# Responsive Operations for Key Services (ROKS): A Modular, Low SWaP Quantum
Communications Payload

Craig D. Colquhoun<EMAIL_ADDRESS>Craft Prospect Ltd,
Suite 12 Fairfield, 1048 Govan Road, Glasgow, UK, G51 4XS  Hazel Jeffrey
Craft Prospect Ltd, Suite 12 Fairfield, 1048 Govan Road, Glasgow, UK, G51 4XS
Steve Greenland  Craft Prospect Ltd, Suite 12 Fairfield, 1048 Govan Road,
Glasgow, UK, G51 4XS  Sonali Mohapatra  Craft Prospect Ltd, Suite 12
Fairfield, 1048 Govan Road, Glasgow, UK, G51 4XS  Colin Aitken  Craft Prospect
Ltd, Suite 12 Fairfield, 1048 Govan Road, Glasgow, UK, G51 4XS  Mikulas
Cebecauer  Craft Prospect Ltd, Suite 12 Fairfield, 1048 Govan Road, Glasgow,
UK, G51 4XS  Charlotte Crawshaw  Craft Prospect Ltd, Suite 12 Fairfield, 1048
Govan Road, Glasgow, UK, G51 4XS  Kenny Jeffrey  Craft Prospect Ltd, Suite 12
Fairfield, 1048 Govan Road, Glasgow, UK, G51 4XS  Toby Jeffreys  Craft
Prospect Ltd, Suite 12 Fairfield, 1048 Govan Road, Glasgow, UK, G51 4XS
Philippos Karagiannakis  Craft Prospect Ltd, Suite 12 Fairfield, 1048 Govan
Road, Glasgow, UK, G51 4XS  Ahren McTaggart  Craft Prospect Ltd, Suite 12
Fairfield, 1048 Govan Road, Glasgow, UK, G51 4XS  Caitlin Stark  Craft
Prospect Ltd, Suite 12 Fairfield, 1048 Govan Road, Glasgow, UK, G51 4XS  Jack
Wood  Craft Prospect Ltd, Suite 12 Fairfield, 1048 Govan Road, Glasgow, UK,
G51 4XS  Siddarth K. Joshi<EMAIL_ADDRESS>Quantum Engineering
Technology Labs & Department of Electrical and Electronic Engineering,
University of Bristol, UK  Jaya Sagar  Quantum Engineering Technology Labs &
Department of Electrical and Electronic Engineering, University of Bristol, UK
Elliott Hastings  Quantum Engineering Technology Labs & Department of
Electrical and Electronic Engineering, University of Bristol, UK  Peide Zhang
Quantum Engineering Technology Labs & Department of Electrical and Electronic
Engineering, University of Bristol, UK  Milan Stefko  Quantum Engineering
Technology Labs & Department of Electrical and Electronic Engineering,
University of Bristol, UK  David Lowndes  Quantum Engineering Technology Labs
& Department of Electrical and Electronic Engineering, University of Bristol,
UK  John G. Rarity  Quantum Engineering Technology Labs & Department of
Electrical and Electronic Engineering, University of Bristol, UK  Jasminder S.
Sidhu<EMAIL_ADDRESS>SUPA Department of Physics, University of
Strathclyde, Glasgow, G4 0NG, UK  Thomas Brougham  SUPA Department of Physics,
University of Strathclyde, Glasgow, G4 0NG, UK  Duncan McArthur  SUPA
Department of Physics, University of Strathclyde, Glasgow, G4 0NG, UK  Roberto
G. Pousa  SUPA Department of Physics, University of Strathclyde, Glasgow, G4
0NG, UK  Daniel K. L. Oi  SUPA Department of Physics, University of
Strathclyde, Glasgow, G4 0NG, UK  Matthew Warden
<EMAIL_ADDRESS>Fraunhofer Centre for Applied Photonics,
Technology and Innovation Centre, 99 George Street, Glasgow, G1 1RD, UK
Eilidh Johnston  Fraunhofer Centre for Applied Photonics, Technology and
Innovation Centre, 99 George Street, Glasgow, G1 1RD, UK  John Leck
Fraunhofer Centre for Applied Photonics, Technology and Innovation Centre, 99
George Street, Glasgow, G1 1RD, UK

###### Abstract

Quantum key distribution (QKD) is a theoretically proven future-proof secure
encryption method that inherits its security from fundamental physical
principles. With a proof-of-concept QKD payload having flown on the Micius
satellite since 2016, efforts have intensified globally. Craft Prospect,
working with a number of UK organisations, has been focused on miniaturising
the technologies that enable QKD so that they may be used in smaller platforms
including nanosatellites. The significant reduction of size, and therefore the
cost of launching quantum communication technologies either on a dedicated
platform or hosted as part of a larger optical communications will improve
potential access to quantum encryption on a relatively quick timescale.

The Responsive Operations for Key Services (ROKS) mission seeks to be among
the first to send a QKD payload on a CubeSat into low Earth orbit,
demonstrating the capabilities of newly developed modular quantum
technologies. The ROKS payload comprises a quantum source module that supplies
photons randomly in any of four linear polarisation states fed from a quantum
random number generator; an acquisition, pointing, and tracking system to
fine-tune alignment of the quantum source beam with an optical ground station;
an imager that will detect cloud cover autonomously; and an onboard computer
that controls and monitors the other modules, which manages the payload and
assures the overall performance and security of the system. Each of these
modules have been developed with low Size, Weight and Power (SWaP) for
CubeSats, but with interoperability in mind for other satellite form factors.

We present each of the listed components, together with the initial test
results from our test bench and the performance of our protoflight models
prior to initial integration with the 6U CubeSat platform systems. The
completed ROKS payload will be ready for flight at the end of 2022, with
various modular components already being baselined for flight and integrated
into third party communication missions.

##  I INTRODUCTION

Responsive Operations for Key Services (ROKS) is a UK Space Agency funded
mission that aims to launch a 6U CubeSat with quantum key distribution (QKD)
and cloud-detection capabilities FLIWhitePaper  into Low-Earth Orbit (LEO).
ROKS has supported the progression of QKD instrumentation from apparatus
spanning optical test benches in university laboratories, to miniaturised,
modular, space-ready subsystems.

Craft Prospect Ltd (CPL) and partners have designed, manufactured, assembled,
tested, and now integrated several modules for this mission CassSmallsatPaper
, though with reconfigurability and interoperability in mind for future flight
opportunities. This approach enables ROKS subsystems to be supplied either
individually or bundled for a range of satellite form factors. The individual
capabilities of one of CPL’s modules will be demonstrated in the upcoming
Canadian space agency mission QEYSSat QEYSSATPaper  \- a variant of the JADE
quantum source produced for ROKS will be driven by a QRNG board developed at
University of Waterloo as a secondary payload on the satellite, demonstrating
its ability to interface with different driving electronics, and with
different QRNGs QRNGPaper  from a range of suppliers.

The remainder of the paper is laid out as follows: the rest of this section
gives the motivation for using orbital CubeSats for QKD, Section II lists and
describes each of the modules and their purposes in the mission, Section III
presents some key module tests and results, Section IV describes some of the
ongoing Optical Ground Station work at University of Bristol, some of the
lessons learned as part of the test and integration process are listed in
Section V, and Section VI concludes the paper.

###  I.1 Why QKD?

Encryption, the use of secret keys to encrypt or decrypt information, is now
ubiquitous on the internet, to the point that most people use encryption on a
daily basis. Two of the most common types of encryption used to secure data
online are RSA RSA  which relies on the multiplication of two large prime
numbers for its key generation, and AES which involves performing several
matrix operations to do the same. There are algorithms for quantum computers
that threaten the security of these methods - Shor’s algorithm ShorPaper
significantly reduces the complexity of prime number factorisation when
compared with classical computers, and Grover’s algorithm GroverPaper
effectively halves the bit length of keys for brute forcing attacks. The
former poses a significant threat NISTPostQuantumCrypto  to RSA encryption of
all currently used bit lengths, and the latter greatly reduces the amount of
time it would take to correctly guess an AES key assuming no vulnerabilities
for the method would be discovered.

With technological advances improving the scalability, fault tolerance, and
commercial availability of quantum computers; it is only a matter of time
until cyber criminals and state actors gain access to them, and until these
encryption methods are rendered insecure. Such nefarious actors need not wait
until capable quantum computers are available to decrypt confidential
information, they may already be storing encrypted data to decrypt later, so
quantum-secure encryption methods are critical at present
NISTPostQuantumCrypto . Quantum key distribution (QKD) is an encryption method
that relies on the quantum mechanical properties of light for its key
generation rather than mathematical complexity, making it theoretically secure
against attacks from quantum computers QKDPrivacyNoisy ; Sidhu2021advances .

While other QKD protocols use the polarisations of entangled photons or time
binning to distinguish between photons, or qubits, that represent a 0 or 1, in
ROKS a symmetric BB84 cointossing  protocol is used. In the quantum
communications channel, weak coherent pulses ($<$1 photon per pulse average)
are generated in one of four linear polarisations at random: horizontal (H)
$\mathrlap{\leftarrow}\rightarrow$, vertical (V) $\updownarrow$, diagonal (D)
$\mathrlap{\swarrow}\nearrow$, or anti-diagonal (A)
$\mathrlap{\searrow}\nwarrow$. These polarisations are paired off into two
polarisation bases with their orthogonal counterparts - the H-V basis
$\mathrlap{\leftarrow}\mathrlap{\hskip 4.5pt\updownarrow}\rightarrow$, and the
D-A basis $\mathrlap{\mathrlap{\swarrow}\nearrow}\mathrlap{\searrow}\nwarrow$
\- within which one polarisation corresponds to a 1 and the other to a 0. All
properties of the photons other than their polarisations should be identical
so the photons are otherwise indistinguishable from one another.

After the bits have been sent a reconciliation process occurs over classical
communication channels, during which only the basis in which each photon was
sent is transmitted so the receiver can determine if the photon was detected
in the correct polarisation basis. If the basis is correct, the qubit has
almost certainly been correctly measured so it is kept; if the basis is
incorrect, the qubit will only be correctly measured 50% of the time, so this
bit is ignored and does not contribute to the final secret key. Because only
single photons are transmitted, any eavesdroppers can be detected by decreases
in the numbers of detected photons, which can also be stated as an increase in
the quantum bit error rate (QBER). Eavesdroppers also cannot reliably
reproduce photons they pick off, because there is a 50% probability they
measure each photon in the incorrect polarisation basis.

As well as ‘signal states’ that are kept and contribute to the final secret
key, ‘decoy states’ DecoyQKD  are also used for added complexity in case of
eavesdroppers. Decoy states contain all the same information as signal states,
just at a different intensity. During the reconciliation process over the
classical communications channel, the transmitter declares which pulses were
signal and which were decoy.

Some BB84 QKD solutions are commercially available, either coupled into free-
space or fibre-coupled. The distances over which these solutions can be used
are limited by atmospheric losses FreespaceQKD  and fibre attenuation FibreQKD
, respectively. Using satellites from various Earth orbits should overcome
these limits CubesatQKD ; Sidhu2020_npjQI ; sidhu2021key  as this can offer
global reach, and the atmosphere exists at relatively low altitudes for some
of which its density is reduced, limiting atmospheric losses QUARCPaper ;
Rarity_atmosphere . The ROKS mission aims to demonstrate the feasibility of
supplying quantum keys from CubeSats in LEO Islam2022finite , with the
potential for constellations in future to reduce lapses in coverage at
compatible ground stations Gundogan2021npjQI ; gundogan2021topical ;
Wallnofer2022_NC .

###  I.2 MISSION GOALS

ROKS is a demonstrator mission both for the devices developed for the
platform, and the services that these devices can provide. The main mission
goals are to demonstrate that QKD technology can be successfully implemented
on a CubeSat Sidhu2020_npjQI ; Satquma2021documentation , and to demonstrate
that onboard intelligence can be used to deliver mission autonomy and improve
utility of a potential CubeSat QKD service.

##  II MISSION PAYLOAD

The ROKS payload consists of 5 modular hardware subsystems, described below.
The quantum source (JADE) and acquisition, pointing, and tracking (APATITE)
components were designed and manufactured in conjunction with mission partners
University of Bristol (UoB) and University of Strathclyde (UoS). The optical
telescope (GARNET) was developed by Fraunhofer Centre for Applied Photonics
(FhCAP), and licensed to CPL for commercialisation. Each of these modules is
shown in its position in the CubeSat structure in Fig. 1.

Figure 1: A Computer Render of the 6U Structure with Key Components
Highlighted

JADE: The JADE quantum source combines optical components that produce quantum
signal pulses (GNEISS), with the electronics used to drive them. The JADE PCB
includes a Zynq FPGA, a series of quantum random number generators (QRNGs),
several laser diode driver chips intended for pulsed operation, sensors to
provide telemetry for the module, and interfaces to other modules. Operational
flexibility is provided by the ability to tune FPGA parameters during runtime
- the user may tune the pulse rate, individual pulse widths and laser
currents, or whether to pulse using numbers form the QRNGs or to pulse
arbitrary patterns for test and calibration purposes. The specified pulse rate
for the ROKS mission is 100 MHz, with 1 ns FWHM pulse width, generating signal
states of 0.8 photons per pulse at the telescope exit and two decoy states of
0.4 and 0 photons per pulse. An image of the JADE module internals is shown in
Fig. 2(a).

GNEISS: The optical component of the JADE module generates weak coherent
pulses of 785 nm light in four linear polarisations. Thermal control of each
laser diode is used to tweak the wavelengths for indistinguishability between
photon sources. There is also a photodiode for diagnostic purposes, and an 830
nm alignment laser source that is used in the APT subsystem. Crucially, all
the beams produced in the module are coupled into a single optical fibre so
that they all share the same spatial mode upon leaving the JADE module.

APATITE: The APATITE acquisition, pointing, and tracking system uses a series
of dichroic mirrors to separate the alignment beam from the quantum source
beams generated by JADE, and combine the quantum source light with a downlink
beacon laser beam. The alignment beam is directed onto a camera sensor as
reference for where the quantum source is pointing - the camera sensor will
also be used to detect uplink beacon light from compatible optical ground
stations (OGSs). The downlink beacon beam serves three purposes: it provides
an alignment signal for OGSs; it serves as a polarisation reference so the OGS
may calibrate its optics to optimise detection of the quantum source
polarisation states; and it supplies timing and synchronisation information
during the quantum key transmission phase. The alignment of the downlink
beacon, the quantum source, and the uplink beacon beams is managed using a
MEMS mirror. The optomechanical parts, camera board, and APATITE driver board
can be seen inside the enclosure in Fig. 2(b). Any light that leaves APATITE
has to pass through the satellite’s telescope.

GARNET: The optical telescope was developed by the Fraunhofer Centre for
Applied Photonics (FhCAP) and licensed to CPL. This Schmidt-Cassegrain
reflecting telescope was designed specifically for ROKS but it can be used for
any optical communications CubeSat in principle. GARNET (shown in Fig. 2(c))
occupies a volume of 1.5 U, has a 90 mm (80 mm clear) aperture and provides
30x magnification to outgoing beams with low distortion with a $\pm$0.25∘
field of view. Its exit pupil is located externally to the telescope to allow
convenient interfacing with a steering mirror - on ROKS it interfaces directly
with the front of APATITE. The telescope housing is designed to minimise
contact with the CubeSat platform to reduce environmental stresses experienced
by the optics.

OPAL: The optical payload (OPAL) encompasses JADE, APATITE, and GARNET as the
full optical subsystem, as shown in Fig. 2(d). This is defined as a bundle
that may be offered for future commercial flight opportunities rather than its
individual parts.

Figure 2: Images of the Modules Contained in the Optical Payload (OPAL)
Segment of ROKS (a) JADE Quantum Source Module; (b) APATITE Acquisition,
Pointing, and Tracking Module; (c) GARNET Optical Telescope; (d) All the OPAL
Modules Connected in a Test Bench Configuration

FLI-NT: The Forwards Looking Imager (FLI), CPL’s first commercially available
product, is a camera sensor instrument powered by a Zynq FPGA embedded with a
deep learning algorithm for user-defined EO feature classification. Expected
use cases include wildfire monitoring, ship tracking, and, as in the case of
ROKS, cloud detection. The FLI-NT name is given to the ROKS model of FLI
because it has been designed and trained for the more challenging purpose of
detecting clouds at night time (NT), on the dark side of Earth. The module
‘looks’ ahead of the CubeSat’s location, identifies whether clouds will block
line-of-sight with the target OGS, and allows the onboard computer to either
prepare for QKD execution, or to direct the CubeSat’s resources elsewhere,
generating more random numbers for the next pass for instance. FLI-NT (shown
in Fig. 3) is configurable to offer up to 120 s look-ahead for the ROKS
mission.

Figure 3: FLI-NT Module

OBSIDIAN: The OBSIDIAN onboard computer interfaces with all the previously
described subsystems using a Zynq-based FPGA running Bright Ascension’s GenOne
flight software with CPL components for responsive scheduling and QKD. This
module also has high-speed throughput to the rf transceiver for the
reconciliation stage of the QKD process and telemetry transmissions. OBSIDIAN
is shown in Fig. 4.

Figure 4: OBSIDIAN Onboard Computer

##  III MODULE TESTING

Prior to the payload integration phase of the AIT process, modules were
calibrated and tested on an individual basis.

###  III.1 JADE Quantum Source

Pulse test and calibration: During JADE electronics board bring-up, various
test points on the PCB and secured components are probed to confirm that the
expected signals and voltages are being produced. One test that can
efficiently identify if there are problems on the board is to examine optical
pulses produced by bare laser diodes connected to the drivers, giving an end-
to-end measure of the driving electronics and the firmware flashed onto the
FPGA. A successful run of this test is shown in Fig. 5, where nominal 100 MHz
optical pulses that are representative of what will be generated by the JADE
quantum source laser diodes can be observed on the oscilloscope screen.

Figure 5: JADE Test Apparatus

The full system is tested once the JADE PCB and GNEISS are integrated.
Although an effort is made to optimise the coupling efficiency of all the
laser beams in GNEISS into the fibre, these efficiencies may fluctuate while
the optical glue cures. The difference in coupling efficiencies is amended
during the JADE calibration process by connecting the GNEISS fibre directly to
a single photon counting module (SPCM) and timetagger system; pulsing all the
laser diodes in a test pattern and tweaking the current of each laser until
the photon numbers are equal. An accumulative histogram showing pulses in the
test pattern at 20 MHz is shown in Fig. 6. A modulation frequency of 20 MHz is
used to so that the signals are unaffected by SPCM dead time during
calibration of the pulse heights or photon numbers. In this case an attempt
was made to equalise the pulse heights.

Figure 6: An Accumulative Histogram Showing JADE Pulses at 20 MHz

Polarisation testing: Since polarisations are critical to BB84 QKD,
polarisation testing occurs at several stages throughout the integration
process to ensure polarisation-dependent effects are minimised. Polarisation
rotations are normal and expected, but if anything in ROKS causes shifts or
losses in some polarisations more than others this will increase the error
rate of our QKD system. Ideally all polarisations should be separated by 45∘.
Once GNEISS and JADE are integrated the quantum source laser diodes can only
be pulsed, so the signal can not easily be detected by a polarimeter.

CPL uses two different apparatus for polarisation testing - one for simple
readout of the relative polarisations generated by JADE, and another to split
the polarisation states into four different beam paths, closely resembling the
instrument that will be used in ROKS-compatible OGSs. In both cases, two
quarter wave plates (QWPs) are used to linearise the polarisation (as
ellipticity can be introduced in the fibre and optics) and a half wave plate
(HWP) is used to rotate that linear polarisation. For the relative
polarisation test the wave plates are placed in front of a polarising beam
splitter (PBS), the HWP is motorised, rotating continuously. An SPCM is placed
on the reflected arm of the PBS, detecting changes of intensity as the HWP is
rotated. Given a known test pattern, the H, V, D, and A photons can be
identified in the signals generated in the detector. The photons counted in
each pulse are plotted against HWP angle, as shown in Fig. 7. Here it can be
seen that the polarisations are all separated by 22.5∘ of HWP rotation,
corresponding to 45∘ polarisation separation.

Figure 7: A Relative Polarisation Measurement Directly from JADE

The polarisation readout instrument is more elaborate, using a combination of
PBSs and non-polarising BSs to direct the light to one of four SPCMs. In this
case, the wave plates are set to angles that maximise the signal for a
polarisation state at each detector, then locked in position. Figure 8
presents four regions of interest (ROIs) on an accumulative histogram showing
photon detection statistics from a test pattern. As these results were
obtained from the port maximised for diagonal photon detection, it is expected
that the D ROI should have the most photon counts, the A ROI should have
almost no counts, and the H and V should have equal counts each approximately
50% of the D count, coming from the opposite polarisation basis. Figure 9
shows the sum of photons in each pulse against time, illuminating that these
proportions are correct and that the system is working as expected.

Figure 8: An Accumulative Histogram Result Obtained Using Polarisation Readout
Apparatus Figure 9: The Sums of the Histograms Shown in Fig. 8. The Lines for
H and V Are Almost Completely Overlapped

Thermal testing: Several thermal cycles are performed on each ROKS module to
ensure that all functions work nominally throughout the operating temperature
range (-20∘C - 50∘C); to check that the modules behave normally when hot and
cold starts are performed at the extremes of the operating temperature range;
and to test that the modules switch on and behave normally after being cycled
through the survival temperature range (-30∘C - 80∘C). All ROKS modules passed
this battery of thermal tests, key data was collected at the operating
temperature extremes, and telemetry was collected throughout the tests where
possible.

The same JADE functional tests as above were performed at the operating
temperature extremes. The photon output directly from JADE is shown in Fig. 10
for various temperatures - there is a clear change in fibre coupling
efficiency, and perhaps the optical power produced by the laser diodes, at
different temperatures. The most surprising observation from these results is
that from room temperature to -20∘C, the number of photons detected from
diodes D and A almost doubles. None of the observed drifts pose a threat to
QKD security, the change in photon number vs temperature must be well
characterised for the flight model so the laser diode modulation current can
be adjusted based on JADE telemetry to tune the photons per pulse.

Figure 10: Accumulative Histogram Results for the Light Directly Output by
JADE for 21∘C, -20∘C, and 50∘C

In addition to the fibre coupling fluctuations, the change in temperature also
induces a change of birefringence in the fibre, leading to polarisation
drifts. Polarisation drifts experienced by light traversing the fibre can be
seen by comparing Fig. 8 and Fig. 11. At 50∘C, there is now a difference in
photon numbers between H and V, which previously had equal outputs through the
diagonal port on the polarisation readout apparatus. In addition, an increase
in the A signal can be observed which would lead to increased errors if not
accounted for using the waveplates in front of the instrument. Again, this
drift does not pose a problem for ROKS, it simply needs to be well
characterised prior to launch, and can be corrected for in the ground station.
Additionally, this test was performed using a 1 m long optical fibre inside
the thermal chamber and a 30 cm long fibre will be used in flight, so this
polarisation rotation will be less pronounced during the mission.

Figure 11: A Polarisation Readout Result Taken at 50∘C from the Same Port as
in Fig. 8

###  III.2 APATITE APT Subsystem

Camera sensor / alignment laser testing: Camera sensor tests are often the
first performed for an APATITE module because of how diagnostically useful the
camera is for further testing.

Figure 12 shows an image taken from the camera sensor of the alignment beam
from JADE entering APATITE from the back of the module, and a simulated uplink
beacon entering APATITE from the front. The beamsteering software will attempt
to overlap the uplink beacon with the alignment beam, or align it to have a
user-defined offset. A back-reflection of the alignment beam appears in the
top-left of the image, for which there are fixes that can be applied to
prevent errors from occurring in the beamsteering software: since the
alignment beam is static, the back-reflection is also static, so an ‘ignore
region’ could be set. Otherwise, because the back-reflection is much dimmer
than the two main beams, the camera sensor’s exposure duration could be
reduced so both of the bright beams can be detected but the back-reflection
cannot.

Figure 12: An Image Taken by the APATITE Camera Sensor Showing the Alignment
Laser (Bottom) and Simulated Uplink Beacon (Top) Spots

Beam divergence measurements: Requirements for APATITE state that the quantum
source and downlink beacon laser beams should have full angle FWHM divergences
before the telescope of 12 $\upmu$rad and 3 mrad, respectively. Divergence is
measured by capturing images of a laser beam at several points along the beam
propagation axis, using the distance along the beam and the beam width at
various points to calculate the divergence. This technique also enables
identification of beam astigmatism, and whether the beam has been truncated in
the APATITE box due to misalignment. An example divergence measurement is
shown in Fig. 13 \- this particular laser beam clipped the edge of an optic,
causing an interference pattern most prominent 120 cm away from APATITE (Fig.
13(b)), and causing the beam to converge on the x axis when it should diverge,
most prominent 220 cm away from APATITE (Fig. 13(c)). The most recently
manufactured optomechanical mounts in APATITE provide correct alignment of all
laser beams, eliminating this issue.

Figure 13: Quantum Source Beam Profile at (a) 20 cm, (b) 120 cm, and (c) 220
cm from the Front of APATITE

Polarisation testing: A relative polarisation test is performed with quantum
source light generated by JADE and aligned through APATITE. Figure 14 shows
the results of this test, overlapped with the results shown in Fig. 7 (data
smoothing has been applied for this figure to improve visibility of results).
It was discovered that the earlier model of APATITE causes a rotation of $\sim
8^{\circ}$ only in the D and A polarisations. From measurements performed on
each of the optics individually and in combination, the root cause was
identified to be the same optical clipping that induced interference in the
beam profile seen in Fig. 13(b).

Figure 14: Relative Polarisation Shifts for Each of the Four Polarisation
States After Passing Through APATITE

Downlink beacon testing: The downlink beacon laser is pulsed for 10 ns at a
rate of 100 kHz, so it operated on a 0.1% duty cycle. An image of the optical
pulse profile detected using a fast photodiode connected to an oscilloscope is
shown in Fig. 15. Using an optical power meter that averages the power
readings, the measured power was 45 mW - this corresponds to a peak power in
the pulses of approximately 55 W.

Figure 15: An Oscilloscope Measurement Showing the Optical Pulse Generated by
the Downlink Beacon Laser in APATITE

The APATITE camera sensor was operated during use of the downlink beacon,
producing the image in Fig. 16. Unfortunately the back-reflections caused by
the downlink beacon beam would blind the camera sensor to the uplink beacon
from the ground station. The most recent iteration of the mechanical part for
which these back-reflections occur will be coated in Vantablack with the
expectation to significantly reduce this effect.

Figure 16: An Image Taken from the APATITE Camera Sensor While the Downlink
Beacon was in Operation

Thermal testing: The APATITE camera sensor and MEMS mirror were tested at
several points throughout the operating temperature range and after hot and
cold starts at the extremes of this range. Both were found to operate normally
without any issues under all the conditions experienced during the tests. An
image of APATITE placed in the thermal chamber is shown in Fig. 17.

Figure 17: APATITE During Thermal Testing, All its Harnesses are Passed into a
Feed-through Port in the Side of the Thermal Chamber

A visible test beam was attached to the fibre coupler to which JADE is usually
connected, and aligned through the front window of the thermal chamber. The
visible test beam was operated at room temperature, then at both of the
extremes of the operating temperature range. Markers were placed to indicate
where the beam was incident at each temperature allowing for the deflection
angle to be measured. From 22∘C to 50∘C the beam was deflected by
approximately 2.13 mrad, and from 22∘C to -20∘C the beam was deflected by 1.81
mrad in the opposite direction. It is presumed that these deflections were
predominantly caused by thermal expansion and contraction of the aluminium
parts as the temperature changed. In GARNET, the beams leaving APATITE will be
magnified by a factor of 30 leading to a factor of 30 reduction in pointing
errors, so over the full operating temperature range the resulting beam is
expected to deflect by $\sim 131\,\upmu$rad over the full operational
temperature range.

###  III.3 GARNET Optical Telescope

Before the construction of GARNET, mechanical assemblies and bonding materials
were characterised across the specified thermal range of ROKS, to confirm that
the process of gluing components into their mounts would not lead to adverse
effects on the optical performance of the telescope. A closeup of one of the
test samples is shown in Fig. 18.

Figure 18: A Closeup Image of an Adhesive Test Performed at FHCAP

Once GARNET was assembled a test setup was used to ensure that the beam
expansion by the telescope is uniform and that alignment of the telescope
mirrors does not modify the mode. The test setup is shown in Fig. 19, where
some light sources are obscured by a frosted glass sheet. This assembly should
provide uniform backlit illumination of the resolution test target, which can
then be observed through the telescope.

Figure 19: A Test Bench for GARNET Distortion Testing at FHCAP

Finally, once GARNET was delivered to CPL, APATITE was secured to the back of
it and an OPAL test bench was constructed, as shown in Fig. 20. On the left is
the OPAL assembly, on the right is the polarisation readout apparatus. A pair
of lenses to shrink laser beam after GARNET has magnified them, can be seen in
the centre. This test bench is being used to characterise the loss of optical
power through the telescope due to a central obstruction required for the
design, and will later be used to analyse the polarisations produced by the
OPAL system from end-to-end.

Figure 20: OPAL Polarisation Test Bench Early in the Build Process

###  III.4 FLI-NT Intelligent Imaging Subsystem

Since the sole purpose of the FLI-NT module is to classify features in images,
the baseline functional test after assembly is simply to record images. An
example FLI-NT test bench that was used for this purpose can be seen in Fig.
21.

Figure 21: The Electronics for the FLINT Module Set Up on a Test Bench

Images were successfully recorded during initial functional testing, at the
extremes of the operational temperature range, and after hot and cold starts
at these extremes. A sample cloud image that has been classified by a newly
assembled FLI-NT module is shown in Fig. 22 \- this variant of FLI uses a
monochrome sensor optimised for the detection of infrared light, since it will
be used to image clouds at night time, but false colour is added during the
feature classification stage.

Figure 22: A Sample Cloud Image After Being Classified by FLI-NT During
Functional Testing

###  III.5 OBSIDIAN Onboard Computer

OBSIDIAN interfaces with all the other subsystems in ROKS, so initial tests
involved integration with other modules on an individual basis to ensure
OBSIDIAN could command them and capture telemetry data. After these tests were
successfully executed, multiple subsystems were connected to OBSIDIAN and
tested simultaneously. An image of one such test is shown in Fig. 23, where
FLI-NT, JADE, and APATITE are electrically connected to OBSIDIAN. These tests
also proved successful.

Figure 23: The Test Bench with All CPL Subsystems Connected for Integration
Testing

Finally a thermal test was performed with JADE and FLI-NT connected to
OBSIDIAN. It was demonstrated that all the modules worked together throughout
the operational temperature range, even after hot and cold starts at the
extremes of this range.

###  III.6 Platform Fit Checking

Once all the subsystems were confirmed working both individually and together,
they were secured to a flight-representative 6U platform, shown in Fig. 24, to
ensure that the modules and their harnesses would fit inside the CubeSat. Keep
out zones and modules that await delivery are blocked out using 3D printed
boxes of the same dimensions. Testing on the system as a whole in this form
factor is imminent at the time of publication.

Figure 24: All the Prepared Modules Inside the 6U CubeSat Structure

##  IV Optical Ground Station

Mission partners at UoB are constructing and testing a portable OGS apparatus
for ROKS. The OGS encompasses a commercial off-the-shelf telescope with custom
receiver optics mounted on the back, as shown in Fig. 25, and an optical
breadboard where transmission optics can be affixed. Collaborative end-to-end
QKD tests were initially planned between CPL and UoB for this phase of ROKS,
however COVID-19 posed significant challenges for integrated testing and in-
person visits. The team worked around these issues by constructing apparatus
at CPL for benchtop QKD demonstrations, as seen in Fig. 20, and by setting up
experiments with the telescope at UoB using downlink beacon representative
lasers rather than a quantum source. The experiments at both facilities were
successful, and integrated end-to-end testing is still anticipated.

Figure 25: The Optical Ground Station Apparatus used at University of Bristol

##  V LESSONS LEARNED

The work presented in the previous sections was the culmination of an intense
eight month development period that saw engineering models of numerous
products advance to flight representative models and integrated into a single
payload. What would already be a significant undertaking for a relatively
small team was made more challenging by partial lockdowns due to COVID-19, an
ongoing chip shortage leading to part sourcing and stockpiling, and longer
than usual lead times for components generally.

Furthermore, it was found upon receipt that some optics did not meet the needs
of the mission, and in some cases did not meet the supplier specifications.
Optics are particularly challenging for this project given the unusual
requirement that relationships between four different polarisations must be
maintained across several optical components. Clarity of what is expected of
suppliers and their products from the outset, an improved goods-in procedure,
and enhanced testing capabilities became critical to avoiding similar issues
with suppliers.

The ROKS team is multidisciplinary, and no one person has an intimate
knowledge of all aspects of any system. Test documentation was crucial to
reduce the people-hours spent on testing and troubleshooting, and to ensure
there were no single points of failure. This prevented team members from being
blocked from completing tests of troubleshooting procedures when they came
across aspects of mechanical or optical hardware, electronics, or software
with which they were unfamiliar, lessening the impact of team absences.

Many of the lessons learned during this phase of the project were due to an
inexperienced team facing its first extensive integration period. Many of the
issues encountered were normal integration challenges, and implementing or
tweaking procedures on-the-fly led to a reduction of issues or improvements to
the outcomes as time went on. A ‘lessons learned’ session was organised with
the team at CPL to discuss and identify areas for improvement, to ease MAIT
stages in future projects. The key messages were as follows:

  * •

Have spares of every part or component delivered

  * •

Photograph or video every process

  * •

Serial number all parts and assemblies

  * •

Keep record cards with modules and assemblies

  * •

Record test configurations

  * •

Invest in test setups

  * •

Maintain test setups and keep them tidy

  * •

Automate repeatable functional tests

  * •

Start end-to-end testing early

  * •

Prototype early and often

  * •

Check supplied parts against their specs

  * •

Plan testing to an appropriate level before executing

  * •

Design parts and assemblies for handling

  * •

Communicate continuously throughout the team

  * •

Plan for forgetfulness and thoughtlessness

  * •

Verify and update budgets frequently

  * •

Record steps taken in detail

  * •

Plan for subcontractor production issues

  * •

Plan for multiple models

  * •

Speak up if work becomes overwhelming

##  VI CONCLUSIONS

The progress made on ROKS payload development represents a significant
milestone for Craft Prospect Ltd and has resulted in the production and
integration of several flight modules for a range of subsystems. These modules
will be made available as configurable products across quantum technology and
onboard intelligence. Lessons learned and shared throughout the team will be
invaluable in extensive future missions that will span from feasibility
studies through to payload and system delivery.

The baseline results in Section III show that the flight models behave as
expected throughout the temperature range anticipated in LEO. These flight
models are subject to continuous tweaks and improvements to ensure the success
and longevity of the ROKS mission. The integration and testing work will
continue, full payload thermal and vibe will ensue once the platform bus
system has been delivered, and a flight readiness review is forecast to occur
by the end of 2022.

##  VII ACKNOWLEDGEMENTS

The ROKS team would like to extend its appreciation to all partners and team
members past and present. In particular CPL acknowledge the support and
expertise of Fraunhofer Research UK, Bright Ascension Ltd, Orbital
Astronautics Ltd, University of Strathclyde, and University of Bristol, with
support from ESA, Innovate UK, UK Space Agency, Quantum Communications Hub,
and all private contributors and industrial supporters of the mission.

The authors would like to thank the UK Space Agency for the continued funding
(National Space Innovation Program NSIP-07) and support that facilitated this
work.

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