The e-POP GAP project

The CASSIOPE (the CAScade Smallsat and IOnospheric Polar Explorer) is a Canadian satellite scheduled for launch in 2009. It is a hybrid mission designed for a wide range of tasks including space-based communication and observations of the Earth’s atmospheric environment. A dedicated suite of eight scientific instruments, called the e-POP (the Enhanced Polar Outflow Probe), will investigate space storms in the upper atmosphere and provide GPS-based navigation information. The University of Calgary's Institute for Space Research leads the development of the e-POP. The project is co-funded by the Canadian Space Agency (CSA) and the Natural Science and Engineering Research Council (NSERC).

GAP (GPS Attitude and Profiling) is one of the eight scientific instruments. GAP has two components: GAP-A for position, velocity, attitude and time determination, and GAP-O for atmosphere occultation. Dr. Richard Langley, a Professor in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, leads the GAP instrument as the Principle Investigator.

GPS Attitude System for the e-POP Platform

Over the last decade, GPS receivers have been successfully used for attitude and orbit determination on microsatellites and minisatellites in low Earth orbit. As a result, it has been a trend in space missions to use cost-effective GPS receivers for space science and engineering experiments. The use of commercial components for spacecraft GPS receivers has been experienced on some other space missions. However, it is so far restricted to low-grade single-frequency receivers and a limited range of correlator chipsets. The use of a fully commercial, geodetic grade dual-frequency receiver with no heritage in space applications has been recently considered for space missions.

A rapid, precise and reliable GPS-based attitude determination system for satellites should be able to compete with existing space-deployed attitude systems such as star sensors. The precision of spacecraft GPS attitude determination is mostly at the 0.5-1.0 degree level. In terms of attitude precision attainable from GPS attitude determination system, multipath and the baseline length between the antennas will be the principal limiting factors.

We developed a real-time GPS attitude determination system using commercial, geodetic grade dual-frequency GPS receiver, slightly modified for the e-POP platform onboard the Canadian CASSIOPE spacecraft to be flown in low Earth orbit.

GAP Design

The design of GAP is based primarily on the use of commercial-off-the-shelf (COTS) GPS receiver technology. Early in the mission design, it was decided to base the GAP instrument on a COTS dual-frequency receiver rather than a space qualified one. The decision was based primarily on economics. NovAtel’s OEM4-G2L dual-frequency receivers have been selected as the candidate hardware for this project. A series of tests were carried out to help determine the viability of using COTS GPS receivers for a satellite mission.

Conceptual view of the CASSIOPE spacecraft

The GAP instrument has been designed and constructed in collaboration with Bristol Aerospace. The GAP interface card EM (Express Module) used to interface the e-POP data handling unit with the GPS receiver cards is based on Bristol Aerospace STARS controller architecture with spaceflight heritage and an added FPGA (Field Programmable Gate Array). Some other components (such as patch antennas) also have spaceflight heritage.

A total of five GPS receivers on the satellite will be used for high precision navigation, attitude determination and radio occultation measurements. The four antennas to be used for navigation and attitude determination, together with their associated equipment called GAP-A, are mounted on the zenith-facing side of the spacecraft and one antenna for occultation, together with its associated equipment called GAP-O, on the anti-ram (i.e., anti-velocity) side of the spacecraft. GAP-A collects and processes simultaneous observations from three of the GPS receivers. Four receiving antennas including one spare will be mounted on the spacecraft. These antennas are mounted in locations to minimize multipath reflections and maximize the baseline length between the antennas. GAP-O consists of a dual-frequency GPS receiver, with a switchable spare, to collect GPS occultation data at a 20 Hz data rate sufficient for ionospheric tomography science.

GAP-A antenna/baseline geometry

GPS Attitude Determination

The DD (double-differenced between satellites and receivers) carrier-phase observations are used for spacecraft attitude determination in our approach while the DD pseudorange observations are used for estimating nominal baseline components and float ambiguities. GPS0 is considered as the base station while GPS1 and GPS2 are considered as the rovers. GPS3 is the spare antenna. At each epoch, the navigation solution of GPS0 is taken as the Earth-centred Earth-fixed (ECEF) position of the spacecraft. The system is based on the general purpose UNB RTK (real-time kinematic) engine which has been used for various scientific and engineering applications. It includes differential carrier-phase ambiguity resolution and position/velocity estimation. The attitude of the spacecraft is determined by estimating the rotation matrix between the body-fixed and ECEF frames using two baseline vectors (i.e., GPS1-GPS0 and GPS2-GPS0) and one vector orthogonal to them (i.e., the cross product of the two baseline vectors).

GPS attitude determination

Performance Test

To demonstrate the capabilities of the attitude software, three different hardware systems were used as GAP software test beds, including a laptop computer, the Bristol SPP (System Platform Processor) controller, and the GAP interface card EM. The Bristol SPP controller is a multipurpose controller board developed for sounding rocket missions. It features a Motorola DSP56309, 128 KB SRAM (Static Random Access Memory) memory, Flash Memory, and two RS-232 serial ports. The GAP interface card EM is based on the Bristol STARS controller architecture with added FPGA.

A 3-axis motion table was built using stepper motors and stepper motor controllers (Pontech STP100). Also, an Ethernet-to-serial controller (Sollae EZL-400s) was integrated in the test bed. This add-on device enabled the motion table to be accessed and controlled from a remote place. The rotation angles measured by each stepper motor can be used as the reference of attitude solutions computed using the three GPS receivers. To accomplish this end, the stepper motors and GPS receivers should be synchronized in time.

GAP software test bed configuration

Concluding Remarks

Out of a dedicated suite of eight scientific instruments for the e-POP mission, GAP provides an accurate absolute time reference, spacecraft position and velocity information to the data handling unit. Also, it will perform spacecraft 3-axis attitude determination. Due to the limited resources of the spacecraft available for GAP real-time attitude determination, its operation may take place only over a short time period. For that reason, our approach to resolve carrier-phase ambiguities is based on epoch-by-epoch ambiguity resolution, which resolves ambiguities instantaneously at the current epoch. At each epoch, two baseline vectors are computed by the UNB RTK engine, and then they are converted into the attitude solutions. The tests conducted so far provide good evidence for a proper functioning of the attitude software.

Further testing of GAP has been carried out at the University of Calgary's Institute for Space Research, which is leading the development of e-POP, and at Bristol Aerospace. Subsequently, additional testing took place during the spacecraft assembly, integration and test program for the e-POP payload at the Canadian Space Agency’s David Florida Laboratory in Ottawa.

Motion Tables in Action (Videos)

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News Articles

• NB Telegraph-Journal

• News@UNB

• UNB GGE News

References

• Langley, R. B. and D. Kim (2008). “An update on the GAP instrument.” Viewgraphs presentation. The e-POP/CASSIOPE Science Team Meeting #12, Ottawa, 19-21 November.

• Langley, R. B. and D. Kim (2007). “The CASSIOPE satellite ionospheric profiling experiment.” Viewgraphs presentation. URSI 2007, North American Radio Science Meeting, Ottawa, 22-26 July 2007.

• Kim, D. and R. B. Langley (2007). “GPS RTK-based attitude determination for the e-POP platform onboard the Canadian CASSIOPE spacecraft in low earth orbit.” Proceedings of ENC-GNSS2007, Geneva, Switzerland, 29 May -1 June, pp. 980-991.

• Serrano, L., D. Kim and R. B. Langley (2006). “Performance analysis of the NovAtel OEM4-G2L receiver for low earth orbit satellite tracking.” University of New Brunswick. Technical report for Bristol Aerospace, Ltd., 8 May.

• Kim, D. and R. B. Langley (2006). “GPS Attitude, Positioning and Profiling (GAP) – Real-time attitude software.” Viewgraphs presentation. e-POP Instrumenter’s Meeting, Royal Military Collage, Kingston, ON, Canada, 22 February.

• Serrano, L., D. Kim and R. B. Langley (2006). “GPS Attitude, Positioning and Profiling (GAP) – Performance analysis of the NovAtel OEM4-G2L receiver for LEO satellite tracking.” Viewgraphs presentation. e-POP Instrumenter’s Meeting, Royal Military Collage, Kingston, ON, Canada, 22 February.

• Langley, R. B. and D. Kim (2005). “Report on sensitivity of GPS antenna covered with AZ technology RM-400 coating.” University of New Brunswick. Technical report for Bristol Aerospace, Ltd., 27 July.

• Langley, R. B., D. Kim and L. Serrano (2005). “e-POP GAP radio science update.” Viewgraphs presentation. e-POP STM#7, University of British Columbia, Vancouver, BC, Canada, 2-3 June.

• Langley, R. B. and D. Kim (2005). “Spaceborne GPS: The UNB experience.” Viewgraphs presentation. SWIFT – GPS Occultation Instrument Science Meeting, Canadian Space Agency, Saint Hubert, QC, Canada, 9 February.

• Langley, R. B., O. Montenbruck, M. Markgraf and D. Kim (2004). “Qualification of a commercial dual-frequency GPS receiver for the e-POP platform onboard the Canadian CASSIPOE spacecraft.” Proceedings of NAVITEC '2004, the 2nd ESA Workshop on Satellite Navigation User Equipment Technologies, ESTEC, Noordwijk, The Netherlands, 8-10 December 2004, pp. 397-405.

• Kim, D., C. Kang and R. B. Langley (2004). “Users’ guide for software interface of the GAP interface card with NovAtel OEM4 family of receivers.” University of New Brunswick. Technical report for Bristol Aerospace, Ltd., July.