Phoenix Miniature GPS Receiver

Receiver Description

"Phoenix" is a follow-on of the Orion GPS receiver for space applications that uses the GP4020 chip to achieve a higher integration level and lower power consumption. The GP4020 chip combines a GP2021 twelve channel L1 C/A correlator with an ARM7TDMI microprocessor. The Phoenix receiver makes use of the industrial SigTec MG5001 board but run's a proprietary software of DLR/GSOC for space and high-dynamics applications.

Fig. 1 Phoenix GPS receiver

At a size of 50 x 70 mm and a power consumption of less than 0.8 W, the receiver is ideally suited for micro-satellites with restricted onboard resources. In case of very tight power budgets, the receiver can also be operated intermittedly. A rapid signal acquistion is assured by aiding the receiver with approximate orbital information in the form of NORAD twoline elements. Critical data like GPS almanac and broadcast ephemerides as well as user orbit parameters can be stored in a buffered non-volatile memory and used to initialize the receiver after a reboot. Likewise the current time is maintained by a real-time clock during deactivation of the main power supply. This allows a hot start of the receiver with typical times-to-first-fix (TTFF) of less than 30s. The Phoenix receiver has been tuned for highly accurate tracking, navigation and timing. It offers

the synchronization of all measurements with integer GPS seconds,
a pulse-per-second signal aligned with the integer GPS second to the order of 0.2 us,
the use of a 3rd order PLL with FLL assist for accurate carrier tracking at high dynamics,
the provision of carrier phase measurements with integer ambiguities for precise relative navigation,
the use of a carrier aided narrow-band DLL for code tracking and low-noise pseudorange measurements,
as well as the computation of carrier phase smoothed pseudoranges and carrier based range-rate.

The Phoenix GPS receiver is available in various versions, which basically differ by the employed receiver software. Aside from the standard receiver, which is restricted to terrestrial applications, a space (-S) version and a high dynamics (-HD) version are available. These employ specific trajectory models to enable a safe and rapid signal acquisition under rapid motion of the host vehicle. For satellites in low Earth orbit, aiding is provided by an analytical orbit model using twoline elements, whereas a set of piecewise polynomials is employed to approximate the trajectory of ballistic vehicles (sounding rockets, re-entry capsules) in the HD version. Various commands specific to each of these versions are provided to load, dump and use the respective aiding information. The two versions also differ by their choice of FLL/PLL loop settings that are adapted to the specific application needs. A narrow bandwidth of the carrier tracking loop is chosen in the Orion-S receivers to achieve the most accurate carrier phase measurements under typical line-of-sight accelerations of 1 G. Wide bandwidth settings, in contrast are chosen in the HD receivers to accommodate the extreme dynamics of a powered flight and the re-entry shock. In addition, an onboard prediction of the instantaneous impact point (IIP) in the Phoenix-HD receiver.

Benefits of the Phoenix/MG5001 board include a high micro-processor performance which allows navigation rate up to 5 Hz without further code modifications. The CPU and memory provide enough spare capacity for adding advanced receiver features or custom specific software extensions. A revised software can be uploaded at any time via the serial interface thus increasing the flexibility of the in-orbit receiver operation. Raw code measurements exhibit a typical noise level of less than 0.4 m and carrier-phase measurements are accurate to better than 0.7 mm.

Fig. 2 Phoenix-S measurement noise as a function of carrier-to-noise ratio (click image for enlarged view)

Despite a high raw data quality, the achievable single-point navigation accuracy is typically limited to 10m (3D rms) due to broadcast ephemeris errors and unaccounted ionospheric path delays. To cope with these limitations, an eXtended Navigation System (XNS) for the Phoenix receiver has been developed. The navigation system makes use of an extended Kalman filter which provides a dynamical smoothing of the raw GPS measurements. Benefits of the dynamical filtering include a substantial noise reduction and the capability to bridge periods of poor GPS visibility or outages through numerical propagation of the orbit. Furthermore, the filter supports the elimination of ionospheric path delays. The Phoenix-XNS receiver can thus achieve a real-time navigation accuracy of about 1m. The XNS functionality is embedded into the standard firmware and can be executed on Phoenix receiver boards with increased (512kB) RAM memory.

Available accessories for the Phoenix GPS receiver include a Win-XP console program, a light weight low-noise amplifier, passive patch, helix and blade antennas, as well as an evaluation kit with housing and interface board for ground testing.

Fig. 3 Low noise amplifier (+28dB) for the Phoenix GPS receiver.

For use in space applications the Phoenix receiver has been tested to tolerate a maximum total ionizing dose of at least 10 krad and to operate in vacuum within a temperature range of -20°C to +50°C. The Phoenix receiver has been selected for numerous sounding rocket projects (Texus, Maxus, Shefex) and micro-satellite missions (PROBA-2, PRISMA, Flying Laptop, X-Sat, TET, ARGO). In addition, Phoenix receivers are employed on the MAST, COMPASS-1 and UWE-3 cubesats. Delivery of unrestricted Phoenix receivers is subject to authorization by the German Bundesamt für Wirtschaft und Ausfuhrkontrolle (BAFA).

Phoenix Miniature GPS Receiver

Receiver Description

Further Reading