GNSS in Space, Part 2: Formation Flying Radio Frequency Techniques and Technology

PRISMA satellite (Photo courtesy of Swedish Space Corporation (SSC) and Intespace)

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In the November/December issue of Inside GNSS, the first part of this column described the upcoming scientific missions that fly two or more smaller satellites in close formation to create large spaceborne instruments.

The final part of this series explains the GNSS techniques and technologies employed to achieve very accurate relative positioning and orientation of the spacecraft at lower altitudes as well as a similar approach used at higher altitudes for relative positioning by means of RF carrier phase measurement techniques.

FF Missions Metrology Requirements
. . .In general, all the proposed scientific missions discussed here have FF elements with closed-loop formation control in non-Keplerian orbits, typically at L2 (the second Lagrange Point about 1.5 million kilometers from the Earth) or HEO. They all have demanding accuracy requirements and are sufficiently consolidated.

. . .

From the overview of the FF mission metrology requirements, we can identify four main development lines in the frame of spacecraft formation flying.

Earth Observation Missions. These missions, in LEO orbit, will respond to the demand for highly accurate Earth models on a global space and time scale. Two or more satellites of identical type and build are flown at close distances to synthesize three-dimensional baselines between the satellites that can be reconfigured during the mission lifetime.

. . .

Dual Spacecraft Telescopes. These instruments aim at spectral investigation of sources that are too faint for study with the current generation of observatories (e.g., Chandra, XMM-Newton). The typical mission profile seeks orbits with a low level of perturbations, stable thermal environment, lack of eclipses, and wide sky visibility.

. . .

Multi-Spacecraft Telescopes. The third type of application addresses the use of multiple spacecraft telescopes. Researchers have identified interferometry in the infrared and visible wavelength regions as the key technology to support new astrophysical discoveries and the direct search for terrestrial exoplanets.

. . .

Long-Range and RdV Missions. The last type of application involves long-range and rendezvous (RdV) missions. These types of missions require an RF sensor technology, combined with the navigation algorithms of the GNC system, during the long-range phase while the satellites are far apart. The chaser vehicle must be able to detect, acquire, and track the relative position of the target spacecraft to close on, and then perform, the final approach and docking.

. . .

The RF Metrology Subsystem
On FF missions, the FFRF subsystem is responsible for the relative positioning of two to four satellites. It generates relative position, velocity, and line-of-sight (LOS) data as inputs to a GNC subsystem for which it provides coarse measurements.

. . .

Measurement Principle and Factors

To allow the determination of relative position and relative speed, the FFRF subsystem provides the following fine-mode information every second:

• intersatellite scalar distance (specified precision = one centimeter)

• intersatellite velocity with a precision of a few millimeters

• azimuth and elevation of line of sight between two satellites (specified accuracy = one degree)

• azimuth and elevation variations

• clock bias between the two satellites.

. . .

Terminal Architecture
The FFRP terminal’s architecture is largely based on results of an ESA Technology Research Program study and CNES architecture studies. It reuses some of the components and software from a 12-channel, L1 spaceborne GPS receiver.

. . .

Multipath Calibration

Signal reflections caused by the satellite structure surrounding FFRF antennas will be the major source of error on FFRF LOS and distance measurements. These multipath errors can reach several centimeters on phase measurements—resulting in a significant degradation of precision.

. . .

Future Trends
As we have seen, a high number of developments made for FFIORD will be directly reusable for other FF missions.

. . .

Conclusion
GNSS constellations are the most practical way to perform RF formation flying in LEO, and autonomous two-way transmission of GNSS-like S-band signals is a better way to perform FFRF in HEO or within the Lagrange points. PRISMA is a unique opportunity in Europe, both technically and programmatically, to validate under real conditions the basic feature of any non-LEO future FF mission—the RF-based autonomous metrology, using GPS-C/A-like signals and techniques.

By early 2009, an autonomous RFFF sensor shall be flying onboard the PRISMA satellites. This sensor will use GPS-like signals in S-band. Later, in 2012, the ESA PROBA-3 and CNES Simbol-X spacecraft will demonstrate the technology in scientific missions in HEO orbit.

However, to achieve the required accuracy, IAR on carrier phase will be needed. For this to succeed, multipath errors will have to be mitigated by calibrating the multipath on the ground to make in-flight corrections.

For the complete story, including figures, graphs, and images, please download the PDF of the article, above.

Additional Resources
[1] Bourga, C., et al., “Autonomous Formation Flying RF Ranging Subsystem,” Proceedings of ION GNSS 2003, Portland, Oregon, USA, September 2003
[2] Cledassou, R., Ferrando, P., “Simbol-X: An Hard X-Ray Formation Flying Mission,” Focusing Telescope in Nuclear Astrophysics Gamma Wave Workshop, Bonifacio, France, September 2005
[3] Garcia-Rodríguez, A., Formation Flight (FF) Radio-Frequency (RF) Metrology. ESA/ESTEC Technology Dossier, Issue 1.2, Noordwijk, the Netherlands July 2008
[4] Godet, J. et al., “Improving Attitude Determination of Satellites,” Internationnal Workshop on Aerospace Applications of the GPS, Breckenridge, Colorado, USA, February 2000
[5] Harr, J., et al., “The FFIORD Experiment: CNES’ RF Metrology Validation and Formation Flying Demonstration on PRISMA,” 3rd International Symposium on Formation Flying, Missions and Technologies, Noordwijk, the Netherlands, April 2008
[6] Issler, J., et al., “Lessons Learned from the Use of GPS in Space; Application to the Orbital use of GALILEO,” Proceedings of ION GNSS 2008, September 2008
[7] Lestarquit, L., et al., “Autonomous Formation Flying RF Sensor Development for the PRISMA Mission,” Proceedings of ION GNSS 2006, Fort-Worth, Texas, USA, September 2006
[8] Persson, S. et al., PRISMA: An Autonomous Formation Flying Mission, Small Satellite Systems and Service Symposium, Chia Laguna, Sardinia, Italy, September 2006
[9] PROBA-3 <http://www.esa.int/techresosurces/index.html>

Manufacturers

PRISMA is a Swedish National Space Board (SNSB) mission, undertaken as a multilateral project with additional contributions from CNES, the German DLR, and the Danish DTU. The prime contractor is the Swedish Space Corporation (SSC), Solna, Sweden, responsible for design, integration, and operation of the space and ground segments, as well as implementation of in-orbit experiments involving autonomous formation flying, homing and rendezvous, and three dimensional proximity operations. It employs Phoenix GPS receivers developed by DLR that incorporates the GP4020 chip from Zarlink Semiconductors, Ottawa, Ontario, Canada.

The FFRF subsystem development is currently in phase C/D, with Thales Alenia Space-France, Toulouse, France, as the prime contractor on both the subsystem and FFRF terminal level. In turn, TAS-F is relying on the following subcontractors:

• Thales Alenia Space España (TAS-E, Madrid, Spain) for development of the RF modules of the FFRF terminal (RF front end, RF transmitter section, RF receiver section), which incorporate a digital technology and software coming from the TAS TOPSTAR 3000 spaceborne GPS receiver.
• GMV (Madrid, Spain) for development of the navigation software, including implementation of PVT algorithms
• Thales Avionics (France) for development of the FFRF terminal signal processing software
• Saab Space (Göteborg, Sweden) for the S-band helix antennas
• an OXCO from TES Electronic Systems, of Bruz, France, is currently being qualified; an OXCO from Composants Quartz et Electronique (Temex), Mougins, France is also being used.

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