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The Caltech Space Solar Power Demonstration One
A. Fikes (aﬁkes@caltech,edu)*, E. Gdoutos (email@example.com)†, M. Klezenberg (firstname.lastname@example.org)⋄
A. Ayling*, O. Mizrahi*, J. Sauderx, C. Sommer†, A. Truong†, A. Wen†, A. Wu*, R. Madonna‡,
H. Atwater⋄, A. Hajimiri*, S. Pellegrino†
Abstract—This paper describes Caltech’s Space Solar Power
Demonstration One (SSPD-1) payload and upcoming mission
on Momentus Space Vigoride 5. SSPD-1 is comprised of three
experiments each of which demonstrates the performance of a
key technology piece in the space environment. We describe the
goals of SSPD-1. The three experiments - Alba, DOLCE and
MAPLE are discussed. The launch of SSPD-1 is scheduled for
November 6, 2022 on Space X’s Transporter 6 mission.
Index Terms—Space solar power, Photovoltaic devices, wireless
power transmission, deployable structures
The Caltech Space Solar Power Project (SSPP) is a multi-
year, multi-disciplinary research and development effort on
space solar power with the goal of achieving an engineering
and economically feasible system to power terrestrial electrical
energy from energy gathered in space. A key tenet of SSPP
to minimize the mass on-orbit. This tenet leads to a unique
instantiation of the space power station as describe in  and
. Fig 1 depicts our concept.
Fig. 1. SSPP employs a modular, scaleable architecture based on an ultra-light
weight unit of functionality called a tile.
The three principle authors are listed ﬁrst alphabetically. Contributing
authors are list alphabetically. The three Principle Investigators are listed last
in alphabetical order.
* - Dept. of Electrical Engineering, California Institute of Technology,
†- Graduate Aerospace Laboratories, California Institute of Technology,
⋄- Dept. of Applied Physics, California Institute of Technology, Pasadena,
Jet Propulsion Laboratory, Pasadena, U.S.A.
‡- System Engineering Consultants, Columbia, U.S.A.
The tile is a multilayered structure with photovoltaic (PV)
material on both surfaces, antennas underneath one of the
PV layers and a layer hosting CMOS integrated circuits and
routing for reference signals and timing for phase control over
the antennas and DC to microwave power conversion. The tile
has all the functionality needed to convert solar energy into
microwave energy and radiate that energy to a desired location.
The tiles are fabricated into strips of lengths ranging from a
few meters to 60 meters, and these are laid up into a carbon
ﬁber structure that is attached to a deployment mechanism
which, in turn is attached to a spacecraft. The carbon ﬁber
structure supports the active, ﬂexible strips and enables the
strips to be folded and coiled into the deployment mechanism
for launch stowage. Our current space vehicle design has a
mass of approximately 430 kg. A power station is made up
of many space vehicles either mechanically attached by their
booms or autonomously formation ﬂying.
One of the intermediate objectives of SSPP is to demonstrate
the technologies central to our concept  in space. Space
demonstrations reduce risk by verifying that the technologies
perform in the environment they are designed to operate in and
demonstrates that the functional interfaces within the system
operate correctly. We envision a series of demonstrations of
increasing complexity to gain further conﬁdence in the design
and scalability of the technologies. Our ﬁrst such demon-
stration is Space Solar Power Demonstration One (SSPD-1).
We note that there was recent space demonstration dedicated
to space solar power led by P. Jaffe . Jaffe’s ”sandwich”
module was hosted on the U.S. Air Force X-37B space plane
and spent over a year in low earth orbit.
We established several ground rules at the start of SSDP-
1. First, the payload consists of three independent experi-
ments so that each technology could be individually tested.
By decoupling the dependencies that occur if we were to
build and ﬂy a scaled integrated demonstrator, we can verify
the performance of the core technologies without potentially
confounding factors due to inter-dependencies. Second, we
execute the development, assembly, integration and test of
SSPD-1 to NASA Class C/D mission standards . Our
mission is driven by technical objects (Class C), but we have
a higher risk tolerance than other classes (Class D), relatively
low complexity (Class D), and have programmatic constraints
(Class D). Operating as a Class C/D mission, enables us
to speed development by not having to comply with many
standards and TORs found in more mission critical payload
development programs. We still maintain rigorous testing
requirements to ensure safety of ﬂight and mission success.
Third, the individual payload components are developed in
parallel holding to their own internally managed schedules
that are anchored to a key milestone schedule taking us from
inception to launch. Fourth, SSPD-1 is a hosted payload on
a satellite. The hosting option saves us from having to buy
a satellite and all the services, including launch, necessary to
operate on orbit. This allows us to focus our small team on
developing and delivering the key payload components and
performance that are relevant to SSPP. This tenet lead us to
engage with Momentus Space for a hosted payload on their
II. SSPD-1 PAYLOAD DESCRIPTION
SSPD-1, depicted in Fig. 2, is a hosted payload comprised
of three experiments and a supporting avionics unit. In keeping
with our ground rules, we have partnered with Momentus
Space to host SSPD-1 on their Vigoride spacecraft scheduled
to launch on the Falcon 9 Transporter mission in November
2022. SSPD-1 will spend 7 months in a low Earth, sun
synchronous orbit at an altitude of approximately 500 km.
Six of the seven months is dedicated to the SSPD-1 mission.
Fig. 2. Rendering of the SSPD-1 payload components on Vigoride 5.
(Courtesy of Momentus Space)
Each of the three experiments addresses one of the key
technologies being developed in SSPP . Alba is an exper-
iment dedicated to characterizing research and developmental
photovoltaic (PV) devices. The Deployable on-Orbit uLtra-
Light Composite Experiment (DOLCE) will demonstrate Cal-
tech’s stowage and deployment technology and characterize
the structure’s ﬂatness and response to disturbances at a 1.7
m x 1.7 m scale. The Microwave Array for Power Transfer
LEO Experiment (MAPLE), employs our custom CMOS radio
frequency integrated circuits (RFICs) and ﬂexible array tech-
nology to perform wireless power transfer (WPT) experiments.
The three experiments are supported by an Avionics suite.
Alba’s objective is to characterize the performance of re-
search solar cell samples on orbit, with several classes of
emerging PV technologies represented among 32 test devices.
Alba (see Fig. 3) hosts devices ranging from approximately
3 mm on a side up to 20 mm x 20 mm in size. The
manifest of PV devices includes ﬂexible ultralight perovskite
cells, rigid perovskite cells, InP and GaAs nanowire cells,
diffused-junction III-V cells, luminescent solar concentrator
(LSC) cells, CIGS cells, thin Si cells, and a conventional
triple-junction III-V cell as a control device. Some cells were
sourced from Caltech research efforts, but the majority were
provided by a broad range of collaborators at other academic,
institutional, or commercial research labs.
Fig. 3. Alba interior (top) and ﬂight cover (bottom)
Each PV device is mounted on a cell carrier circuit board
assembly. The underlying instrumentation circuitry is that of
the Aerospace Measurement Unit (AMU) developed by the
Aerospace Corporation, which has ﬂight heritage including
high-altitude balloon tests and cubesat missions.   Each
cell carrier comprises an independent AMU circuit, containing
a microprocessor, DAC, ADC, and other electronics to enable
measurement and recording of the cells’ current-voltage (I-
V) characteristics (See Fig. 4) To maximize the number of
cells that could be included in the experiment, three different
form factors of the AMU circuits were fabricated, which
work in conjunction with cell-speciﬁc interposer PCBs to
provide mounting pads for each cell technology. All cell carrier
positions include temperature sensors located beneath the cells,
and resistive heaters for optional thermostatic regulation of the
cells at elevated temperatures. The outermost (8) cell carriers
also contain sun angle sensors to enable precise calibration of
the results. The circuits are operated over an I2C bus controlled
by the the ﬂight computer for data acquisition and down link
to the ground. For risk diversiﬁcation, the 32 devices are split
into two banks of 16, each with separate I2C and power buses.
Alba is able to collect data once the Vigoride spacecraft has
completed its commissioning tasks and is declared operational.
There is approximately one month of time that Alba will
have to collect data before the DOLCE payload is deployed.
During that month, we cannot impose pointing requirements
on Vigoride as it is servicing other customers and performing
some engineering tests, so any data we collect will be on
an opportunistic basis. Alba will essentially free run and
collect data during this time. The ﬂight computer will use
Fig. 4. A sample of a characterization circuit board with a PV device installed.
Alba’s sun angle sensor data to determine if a given data set
should be saved for down link or discarded. Once Vigoride
is dedicated to SSPD-1 and DOLCE deploys, Alba will
experience shadowing on many of its samples when the sun is
at normal incidence to the deck (Fig. 5). to compensate for the
shadowing, Vigoride will make attitude adjustments insuring
all the PV samples are illuminated every day. Fig. 5 shows the
preferred attitude for the Vigoride deck and the PV samples
that receive illumination at each angle. Green indicates near
normal incidence for sunlight, red indicates shadowing, and
yellow and orange indicate less the desirable incidence angles.
Vigoride has a pointing accuracy of less than 5 °so Vigoride
should provide adequate pointing.
Fig. 5. Preferred Vigoride attitude to mitigate shadowing effects on Alba.
Alba has a mass of approximately 3.6 kg and is housed
in a 494 mm long x 110 mm wide x 100 mm tall structure.
The experiment will operate throughout most phases of the
ﬂight, collecting data opportunistically when the cells are illu-
minated. During phases when Alba is the primary experiment,
the Vigoride spacecraft will adjust its attitude relative to the
sun to provide continuous normal-incidence illumination on
the test cells.
DOLCE’s objectives are to demonstrate that the deployment
mechanism, which is central to the SSPP packaging concept
, functions properly in the space environment, and to
understand the shape and behavior of the deployed, ultralight
weight carbon ﬁber structure in zero-g after deployment and
when subject to disturbances such as spacecraft vibration due
to maneuvering or thermal gradients on the structure.
Figure 6 depicts DOLCE in the stowed conﬁguration. The
structure is folded and coiled inside of the mechanism. Starting
at the top of the mechanism, there is a camera and LED light
assembly that is erected vertically on three carbon ﬁber booms.
The cameras provide both video and still imagery for use in
characterizing the structure during and after deployment. There
is also a camera on the lid of the Avionics Box with a wide
angle lens viewing toward the zenith relative to the payload
deck. This camera can be used to capture deployment imagery
and is the backup camera for the experiment should the camera
boom fail to deploy. Below the camera and light assembly
is a vertical aluminum plate which is the back side of one
of the rollers that enable the structure to uncoil. The rollers
are hinged at the bottom and, when deployed, rotate about
the hinge permitting the structure to unfold. Kapton sheets
(orange ﬁlm) apply a constraining pressure on the structure as
it is uncoiled during deployment to better control the uncoiling
and to ensure the structure remains folded until the rollers are
deployed. There are four diagonal booms (two are visible in
Fig. 6) which deploy prior to uncoiling the structure. These
booms support a cord attached to the structure. The cord
supplies a constant tension on each corner of the structure
to facilitate a smooth uncoiling. Note that the cords do not
pull the structure out of the coil, the structure is pushed out
by motors driving the rollers.
Fig. 6. DOLCE in stowed conﬁguration. Two of the four diagonal booms are
visible on the lower portion of DOLCE.
The DOLCE deployment sequence is shown in Figures 7
- 9. The DOLCE ﬂight model is being deployed in the
laboratory prior to going into environmental test. Figure 7A
shows DOLCE in stow conﬁguration. The camera boom is
then deployed (Fig. 7). Imagery from the four cameras is taken
and down linked for veriﬁcation of the deployment and the
next phase of deployment begins. The four diagonal booms
are extended to about half of their ﬁnal length (Fig. 8 A), and
again imagery is captured and down linked. The structure is
then unrolled by ﬁve motors inside the deployment mechanism
(Fig. 8B), with imagery being taken and sent to the ground.
Once the uncoiling stops, the booms are extended to their
full length (Fig. 9 A). Finally, the rollers on the deployment
mechanism are released allowing the 1.7m x 1.7m structure
to unfold (Fig. 9 B). The deployment is captured by video for
later analysis. This completes the deployment.
Fig. 7. DOLCE in stowed conﬁguration (A)and with camera boom deployed
Fig. 8. Diagonal boom extension (A) and structure uncoiling (B).
Fig. 9. Full extended booms (A) and structure deployment (B).
DOLCE weighs approximately 33 kg and measures 730 mm
in height, 355 mm in length and 355 mm in width in its stowed
MAPLE’s objectives are to demonstrate beam focusing
and steering using the SSPP developed CMOS RFICs and
ﬂexible antenna arrays in the space environment . MAPLE
is contained in a 6U cubesat frame and weighs approximately
2.6 kg. The payload with its MLI blankets is depicted in
Fig. 10). Figure 11 shows the interior of MAPLE. A 32 dipole
antenna ﬂexible array, driven by two CMOS RFICs, is located
at one end of the 6U frame along with a camera. The array
radiates microwave energy at 9.884 GHz. A microprocessor
and reference signal generator are co-located with the CMOS
RFICs. The microprocessor provides command and control for
the experiment and communications with the ﬂight computer
for data transfer, telemetry and commanding. At the opposite
end of the 6U frame is an aperture covered by a sapphire
window and a rectenna array. Along the right side of the
frame near the aperture is another rectenna array and camera
is mounted on the wall opposite from the rectennas.
Fig. 10. MAPLE payload with low emissivity covering. Sapphire viewing
window is visible.
Fig. 11. MAPLE interior showing the Tx array and two rectenna arrays.
The purpose of the rectenna arrays is to demonstrate the
beam steering and focusing capabilities of the transmitting
array and processor. At various times during the ﬂight, the
array will be powered on and the beam focused on one of
the rectenna arrays which measures the beam pattern. The
cameras are used to record the lighting of an LED attached to
the rectennas to offer photographic evidence that power was
Fig. 12. Flight Avionics Box with various connectors labled. Note camera
on the top of the box.
transmitted. MAPLE has several experimental modes intended
to test different attributes of the transmitter hardware. These
experimental modes are intended to test sustained operation,
repeatability of array focusing, and system aging.
D. Avionics and Software
All the SSPD-1 components are supported by an avion-
ics suite (Fig. 12) consisting of a Xiphos Q7 single board
computer (SBC), three Raspberry Pi processors for camera
control, ﬁve motor controllers for DOLCE, two custom inter-
face boards, and a GOMSpace electrical power system. The
SBC interfaces with Momentus’ ﬂight computer to receive
commands and pass back telemetry and mission data via
an RS-485 interface. The SBC transmits commands to the
other SSPD-1 components and receives their telemetry and
mission data. The computer also controls the power supply
and can selectively power on or off the other three payload
Our ﬂight software is based on ASI Inc.’s MAX framework.
MAX is hosted on the SBC running on a Linux operating
system. Our ﬂight software has approximately 150,000 lines of
code. MAX provides tools for fault detection and remediation,
command and control, maintaining multiple boot images,
uploading new software from the ground, and performing all
the other functions needed to make the payload operate as
SSDP-1 has gone through environmental testing and inte-
gration to the Vigoride 5 space vehicle. Vigoride 5 has been
transported to the Space X processing facility at Kennedy
Space Flight Center at Cap Canaveral, Florida, where it will be
mounted onto a port on the EPSA (EELV Payload Secondary
Adapter) ring of the Falcon 9 for launch on the Transporter 6
mission scheduled for November 6, 2022. .
This work is generously supported by the Caltech Space
Solar Power Project. The Alba team thanks Colin Mann and
Don Walker of the Aerospace Corporation, and the numer-
ous collaborators who contributed solar cells for the ﬂight
experiment. The DOLCE team thanks Mr. Don Baxter and
Sam Foroozan for their advice and guidance on the electrical
system design for the payload, and Mr. John Maciejewski for
his assistance in assembling the Avionics Box and DOLCE
cable harness. The MAPLE team thanks Dr. Damon Russell
for his advice on various aspects of designing and assembling
electronic payloads for space. The SSPD-1 team thanks Mr.
Derek Johnson and Mr. Szymd at ASI, Inc. for their contribu-
tion to the ﬂight software and Mr Len Day for his advice and
guidance on software development and testing. The SSPD-1
further thanks the Momentus Space team for their eagerness
and support in getting SSPD-1 to ﬂight.
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