless
communications, the positioning of the payloads and subsystems into the internal structure in
a rack way, and a miniaturized Earth observation camera shown in Fig. 1(b).
The APIS camera had two objectives during the mission lifetime. The first one was to evalu-
ate the space radiation damage on commercial off-the-shelf (COTS) components and the second
one to verify the functionality of the passive athermalization mechanical system. Both objectives
would be evaluated through image analysis.
The restrictive requirements on size and weight resulted in a compact device with APIS
external envelope dimensions of 57 mm ×46 mm ×35.7 mm and 120 g weight (see Fig. 2).
On the other hand, the low power consumption requirement was met using complementary
metaloxidesemiconductor technology in the focal plane.
APIS performed Earth observations in the visible spectrum from an elliptical orbit of 600 ×
800 km with an inclination of 97.8 deg. The expected mission lifetime was established in one year.
The nanosatellite incorporated a pyrotechnic shutter (as shown in Fig. 3) in order to avoid
direct Sun light into the camera during the launch and commissioning phases.
2 Instrument Concept
This section describes the three main parts that make up the APIS camera concept: the optical
system, the electronics design, and the mechanical design.
Fig. 1 (a) OPTOS satellite in launch configuration with the panels folded and (b) OPTOS internal
structure with APIS assembled in the upper left part.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-2 JulSep 2019 Vol. 13(3)
2.1 APIS Optical System
The optical system was a refractive objective working in the visible spectral band (450 to
650 nm). It was designed using the optical design software CODE V from Synopsis®Inc.
The objective consisted of a triplet with a conic surface and two meniscus lenses close to the
focal plane (see Fig. 4). This configuration reduced residual aberrations in the image, such as
astigmatism, spherical, and field curvature.
The system was designed with an external aperture stop and three different glasses from
the Schott catalog: NFK51, NSF57, and NLASF41.
Figure 4shows the optical layout which includes the protective window of the selected
detector for APIS camera (the IBIS5-B-130013 CMOS image sensor).
The optical system with a 2.5 f-number (20-mm focal length and 8-mm pupil diameter) was
optimized, using the modulation transfer function (MTF) as merit function, for an asymmetric
field of view (FOV) of 12.16 deg ×9.25 deg and a pixel size of 6.7 μm. This FOV was
equivalent to a rectangular Earth swath between 128 km ×97 km and 170 km ×129 km.
It also provided a resolution between 201 and 268 m, depending on the orbit height.
Some additional studies, such as thermal behavior, tolerances, and sensitivity analysis, sig-
nal-to-noise ratio (SNR) and ghost images, were performed during the critical design phase
Fig. 3 Shutter closed in the upper part of the OPTOS internal structure.
Fig. 2 APIS camera compared to a 5 euro cent coin.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-3 JulSep 2019 Vol. 13(3)
because of their implications in the mechanical design and manufacturing phases. The main
conclusions obtained from these studies are detailed from Secs. 2.1.1 to 2.1.4.
2.1.1 Thermal-vacuum behavior
APIS would be subject to vacuum conditions and temperature changes during flight operation
with the consequent performance degradation due to the environment. The APIS behavior due to
these environmental changes had to be analyzed in order to guarantee its optical performance in
flight.
The thermal-vacuum behavior was simulated with CODE V for APIS operational conditions:
vacuum and a 20°Ctemperature range. For these conditions, the image degradation was sep-
arately evaluated as nominal focus shifts (26.5 μmdue to vacuum and 18 μmdue to temper-
ature range). The result of the analysis was focus shifts greater than the APIS focus depth
(7μmin average for the spectral band). As a consequence, it was proposed that the focus shift
due to vacuum conditions be compensated at the laboratory before APIS launch. However, the
thermal focus shift expected during the APIS flight operation indicated that an athermalization
mechanical system was required.
2.1.2 Tolerances and sensitivity analysis
The objective of this analysis (performed with CODE V) was to define a manufacturing and
assembly tolerances budget. It also determined the best set of compensators that could simulate
the assembly and necessary alignment process to recover the image quality of the instrument.
The APIS tolerances budget was highly critical (taking into account the reduced dimensions
of the system) as the second and third lenses are the most critical elements. Therefore, the focal
plane was chosen as the best tolerances compensator. In particular, linear displacement along the
optical axis and tilts of the APIS focal plane should be able to be adjusted during the APIS
alignment phase. These requirements about the assembly and alignment process were considered
in the mechanical design.
2.1.3 Signal-to-noise ratio study
This study was performed to evaluate the performance of the electro-optical instrument. In par-
ticular, in the case of an instrument for Earth observation, the signal comes from the scene that is
considered as an extensive source of certain radiance. Therefore, three kinds of scenes (assuming
different latitudes and illumination conditions) were taken into account for the APIS signal
evaluation: desert/snow/ice areas (high reflectance), oceanic areas (low reflectance), and land
areas. Regarding the noise evaluation, only four sources of noise were considered: photonic
noise, readout noise, digital noise, and dark noise. The criterion followed in the analysis was
to obtain an SNR value better than 100:1 for average illumination conditions (typical value for
an electro-optical system).
The irradiance on the detector depends on the scene radiance, the optical transmission, and
the f-number of the instrument. The low f-number of APIS already indicated that a high
Fig. 4 APIS camera optical layout.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-4 JulSep 2019 Vol. 13(3)
irradiance on the detector would be expected. In addition, the integration time regulates the
energy received in each pixel.
The frame period of the IBIS5-B-1300 sensor depends on the shutter type (see Ref. 13).
In the snapshot shutter mode (selected for APIS camera), the minimum read out time of
the full resolution at nominal speed (40 MHz pixel rate) is obtained with an integration time
of 1 ms. This minimum value of the integration time produced saturation of the detector for
the scenes with high reflectance (desert/snow/ice areas).
Therefore, the main result of this SNR evaluation was that a neutral-density filter (with
T15%) should be used at the entrance of the instrument to avoid saturation of the detector.
In this way, an SNR value better than 100:1 and an optimal detector response (dynamic
range) could be obtained by considering a different integration time for each scene.
The selected filter was the NG5 from the Schott catalogue with 3.3 mm thickness. This neu-
tral-density filter was used as substrate of the bandpass filter required for the APIS camera that
was specified and designed by the Laboratorio de Instrumentación Espacial (LINES) at INTA.
The design consisted of a combination of multilayer edges filters on both filter surfaces. The
transmittance of the final bandpass/neutral-density filter fulfilled the requirement obtained from
the SNR study.
2.1.4 Ghost images
This study was performed with the Advanced Systems Analysis Program (ASAP) from Breault
Research Organization Inc. The objective of the study was to analyze the influence in the APIS
point spread function (PSF) produced by retroreflections on the optical surfaces. The study con-
sisted of finding the most relevant energetic paths and analyzing the relation between the energy
collected by the ghost images and the PSF of the system.
The main contributions to the generation of ghost images were related to the interaction of
the CMOS reflections and the flat surfaces (filter and detector window), and retroreflections of
the meniscus lenses. The study verified that using high antireflection coatings in the optical
components (T>99%) resulted in a flux ratio of the ghost images/signal of 103.
Figure 5shows the global PSF flux in the focal plane (with the ghost process included) for
a 100-W energy incident. Figure 6shows the corresponding flux of the ghost image process
separately.
Therefore, the influence of the ghost images was negligible. Ghost images were on the same
order as the inherent noise of the system.
2.2 APIS Electronic Design
The selected detector for the APIS camera was the IBIS5-B-1300 model from Cypress Inc. (cur-
rently ON Semiconductor®) in a monochrome version. The IBIS5-B-1300 is a CMOS image
Fig. 5 Global PSF including the ghost image process (0.657046 W total flux).
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-5 JulSep 2019 Vol. 13(3)
sensor that integrates several functionalities. It performs as an analog-image acquisition system,
a digitizer, and a digital signal processing system. The main key performance parameters of
the sensor can be seen in Ref. 13.
The APIS camera was optimized for a focal plane size of 4.3 mm ×3.2 mm corresponding to
a region of interest (ROI) of 640 ×480 pixels in order to boost the readout speed. The sensor
worked in snapshot shutter mode.
The electronic design of APIS was divided in two boards as shown in Fig. 7: the APIS focal
plane board, which includes the CMOS image sensor and a temperature sensor, and the APIS
main board with the rest of the electronic devices.
The APIS focal plane board was connected to the APIS main board by a flexible cable with
10 data lines, 14 control lines, and 2 power lines.
2.3 APIS Optomechanical Design
The APIS optomechanical design had five important design challenges. (1) Geometrical: the
APIS camera had to be installed inside a cubic prism volume of 65 mm ×52 mm ×36.4 mm.
(2) Weigh: the APIS camera had to weigh less than 150 g. (3) Tolerances: critical position tol-
erances of the optical elements had to be maintained. (4) Thermal: the mechanical design had to
Fig. 6 Ghost image process (0.000476 W total flux).
Fig. 7 APIS electronics diagram.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-6 JulSep 2019 Vol. 13(3)
absorb the optical distance changes induced by the thermal expansion (passive athermalization
system). (5) Stray-light protection: the APIS camera had to avoid the stray-light radiation by
means of baffles at the entrance of the instrument and in the detector area.
2.3.1 Mechanical subassemblies
The most important subassemblies of the APIS optomechanical design can be identified in Fig. 8
by different colors. The platform subassembly is formed by the base part (dark red part) that
allows the system to be attached to the lateral wall of OPTOS CubeSat, the sliding part (green
part), and two linear sliding elements to join them. Also shown is the optical subassembly, which
includes the five mounted lenses, the tube to hold them (dark blue part) and the support for all of
it (yellow part) with the venting slots. Next is the focal plane subassembly, which is formed by
the electronic card with the detector itself. This subassembly provides the system with the ability
to tilt the focal plane array with respect to orthogonal axes by means of a gimbaled platform.
Finally, the baffle subassembly formed by the baffle itself (including the aperture stop) and
the bandpass/neutral density filter that was glued on it.
2.3.2 Focal plane movements and adjustments
In order to comply with the critical alignment requirements of the focal plane (movement ranges/
accuracies) during the assembly integration and verification (AIV) phase, the optomechanical
design provided the system with the ability to change optical-axis distance and two tilting angles
of the focal plane. The linear movement along the optical-axis was achieved by the relative
displacement between the two main parts of the platform subassembly. The use of linear guide-
ways (R1-030 model), 30 mm long from Schneeberger GmbH and an ultrafine adjustment cou-
ple thread/screw (F3SS8 model with 250 μmrev) from Thorlabs Inc. allowed a micrometric
displacement. Moreover, a tilting movement was achieved by using two small bearings and
the aforementioned ultrafine adjustment couple thread/screw for each rotation axis.
We have also used these linear guideways in the full disk telescope refocusing mechanism14
for the Solar Orbiter ESA Mission, successfully developed at our laboratory.
2.3.3 Lens mounting
The critical positioning tolerances together with the small diameter of the lenses (10 mm in
average) led to identifying the lens mounting as one of the most critical problems of this design.
The best solution was to glue every lens to an individual mount (AL-6082-T6 material) with
RTV560 red silicone. The gluing process was carried out on a centering machine to check the
centering and tilting of each lens during the glue curing time. All the mounts had the same
diameter for an easier assembly into the lens tube (dark blue part in Fig. 8).
Moreover, eight positioning notches were performed in the mount of the most critical lens (conic
lens) in order to be able to rotate the lens through the venting slots during the alignment phase.
Fig. 8 APIS optomechanical design: (a) CAD view and (b) exploded view.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-7 JulSep 2019 Vol. 13(3)
2.3.4 Athermalization system
The passive athermalization system was accomplished by selecting different materials for the
parts that held the lenses pack. In particular, the tube in which all the five lenses were mounted
in cascade was made of INVAR (dark blue part in Fig. 8) which has a practically zero coefficient
of thermal expansion. In addition, this lens barrel was mounted into an aluminum part (yellow
part in Fig. 8). This placement produced a distance variation between the apex of the fifth lens
and the focal plane that remained within the focus depth of the system for a temperature range of
40 K. That means that the proposed mechanical design was able to absorb the optical focus shift
(see Sec. 2.1.1) due to the changes in the optical properties of the elements affected by
temperature.
The experimental verification of this can be seen in Sec. 4.5 (Table 6). The maximum change
in the APIS focus produced during a 40 K thermal excursion (in vacuum conditions) was 4.9 μm
(below the focus depth).
2.3.5 Baffles
In order to reject unwanted light into the FOV, the entrance baffle was a piece with an internal
cone 15 deg wide configured as a sequence of discs equally separated (gray color at the entrance
of APIS in Fig. 8).
The detector baffling was obtained by a protrusion of the main part that supported the lens
pack (yellow part in Fig. 8). This part was internally milled to fit around the detector chip and to
reject incoming lateral light.
2.3.6 Stray-light analysis
The APIS camera included a real aperture stop. This stop guaranteed a good performance of
APIS with respect to the stray-light rejection factor because it limited the light collected by the
CMOS. However, a stray-light analysis of the optomechanical design had to be performed to
check the amount of unwanted scattered light reaching the CMOS area.
The simulation was performed with ASAP considering a 1-W emitting Lambertian source
covering the whole entrance pupil of the system. The ghost images and the preliminary stray-
light behavior of APIS were studied considering the most relevant coatings and paintings of both
optical and mechanical parts. Three kinds of coatings with different scattering properties were
taken into account in the simulation. The Chemglaze 306 coating was applied to all internal parts
of each optomechanical support and to those external parts that would be exposed directly to the
optical beam, such as the baffles. The mechanical interface and the rest of the mechanical com-
ponents were coated with black diffuse, which shows a much higher scattering level for different
angles of incidence. The third coating for polished reflective surfaces was applied on some
mechanical parts, such as adjustment mechanisms, screws, and the external surfaces of the
mechanical structure.
The merit function used to analyze in detail the behavior of the stray-light in the APIS camera
was the point source transmittance (PST) curve. The PST associated to the stray-light process
was calculated as the ratio of the flux obtained on the focal plane for an incident on-axis colli-
mated beam to the flux collected on the focal plane for an incident collimated beam outside the
FOV of the instrument, i.e., from 7 deg to 50 deg (angles outside the maximum semi-FOV of
APIS camera).
In the simulation, we separated the flux produced by the beams inside the FOV of the
instrument, which produce the image in the CMOS, with respect to the flux produced by
the beams scattered in any mechanical surface, whicharerelatedtothestray-lightprocess.
The results showed that the relative flux received in the image plane due to the scattering
in the mechanical surfaces was 108times the energy corresponding to the normalized flux
directly collected in the CMOS area (inside the FOV). This means that the baffling proposed in
the optomechanical design effectively attenuates the unwanted scattered radiation. Figure 9
shows the rejection factor [log (PST)] as a function of the incident angle (only angles out
of the FOV are represented).
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-8 JulSep 2019 Vol. 13(3)
3 Radiation Test
As it was mentioned in Sec. 1, one of the objectives of the APIS camera was to evaluate the
degradation of the commercial glasses and detector during the mission. Therefore, it was nec-
essary to study the behavior of these elements under radiation levels similar to the expected
conditions for the OPTOS mission.
The foreseen radiation environment for OPTOS is dominated by the contribution of the Van
Allen proton belt. It consists of high-energy protons in the range from 0.01 to >500 MeV.
Therefore, a series of ionizing and nonionizing irradiation testing campaigns were performed
in order to reproduce the accumulated effects during the mission. The ionizing damage was
studied by exposing three optical bread board (BB) models and two BB focal plane boards
to a Co60 gamma radiation field. In addition, combined ionizing and nonionizing effects were
analyzed using a quasimonoenergetic proton source. In this case, another two BB focal plane
boards were irradiated with particle fluences equivalent to the estimated mission level.
3.1 Gamma Radiation Test on APIS Optical BB Models
The APIS camera included optical glass elements that could be affected by the ionizing radiation.
The behavior in radiation environment was already studied for the optical glasses NFK5115 and
NSF5716 within the LINES research activity for assessing and using COTS glasses in space
environment. The behavior for NLASF41 glass was unknown. Therefore, gamma radiation test
of this glass and the whole lens set were performed in order to correlate the transmittance loss
observed during the mission lifetime.
To estimate the expected transmittance losses during the mission life, three different optical
BB models (BB1, BB2, and BB3) were exposed to 5, 10, and 15 krad doses, respectively (cor-
responding to 1, 2, and 3 mission years). They were made up of optical parallel-plane plates of
8 mm diameter and thickness that closely simulate the elements used in the APIS camera optical
design. The doses were calculated according to the mission orbital parameters and the standard
models AP8/AE817 for the Van Allen belts. The ESP/PSYCHIC1820 model for the probability of
a solar particle event occurrence was also used. As a conservative approach, AP8/AE8 models
were used considering a worst-case scenario of maximum solar activity.
The BB models were exposed to gamma radiation in the Co60 pool at the NAYADE facility21
belonging to the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas
(CIEMAT). The irradiation conditions were 15.7°C temperature, 4.86 krad (Si)/h dose rate, and
9.5% of maximum dose nonuniformity.
The NLASF41 glass showed higher transmission losses than NFK51 and NSF57 glass. The
absorption induced by gamma radiation on NLASF41 glass followed the same behavior already
observed on NFK51 and NSF57 glass. That means the transmission loss increases as a function
of radiation dose following an exponential curve.
Fig. 9 Logarithm of the irradiance in the focal plane due to the stray-light process.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-9 JulSep 2019 Vol. 13(3)
Transmission measurements versus wavelength of the BB models were carried out before
and just after each irradiation test with a Perkin-Elmer Lambda 850 spectrophotometer (0.08%
accuracy).
Table 1shows the average transmittance data of the three optical BB models before irradi-
ation and the transmittance data difference of each optical BB model before and after irradiation
for several wavelengths into the work range.
Therefore, optical transmittance losses (on average) between 7% and 20% (corresponding to
one and two mission years) would be enough to observe a performance degradation in the APIS
image data and therefore to achieve the scientific goal of the instrument.
3.2 Gamma Radiation Tests on APIS BB Focal Plane Boards
Two different BB focal plane boards (M01 and M02) were also exposed to gamma radiation in
the Co60 NAYADE facility. The irradiation process was performed in six accumulated dose steps:
1, 2.5, 5, 10, 15, and 21 krad, respectively. The irradiation conditions were the same as the ones
mentioned in Sec. 3.1.
Electrical characterizations were carried out with the BB focal plane boards before irradiation
and after each irradiation step using the IBIS5 evaluation system provided by Cypress Inc. (see
Ref. 13). A dark current analysis was performed in order to see the degradation of the active
pixels by effect of the radiation. Degradation pixels were considered to have a charge level in
dark current >1.91% of the full well charge (62;500 e).
The measurements showed an increase on pixel degradation from 10 krad of accumulated
dose on both specimens. Figure 10(a) shows the abrupt increment of the pixel degradation by
effect of the gamma radiation from 10 krad, and Fig. 10(b) shows the zone between 0 and 5 krad
in more detail.
Table 1 Transmittance loss data of the optical BB models.
Wavelength (nm)
450 500 560 620 670
Average %T BB models (before irradiation) 56.9 58.4 59.3 59.6 59.7
Δ%TBB1 at 5 krad 12.0 10.4 7.3 4.0 2.1
Δ%TBB2 at 10 krad 29.6 27.0 21.2 13.8 9.2
Δ%TBB3 at 15 krad 38.7 36.3 29.4 19.9 13.7
Fig. 10 (a) Pixels with charges in dark current versus gamma radiation dose and (b) detail of
the zone from 0 to 5 krad.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-10 JulSep 2019 Vol. 13(3)
In addition, an image quality analysis was carried out in order to evaluate the contrast in the
images obtained by the CMOS sensor with respect to the gamma irradiation dose. The setup
was formed by an LED (λ¼565 nm), a target (Foucault Test), an optical system, and the CMOS
image sensor. Figure 11(a) shows the Foucault test image obtained before the CMOS gamma
irradiation, and Fig. 11(b) the Foucault test image after 21 krad of accumulated gamma dose.
The procedure was to calculate the maximum and minimum intensity of the peak patterns
obtained for the image central row. The Michelson contrast (visibility) was only estimated using
the four central peaks. Table 2shows the average contrast of the two samples as a function of
the gamma radiation dose.
The obtained results guaranteed that changes in the IBIS5-B-1300 sensor could be consid-
ered negligible with doses of up to 10 krad of gamma radiation.
3.3 Proton Radiation Tests on APIS BB Focal Plane Boards
Another two BB focal plane boards (M03 and M04) were exposed to proton radiation in order to
study the effects of the nonionizing radiation damage combined with ionizing effects. These
irradiation tests were carried out at the high-energy proton beam line of radiation effects facility,
at Jyväskylä University (Finland).22 In this facility, protons in the energy range from 10 to
50 MeV can be delivered with a homogeneous beam size of 10 ×10 cm2. The irradiation process
was performed at the maximum available energy of 50 MeV and in six accumulated fluence
steps up to a total proton fluence of 1.9 ×1011 cm2and an equivalent total ionizing dose of
29.3 krad (see Table 3).
The results showed a similar behavior of the sensor as the one obtained with the gamma
irradiation. From the 9.5 ×1010 proton fluence step (14.6-krad ionizing equivalent dose), a
Fig. 11 (a) Foucault test image at 0 krad gamma dose and (b) Foucault test image at 21 krad
gamma dose.
Table 2 Image contrast versus gamma radiation dose.
Gamma dose
(krad)
Average contrast
M01 sample
Average contrast
M02 sample
1 0.99 0.99
2.5 0.99 0.99
10 0.92 0.93
15 0.82 0.82
21 0.72 0.65
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-11 JulSep 2019 Vol. 13(3)
strong increase of pixels degradation in dark current was observed. Moreover, 2 months after the
last irradiation step further increase in pixel degradation was observed. This fact indicated an
activation in the time of the samples irradiated with high-energy protons. Table 4shows the
number of pixels with charges in dark current detected in the M04 sample after the irradiation.
(The M03 sample data are omitted because of a problem during the irradiation).
On the other hand, the image contrast at the maximum proton fluence (1.9 ×1011 ) only
showed slight variations as can be seen in Table 5and Fig. 12.
Therefore, the degradation in the sensor for proton fluences <1011 (pþcm2)or<15 krad
ionizing equivalent dose can be considered negligible.
Table 3 50 MeV proton fluence data.
Step
50 MeV proton
fluence (pþcm2)
Equivalent accumulated
dose [krad (Si)]
11.58 ×1010 2.44
23.17 ×1010 4.88
34.75 ×1010 7.32
49.5×1010 14.6
51.43 ×1011 21.9
61.9×1011 29.3
Table 4 Number of pixel with charges in dark current versus proton
fluence for M04 sample.
50 MeV proton
fluence (pþcm2)
Degradation for M04 sample
after irradiation
(no. of pixels)
Degradation for M04 sample
2 months after irradiation
(no. of pixels)
0 131
1.58 ×1010 470
3.17 ×1010 884
4.75 ×1010 1361
9.5×1010 8004
1.43 ×1011 141,473
1.9×1011 745,335 1,023,394
Table 5 Image contrast versus proton fluence.
50 MeV proton
fluence (pþcm2)
Average contrast
M04 sample
1.58 ×1010 0.98
3.17 ×1010 0.97
4.75 ×1010 0.97
9.5×1010 0.95
1.43 ×1011 0.93
1.9×1011 0.91
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-12 JulSep 2019 Vol. 13(3)
4 Assembly Integration and Verification Phase
The AIV phase of the APIS camera was performed in the LINES ISO-7 cleanroom at INTA. This
cleanroom incorporated a 100 class portable clean area and a vacuum chamber, among other
ground support equipments.
Three APIS camera units were manufactured: a flight unit and another two units for verifi-
cation purposes during the AIV phase. This phase was divided in five important steps that are
detailed from Secs. 4.1 to 4.5. The MTF of the APIS camera (as the merit function for performance
verification) was measured in each of these steps. Previously, a theoretical MTF study had been
developed in order to estimate the expected MTF value in each step throughout the AIV phase.
4.1 Optical Characterization
The alignment and optical characterization of APIS optical units (without focal plane) were
performed with the help of a visible MTF bench from Image Science Ltd. The evaluation of
the optical performance was carried out through the measurement of the MTF at the Nyquist
frequency (74.6 line pairs/mm) on and off axis. For this AIV phase step, the theoretical MTF
had been calculated considering the contribution of three partial MTFs (diffraction, aberration,
and alignment/manufacturing tolerances). This theoretical MTF value at the Nyquist frequency
was 0.76. A thorough alignment of the lenses for each unit achieved experimental MTF values
of 0.76 0.02.
The focal of the three optical units was also calculated from geometrical relationships
between the positions of two pinhole images acquired in paraxial approximation. The focal
values obtained were within an interval of 20 mm 1%.
4.2 CMOS Sensor Characterization
The three sensors were characterized in order to set the best configuration parameters for each
one. Of particular importance were two parameters: DAC_FINE is used to tune the difference
between odd and even columns and DAC_RAW is used to add a general offset (both even
and odd columns) to the fixed pattern noise corrected pixel value. These parameters were
independently adjusted considering the sensorsbehavior in both dark current and flat field
simultaneously.
Some other parameters were selected during the characterization as the best default configu-
ration for the sensors. For example, a synchronous (snapshot) shutter to have the light integration
on all pixels in parallel; a slow frame/line calibration mode in order to cancel the thermal-KTC
noise (reset noise) during the calibration of the output amplifier; a unity gain mode and the
internal clock granularities to control the column/pixel readout; and the synchronous shutter
sequencer.
Fig. 12 (a) Foucault test image before proton irradiation and (b) Foucault test image at 1.90 1011
proton fluence.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-13 JulSep 2019 Vol. 13(3)
4.3 Electro-Optical Characterization
The alignment of the APIS focal plane was performed using a 300-mm focal length focusing
autocollimator from Trioptics GmbH with 25-mm focusing range. We established the equiv-
alent relationships between a through-focus, performed around the autocollimator focus, and the
corresponding APIS camera focus. To do that, a specific distance between instruments was set.
The theoretical MTF at this step of the AIV phase had been calculated adding the MTFs
associated with the detector (footprint, spectral effects, array flatness, and tilt). This theoretical
MTF value at Nyquist frequency was 0.46.
The alignment procedure of the APIS focal plane consisted in acquiring a number of images
of the autocollimator tilted-cross hair. For each image, the cross hair was placed at different
positions around the autocollimator focus. Then, an analysis software, developed in MATLAB,
processed these images. The analysis was based on the sloping slit method,23,24 which allowed
finding the best focus position for each APIS camera unit. For each image, the line spread func-
tion (LSFData) of each cross hair arm was obtained and fitted to a Gauss function (LSFGauss).
Figure 13(a) shows the cross hair image of the APIS camera with its labeled arms and Fig. 13(b)
shows an example of LSFData and LSFGauss.
Then, the full width at half maximum (FWHM) for each LSFGauss was calculated and
these values were fitted to a parabolic function with respect to the autocollimator position [see
Fig. 14(a) as an example]. In addition, MTF curves were calculated as the module of the LSF
Fourier transform (jffLSFgj) for each image: MTF1 ¼jffLSFData gj and MTF2 ¼jffLSFGaussgj.
Finally, the MTF values at the Nyquist frequency were fitted to a parabolic curve and plotted as
a function of the autocollimator position [see Fig. 14(b) as an example].
Fig. 13 (a) Image of the tilted-cross hair and (b) example of LSFData and LSFGauss .
Fig. 14 (a) Example of the LSF FWHM fitting and (b) example of the MTF at Nyquist frequency
fitting.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-14 JulSep 2019 Vol. 13(3)
Therefore, the autocollimator position with the minimum FWHM value and the maximum
MTF value showed the best focus position of the APIS camera (regarding the equivalent relation-
ships between through-focus).
The alignment procedure of the focal plane for each APIS camera unit achieved MTF
values 0.40. This experimental electro-optical MTF value at room conditions agreed with a
theoretical final MTF in orbit of 0.17 (added MTFs such as atmospheric, jitter, and operational
perturbations).
4.4 Vibration Test
The object of this test was to validate the APIS electro-optical design and demonstrate the
availability of the specimen to withstand the stress, accelerations, and accumulated fatigue dam-
age resulting from the specific vibration environment. For this purpose, one of the APIS camera
units was assembled on the OPTOS structural thermal model (STM) that was subjected to its
own vibration test. OPTOS STM was vibrated into the Poly-Picosatellite Orbital Deployer
(P-POD).
Figure 15(a) shows the OPTOS STM model with APIS camera unit assembled on it and
Fig. 15(b) shows the OPTOS STM model (into the P-POD) on top of the vibration platform.
The test was carried out in the Mechanical and Environment Test laboratory at INTA. The
following sequence for each axis was applied: low-level sine vibration, high-level sine vibration,
low-level sine vibration, high-level random vibration, and low-level sine vibration.
An electro-optical characterization was carried out before and after the test. In addition, dark
current and flat field analysis for different integration times were performed before and after the
test. The results showed that the vibration environment test did not modify the optoelectronic
performance of the APIS payload.
4.5 Thermal-Vacuum Test
The object of this test was to check the vacuum induced focus shift, the APIS thermal-vacuum
behavior, and the passive athermalization system. The test was carried out in the thermal vacuum
chamber (TVC) at LINES ISO-7 cleanroom at INTA. This TVC has optical windows that
allowed optical checks during the thermal-vacuum test.
The performed thermal profile, with a warming ramp of 0.2°C/min at 106mbar vacuum
level, is shown in Fig. 16. The temperatures for image acquisition are marked in the figure with
black points. The validation of the passive athermalization system was intended to be performed
into the 20°Cto þ20°Ctemperature range.
Fig. 15 (a) APIS unit assembled on the OPTOS STM and (b) OPTOS STM ready on the vibration
platform.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-15 JulSep 2019 Vol. 13(3)
The APIS camera unit selected for this test was placed inside the TVC, and the focusing
autocollimator was outside the TVC and facing the APIS camera through the optical window.
Previously, the APIS camera focus at room conditions was shifted 44 μm. This value included
the theoretical vacuum focus shift (26.5 μm) and the thermal focus shift due to changing the
temperature from 20°C to 0°C (17.5 μm) according to CODE V.
An electro-optical characterization was carried out at the black points indicated in Fig. 16
following the procedure/algorithm presented in Sec. 4.3. The equivalent relationships between
autocollimator and APIS through-focus were such that 1 mm displacement from the focus of the
autocollimator was equivalent to 4.5 μmdisplacement in APIS focal plane. Therefore, the APIS
camera focus depth (7μm) was equivalent to 1.6 mm in the autocollimator.
The analysis of the results gave an experimental vacuum focus shift of 22.1 μm, which is
according to the theoretical value obtained in Sec. 2.1.1. In addition, the passive athermalization
system kept the APIS camera focus inside the focus depth for the complete temperature range.
Experimental results can be seen in Table 6.
After the AIV campaign, the APIS camera flight model was assembled in OPTOS CubeSat
with the vacuum focus shift applied.
Fig. 16 TVC performed thermal profile.
Table 6 Thermal-vacuum test experimental results.
APIS
temperature (°C)
Pressure
(mbar)
Autocollimator
position from
focus (mm)
APIS position
from focus
(μm)
MTF
value
þ20.85 1039.844.10.42
þ19.75 1064.922.10.40
20.01 1064.620.70.39
9.77 1064.118.50.40
þ0.10 1063.515.80.40
þ10.00 1064.218.90.40
þ20.50 1064.118.60.39
þ20.00 1038.437.80.39
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-16 JulSep 2019 Vol. 13(3)
5 APIS Payload Operation
The OPTOS ground segment was formed by an array of four Yagi antennas and a control center
located at INTA. The control center performed the spacecraft tracking, the mission control,
the commanding, the telemetry processing, the analysis, and the distribution. The APIS
image acquisition was always performed via tele-commands sent from the ground. The tele-
commands allowed changing the value of some CMOS sensor configuration parameters from
the default configuration. The APIS image telemetry included temperature before acquisition,
image data, temperature at the acquisition time, result of the acquisition process, and attitude and
orbit data.
The operation of the APIS payload was divided into two phases: the first one was carried out
with the shutter closed and the second one with the shutter opened. The objective of the first
phase was to check the detector and electronics health and the thermal environment of APIS
payload during one orbit. In addition, this first phase established the image download time and
the image recovery process from the telemetry packages. The procedure during this first phase
was to take dark images changing sensor parameters, such as image size (ROI), ROI origin,
integration times, etc. The objective of the second phase was to take images of different scenes
to study the performance degradation of the images, to perform a georeferencing procedure, and
to determine the linear/angular offsets needed for a correct georeferencing.
6 In-Flight Results
OPTOS CubeSat was successfully launched in November 21, 2013. In the summer of 2014, the
commissioning phase was ended. During this phase, the Survival On Board Software (OBSW)
was validated and the Nominal OBSW was activated and validated in orbit. Therefore, the acti-
vation and management of the payloads and the configuration parameters were ready. After that,
three payloads of OPTOS were operated in a nominal mode (ODM, FIBOS, and GMR).
The first phase of APIS operation started at the end of January 2015 and lasted until October
2015, alternated with the operation of other payloads. The initial action was to check the thermal
environment of APIS payload during one orbit. This point was very important because APIS did
not have thermal control. Therefore, this action was carried out several times during the mission
lifetime. The orbital points where APIS payload reached the minimum/maximum temperatures
were established: Tmin ¼24.95°C(5 min after penetrating into the Sun phase) and Tmax ¼
29.91°C(1 h after penetrating into the Sun phase). The APIS temperatures during the eclipse
phase were higher due to the thermal inertia of the instrument but they were not relevant because
APIS payload would not be operated during the eclipse. During this first phase, dark images
were taken using 8-bit resolution and different ROIs. In that way, the dump time of the images
from the sensor memory to the enhanced processing unit by the CAN bus was decreased at a
cost of reducing the image size. The ROI origins, integration times, and DAC RAW/DAC FINE
parameters were checked. The result was that the detector did not show any performance deg-
radation after almost 2 years in orbit. This fact was in agreement with the results obtained after
the radiation test (see Secs. 3.2 and 3.3), since ODM payload reported 0.15 krad per year of
accumulated radiation dose (102times the simulated dose during the radiation test).
Unfortunately, the activation of the tasks management related with attitude control failed.
At this point, the satellite had been in orbit for more than 2 years. Therefore, the second phase
of the APIS operation started at the end of January 2016 without attitude determination and
control subsystem (ADCS).
The shutter was opened successfully and the first image was taken on February 5, 2016 over
an area of the American state of Wyoming (Medicine Bow National Forest). The parameters
of this image were 610 km orbit height, 324 ×347 pixels (Earth swath of 66 km ×71 km),
204 m resolution, and 2 ms integration time. Figure 17(a) shows the APIS camera first image
and Fig. 17(b) the location of the area in Google Earth.
The sensor temperature of APIS, at the time the image was taken, reported a temperature of
47°C due to the reduction of the eclipse phase. The image showed an apparently good focus (note
the lower spatial resolution and the presence of clouds and cloud shadows in the APIS image),
taking into account the lack of active ADCS, although no quantitative assessment was made.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-17 JulSep 2019 Vol. 13(3)
Even though the acquisition temperature was outside the operating range of APIS, the athermal-
ization system worked satisfactorily.
Taking into account the reported data from the ODM payload, the total ionizing dose mea-
sured near the location of the APIS lenses was estimated to be 0.4 krad (including conservative
margins). The transmission loss of the APIS lenses was theoretically calculated using the soft-
ware application to predict optical performance loss (due to gamma radiation) developed by
LINES laboratory.16 The value obtained was a 1% transmission loss, which was insufficient to
observe optical degradation on glasses through the images (see Sec. 3.1).
The Remote Sensing Group at INTA carried out the image georeferencing procedure. Two
images, MOD09GQ.A2016036.h10v04.005.2016038064612.hdf and MOD09GQ.A2016036
.h09v04.005.2016038064855.hdf, taken the same day by the Moderate Resolution Imaging
Spectroradiometer (MODIS) instrument of NASAs Terra satellite were used in the process.
The two MODIS products were geolocated and mosaicked using simple geolocation software
to obtain a single image with similar spatial resolution to the APIS camera. Finally, the APIS
camera image was compared with that image.
The MODIS data products were retrieved from the online data pool, courtesy of the NASA
Land Processes Distributed Active Archive Center (LP DAAC).25 Figure 18(a) shows the
result of the APIS image georeferencing procedure compared with the MODIS single image
[Fig. 18(b)]. This result indicated an OPTOS satellite rotation of almost 50 deg around the
Z-axis (nadir). This was an expected OPTOS limitation due to the lack of active ADCS.
After the results obtained from the analysis of this first image, APIS continued taking images
of the same size. The integration time was adjusted because the movement of the satellite could
produce motion-blur in the images. Figure 19 shows another three APIS camera images: (a) a
view of the Ontario area, taken on March 18 at a temperature of 47.3°C; (b) Canadian Artic area,
taken on April 1 at a temperature of 35.2°C; and, (c) Canadian Artic area, taken on April 3 at
a temperature of 34.4°C, all in 2016.
Figure 20 shows two more APIS images: (a) North of Russia, taken in May 27 (26.8°C),
with 483 ×632 pixels (Earth swath of 104 km ×136 km) and 216 m resolution at 644 km
orbit height; and (b) the last image of the APIS camera taken in Northern Canada in
September 10 at a temperature of 32°C. The communication with the OPTOS satellite was
lost in September 19, 2016 after almost 3 years in orbit.
7 Conclusions
The OPTOS CubeSat could be a suitable platform for simple Earth observation payloads, keep-
ing in mind that its main purpose was technological research. The APIS camera, in turn, has
proven to be a robust and valuable instrument. As a result, INTA has already begun the develop-
ment of a new APIS camera to support an experimental mission based on CubeSat to measure the
quality of inland waters. The lessons learned from the OPTOS mission will be used to improve
Fig. 17 (a) APIS camera first image and (b) Google Earth location of the zone.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-18 JulSep 2019 Vol. 13(3)
the APIS performance in order to produce useful Earth observation data on such missions and
also to pose and test new technological questions.
The use of a passive athermalization mechanical system in cameras with a small #F has been
experimentally checked in the laboratory and validated in an LEO, through the APIS images,
allowing an operative temperature range greater than 40 K.
Fig. 18 (a) Georeferenced APIS image and (b) MODIS single image.
Fig. 19 (a) Ontario area, (b) Canadian Artic area, and (c) Canadian Artic area.
Fig. 20 (a) North of Russia and (b) the last image of APIS in Northern Canada.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-19 JulSep 2019 Vol. 13(3)
The work described in Sec. 3guaranteed the performance of the IBIS5-B-1300 CMOS
sensor on a 2-year Earth observation mission but not for the three commercial glass types used
in the APIS camera design. The data obtained in flight about the real accumulated radiation
dose (almost 102times smaller), the minimum transmission loss of the APIS camera glass
(around 1% theoretically), and the images taken with the APIS camera have demonstrated the
viability of using these COTS elements in 2-year Earth observation missions without perfor-
mance loss.
Acknowledgments
The Centro para el Desarrollo Tecnológico Industrial (CDTI) supported the initial A & B phases
of the OPTOS project by financing SENER and THALES Companies. The authors would like to
thank Manuel Pérez for the APIS electronic design, Javier Villanueva from Acctiva-Norinstal,
S.L. for the APIS mechanical design, and Eduardo Ballesteros and Antonio Liébana from C. A.
Asociados, S.L. in charge of the optomechanical manufacturing and assistance with the APIS
optomechanical assembly. The authors also thank Concepción G. Alvarado for the specification
and design of the bandpass filter and Elisa Muñoz, Amaia Santiago, and César Arza for their
assistance during the APIS payload operation phase. Finally, the authors thank Antonio Sánchez,
Gonzalo Ramos, Javier Iglesias, Marcos J. Michavila, and Manuel Silva for their support in
the development of this instrument.
References
1. A. Mehrparvar, CubeSat design specification,in The CubeSat Program, CalPoly SLO,
San Luis Obispo (2014).
2. I. Lora et al., INTA picosatellite OPTOS: mission, subsystems, and payload,in 4th Ann.
CubeSat Dev. Workshop, Huntington Beach (2007). http://mstl.atl.calpoly.edu/~bklofas/
Presentations/DevelopersWorkshop2007/Lora_Ivan.pdf.
3. S. Esteve et al., Small satellite platforms for space environment and effects monitoring,in
SEENoTC Workshop Data Sharing, Exp. Flight Oppor. and Lessons Learn., Toulouse
(2008).
4. C. Cazorla et al., Attitude determination and control strategies for the OPTOS picosatel-
lite,in ISU Alumni Conf., Barcelona (2008).
5. INTA, Optical wireless for intra-spacecraft communications,in CCSDS Fall Meeting
Wirel. Working Group, Berlin (2008). https://cwe.ccsds.org/sois/docs/SOIS-WIR/AAA
%20Meeting%20Materials/2008/Fall/CCSDS%20-%20BERLIN%202008%20-%20Optical
%20Wireless%20for%20intra%20Spacecraft%20Communications.pdf.
6. J. M. Encinas, OPTOS STM - results and satellite validation,in 6th Ann. CubeSat Dev.
Workshop, San Luis Obispo (2009). http://mstl.atl.calpoly.edu/bklofas/Presentations/
DevelopersWorkshop2009/5_Encinas-OPTOS.pdf.
7. P. Cabo et al., OPTOS: a pocket-size giant (mission, operation, and evolution),in Proc.
AIAA/USU Conf. Small Satell., The Smaller Elements, SSC09-X-11, Logan (2009). https://
digitalcommons.usu.edu/smallsat/2009/all2009/67/.
8. G. Albaladejo et al., Highly efficient CubeSat platform: project OPTOS,in 59th Int.
Astronaut. Congr. (IAC), Glasgow, IAF Ed. Proc. IAC, Vol. 7, 08.B4.6.A3, pp. 41834191
(2008).https://iafastro.directory/iac/archive/browse/IAC-08/B4/6.A/2081/.
9. C. Martínez et al., OPTOS project: new generation of innovative satellites,in 4S Symp.
Small Satell. Syst. Serv., Funchal (2010).
10. C. Arza et al., OPTOS: STM result, satellite validation and future evolution,in 60th Int.
Astronaut. Congr., Daejeon, IAF Ed. Proc. IAC, Vol. 5, IAC-09.B4.1.8, pp. 34603470
(2009). https://iafastro.directory/iac/archive/browse/IAC-09/B4/1/5391/.
11. V. M. Aragón et al., Researching a robust communication link for CubeSat: OPTOS, a
new approach,in SPACOMM 2011: Third Int. Conf. Adv. in Satell. and Space Commun.,
Budapest, pp. 4550 (2011). http://www.thinkmind.org/download.php?articleid=spacomm_
2011_3_20_30059.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-20 JulSep 2019 Vol. 13(3)
12. C. Martinez, OPTOS,in 5th CubeSat Dev. Workshop, San Luis Obispo (2008). http://mstl
.atl.calpoly.edu/bklofas/Presentations/DevelopersWorkshop2008/2-OPTOS-Cesar_Martinez
.pdf.
13. Cypress Semiconductor Corporation, Document #: 38-05710 Rev. *A, San Jose (2007).
14. M. Silva-López et al., Analysis and evaluation of the Full Disk Telescope refocusing
mechanism for the Solar Orbiter mission,Opt. Eng. 54(8), 84104 (2015).
15. M. Fernández-Rodríguez et al., Modeling of absorption induced by space radiation on
glass: a two-variable function depending on radiation dose and post-irradiation time,
IEEE Trans. Nucl. Sci. 53(4), 23672375 (2006).
16. M. Fernández-Rodríguez, De los efectos del ambiente espacial en las propiedades ópticas
de vidrios y recubrimientos para sistemas espaciales, Chapters 4 and 7, INTA Ministerio de
Defensa, Madrid (2017).
17. J. I. Vette, Trapped radiation environment model program (19641991),in NASA/Natl.
Space Sci. Data Center, NSSDC/WDC-A-R&S 9129, Greenbelt (1991). https://ntrs.nasa
.gov/archive/nasa/casi.ntrs.nasa.gov/19930001815.pdf
18. M. A Xapsos et al., Probability model for worst case solar proton event fluencies,IEEE
Trans. Nucl. Sci. 46, 14811485 (1999).
19. M. A Xapsos et al., Probability model for cumulative solar proton event fluencies,IEEE
Trans. Nucl. Sci. 47, 486490 (2000).
20. M. A. Xapsos et al., Model for cumulative solar heavy ion energy and linear energy transfer
spectra,IEEE Trans. Nucl. Sci. 54(6), 19851989 (2007).
21. Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT),
http://www.ciemat.es/sweb/SEPA/Instalaciones/Html/Pdf/93.pdf.
22. A. Virtanen, Radiation effects facility RADEF,in Proc. Eighth IEEE Int. On-Line Test.
Workshop (IOLTW 2002), Isle of Bendor, p. 188 (2002).
23. T. L. Williams, The Optical Transfer Function of Imaging Systems, Chapter 10, Institute
of Physics Publishing, London (1999).
24. G. D. Boreman, Modulation Transfer Function in Optical and Electro-Optical Systems,
SPIE Press, Bellingham (2001).
25. National Aeronautics and Space Administration (NASA), Land processes distributed active
archive center,https://lpdaac.usgs.gov/data_access/data_pool.
Daniel Garranzo received his BS degree in physics in 2002, with a specialization in physical
and control devices, and his MS degree in 2006 from the Complutense University of Madrid
(UCM). In 2004, he joined the Spanish National Institute for Aerospace Technology (INTA) as
an optical engineer. His experience includes optical metrology, alignment, verification and inte-
gration of space optical instrumentation, and qualification of devices and materials for space
applications.
Armonía Núñez received her BS degree in physics (optics) from the University of Zaragoza in
1989. Since 1994, she has been working as an optical engineer in the area of space optics at the
National Institute of Aerospace Technology (INTA). Her experience includes optical design,
integration, and verification of space optical instrumentation, and characterization and modeling
of optical materials exposed to space environments.
Hugo Laguna received his senior technician in electronic product development degree at the
IES Madrid in 1998 and graduated in optics and optometry at the UCM-Faculty of Optics in
2012. Since 1998, he has been working as an electro-optical engineer at the INTA, first in
the Department of Aeronautical Programs, and from 2008 to the present in the area of space
optics.
Tomás Belenguer has been a member of the INTA technical staff since 1995. He has developed
a career in optical engineering applied to industrial and space optical systems and equipment. He
has more than 20 years of experience in design, integration, and characterization of optical
instrumentation and more than 15 years of experience in space programs. Since 1986, he has
belonged to the Laboratorio de Instrumentación Espacial and he is currently the head of the
Spatial Optics Area at INTA.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-21 JulSep 2019 Vol. 13(3)
Eduardo de Miguel is the head of the Remote Sensing Systems Group at the INTA. He has been
working since 1992 in spaceborne and airborne remote sensing projects. Most of his work has
been related to specification, implementation, and validation of processing and archiving facili-
ties, including HW/SW issues, processing algorithms, and user services. He has also participated
in the definition of space and ground segment requirements for a number of Earth observation
missions.
María Cebollero received her BS degree in physical sciences in 2005 from the Autonomous
University of Madrid (UAM) in the specialty of applied sciences. Since 2006, she has been
working at the INTA in space systems engineering, developing tasks as satellite operator, thermal
engineer, and AIV responsible person.
Sergio Ibarmia received his BS and MS degrees in atomic, nuclear, and molecular physics in
2003 from the UCM. In 2005, he joined to the INTA as a radiation engineer in the Space
Programs Department. His work at INTA has included radiation-shielding design for space
missions, radiation hardness assurance and component testing, and development of high-energy
particles simulation techniques.
César Martínez has been the head of the Small Satellites Program at INTA since 2009. He
earned his BSc degree in aerospace engineering from Polytechnic University of Madrid in
1993, his MSc degree in physics from Open University in 1998, and his PhD in telecommu-
nications from the University of Vigo in 2016. He has wide experience in management and
engineering of space programs.
Garranzo et al.: APIS: the miniaturized Earth observation camera on-board OPTOS CubeSat
Journal of Applied Remote Sensing 032502-22 JulSep 2019 Vol. 13(3)
... This FOV was equivalent to a rectangular Earth swath between 128×97km and 170×129km. It also provided a resolution between 201 and 268m, depending on the orbit height [44]. ...
Conference Paper
Full-text available
There are currently projects implementing nanosatellites or constellations of the CubeSat class nanosatellites. This type of platform is growing at the speed of light, it has several advantages in terms of cost, deployment, and launch, it paves the way for the development of a multitude of missions, such as Earth observation EO, astronomical, environmental, and weather missions. CubeSats are used for technological and scientific demonstration missions but also constitute a training tool in various and varied fields. The enthusiasm of universities and space companies for these nanosatellites on the market is rapidly increasing. This lightweight spacecraft use increasingly compact imagers for image acquisition and Earth observation missions. Indeed, the technological advances made to optical sensors, particularly in terms of the miniaturization of electronic components and optical systems such as CMOS detectors, filters, and electronic modules, have led to the appearance of new solutions. When selecting an imager for a spacecraft mission, it is important to know several parameters, such as the technical specifications, and performance criteria, in terms of spatial, spectral, and radiometric resolutions and swath. The choice of an imager in a satellite mission, whether onboard as a main or secondary payload, must be justified by meeting the defined mission requirements. Therefore, the importance of drawing up a state of art, and grouping together all the optical cameras developed and embedded to date would be an essential tool for CubeSat designers. This paper presents a survey of miniaturized optical cameras, defining the technical specifications of the imagers in the context of a mission analysis of EO CubeSat missions. The work provides guidelines for future CubeSat missions, the performances and constraints of the cameras are presented in such a way as to serve for the choice and decisions according to the requirements fixed initially by the mission.
Article
Full-text available
A probabilistic model of cumulative solar heavy ion energy and LET spectra is developed for spacecraft design applications. Spectra are given as a function of confidence level, mission time period during solar maximum and shielding thickness. It is shown that long-term solar heavy ion fluxes exceed galactic cosmic ray fluxes during solar maximum for shielding levels of interest. Cumulative solar heavy ion fluences should therefore be accounted for in single event effects rate calculations and in the planning of space missions.
Article
A new model of cumulative solar proton event fluences is presented. It allows the expected total fluence to be calculated for a given confidence level and for time periods corresponding to space missions. The new model is in reasonable agreement with the JPL91 model over their common proton energy range of > 1 to > 60 MeV. The current model extends this energy range to > 300 MeV. It also incorporates more recent data which tends to make predicted fluences slightly higher than JPL91. For the first time, an analytic solution is obtained for this problem of accumulated fluence over a mission. Several techniques are used, including Maximum Entropy, to show the solution is well represented as a lognormal probability distribution of the total fluence. The advantages are that it is relatively easy to work with and to update as more solar proton event data become available.
Article
The Full Disk Telescope is part of the Polarimetric Helioseismic Instrument on board the future Solar Orbiter ESA/NASA mission. It will provide full-disk measurements of the photospheric magnetic field vector and line-of-sight velocity, as well as the continuum intensity in the visible wavelength range. Along this mission, it is expected that thermal drifts will induce image focus displacements. Consequently, providing an autofocus system is mandatory to prevent image degradation. The refocusing system is based on an autonomous image contrast analysis and it allows for a lens displacement in order to locate the best focus position. The figure of merit chosen for the image quality evaluation is presented. The influences of attitude instability and mechanical uncertainties are considered in a refocusing process simulation. In addition, an engineering model of the mechanism is tested at flight operating conditions. To check its performance, an optical interrogation system is set up. Determination of accuracy and repeatability of the mechanism positioning is experimentally evaluated and discussed according to the ISO standard. The results show that the proposed refocusing system is sufficiently robust against the expected image shifts and mechanical instabilities. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE).
Article
This paper presents a brief introduction to the concepts of the NATO Standard on the OPTICAL Transfer function of imaging systems.
Article
The major effort that NASA, initially with the help of the United States Air Force (USAF), carried out for 27 years to synthesize the experimental and theoretical results of space research related to energetic charged particles into a quantitative description of the terrestrial trapped radiation environment in the form of model environments is detailed. The effort is called the Trapped Radiation Environment Modeling Program (TREMP). In chapter 2 the historical background leading to the establishment of this program is given. Also, the purpose of this modeling program as established by the founders of the program is discussed. This is followed in chapter 3 by the philosophy and approach that was applied in this program throughout its lifetime. As will be seen, this philosophy led to the continuation of the program long after it would have expired. The highlights of the accomplishments are presented in chapter 4. A view to future possible efforts in this arena is given in chapter 5, mainly to pass on to future workers the differences that are perceived from these many years of experience. Chapter 6 is an appendix that details the chronology of the development of TREMP. Finally, the references, which document the work accomplished over these years, are presented in chapter 7.
Conference Paper
Since the first days of the space conquest, electronic components have changed remarkably. In the seventies, single event effects (SEE), caused by heavy ions and protons, were unknown. The increase in integration density led first to single event upsets and later, with the CMOS technology, single event latchups etc... The end of the cold war crashed the military market and, since that, the growing acceptance of COTS components in space systems has encouraged the major manufacturers to withdraw from the RadHard component production. Therefore, one had to start testing of the SEE durability of COTS components with particle accelerators.
Article
NBK7 and FK51 glass from SCHOTT commonly employed on space optical instrumentation design have been gamma irradiated in order to relate the color centers generated by effect of the radiation to the changes in their optical properties. The effects of gamma radiation on the glass optical properties have been analyzed from transmission measurements and ellipsometric characterization. The absorption bands induced on glass by gamma-radiation were included in the optical model by Gaussian absorption peaks. The variations obtained of the real part of the refractive index and surface roughness were negligible. An exponential function is proposed to describe the absorption increase when the total dose rises. Additionally, a study of the amplitude decrease of these absorption bands with the time is carried out in order to determine relaxation functions. Finally, a two variable function-absorption peak amplitude versus total dose and relaxation time-is proposed in order to link the exponential growth of the absorption peak amplitude with the relaxation function of the absorption bands. The model proposed provides a description of the possible changes produced in the glass optical properties by effect of gamma radiation and predicts the glass behavior with the time
Article
A predictive model of worst case solar proton event fluences is presented. It allows the expected worst case event fluence to be calculated for a given confidence level and for periods of time corresponding to space missions. The proton energy range is from >1 to >300 MeV, so that the model is useful for a variety of radiation effects applications. For each proton energy threshold, the maximum entropy principle is used to select the initial distribution of solar proton event fluences. This turns out to be a truncated power law, i.e., a power law for smaller event fluences that smoothly approaches zero at a maximum fluence. The strong agreement of the distribution with satellite data for the last three solar cycles indicates this description captures the essential features of a solar proton event fluence distribution. Extreme value theory is then applied to the initial distribution of events to obtain the model of worst case fluences.
CubeSat design specification
  • Mehrparvar
A. Mehrparvar, "CubeSat design specification," in The CubeSat Program, CalPoly SLO, San Luis Obispo (2014).