PRISMA radioelectrical mock-up with S-band helix antennas in CNES anechoic chamber (2007 campaign)
GNSS in Space, Part 2: Formation Flying Radio Frequency Techniques and Technology
PRISMA satellite (Photo courtesy of Swedish Space Corporation (SSC) and Intespace)
Working Papers explore the technical and scientific themes that underpin GNSS programs and applications. This regular column is coordinated by Prof. Dr.-Ing. Günter Hein. Contact Prof. Hein at Guenter.Hein@unibw-muenchen.de
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
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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.
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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.
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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.
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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.
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The RF Metrology Subsystem
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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:
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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.
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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.
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:
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