A Miniaturised GPS Receiver for Space Applications

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Copyright © IFAC Automatic Control in Aerospace, Saint-Petersburg, Russia, 2004

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A MINIATURISED GPS RECEIVER FOR SPACE APPLICATIONS Takuji Ebinuma 1 , Martin Unwin 2, Craig Underwood 1 , Egemen Imre 1 1 Surrey

Space Centre, University of Surrey, UK

2 Surrey

Satellite Technology Limited, UK

Abstract: An increasing number of small satellites are proposed for miscellaneous applications, many of which assume the use of Global Positioning System (GPS) for positioning, timing and orbit determination. Some of these satellites are only a few kilograms in mass, which is the same mass as a typical space GPS receiver. Equally demanding are the requirements for power consumption, where I watt is considered excessive, and also low cost. This paper describes the characterisation and adaptation of new small commercial component based GPS receivers for space use, including software performance validation, radiation testing, and preparation for demonstration on a satellite. Copyright © 2004lFAC Keywords: Global positioning systems, Satellite applications, Space vehicles, Navigation, Reliability evaluation.

I. INTRODUCfION

receiver provided position and timing through out the mission and also played a vital role in the rendezvous attempt with Tsinghua-l (Unwin, et al., 2000).

Rapid advances in commercial-off-the-shelf (COTS) electronic technologies in recent years make it feasible to construct small satellites that have a mass of between 1-10 kg (generally referred to as nanosatellites) or even sometimes less than I kg (picosatellites). This opens up the possibility of designing and building satellites with genuine applications as part of an education and training exercise for prospective spacecraft engineers at a price that is affordable to individual academic institutions. In 1997, the Surrey Space Centre embarked upon a program to design and develop a practical, low-cost nanosatellite platform, called Surrey Nanosatellite Application Programme (SNAP) . The first of these nanosatellites, SNAP-I, was a 6.5 kg vehicle, intended to demonstrate the nanosatellite platform concept and to act as an orbital test-bed for several new COTS technologies, including a GPS navigation payload, which weighs 50 g and consumes 2 W under nominal operation. The SNAP-I nanosatellite was launched on June 28, 2000, along with the Chinese rnicrosatellite Tsinghua-I. The GPS

Started in 1999, the CubeSat project has stimulated a boom in student-built small satellites all around the world (Puig-Suari, et al., 2002). The challenge is to develop tiny satellites weighing only I kg and occupying a volume of only 10X 10X 10 cm, which requires extremely tight integration and low power consumption. For these student projects and other similar applications, a miniature GPS receiver has been developed by Surrey Space Centre and Surrey Satellite Technology Limited (SSTL). The receiver is called SGR-05U, which comprises of SSTL's SGR-I0/20 GPS receiver software hosted on an adapted commercially available OEM GPS board. This paper describes the characterisation and adaptation of this new miniature GPS receiver for space use. The navigation performance of the SGR05U was evaluated using a GPS signal simulator with a low Earth orbit scenario. Radiation tests, including total dose and single event effect tests, were

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Table I Physical and electrical parameters of the SGR-05U receiver

performed to characterise the susceptibility of the new hardware. Finally, a brief introduction to the TopSat satellite mission, and its preparation for demonstration of the SGR-05U receiver on TopSat is presented.

Parameter Dimension Weight Operation Temperature Power Supply

2. THE SGR-05U RECEIVER The SGR-05U receiver is based upon a commercial MG5001 OEM GPS board manufactured by an Australian company called Sigtec. At the heart of the MG5001 is Zarlink Semiconductor's GP4020. The GP4020 incorporates a 12-channel correlator module for GPS Lt CIA code and carrier tracking, and a 32-bit ARM7TDMI processor core is integrated onto the same chip. The operation at 3.3V and the integration or correlators and processor, brings the power consumption down compared to previous Zarlink receivers.

range (originally intended for portable GPS products or other mobile wireless products). The checks and changes required for this antenna include: • Manufacturing and quality control for the space environment (material review, suitability under vacuum, temperature range) • Replacing cap with appropriate material for space applications (low outgassing, and resistance to atomic oxygen) • Ensuring correct tuning with new cap • Qualification under vibration • Matching LNA & filtering to GPS receiver

The GP4020 also offers a high level of compatibility with the previous generation Zarlink GP2021 correlator and ARM60P processor. These are used in SSTL's SGR-IO/20 spacebome GPS receivers, and so the receiver fmnware can be readily ported onto the SGR-05U board. An advantage of the new hardware is that the ARM7TDMI core supports 16bit Thumb mode operation in addition to standard 32-bit ARM mode operation. This permits normal 32-bit instruction code to be compressed into a small size, and operated off 16-bit wide memory, thus halving the power consumed by the memory while incurring only a slight degradation in processing speed.

3. RECEIVER PERFORMANCE V ALIDA TION The raw measurement accuracy and the expected onorbit navigation performance of the SGR-05U has been evaluated in a signal simulator test following the generic concept for spacebome GPS receiver testing designed by Montenbruck (Holt, et aI. , 2(03). In addition to the general pseudorange, carrier phase, and Doppler measurements, the SGR-05U receiver provides carrier-smoothed pseudorange and carrier based range-rate measurements. Raw pseudorange, carrier phase, and Doppler measurements exhibit typical noise level of 0.9 m, 1.5 mm, and 0.2 rnIs respectively at nominal signal-to-noise ratios. The carrier-smoothed pseudorange is typically accurate to 0.2 m, while carrier based range-rate is accurate to about 3.0 cm/so By default, the navigation solution is computed every second using carrier smoothed pseudorange and carrier based range-rate measurements. The low Earth orbit navigation performance was evaluated using a Spirent SGR4760 GPS signal simulator in the absence of broadcast ephemeris and ionospheric delay errors. The position errors are generally well below 1 m (l-sigma), and the velocity solutions are accurate to better than 8 cm/s (l-sigma).

Fig. 1. SGR-05U Space GPS Receiver plus antenna.

The resulting receiver, the SGR-05U (Fig. I), is only the core engine of a GPS receiver, and is missing many advanced hardware features of the SGR-10120, but its positioning performance is very similar. It also features very low power consumption (less than 0.8 W) and smaller physical dimensions. General physical and electrical parameters of the SGR-05U receiver are summarised in Table 1. A miniature antenna solution for the SGR-05U has been derived from Sarantel' s PowerHelix antenna

Value 70X45X 10 mm 20g O· C to +50· C 0.5-0.8 W at 5 V

Although the single point navigation solutions meet the accuracy requirements of many space applications, a further reduction of the navigation noise level can be achieved by smoothing the solutions in a simple dynamic filter, which is compact enough to run within the receiver software. A basic dynamic orbit model including only the Earth's J2 perturbation can efficiently remove shortterm noise in single point navigation solutions, especially for velocity terms. The filtered velocity

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errors are accurate to 2-3 cmls (l-sigma), about 70% better than the original single point velocity accuracy . Typical mean and standard deviation values of the filtered navigation errors are summarised in Table 2.

receiver is able to maintain a one-second navigation solution update rate and to reduce the average power consumption at the same time. The implementation of the trickle mode is being considered for future work.

Table 2 Filtered navigation accuracy using a very simple dynamics model

5. RADIATION TESTING

Radial Along-Track Cross-Track

Position [m] +0.16± 0.49 -O.OS± 0.21 -0.00+ O.IS

In collaboration with QinetiQ Ltd and ESA and with some funding from the British National Space Centre (BNSC), Surrey Space Centre embarked on a radiation test campaign in an attempt to characterise the susceptibility of the new chipset to radiation expected in an orbital environment.

Velocity [cmls] +0.20± 1.91 -0.42± 0.72 -0.32 + 0.S3

It should be noted that the receiver performance was evaluated in the absence of broadcast ephemeris and ionospheric delay errors. Typical ionospheric and broadcast ephemeris errors encountered in a 700 km low Earth orbit would limit positioning accuracy to around 10 m, but the velocity accuracy should not be affected by these errors so much.

5.1 Total Dose Testing

Total dose testing was performed at the University of Surrey using a Cobolt-60 gamma radiation source. The small size of the GPS receiver meant that the entire receiver could be radiated at once, enabling the characterisation of the whole receiver. The receiver was operated under successive doses of radiation until it failed .

4. DUTY CYCLE OPERATION

The SGR-05U features a duty cycle operation mode for further reduction of average power consumption. The main idea of the duty cycle operation is to activate the GPS receiver functions only few times per orbit to obtain accurate position and clock information. During the off-duty operation, the main GPS modules, i.e. the RF front-end and correlators, are turned off, and only the microcontroller keeps running to propagate the last navigation information to provide estimated position and velocity of the spacecraft for the user. The propagated spacecraft state is also used to initialise the receiver to allow for hot-start at the next on-duty cycle along with the saved Ephemeris information of the GPS satellites.

5.2 Single-Event Effects Analysis

Subsequently, the susceptibility of the GP4020 to single event effects (SEE) was tested in a campaign at the UCL Belgian Heavy Ion Facility. SEE testing is achieved by frring heavy ions at the device under test and observing susceptibility. The energy and choice of the heavy ions can be varied to simulate the effects of typical radiation experienced in orbit due to protons and cosmic rays.

Table 3 Duty cycle power consumptions Operation Mode On-duty Off-duty

Power consumption 670 mWat5V 305 mWat5V

Table 3 summarises the typical power consumption of the SGR-05U during on-duty and off-duty operations. It generally takes one minute for the receiver to get the first position fix from hot-start. Assuming 5-minutes on-duty and 25-minute off-duty operation, this provides roughly 3 duty cycles in a LEO orbit. Then the average current will be 670x 5 + 305 x 25 30

Fig. 2. Test die "el" mounted inside of the chamber. The GP4020 GPS Correlator was tested for SEE activity using the Heavy Ion Facility at UCL Belgium on July 6, 2002. Two dies were tested identified as "e I" and "e3". For SEE testing, it is necessary to expose the device to the ions, Le. the device had to be de-lidded. To enable proper test of the component, it must be embedded as part of an operational circuit so that radiation induced events

=366 (mW) .

An extreme case of the duty cycle operation is called trickle mode, in which the receiver toggles on- and off-duty cycles within a second. With this mode, the

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can be monitored dynamically. Each device was therefore prepared and mounted in a separate circuit board (Fig. 2). The results of the radiation testing will be fully documented elsewhere, but in summary the preliminary analysis gives the following results: • Total Dose Tolerance : Failure between 11 and 15 kRads, • Single Event Latch-up: The GP4020 will experience no latch-ups below about 15 MeV cm2 mg-I, and a saturated crosssection above that of around 10-4 cm 2 device-I. • Single Event Upset: The GP4020 internal RAM is quite sensitive to upsets with a threshold of around 3 MeV cm2 mg- I and 1.5 x 10-7 cm2 bif l cross-sectional area. • Single Event Functional Interrupts (SEFIs) have also been observed, and statistics gathered.

Fig. 3. TopSat Satellite. testing and simulated perfonnance characteristics have been presented, and the forthcoming flight on the TopSat satellite is described. This technology .is an example showing how continued advances In commercial electronics can be applied to create opportunities for low cost application on the smallest of satellites.

The implications of these results are that the SGR-05U will survive over 10 years in a low Earth orbit, and the GP4020 is unlikely to latch-up (although latch-up protection should be accommodated). Upsets may be reasonably common, and the GP4020 may suffer the occasional SER requiring a reset (or a self-reset to be induced by the system watch-dog). Taking into account these effects, the receiver should still be usable for low cost satellites. A "professional" version of the SGR-05U is being developed at SSTL, referred to as the SGR-05P. Amongst other enhancements, the SGR-05P will carry EDAC protected memory to improve robustness under radiation, but will otherwise have a similar performance.

6. DEMONSTRATION ON TOPSAT The first experimental demonstration of the SGR05U is expected to be on the UK satellite TopSat due to be launched in 2005. (See Fig 3.) The TopSat is a mission to demonstrate the use of small low-cost platforms for high-resolution imaging (Cawley, 2003; Brooks, 2001; Wicks, et ai. , 2001). The SGR05U receiver is currently being integrated into the satellite as an experiment of opportunity. TopSat already carries a multi-antenna SGR-20 receiver that will provide position, and be used for attitu~e determination experimentation. The SGR-05U IS flown in parallel, i.e. using a separate antenna, and its connection over the satellite's CAN-bus will permit the SGR-05U to be used as a back-up or low power alternative to the SGR-20.

7. CONCLUSIONS

ACKNOWLEDGEMENTS This work has been funded by SSTL, SSC, BNSC, and QinetiQ and ESA are also acknowledged for support in the radiation testing. Members of the GPS team have helped with this work, and thanks are also due for the opportunity on TopSat.

REFERENCES Brooks, P. (2001). TopSat - High Resolution Imaging from a Small Satellite. 15th Annual USU Conference on Small Satellites, SSCOI-I-2 Cawley, S. (2003). TopSat: Low Cost High Resolution Imagery From Space. 4th lAA Symposium on Small Satellites for Earth Observation,IAA-B4-0801. Holt, G.N., E.G. Lightsey and O. Montenbruck (2003). Benchmark Testing for Spaceborne Global Positioning System Receivers. 2003 AIAA Guidance, Navigation, and Control Conference and Exhibit, AIAA-2oo3-5666. Puig-Suari, J., C. Turner and R.J. Twiggs (2001). CubeS at: The Development and Launch Support Infrastructure for Eighteen Different Satellite Customers on One Launch. 15th Annual USU Conference on Small Satellites, SSC01-VIIIb-5. Unwin, M.J., P.L. Palmer, Y. Hashida and C. Underwood (2000). The SNAP- l and Tsinghua-I GPS Formation Flying Experiment. ION GPS2000, pp. 1608-1611. Wicks, A., S. Janson and J. Harrison (2001). An EO Constellation based on the TopSat Microsatellite: Global Daily Revisit at 2.5 meters. 15th Annual USU Conference on Small Satellites, SSCOl-I-6

This paper has described the development of a new receiver, SGR-05U. The design approach, radiation

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