Libre Space Foundation (LSF) is devoted to designing and building open-source space technologies. We always support and promote space exploration, scientific research and knowledge. For the past year, we have been working hard on the QUBIK mission to create a platform on which a series of communications experiments will be conducted, upon the mission’s launch.
Ιn a nutshell
For the QUBIK project, Libre Space Foundation has designed and developed two open-source PocketQube Satellites, QUBIK-1 and QUBIK-2. As part of the QUBIK mission, LSF has also designed and built PicoBus; the first, open-source PocketQube deployer. The QUBIK PocketQubes along with AMSAT-EA‘s GENESIS-L and GENESIS-N and Fossa Systems‘ FOSSASAT-1 and FOSSAT-2 were integrated into the deployer and are part of the Firefly Aerospace’s DREAM payloads program on its inaugural Firefly Alpha launch.
The Story so far
Over the past year, the PicoBus, as well as QUBIK-1 and QUBIK-2, underwent all the necessary, standard testing and verification campaigns.
For example, by the first months of 2020, PicoBus had undergone the thermal vacuum test and the vibration test. From there, it was time for assembling the PocketQubes and the deployer, and then the bake out was in order. Shortly after, the Acceptance campaign took place for PicoBus, and so did the Protoflight Campaign for QUBIK-1 and QUBIK-2. You can read more about the extensive testing and the processes that took place earlier in the year here.
While the hardware was undergoing all the testing, the software was polished and tested too, aiming at enabling a series of fascinating communications experiments.
The two PocketQubes, QUBIK-1 and QUBIK-2 are designed to have a short life span of 3-4 weeks. Though this period of time is relatively short, it is enough for LSF to run a series of communications experiments. These are primarily scheduled to take place during the Launch and Early Orbit Phase (LEOP) of the operations and the experiments revolve around satellite identification and tracking. LEOP is a critical phase for satellites and spacecraft operations; since the sooner a team identifies and picks up communication with the satellite, the better it is for the entire operation. Identifying and tracking a satellite early on, almost upon deployment, helps the team to proceed fast and tackle possible problems arising. It is the significance of early identification and tracking and the impact this has on saving missions and operations that prompted LSF to decide to focus the scheduled experiments on that specific Phase of operations. By forming a series of assumptions about Satellite identification and tracking, LSF has set the objectives and several communications experiments that need to be conducted in search of possible solutions.
Thus, QUBIK-1 and QUBIK-2 will set out to help explore the following objectives:
- To unambiguously identify satellites as soon as possible after deployment
As we mentioned above, early satellite identification and tracking are of primary importance as they allow the operations team to act fast and address possible issues early on. Be it communications or altitude-related issues they should be dealt with swiftly, especially since failure to do so promptly, might render an operation unsuccessful. QUBIK-1 and QUBIK-2 are designed to enable easy RF Identification during the LEO Phase.
- Generate or update existing orbital elements based on Doppler curve tracking of satellite transmissions
The process of identification and tracking becomes even more complicated. As is often the case, nanosatellites and microsatellites are deployed in numbers, many together, from the same launch vehicle and flying in the same orbit. Distinguishing between them and tracking is a daunting task, puzzling the teams as the operators of these satellites often rely only on external tracking alone. They rely on the only available public resource providing orbital elements, and that is the Combined Space Operations Center (CSpOC) (through their space-track.org dissemination website.) Nevertheless, things get even more perplexed for operators as in addition to crowded deployments the small radar cross-section makes identification via CSpOC quite a challenge. Both of these parameters are the cause of significant delays in the publishing of the initial orbital elements. It can take up to a few weeks to identify a satellite accurately, and sometimes they might not be identified at all.
For dealing with the issues of misidentification and no-identification at all, for the QUBIK PocketQubes passive Doppler tracking will be utilised. This will be facilitated through SatNOGS (the global network of satellite ground stations) in order to independently determine orbital elements during the Launch and Early Orbit Phase.
- Extensively explore objectives 1 and 2 and do so in a way that adheres to the Principles of the Libre Space Manifesto.
All LSF’s operations and projects are led forward by the Principles of the Libre Space Manifesto, which constitutes the operational framework of all LSF processes. This means that the objectives mentioned above will be explored accordingly; in a scalable and open-source way with the data collected being distributed openly for everyone to have access.
- Create a reusable, open-source PocketQube Platform.
At LSF, we have been focusing our attention to the PocketQube format, and for this, we have worked hard to build a reusable, open-source ProcketQube Platform with a complete stack of open-source technologies developed for it. This is because we see potential in the PocketQube format as it is versatile enough and low-cost, able to facilitate a wide variety of experiments, payloads and to have a wide range of technology developed around it.
QUBIK-1 and QUBIK-2 will be used for testing, aiming at exploring the afore-mentioned objectives to the fullest. These will be investigated in-depth through a series of communications experiments regarding Identification and Tracking of Satellites.
This part of the experiments focuses on radio beacons and Telemetry transmissions, and they include:
Digital modulation schemes may use a preamble or a postamble in order to provide narrow-band transmissions which can help facilitate tracking from RF spectra. Distinguishing between satellites can be achieved by estimating the differences in preamble/postamble length. However, since preambles in most framing schemes are often the same, this constitutes preamble not an ideal solution to allow unambiguous identification.
By demodulating and then decoding the signals captured, this approach allows for identification via a call-sign or address within the packets. It requires full demodulation and decoding data chain. The assumptions required about SNR, RX sensitivity and RF collisions on narrow, non-spread-spectrum modulations will only compromise demodulation and decoding.
Note that this is one of the experiments that LSF will be running as this experiment does not require any changes to be made in the existing workflow. Thus, it will be introduced in QUBIK as a control-case (since the team already utilizes it).
Identifying a satellite by estimating the differences in beacon length. In crowded deployments, as is the case of QUBIK-1 and QUBIK-2, this is not a scalable option.
Differences in beacon cadence can also help distinguish between satellites, through estimating how often a beacon is transmitted. Though this approach helps prevent beacons and satellites from overlapping it can not be applied in crowded operations.
Barker codes can be utilised to provide monotonic identification right from the RF level by performing only cross-correlation at the raw signal. Barker codes require only bit-level changes, and they can be used to facilitate lower SNR identification and decoding, identifying a spacecraft right from the PHY. However, for this approach to work, it would require ground stations to have multiple decoders operating in parallel. As the number of the decoders would have to be possibly equal to the number of different code sequences, this approach is not scalable for us to pursue.
Spread spectrum low power beacon
For this experiment, the RILDOS proposed protocol was chosen, transmitting a beacon with low transmission power. The basic idea which we will explore is to use the spread sequences of the RILDOS protocol and retrieve a message from a satellite even in a negative SNR environment. What is more, with this experiment LSF will attempt exploring possible techniques that can be applied on an SDR-based ground station estimating the frequency drift between the spacecraft and the ground station. The techniques applied will be estimated and evaluated on the grounds of their accuracy and whether the RILDOS protocol can be used for both identification and tracking.
Keep in mind that even though RILDOS requires 2 Mbps, in this experiment and due to hardware restrictions, RILDOS will be tested in lower bandwidth. In this case, the basic features of the protocol will remain intact while the performance in terms of BER is expected to be affected and possibly degraded.
This too, will be one of the experiments that are scheduled to be conducted in QUBIK-1 and QUBIK-2.
The tracking of the QUBIK PocketQubes through their Doppler curves must be achieved by measuring transmitter frequency as a function of time. This can be determined directly either from waterfalls, the artifacts provided by the observations made by the SatNOGS Network or by demodulating transmissions. Tracking waterfalls does work well for lower signal-strengths, but when it comes to digital modulation schemes, it gets harder to obtain the transmitter frequency. This becomes difficult when transmissions of satellites overlap.
Waterfalls can facilitate satellite tracking for any modulation scheme with narrow-band features. CW would provide the highest accuracy at the expense of low bit rates.
By using a preamble or a postamble, digital modulation schemes can provide narrow-band features in the waterfalls. Examples of this approach are the BPSK transmissions of Funcube satellites or the FSK transmissions like those of UWE-4 or Firebird-4.
The residual carrier is a strong carrier found on top of the modulation spectrum mask as it runs for the entire frame duration. Usually, the residual carrier is utilised to drive the PLL that tracks any frequency drift that can be retrieved either from the RF level or visually.
Since the RF IC of QUBIK (AX5043) does not support an optional residual carrier, LSF will opt for an alternative yet innovative approach. By using QPSK with special precoding, it can be forced to use only two of the QPSK symbols. This produces a DC biased BPSK, which eventually will have a carrier at the center of the spectrum mask. What makes this approach possible is the absence of a DC block filter on AX5043.
If you want to have a look at the experiments in more detail, you can read the documentation.
The latest Updates: Final QUBIK-1 and QUBIK-2 Testing and PICOBUS Integration
The QUBIK team has been developing and testing the software throughout the summer, updating and implementing features; following strict procedures so that the QUBIK project to be ready for the final testing and integration. The last weekend of October 2020, was an exciting one, for us at Libre Space Foundation because not only did the final testing of the PocketQubes take place but also because of the integration of the PocketQubes in the PicoBus deployer.
Both procedures took place at the LSF Headquarters, at hackerspace.gr, in the centre of Athens, Greece and were carefully orchestrated and successfully carried out while being streamed live on the LSF YouTube Channel.
The PicoBus Deployer with the six integrated PocketQubes will be shipped to the US.
Firefly’s Aerospace Alpha launch is expected to happen as early as Dec. 22 with room in the schedule to launch as late as Jan. 31.
For us, at Libre Space Foundation, the QUBIK is an aspiring project of which we are immensely proud. It is a self-funded initiative to create an ideal environment for testing and conducting communications experiments. All the technology stacks used and the tools created are open-source. The solutions to be found (if any) will further enhance and enable a wide variety of experiments, payloads, technology development and missions.
All these are governed by and conform to the Libre Space Manifesto, and its Principles found etched on the QUBIK PocketQubes.