Optical Clock Technologies for Global Navigation Satellite Systems

Abstract

Future generations of global navigation satellite systems (GNSS) can benefit from optical technologies. Especially optical clocks could back-up or replace the currently used microwave clocks, having the potential to improve GNSS position determination enabled by their lower frequency instabilities. Furthermore, optical clock technologies - in combination with optical inter-satellite links - enable new GNSS architectures, e.g. by synchronization of distant optical frequency references within the constellation using time and frequency transfer techniques. Optical frequency references based on Doppler-free spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. Compact and ruggedized setups have been developed, showing frequency instabilities at the 10^(-15) level for averaging times between 1s and 10.000s. In this paper, we introduce optical clock technologies for applications in future GNSS and present the current status of our developments of iodine-based optical frequency references.

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GPS Solutions (2021) 25:83
https://doi.org/10.1007/s10291-021-01113-2
ORIGINAL ARTICLE
Optical clock technologies forglobal navigation satellite systems
ThiloSchuldt1 · MartinGohlke1· MarkusOswald1,2· JanWüst1· TimBlomberg1· KlausDöringsho3,4·
AhmadBawamia3· AndreasWicht3· MatthiasLezius5· KaiVoss6· MarkusKrutzik3,4· SvenHerrmann2·
EvgenyKovalchuk3,4· AchimPeters3,4· ClausBraxmaier1,2
Received: 5 October 2020 / Accepted: 3 March 2021 / Published online: 5 April 2021
© The Author(s) 2021
Abstract
Future generations of global navigation satellite systems (GNSSs) can benefit from optical technologies. Especially opti-
cal clocks could back-up or replace the currently used microwave clocks, having the potential to improve GNSS position
determination enabled by their lower frequency instabilities. Furthermore, optical clock technologies—in combination with
optical inter-satellite links—enable new GNSS architectures, e.g., by synchronization of distant optical frequency references
within the constellation using time and frequency transfer techniques. Optical frequency references based on Doppler-free
spectroscopy of molecular iodine are seen as a promising candidate for a future GNSS optical clock. Compact and ruggedized
setups have been developed, showing frequency instabilities at the 10–15 level for averaging times between 1s and 10,000s.
We introduce optical clock technologies for applications in future GNSS and present the current status of our developments
of iodine-based optical frequency references.
Keywords Optical clock· Iodine reference· Space instrumentation· Future GNSS
Introduction
Over the last decades, optical clock technologies evolved,
recently demonstrating frequency instabilities at the 10–18
level for integration times of a few thousand seconds (Ushi-
jima etal. 2015; McGrew etal. 2018). While becoming more
and more widespread technology in and outside laboratories
on Earth, also space applications—including GNSS—can
benefit from the recent advancement of optical technologies.
Optical clocks surpass the performance of the currently used
GNSS microwave clocks by several orders of magnitude.
On the one hand, optical clocks could back-up or replace
the currently used microwave clocks (Droz etal. 2006), on
the other hand, optical clock technologies—in combination
with optical inter-satellite links—enable new GNSS archi-
tectures. One example is the proposed Kepler architecture
(Giorgi etal. 2019) which foresees 24 satellites in medium-
earth orbits (MEO, in three orbital planes, similar to the cur-
rent Galileo system), together with 6 satellites in low-Earth
orbit (LEO). All Kepler satellites carry optical frequency
references which are intra-system synchronized using bi-
directional optical inter-satellite links together with time and
frequency transfer techniques. In the current baseline, the
satellites are equipped with frequency references based on
optical resonators, providing the required high short-term
stability where the round-trip time of the light within one
orbital plane (of about 0.1s) is relevant. The LEO satellites
additionally carry mid- to long-term stable absolute opti-
cal clocks based on Doppler-free spectroscopy of molecular
iodine for the definition of the system time.
In the following, we first give a short overview of opti-
cal clock technologies for space, together with their current
technology development status. We then detail our develop-
ments with respect to iodine-based optical frequency ref-
erences for applications in space and present the mission
* Thilo Schuldt
Thilo.schuldt@dlr.de
1 German Aerospace Center (DLR), Institute ofSpace
Systems, Bremen, Germany
2 Center ofApplied Space Technology andMicrogravity,
University ofBremen, Bremen, Germany
3 Ferdinand-Braun-Institut gGmbH, Leibniz-Institut Für
Höchstfrequenztechnik, Berlin, Germany
4 Institute ofPhysics, Humboldt-Universität zu Berlin, Berlin,
Germany
5 Menlo Systems GmbH, Martinsried, Germany
6 SpaceTech GmbH, Immenstaad, Germany
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concept COMPASSO, a proposed DLR in-orbit verification
mission on the International Space Station (ISS).
Please note that we refer to a “clock” for a system which
delivers a countable frequency signal in the radio frequency
range, and where a time signal can be derived from, as it is
e.g., the case in GNSS applications. When being based on an
absolute and known frequency, such a system is often also
referred to as a “frequency standard”.
Optical clock technologies forspace
applications
Optical clock technologies include a variety of different
implementations, all having their own assets and drawbacks.
Looking at space applications, in addition to the demon-
strated performance such as frequency stability and accu-
racy, it is necessary to consider the size, weight, power con-
sumption, robustness, and reliability of the optical reference.
Ultimate frequency stability at the 10–18 level is shown
by optical lattice and single-ion clocks in rather complex
laboratory setups (Ushijima etal. 2015; McGrew etal.
2018; Delehay and Lacroute 2018). Technology develop-
ment with respect to transportable setups has been initiated
(Koller etal. 2017; Cao etal. 2017; Brewer etal. 2019; Han-
nig etal. 2019), and a compact setup of a 88Sr lattice clock
has been realized, where space-related design criteria have
been considered (Bongs etal. 2015; Origlia etal. 2018).
Such optical clocks require several lasers, a vacuum chamber
and a cavity pre-stabilization of the clock laser to achieve
their outstanding frequency instability enabled by millihertz
linewidth transitions.
Optical atomic beam standards, e.g., using Ca or Sr atoms
show a lower complexity and can be realized in more com-
pact setups. However, similar to optical lattice and single-
ion clocks, they require a vacuum chamber and use cavity
pre-stabilization of the clock laser. With a compact setup of
a Ca beam standard, frequency instabilities of 1.8 × 10–15 at
an integration time of 1600s have been demonstrated (Shang
etal. 2017). A compact and ruggedized Sr beam standard
for application on a sounding rocket is currently developed
at HU Berlin (Gutsch etal. 2019).
Gas-cell-based optical frequency references have mod-
est complexity without need of a vacuum chamber or pre-
stabilization and can be realized with small dimensions,
weight and power budgets. They typically employ modula-
tion transfer spectroscopy (MTS) or frequency modulation
spectroscopy (FMS) of optical transitions with linewidths
of the order of MHz. With a compact Rubidium-based fre-
quency reference using MTS near 420nm, frequency insta-
bilities of 2.1 × 10–15 at an integration time of 80s have been
claimed, deduced from the error signal (Zhang etal. 2017;
Chang etal. 2019). Iodine-based frequency references near
532nm have been realized for many decades, resulting in
compact and ruggedized setups, also with respect to appli-
cations in space (Nyholm etal. 2003; Leonhard and Camp
2006; Zang etal. 2007; Argence etal. 2010; Schuldt etal.
2017; Döringshoff etal. 2017), showing frequency insta-
bilities at the 10–15 level for integration times between 1
and 1000s. A very compact setup has been successfully
flown on a sounding rocket, together with a frequency comb
(Schkolnik etal. 2017; Döringshoff etal. 2019). Frequency
stabilization to iodine transitions near 515nm is investi-
gated within the Japanese proposed space gravitational wave
detector DECIGO (Deci-Hertz Interferometer Gravitational
Wave Observatory) where a compact setup has been realized
(Suemasa etal. 2017) and by a French collaboration using
a frequency tripled output at 514nm of Telecom laser tech-
nology at a wavelength near 1542nm (Barbarat etal. 2018).
The Rb two-photon transition (TPT) at 778nm is often
used as a frequency standard, providing Doppler-free spec-
troscopy. No laser pre-stabilization and no vacuum chamber
are required. While several setups have been realized in the
past, recent developments include a compact setup for appli-
cations as a successor to the atomic frequency standard in
GPS with demonstrated frequency instabilities at the 10–15
level (Martin etal. 2018). Furthermore, an integrated Rb
clock has been realized, using a micro-fabricated rubidium
gas cell in combination with a microcomb (Newman etal.
2019; Maurice etal. 2020).
Optical clocks require a frequency comb to transfer the
stability of the clock laser to a radio frequency and thus to
provide a countable clock signal. Developments for space
applications are already initiated, and compact frequency
combs have been successfully flown on a sounding rocket
(Lezius etal. 2016; Döringshoff etal. 2019; Pröbster etal.
2021).
Table1 summarizes the key figures of the technolo-
gies detailed above. For comparison, it also includes the
space-grade microwave references currently used on Galileo
[rubidium atomic frequency reference (RAFS) and passive
hydrogen maser (PHM)] where data are taken from the pub-
licly available data sheets. For the optical references, the
given values on performance, i.e., frequency stability, are
taken from the corresponding publications, together with
the values on size, weight and power (SWaP) budgets, if
given. As the technology development status of the tech-
nologies is quite different—ranging from transportable and
compact setups to implementations dedicated for space—the
entries cannot directly be compared. It is e.g., assumed that
within a dedicated development, the budgets of lattice and
ion clocks can be significantly reduced with respect to the
current transportable setups (Takamoto etal. 2020). Also,
long-term stability of the optical reference is often not yet
investigated. However, the summary in Table1 can be taken
as the basis for necessary trade-offs, e.g., between required
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Table 1 Summary of the key figures of the different optical clock technologies, together with the corresponding figures of the Galileo RAFS and PHM
a Values on frequency instabilities deduced from the error signal (Zhang etal. 2017).
b The values for the frequency comb are explicitly given for the optical references (10kg, 66W, 7l) (Döringshoff etal. 2019; Pröbster etal. 2021). It is assumed that the values on mass and
power consumption can be further reduced in a design upgrade of the frequency comb
c The SWaP budgets for the optical clock technologies include the laser(s)
d Estimation based on current state-of-the-art implementation techniques using a similar design as for the iodine references
e Estimation based on the QUEEN study by HU Berlin, which includes 2 ECDL-MOPA laser systems and is not yet designed for highest frequency stability at the 10–15 level
f At + 10°C baseplate; 70W at -5C baseplate
g Iodine reference successfully flown on a sounding rocket (not designed for highest performance), component level space heritage within other space missions (LISA, LISA Pathfinder, GRACE
follow-on, NGGM)
Galileo RAFS Galileo PHM Ca beam I2 MTS Rb MTS Rb TPT Sr Lattice clock Ca single ion clock
References Orolia datasheet
(2016) Leonardo data-
sheet (2017) Shang etal.
(2017)Schuldt etal.
(2017); Döring-
shoff etal.
(2019)
Zhang etal.
(2017)Martin etal.
(2018)Bongs etal.
(2015); Origlia
etal. (2018)
(Delehay and Lac-
route 2018; Cao
etal. 2017)
Frequency stabil-
ity (in RAV
@ integration
time τ)
1s 3 × 10–12 2 × 10–12 5 × 10–14 6 × 10–15 1 × 10−14a 4 × 10–13 n/s n/s
10s 1 × 10–12 3 × 10–13 2 × 10–14 3 × 10–15 4 × 10−15a 1 × 10–13 1 × 10–16 6 × 10–15
102s 3 × 10–13 7 × 10–14 5 × 10–15 2 × 10–15 3 × 10−15a 4 × 10–14 4 × 10–17 2 × 10–15
103s 6 × 10–14 2 × 10–14 2 × 10–15 2 × 10–15 n/s 1 × 10–14 1 × 10–17 6 × 10–16
104s 3 × 10–14 7 × 10–15 n/s 3 × 10–15 n/s 5 × 10–15 4 × 10–18 2 × 10–16
105s Long-term
drift < 10–10 /
year
Long-term
drift < 10–15 /
day
n/s < 2 × 10–14 n/s n/s n/s n/s
106s n/s n/s n/s n/s n/s n/s
Longest reported
(continuous)
τ (s)
1600 700,000 600 180,000 30,000 30,000
Clock transition frequency/wave-
length 6.8GHz 1.4GHz 657nm 532nm 420nm 778nm 698nm 729nm
Clock transition natural linewidth 0.4kHz 300kHz 1450kHz 330kHz 6mHz 140mHz
SWaP Budgetsb,c Mass (kg) 3.4 18.2 n/s 21 + 10b10d + 10b12e + 10b < 250 n/s
Power (W) 35 60fn/s 44 + 66b20d + 66b25e + 66bn/s n/s
Volume (l) 3.2 26.3 300 + 7b33 + 7bn/s 8e + 7b < 1000 540
Complexity # Lasers n/a n/a 2 1 1 1 5 6
Vacuum chamber Yes No No No Yes Yes
Cavity pre-stabi-
lization n/a n/a Yes No No No Yes Yes
TRL 9 9 4 4-5g 4 4 4 4
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frequency stability, SWaP budgets and robustness/complex-
ity, and also concerning the necessary time frame to develop
a space-qualified optical clock.
In Table1, also the technology maturity is assessed and
quantified by the so-called Technology Readiness Level
(TRL). The highest level (TRL9) is reached for flight-proven
components and systems with demonstrated performance
in space operation. The lowest level (TRL1) corresponds to
the observation and reporting of basic principles. Functional
verification in a typical laboratory environment corresponds
to TRL4, the full-scale engineering model with successful
environmental testing to TRL6.
At DLR and the University of Bremen, iodine-based
frequency references using modulation transfer spectros-
copy have been investigated for several years with respect
to applications in space, including missions to measure the
earth’s gravity field (Nicklaus etal. 2017) and space-borne
gravitational wave detection (Schuldt etal. 2019). Such ref-
erences can be realized compact and ruggedized, with small
dimensions, mass and power consumption. Typically, the
a10 component of the R(56)32–0 transition in 127I2 near a
wavelength of 532nm is used for frequency stabilization.
It is a standard frequency, recommended by the Interna-
tional Bureau of Weights and Measures (Bureau Interna-
tional des Poids et Mesures,BIPM) with a relative standard
uncertainty of 8.9 × 10–12 (Riehle etal. 2018). As the laser
is operated at 1064nm, iodine-based frequency references
can rely on space heritage of the laser and laser components,
developed e.g., within the missions LISA (Laser Interferom-
eter Space Antenna), LISA Pathfinder and GRACE (Grav-
ity Recovery and Climate Experiment) follow-on as well as
mission concepts developed within the NGGM (Next Gen-
eration Gravity Mission) program by the European Space
Agency (ESA). Furthermore, commercial laser communica-
tion terminals operate at the same wavelength. Iodine-based
optical frequency references are seen as a promising can-
didate for an optical clock for future GNSS showing lower
frequency instabilities as the currently used PHM. Relying
on an extensive heritage, a flight model fulfilling the Gali-
leo requirements could be realized on short timescale and
at moderate costs. As a perspective, this technology is seen
as an initiating step toward a routinely applied and reliable
optical clock technology in space, also paving the way for
future ultra-high performance optical single-ion and lattice
clocks in space.
Absolute frequency references based
onDoppler‑free spectroscopy ofmolecular
iodine near532nm
Molecular iodine offers very strong absorption lines near
532nm which can easily be accessed with a laser system at
a wavelength of 1064nm using second harmonic generation
(SHG), also realized in compact and ruggedized setups for
applications in space.
Over the last years, DLR—in collaboration with the Hum-
boldt-Universität zu Berlin and the University Bremen—has
developed several setups of iodine-based frequency refer-
ences with a roadmap toward space applications. The opti-
cal setups for modulation transfer spectroscopy are realized
using an adhesive bonding technology where the optical
components are joint to a baseplate made of glass (or glass
ceramics, respectively) with a space-qualified two-compo-
nent epoxy, see Fig.1. This ensures the high thermal and
mechanical stability of the optical system needed for opera-
tion in space.
With a setup on Elegant Breadboard (EBB) level (Fig.1,
left), frequency instabilities of 6× 10–15 at 1s integration
time and below 3× 10–15 for integration times between
100s and 10,000s have been demonstrated (Schuldt etal.
2017). This is, to our knowledge, the best published per-
formance for such an iodine-based frequency reference.
The frequency instabilities of the EBB setup—given in
Fig. 1 Several implementations of the iodine spectroscopy unit, real-
ized at DLR in collaboration with the universities HU Berlin and
Bremen. Left: Elegant Breadboard (EBB, 25 cm × 55 cm) (Schuldt
et al. 2017); Middle: Engineering Model (EM, 18 cm × 38 cm)
(Döringshoff etal. 2017); Right: Setup used on the sounding rocket
mission JOKARUS (15cm × 25cm) (Schkolnik etal. 2017; Döring-
shoff etal. 2019)
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Allan deviation—are shown in Fig.2, evaluated from a
beat measurement with an ultra-low expansion (ULE) cav-
ity setup. A linear and a second-order polynomial drift
have been removed from the corresponding time records
where the linear drift is attributed to the iso-thermal creep
of the cavity. The Allan deviation shows white frequency
noise for integration times between 1 and 10s and Flicker
noise of about 3× 10–15 at integration times > 1000s. For
integration times up to 1000s, the frequency instabilities
are 1–2 orders of magnitude lower than the one of the
current Galileo clocks, i.e., RAFS and PHM. At longer
integration times, the iodine reference approaches the per-
formance of the Space Hydrogen Maser (SHM), an active
hydrogen maser which is currently implemented within
the ACES (Atomic Clock Ensemble in Space) mission on
the ISS (Goujon etal. 2010). As our measurement of the
iodine performance is most probably limited by the cav-
ity reference, we started an investigation of the long-term
stability of the EBB setup which is currently ongoing. A
first frequency stability evaluation where the iodine EBB
is compared to a hydrogen maser via an optical frequency
comb is shown in Fig.2 (blue curve). It is based on a
16days continuous operation of the iodine reference.
The engineering model (EM) setup was further devel-
oped with respect to compactness and uses a specifically
designed compact iodine cell, see Fig.1, middle (Döring-
shoff etal. 2017). The EM spectroscopy unit was subjected
to thermal cycling from -20°C to + 60°C and vibrational
loads with sine vibration up to 30g and random vibration
up to 25.1 grms. The frequency stability was measured before
and after the tests where no degradation was observed. The
frequency offset between the EBB and EM setups is below
1.5kHz with a reproducibility below 250Hz (Döringshoff
etal. 2017). While gas cells typically use a cold finger for
setting the pressure inside the cell, the EM setup was also
operated with a gas cell filled at an unsaturated vapor pres-
sure of about 1Pa, showing similar performance. This
allows lowering the complexity of the setup by omitting a
temperature control and reducing the SWaP budgets.
An iodine-based frequency reference has been success-
fully operated on a sounding rocket (Schkolnik etal. 2017;
Döringshoff etal. 2019) as part of the JOKARUS mis-
sion, showing autonomous operation during the 6min long
space flight. The setup was optimized with respect to the
specific sounding rocket requirements, especially regarding
dimensions. The compact spectroscopy unit with a 15cm
long iodine cell, is shown in Fig.1, right, using a micro-
integrated extended cavity diode laser (ECDL) including a
semiconductor power amplifier as light source (Kürbis etal.
2020). A short-term instability of the iodine reference of
1.5 × 10–13/√τ has been demonstrated (Döringshoff etal.
2019), probably limited due to its very compact design,
intermodulation noise and the lack of intensity and RAM
stabilization.
Within the ongoing project ADVANTAGE (Advanced
Technologies for Navigation and Geodesy), a next iteration
of the iodine setup is developed, based on the EBB-, EM-
and JOKARUS developments. The next step toward space
instrumentation is carried out by a system level design where
all components for the laser system, and the spectroscopy
are integrated within one physical box. For the design, an
absolute temperature of 15°C with a stability of ± 5°C is
Fig. 2 Measured frequency
stability of the Iodine-EBB ref-
erence, given in Allan deviation.
Shown is a beat measurement
with a cavity setup (Schuldt
etal. 2017), together with the
frequency stabilities of the
current Galileo clocks (RAFS,
PHM) and the active hydrogen
maser for ACES (SHM). Also
shown is a beat measurement
with a hydrogen maser (first
measurement, not yet opti-
mized, the degradation > 1000s
is believed to be an artefact).
The measurement is limited by
the H-maser performance for
integration times < 1000s
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assumed for the spacecraft interface plate. An actively con-
trolled thermal shield within the iodine reference guarantees
an operation of the spectroscopy board at (22 ± 0.1)°C. The
laser is fiber-coupled to this unit in the current design, where
a Nd:YAG solid state laser is assumed baseline. In a design
upgrade, a compact ECDL module will be integrated within
the unit. Figure3 shows a photograph of the integrated spec-
troscopy board, the corresponding optical layout for modula-
tion transfer spectroscopy and a CAD model of the overall
system level design of the reference.
The design of the ADVANTAGE setup is taken as basis
for an optical reference developed for future GNSS. The
corresponding schematic is shown in Fig.4 where the fre-
quency reference is split into two functional units: the iodine
spectroscopy unit (including laser, fiber-optic components
for beam preparation and frequency doubling) and the iodine
control electronics (including laser driver, temperature
controllers, AOM and EOM drivers, servo control loops for
intensity, RAM and frequency stabilization). The stable light
at a wavelength of 1064nm is input to an optical frequency
comb, which delivers a stable10MHz clock signal.
A component-level breakdown, based on the design
shown in Fig.4 results in an overall mass of 20.1kg (includ-
ing a 20% component level margin), a power consumption
of 43.1W (including a 10% component level margin) and a
volume of 32.3l (without margin) of the optical reference
(without frequency comb).
In‑orbit verication mission
As part of a general technology development roadmap for
future Galileo, DLR plans an in-orbit verification mission,
called COMPASSO. This mission will demonstrate optical
Fig. 3 ADVANTAGE setup
of the iodine reference. Top:
Photograph and schematic of
the spectroscopy board using a
22cm long iodine cell in 4-pass
configuration. Bottom: CAD
at system level, showing the
spectroscopy board within a
thermal shield. The fiber-optic
components for the laser system
(including AOM and EOM)
are mounted to the bottom
side of the thermal shield, the
SHG modules at one of its side
plates. The laser is fiber coupled
to this unit
Fig. 4 Layout of the iodine
reference with interfaces to the
frequency comb, which deliv-
ers the stable10MHz output
signal. ECDL extended cavity
diode laser, AOM acousto-
optical modulator, EOM electro-
optics phase modulator, SHG
second harmonic generation,
VHBG volume holographic
bragg grating, RAM residual
amplitude modulation, NC
noise-canceling photo-detector,
PD photo-detector, DDS direct
digital synthesizer, MO master
oscillator, PA power amplifier,
PID proportional-integral-
derivative servo control, TM/TC
telemetry and telecommand
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clock and optical link technologies on the Bartolomeo plat-
form (Steimle etal. 2019) which is externally attached to the
Columbus module of the ISS. Based on the mission concept
presented by Schuldt etal. (2019), the mission feasibility
of COMPASSO is currently investigated within a Phase A
study at DLR where a mission lifetime of 2years is assumed.
The payload consists of two iodine-based frequency refer-
ences as detailed above, together with an optical frequency
comb, a microwave frequency reference and an optical laser
communication and ranging terminal, cf. the architecture
shown in Fig.5. The optical link is used for time- and fre-
quency transfer and synchronization of a ground-based (micro-
wave or optical) frequency reference to the space-based optical
frequency references. Furthermore, it enables data communi-
cation and high-accuracy ranging.
The two iodine-based frequency references can be stabi-
lized to the same or to different (nearby) ro-vibronic transi-
tions. Their frequency stabilities are evaluated by compar-
ing both references in the optical frequency range, i.e., near
282THz (corresponding to a wavelength of 1064nm). The
optical frequency comb can be referenced to the iodine ref-
erence and transfers its frequency stability from the optical
frequency range to the radio frequency range. Furthermore, the
frequency comb can be referenced to an onboard microwave
reference (e.g., a Galileo PHM or RAFS) and thus enables
multiple comparison measurements with which the frequency
stability in the relevant time period of the references can be
evaluated. Using the two-way optical laser communication
and ranging terminal (LCRT) the performance of the optical
references onboard the ISS can additionally be compared to
ground-based clocks.
Acknowledgements This work is supported by the Helmholtz-Gemein-
schaft Deutscher Forschungszentren e.V. under grant number ZT-0007
(ADVANTAGE, Advanced Technologies for Navigation and Geod-
esy) and by the German Space Agency DLR with funds provided by
the Federal Ministry of Economic Affairs and Energy (BMWi) under
grant numbers 50QT1102, 50QT1201, 50WM1646 and 50NA1905.
The iodine reference is investigated within the “Iodine Optical Clock
(IOC)” project under lead of SpaceTech GmbH, Immenstaad, Germany,
with funding by the EU Horizon 2020 program.
Funding Open Access funding enabled and organized by Projekt
DEAL.
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GPS Solutions (2021) 25:83
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Thilo Schuldt joined the German
Aerospace Center (DLR), Insti-
tute of Space Systems (Bremen)
in 2013 and is group leader for
“System Enabling Technolo-
gies”. He received his Ph.D.
from the Humboldt-Universität
zu Berlin for his work on the
space-based gravitational wave
detector LISA. Thilo Schuldt has
been working on many ESA and
DLR funded projects in optical
metrology, focusing on optical
frequency references and high-
sensitivity laser interferometry.
Martin Gohlke joined the German
Aerospace Center (DLR), Insti-
tute of Space Systems (Bremen)
in 2012. He worked on the opti-
cal ground support equipment
for the GRACE follow-on mis-
sion in cooperation with DLR
institutes and space companies
(Airbus, STI and JPL). LISA
related topics such as laser inter-
ferometry, optical frequency ref-
erences and temperature sensors
are part of his current activities.
Markus Oswald is doing his
Ph.D. on iodine frequency refer-
ences for space at the University
of Bremen. After his master’s
degree from the HTWG Kon-
stanz – University of Applied
Sciences, in 2014, he worked on
an FPGA-based digital laser
locking system. Within the scope
of his Ph.D. program, he has
contributed to the development
and realization of various iodine
frequency reference setups for
DLR and ZARM.
Jan Wüst joined the German
Aerospace Center (DLR), Insti-
tute of Space Systems (Bremen)
in 2019. He received his master’s
degree from the HTWG Kon-
stanz – University of Applied
Sciences. The thesis was about
residual amplitude modulation in
modulation transfer spectroscopy
for frequency stabilized laser
systems. Within the ADVAN-
TAGE project, he is responsible
for the mechanical and thermal
design of the iodine spectros-
copy subsystem.
Tim Blomberg joined the Ger-
man Aerospace Center (DLR),
Institute of Space Systems
(Bremen) in 2018. He received
his master’s degree from the
RWTH Aachen University for
the development, integration and
test of an isostatic mounting for
the iodine spectroscopy board of
ADVANTAGE. Assembly, inte-
gration and test activities within
ADVANTAGE are part of his
current responsibilities.
Klaus Döringshoff joined the
Ferdinand-Braun-Institut, Leib-
niz-Institut für Höchstfrequenz-
technik (Berlin) in 2020, and
works on the development of
optical frequency references
based on high resolution laser
spectroscopy. He received his
Ph.D. from the Humboldt-Uni-
versität zu Berlin in 2018 for his
work on the optical frequency
references based on ro-virbronic
transitions in molecular iodine.
Ahmad Bawamia received a
bachelor’s degree in mathemat-
ics and physics at Université
Bordeaux I in Bordeaux, France
in 2001 and his master’s in opto-
electronics engineering at école
nationale supérieure des sciences
appliquées et de technologie in
Lannion, France in 2004. He
then joined the Ferdinand-
Braun-Institut, Leibniz-Institut
für Höchstfrequenztechnik
(FBH) in Berlin, Germany in
2005, working on the improve-
ment of the spatial beam charac-
teristics of broad-area semicon-
ductor lasers. He obtained his doctoral degree at the technical
university of Berlin in 2011. Since 2010, Ahmad is a full-time scientist
at FBH, where he specialized in the development of micro-integrated
semiconductor laser modules for precision spectroscopy applications,
especially in space.
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Andreas Wicht heads the Joint
Lab Quantum Photonic Compo-
nents at FBH. He is an expert in
the development of micro-inte-
grated diode laser modules for
applications that require stable
lasers, including quantum optics
with ultra-cold atoms. After
receiving his Ph.D. from the
University of Hannover, he
joined Stanford University as a
postdoc in 1999 to work on pre-
cision atom interferometers.
Between 2002 and 2008 he was
affiliated with the Universität of
Düsseldorf as an assistant pro-
fessor. In 2008 he joined the FBH to establish the Joint Lab Quantum
Photonic Components, a Joint Lab between the FBH and the Hum-
boldt-Universität zu Berlin.
Matthias Lezius is group leader
and senior scientist for “Space
Combs” at Menlo Systems
GmbH in Martinsried. He
received his Ph.D. at the Inns-
bruck University in Austria for
laser-interaction with rare-gas
clusters. Since 2010 he has man-
aged various DLR and ESA
funded projects advancing the
technology readiness, robustness
and compactness of optical fre-
quency combs. These combs
have subsequently been used for
scientific experiments on
TEXUS sounding rockets.
Kai Voss is the head of optical
instruments group at SpaceTech
GmbH (STI). He received his
Phd. in physics in 2004 and is
working on laser-optical systems
for space since 2011. He has
been the system engineer at STI
for the Laser ranging interferom-
eter on GRACE FO, has led sev-
eral technology studies and hard-
ware projects.
Markus Krutzik is head of the
Joint Lab Integrated Quantum
Sensors (IQS) operated by Ferdi-
nand-Braun-Institut, Leibniz-
Institut für Höchstfrequenztech-
nik (FBH) and the
Humboldt-Universität zu Berlin.
R&D activities focus on the
development of integrated
atomic systems for frequency
metrology, timing and field sens-
ing applications. He got his
Ph.D. in 2014 (HU Berlin),
worked at the University of Cali-
fornia, Berkeley, and NASA Jet
Propulsion Laboratory,
Pasadena.
Sven Herrmann is a research
scientist at the Center of Applied
Space Technology and Micro-
gravity at the University of
Bremen. He received his Ph.D.
from the Humboldt-Universität
zu Berlin for his work on optical
cavity-based frequency refer-
ences for tests of fundamental
physics. He has since contrib-
uted to several DLR and ESA
funded activities in the field of
fundamental physics, quantum
sensing and cold atoms.
Evgeny Kovalchuk is a research
scientist at the Humboldt-Uni-
versität zu Berlin, where he
received his Ph.D. for his work
on optical parametric oscillators
for precision IR spectroscopy
and metrology. His main
research interests are laser stabi-
lization, femtosecond combs,
optical clock development, pre-
cision measurements, optical
frequency metrology and phase
noise measurements.
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GPS Solutions (2021) 25:83
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Achim Peters is professor for
Optical Metrology at the physics
department of the Humboldt-
Universität zu Berlin. Since 2008
he is also affiliated with the Fer-
dinand-Braun-Institut, Leibniz-
Institut für Höchstfrequenztech-
nik (FBH) and the Joint Lab
Quantum Photonic Components.
He obtained his Ph.D. in physics
at Stanford University in 1998
and has since then worked on
many projects pursuing a variety
of precision measurements as
well as advancing the related
technologies.
Claus Braxmaier is chair of
Space Technology, University of
Bremen, in cooperation with
DLR. Braxmaier is head of the
DLR-department “System Ena-
bling Technologies” in Bremen.
He received a Ph.D. from the
University of Konstanz and has
been working on many ESA and
DLR funded projects in space
sciences, gravity missions, cold
atom optics, quantum metrology,
frequency references and laser
interferometry.
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

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