A Look Back the First Use of COTS GNSS Receivers in Space
Lessons Learned from CanX-2 Nanosatellite Success Continues to Drive Advancements
A decade ago, the CanX-2 nanosatellite, part of the Canadian Advanced Nanospace eXperiment (CanX) program to develop a series of low-cost, ultra-light-weight orbital research platforms, launched into low Earth orbit. It was the first student designed, built and operated CubeSat mission to successfully operate a Commercial-Off-The-Shelf (COTS) GNSS receiver in space.
While the CanX-2 experiments have long been complete, the foundational knowledge and successes gained with the GNSS receiver were an important forerunner to today’s rapidly growing low cost small satellite (smallsat) and nanosatellite market.
In a report titled, “Small Satellite Launch Services Market, Quarterly Update Q1 2018, Forecast to 2030,” the Global Aerospace & DefenseResearch Team at Frost & Sullivan estimates that more than 30 commercial smallsat operators are planning to launch up to 4,425 satellites over the next three to five years. To put that progression in perspective, less than 100 nanosatellites were launched prior to 2014, and while considerably more launched between2014-2018, those total numbers ranged in the hundreds, not thousands. Further, the report anticipates a demand for 11,631 small satellites between 2018-2030.
We take a look back at CanX-2 demonstration technologies that set the groundwork for the current small satellite movement and a new era in small, cost effective space exploration systems.
The seminal mission of the GNSS receiver was aboard CanX-2 to demonstrate that a nanosatellite is a suitable platform for a scientific radio occultation experiment.
Radio occultation is an established technique for gathering meteorological data, which is then used in global weather models to increase forecast accuracy and could be used to monitor climate change.
The approach relies on a space-based GNSS receiver processing signals from GNSS satellites. Once measurements between the satellites are collected, an algorithm measures the refraction of GNSS signals passing through the atmosphere, which varies depending on the concentration of temperature and water vapor.
A decade ago, only specialized space based GNSS receivers were custom equipped to survive the rigors of launch, the temperature extremes of space, operation in a vacuum and the increased radiation from the sun. As well, the extreme velocity of a satellite leads to a much broader range of Doppler shifts for incoming GNSS signals as compared to a receiver on the ground. Therefore, the search algorithm typically took longer to run, find the signals and make a position fix. In most cases, the specialized receivers facilitated multiple channels to perform parallel searches and rapidly acquire the signals with the combined input from multiple antennas.
These customized receivers exceeded cost, weight, size and power draw limits for use in a CubeSat solution. The CanX-2 team sought to prove that COTS technology could work with slight modifications.
Adapting the Receiver
The CanX-2 radio occultation experiment was designed by a team of students at the University of Calgary while the satellite was designed and built from commercial-grade components at the University of Toronto’s Institute for Aerospace Studies (UTIAS) Space Flight Laboratory.
CanX-2 is a triple CubeSat that measures 10 cm x 10 cm x 34 cm and weighs approximately 3.5 kg. It incorporates two high performance computers, a three-axis momentum bias coarse pointing attitude control system, a miniature reaction wheel, a nanosatellite propulsion experiment, a high data rate S-band transmitter, two CMOS imagers, a battery and a series of solar panels.
CanX-2 scientific payloads included an atmospheric spectrometer (to characterize greenhouse gases and atmospheric pollution), a space materials experiment and a NovAtel®, part of Hexagon’s Positioning Intelligence division, OEM4-G2L dual-frequency geodetic grade GNSS receiver for the radio occultation experiment.
The team reduced the receiver’s carrier phase smoothing firmware to the minimum possible level to avoid corruption of the code ranges by ionospheric effects. The elevation mask was opened from the default of five degrees above the horizon to 45 degrees below the horizon to enable tracking through the portion of the sky below the nanosatellite.
CanX-2 was launched into low Earth orbit on April 28, 2008 from Sriharikota, India. It orbits at 635 km above the ground with an inclination of 98 degrees. CanX-2 is also sun-synchronous, so that the satellite always crosses the equator at the same local solar time. The orbital velocity of CanX-2 is approximately 7 km/s. It is controlled by a single ground station at the University of Toronto.
In the radio occultation experiment, GNSS data was collected on orbit and used in conjunction with an orbital propagator, which enabled the calculation of the satellite’s changing position.
CanX-2 was unique in that it attempted to collect radio occultation data using a single GNSS antenna, forcing a compromise between positioning geometry and scientific results. The system needed to find a balance between maximizing the signal power on the occulting GPS satellite by pointing the antenna rearward while still maintaining positioning accuracy with the remaining visible GNSS satellites. The objective of the CanX-2 radio occultation payload was to collect GNSS data of sufficient quality to extract atmospheric profiles working within these two constraints.
Therefore, a fundamental requirement of the GNSS receiver was the ability to turn on intermittently and acquire both a position solution and lock onto the occulting GNSS satellite prior to the start of an occultation event.
To warm start the receiver, the system needed to know the current time, the antenna’s field of view, the receiver’s clock drift, and the Doppler shifts on the GNSS signals, as calculated from the changing positions of the GNSS satellites and the changing position of CanX-2. To improve the process, the radio occultation team would download the satellite data from the Internet, make scripts for the coming week based on radar tracking and then email those scripts to the University of Toronto, where those students would upload them to the satellite.
The scripts would run at a specified time and then the Toronto-based team would download the GNSS data and send all the results to the University of Calgary team the following week.
Dr. Erin Kahr, a former grad student with the Department of Geomatics Engineering at the University of Calgary and key student investigator on the CanX-2 GNSS payload, she now works with the German Space Operations Center in Munich, Germany. She notes, “Even the onboard systems that needed position data were run based on uploaded NORAD data rather than GPS data, because the receiver was so rarely turned on. Even if we had wanted to use the GPS data for starting the receiver, the turnaround time was way too long.”
The team hit some snags along the way. All the trials run from November 2008 to February 2009 failed to track sufficient GNSS satellites for a position fix. At most, three satellites were tracked simultaneously. After some debugging, tests began again two months later. From April to June of 2009, the first positioning data was collected, and the channel assignment scripts were refined to better suit the needs of the radio occultation experiment.
From December 2009 to July 2010, radio occultation data was successfully collected. From July to April of 2010, the focus of GNSS data collection shifted to longer arcs at lower logging rates, suitable for orbit determination work.
Kahr adds, “Remember when the satellite was designed, nobody had ever flown a COTS NovAtel receiver in space so there was no guarantee it would work—but it did. In fact, that’s the real legacy of CanX-2’s receiver. In subsequent missions, like CanX-4 and -5, critical systems relied on the receivers to run and perform well because we’d proven their capability.”
Flying in Formation
The legacy of CanX-2 mission success is evident in several Canadian space programs to this day. For one, all later generation CanX satellites rely on NovAtel receivers.
Built on the UTIAS Space Flight Laboratory Generic Nanosatellite Bus (GNB) platform, both CanX-4 and -5 spacecraft carry identical COTS NovAtel OEMV-1G receivers, with 14 single frequency GPS L1 channels and a single Antcom 2G15A-XM-1-B COTS patch antenna. The receivers were slightly modified and qualified for space flight at the University of Toronto to provide real-time relative positioning information to the sophisticated onboard system.
Subsequently, CanX-4 and -5 were the first nanosatellites to autonomously maintain position in precise formation with sub-metre control accuracy in low Earth orbit. The nanosatellites flew without assistance from ground control in four planned orbital configurations with separations varying from 50 metres to one kilometre at 7 km/s.
CanX-4 and -5 investigators noted in a recent paper: “The receivers onboard CanX-4 and -5 exhibit an excellent tracking capability, with code noise on the order of 20 cm, carrier phase noise on the order of a few mm, and a peak signal strength of up to 51 dB-Hz in keeping with expectations for geodetic grade GPS receivers. The antennas’ fields of view are nearly omnidirectional, allowing the receivers to maintain lock in spite of frequent, dramatic changes in attitude during formation keeping phases.”
The Canadian Aeronautics and Space Institute (CASI) honored the UTIAS Space Flight Laboratory with its 2010 CASI Alouette Award for CanX-2 and the 2018 Alouette Award for the CanX-4 and CanX-5 nanosatellite precision formation flying program. The Alouette Award is given to individuals or groups for outstanding contribution to advancement in Canadian space technology. The selection committee noted that precision autonomous formation flying has practical application in sparse aperture sensing, ground target tracking, precise geolocation and on-orbit servicing.
Next Gen Solutions
As the Frost & Sullivan report pointed out, the demand for smallsats and nanosatellites, like those developed in the CanX program, are expected to grow exponentially in the next decade. Further, the study found that currently most smallsat commercial operators are in North America, with about 50 percent from the United States, 10 percent from Canada, and 25 percent from Europe.
The mission objectives for research institutions, government agencies and private sector companies is broad, though many are focused on using smallsats to clean up space, for scientific observation (e.g. space weather, Earth imaging, weather predications) and to better connect our world.
For example, the UTIAS Space Flight Laboratory’s CanX-7 nanosatellite is testing drag sail technology, a deorbiting system to ensure that small satellites comply with Inter-Agency Space Debris Coordination Committee (IADC) guidelines for space debris mitigation.
Similarly, a NASA Ames team has been working on what it calls an Exo-Brake, a parachute system that will be used for smallsat disposal and to support on-demand sample return from low Earth orbit scientific/manufacturing platforms such as the International Space Station.
In terms of communication, Vivek Suresh Prasad, Space Industry Principal for Global Aerospace, Defense & Security at Frost & Sullivan notes in the Small Satellite Launch Services Market report, “While just more than 40 percent of the total number of operators plan to offer affordable Earth observation solutions, more than 21 percent are targeting narrowband Internet of Things (IoT) and machine-to-machine (M2M) connectivity solutions, in response to the downstream digital transformation of various industries.”
Along with the development of smallsats and nanosatellites, nanosatellite launch vehicle technologies have begun to emerge. The Frost & Sullivan report estimates that the global smallsat launch services market will “soar past the forecasted revenue of $62 billion in the period between 2018 and 2030.”
Prasad noted in a recent story that “more than 40 launch vehicles with a less-than-two-ton payload capacity are under development and are set to be operationalized in the next two to four years.”
For instance, Virgin Orbit’s LauncherOne, on track for inaugural liftoff later this year, is designed to launch small satellite payloads (i.e., up to 300 kg) into low Earth orbit. Similarly, Generation Orbit Launch Services, Inc. (GO) has developed a full-scale, functional prototype of the GOLauncher1 (GO1) hypersonic flight test booster to deliver both nanosats and sub-50 kg microsats to low Earth orbit.
The small satellite market has come a long way since the launch of CanX-2 in 2008—and much of the current momentum can be traced to the successful development and testing of GNSS signal occultation on CanX-2 and subsequent CanX payload experiments.