Global Navigation Satellite Systems have many wide reaching applications through their evolving technologies. The space environment is proving to be a great setting for new applications. Formation flying spacecraft and autonomous navigation in servicing missions are the two applications discussed below. The TanDEM-X mission will be the example for the formation flying spacecraft. Some topics covered for this mission will include hardware deployed, data products created, accuracies/precisions of the products, and applications for the products. With respect to the application of autonomous navigation, the Hubble Space Telescope (HST) Servicing Mission 4 (SM4) and its Relative Navigation Sensor (RNS) is the topic presented.
TerraSAR-X was launched from Baikonur, Kazakhstan on June 15, 2007. Since becoming fully operational in near the beginning of 2008 the spacecraft has been using its active X-band antenna to create high quality radar images of the Earth. Using this active form of remote sensing it can operate without relying on weather and lighting conditions. Some of the key technical features include its one meter per pixel high resolution operating mode. The TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) was launched on June 21, 2010. These similar satellites are in a “514 km altitude sun-synchronous dusk-dawn orbit with 97° inclination and an eleven day repeat cycle (Ardaens et al., 2008).” The two spacecraft have a variable baseline and TanDEM-X has the ability to make small corrections in its following orbit. The small perturbations due to the topographical differences on the Earth’s surface and the atmospheric drag create challenges for maintaining an appropriate baseline as well as avoiding collision of the spacecraft.
|TanDEM-X (image credit: infoterra)|
The hardware deployed for these spacecraft include a real-time onboard autonomous formation keeping module, called TAFF (TanDEM-X Autonomous Formation Flying), which makes use of the MosaicGNSS receivers available on the two spacecraft (Ardaens et al. 2007). The MosaicGNSS receivers are made by the aerospace subsidiary of the European Aeronautic Defense and Space Company (EADS) known as Astrium. They are radiation tolerant GPS that can even process weak signals to determine position, velocity, and time when less than four satellites are acquired for tracking (astrium.eads.net). To plan and carry out the orbit control maneuvers autonomously an additional Inter Satellite Link (ISL) hardware with a cold-gas propulsion system was integrated (Ardaens et al. 2008).
The Ardaens et al. paper conducted a simulation to determine the control performance of TAFF. This was done using both autonomous and ground-in-the loop control. For the ground-in-the loop control the performance was 2.7 meters radial, 26 meters along-track, and 0.4 meters cross-track. The TAFF performed at 1.7 meters radial, 6.5 meters along-track, and 0.3 meters cross-track (Ardaens et al. 2008). This indeed shows a better performance can be gathered by TAFF as stated in Jean-Sebastien Ardaens paper. These performances lead to high quality products made by the TanDEM-X mission. The mission has just recently started with data acqusition to follow in early 2011. The quality features include a vertical accuracy of 2 meters (relative) and 10 meters (absolute) and a horizontal raster of ~12 meters by 12 meters (infoterra.de/tandem-x_dem).
According to Infoterra the first regional DEMs will be available in 2013. Full global coverage is expected to be in the year 2014 (infoterra.de/tandem-x_dem). There are many applications for these digital elevation models. The uses in knowing more about the geomorphology of the Earth’s surface through DEM creation help in determining which areas will be affected by climate change first. The changing thickness of ice sheets and glaciers in the world’s most remote places can be measured by DEMs. Other mass wasting of the Earth’s surface in the form of avalanches and landslides can be studied for hazard control. Flooding of areas can be better modelled at large and small scales through this new data set. Presented was just a few examples but there are many more applications for a high quality DEM that the TanDEM-X mission will produce.
The evolving application of using GNSS technology for autonomous navigation has been recently used for the Hubble Space Telescope (HST) Service Mission 4 (SM4). A paper by Ian Cohen titled, “Relative Navigation Using Reflected GPS Signals” examines the design and implementation of a system that uses autonomous relative navigation. The simulations Cohen conducted for the HST SM4 included using an extended Kalman filter. Cohen used a GPS receiver that ran on a radiation hardened ColdFire Processor at 65.536 MHz with 2 MB of application SRAM. The simulations Cohen performed used a dynamic model of spacecraft maneuvering at close proximity in near circular orbits. This system called for a relative range measurement carried out by differencing the reflected pseudo-range and the direct pseudo-range (Cohen 2008). “Differencing is done to mitigate the common mode errors from the GPS signal, including ionospheric effects, oscillator bias, and front end noise; providing a relatively clean measurement of the relative position (Cohen 2008).” With enough measurement available, the Kalman filter was a sufficient solution (Cohen 2008). However, the algorithm had difficulty in short ranges and in the case of very long ranges signal visibility problems arise (Cohen 2008). The Cohen paper noted that they did not seek to examine the impact of the reflection on the measurement accuracy.
On the 11th of May, 2009 the Space Shuttle Atlantis lifted of the launchpad heading to the highest altitude it ever takes. The RNS system flying in the cargo bay of the Space Shuttle met many goals. In addition to the GPS receiver, the system hardware also consisted of three cameras, an electronics box for recording data, and a microprocessor system for running algorithms. The three cameras had varying optical ranges and the electronics box used several commercial hard drives (Naasz et al. 2010). The Navigator GPS receiver used was optimized for fast signal acquisition and weak signal tracking (Bamford et al. 2004). The threshold is between 22 and 25 dB-Hz, a great sensitivity allowing for better GPS observing (Bamford et al. 2004).
|STS-125 Launches (image credit: NASA GSFC)|
|View of HST from the Atlantis (image credit: STS-125 Crew, NASA)|
The GPS aspect of the system operated very well during the Rendezvous Proximity Operations and Docking (RPOD) procedure (Naasz et al. 2010). During the rendezvous an algorithm tracked HST for 20 minutes and 27 seconds with a peak quality of 99.2% and another algorithm tracked HST for 15 minutes and 31 seconds with a quality of 87.1% during the HST deployment phase (Naasz et al. 2010). In the review of this application there was no mention of the accuracy performed by the GPS found. A change to the relative approach attitude in the form of a 45 degree Shuttle yaw maneuver resulted in the RNS being mis-configured for the initial pose acquisition (Naasz et al. 2010). This is the reason why both algorithms were not in operation for the rendezvous part of the mission. The flight data for the algorithms of the RNS system were compared to truth data from the Shuttle program and the attitude results were within the desired levels but the path results were not (Naasz et al. 2010). The operating environment and its many variables continue to be the biggest difficulty for writing algorithms to perform autonomous navigation.
A team of engineers at NASA’s Goddard Space Flight Center are still working to continue the evolving instruments that can track weak GPS signals above the altitude of the GNSS constellations. This is the same team that developed the technology needed to perform the RNS experiment for the HST SM4. The current developed technology “will serve as the primary navigation sensor on NASA’s Global Precipitation Measurement Mission (GPM)” (www.nasa.gov). Another future mission to use this technology is the Magnetospheric MultiScale (MMS) mission. The next line of technology under development may be able to “acquire the GPS signal even if the spacecraft carrying the receiver is located at lunar distances” (www.nasa.gov). NASA Goddard’s Navigator team and other engineers working with various space agencies are helping reduce mission costs. The autonomy through formation flying and rendezvous/docking enables ground control costs to be lowered. The faster, better cheaper is not just a goal of NASA but ESA as well.
These two applications of GNSS technology in the space environment start to show the promise for the future. The TanDEM-X mission is only in the beginning and the world will see great data products come out of using formation flying spacecraft through the help of GNSS. The Space Shuttle program has ended but many lessons were learned from the STS-125 mission. In years to come it is likely that robotic missions will replace humans in the servicing missions of satellites and hopefully allow humans to reach new heights. The future of human space exploration will continue from Low Earth Orbit (LEO) to beyond, hopefully in the next 30 years. If humans return to the Moon for longer missions and continue on to Mars, there is little doubt that GNSS technology will be involved.
Ardaens, Jean-Sébastien, Simone D'Amico, Dieter Ulrich, and Denis Fischer. "TanDEM-X Autonomous Formation Flying System." 3rd International Symposium on Formation Flying, Missions and Technology (2008): 1-9. Print.
Astrium Mosaic GNSS Receiver LEO, MEO, GEO. EADS, 2010. Web. 26 Dec. 2010.
Bamford, W. et al., “Real-Time Geostationary Orbit Determination Using the Navigator GPS Receiver,” 2005 Flight Mechanics Symposium, NASA Goddard Space Flight Center, Greenbelt, MD, October 18-20, 2005.
Cohen, Ian R., "Relative Navigation Using Reflected GPS Signals," Proceedings of the 2008 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2008, pp. 224-231.
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Kriger, G, A Moreira, H Fiedler, I Hajnsek, and M Werner. "TanDEM-X: A Satellite Formation for High-Resolution SAR Interferometry." IEEE Transactions on Geoscience and Remote Sensing 45.11 (2007): 3317-41. Print.
Naasz, Bo, John V. Eopoel, Steve Queen, C M. Southward, and Joel Hannah. "Flight Results of the HST SM4 Relative Navigation Sensor System." 33rd Annual AAS Guidance and Control Conference 08.280 (2010): 1-24. Print.
TanDEM-X Global Homogeneous DEM. Infoterra, 2010. Web. 27 Dec. 2010.
TanDEM-X Satellite and Mission. Infoterra, 2010. Web. 27 Dec. 2010.
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