The Role of GNSS in SAR Formation Flying
August 16, 2011 By: Rob Middleton, Steve Tsitas, Yong Li, Eamonn Glennon, Peter Mumford, Kevin Parkinson, Nagaraj Shivaramiah, Rui Li, Yuanyuan Jiao, Jinghui Wu, Faisal Khan, Joseph Gauthier, Andrew G. DempsterThe Garada project, funded by the Australian Space Research Program, aims to establish an Australian space engineering capability to provide a disaster-monitoring earth observation system.
Satellite-borne earth observation is very important for Australia and for other countries that need to monitor large, inaccessible areas. For example, in the summers of 2010 and 2011, 95 percent of Queensland, an Australian state more than twice the size of Texas, was disaster-declared as a result of extensive floods and cyclones. It would be impossible to map this from the ground, and mapping from airborne platforms is expensive, dangerous, and environmentally unfriendly. However, it is important to the emergency services, scientists, land-usage and town-planning agencies, manufacturing industry, agriculture, commerce, and insurance industries to know and understand the extent and nature of floods.
In a disaster monitoring context, large areas of the earth must be surveyed in a timely and dependable way. As weather and daylight cannot be relied upon, there is a demand for a remote sensing capability that is independent of daylight, i.e. an active system, and one that can image through cloud, rain, dust, smoke etc. An ideal candidate is Synthetic Aperture Radar (SAR), which can form two-dimensional, photograph-like imagery of large areas (thousands of square kilometres) of the Earth with high resolution (3 m) 24 hours a day and through most kinds of atmospheric obscurants.
In flood disaster monitoring there is a requirement for new and accurate imagery to be delivered every hour with a latency of less than half an hour. However, providing this kind of revisit rate to affected areas requires a large number of satellites, and SAR satellites are usually large, heavy, and expensive. It is the aim of the Garada project to investigate the Australian development of a new class of SAR satellites that use aggressively cutting-edge technologies to provide a low-cost constellation of satellites to give 24 hour, all-weather, high-resolution SAR imagery of the ground.
To enable this, low-power, solid-state radar systems, with the transmit and receive functions of the radar divided into two or more separate satellites (i.e. a bistatic configuration), flying in formation, are being investigated. New technologies also being investigated are flexible “sliver” solar cells, deployable antennas, ultracapacitor power systems, and more. But flying a bistatic formation also places unusual demands on the GNSS receivers.
In almost all formation-flying applications, GPS is used to provide highly accurate relative position of the individual components of the formation. This was always a requirement of the project, and to that end a space-qualified L1/ E1/ L5/ E5 receiver is being developed at ACSER. This should provide much better performance than current L1 space receivers. Algorithms to support this positioning requirement are being developed by project partners Delft University of Technology and Curtin University of technology.
The unique requirement arising from the Garada project as it has developed is for high levels of synchronisation and syntonisation (oscillator frequency matching) on the satellites. One of the important new research projects kicked off by Garada is an examination of how to modify the UNSW Namuru receiver to provide ultra-precise timing.
As luck would have it, a cubesat formation flying mission with Australia’s Defence Science and Technology Organisation (DSTO) came along soon after the award of the Garada contract. For this, standard formation flying positioning is required, and a simpler L1 GPS receiver can be used to determine the relative position between each pair of satellites in the formation. The satellites usually have GPS receivers installed, which produce the raw measurements including pseudoranges and carrier phase measurements. Although use of dual-frequency GPS carrier phase measurements can eliminate the ionospheric error, for the short baselines formed by the satellites, which are often high in the ionosphere, the ionospheric errors can be efficiently removed by the double-difference of the single-frequency carrier phase measurements. This can not only reduce the complexity of hardware design of GPS receivers but also simplify the relative navigation algorithm.
Orbital operation of the satellites introduces high dynamics to the receivers’ tracking loops, and possible frequent cycle slips. The atmospheric delay in orbit is different from ground applications. With the single-frequency carrier phase measurements, the relative navigation algorithm must take into account ionospheric correction for satellites in low earth orbits, especially when differences in the heights of satellites could lead to different values of total electron content (TEC). Moreover, the spacecraft’s dynamics would cause frequent change of visible satellites, the relative navigation algorithms should be carefully designed to deal with the carrier phase measurements in certain time-series data processing techniques, such as Kalman filtering. It is crucial to fix the integer ambiguities of the carrier phase measurements to achieve high relative accuracy (several centimetres). The algorithms for space-borne integer ambiguity resolution should have a high success rate, fast search, and low insensitive to cycle slips.
Both these projects are early in their lives, but initial results show that the new “space” version of Namuru’s Aquarius software produces carrier phase measurements that can be used to produce mm-level relative positioning in simulated space scenarios for formations with useful separations. Hardware for the single- and multiple-frequency versions 3.2 and 3.3 of Namuru will be available Q4 2011 and Q4 2012 respectively.
References
Shivaramaiah, N. C., Mumford, P.& Parkinson K., "Baseband Hardware Design for Space-grade Multi-GNSS Receivers", International GNSS Symposium, Sydney, Australia, to be held 15-17 Nov 2011.
Parkinson K, Mumford, P., Glennon E., Shivaramaiah, N. C.& Dempster, A. G., "A Low cost Namuru V3 receiver for Spacecraft operations", International GNSS Symposium, Sydney, Australia, to be held 15-17 Nov 2011.
Glennon E., Mumford, P., Parkinson K& Shivaramaiah, N. C., "Aquarius Firmware for UNSW Namuru GPS Receivers", International GNSS Symposium, Sydney, Australia, to be held 15-17 Nov 2011.
Eamonn Glennon, Nagaraj Shivaramaiah, Peter Mumford and Kevin Parkinson, "A GPS Receiver Designed for Cubesat Operation" 11th Australian Space Science Conference, Canberra, Australia, to be held 26-29 Sep 2011.
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