GPS Tracking of the IRDT-2 Re-entry Capsule
Introduction
Under contract of ESA and the European Community the German Astrium GmbH is presently preparing the second test flight (Inflatable Rentry and Descent Technology IRDT-2) for the demonstration of a novel reentry technology making use of an inflatable aerobraking shield. The project conducted jointly with the Babakin Space Center, Moscow, aims at the development of a download system for the International Space Station, which is able to return small payloads to the ground independent of the US Space Shuttle. IRDT makes use of technologies originally developed within the Russian Mars program and differs from common recovery systems for reentry capsules or sounding rockets. Instead of a parachute an inflatable heat shield is employed to decelerate the capsule and land it safely on ground.
The IRDT-2 capsule will be launched by a Volna rocket from a Kalmar type submarine in the Baltic sea north of Murmansk and injected into a ballistic trajectory passing across the arctic sea and northern Siberia (Fig. 1). Following deployment of the first shield, the capsule reaches the rentry point at a 100km altitude and a velocity of roughly 7 km/s. Here, a second shield is deployed which introduces a steep descent of the capsule. The actual landing takes place on the Kamshatka peninsula within 25 min after separation.
As part of the IRDT-2 payload, a modified Orion GPS receiver for space applications will be flown by DLR/GSOC and the resulting navigation data will complement other sensors and experiments in the post mission analysis. In view of the short mission duration and the fact, that the IRDT unit will only be switched on at separation from the upper stage, special precautions have to be taken to allow for a hot start of the GPS receiver at boot time even under the high dynamics of the re-entry trajectory. The Mitel Orion receiver has therefore been selected for the IRTD tracking system, since it supports software modifications through the Mitel Architect development system.
Receiver Description
The GPS Orion receiver makes use of the GP2000 chipset, which comprises a GP2015 RF down-converter, a DW9255 SAW filter, a GP2021 correlator and a 32-bit ARM-60B microprocessor. Using a single active antenna and RF front-end, the receiver supports C/A code tracking of up to 12 channels on the L1 frequency. Within the IRDT flight unit, the main receiver board is supplemented by a taylor-made interface unit, which comprises basic support functions (power regulator, backup battery and serial interface converters) as well as a dedicated data handling system. It provides a dedicated micro-controller and an EPROM memory, which are used to store navigation solutions during the flight of the IRDT-2 capsule for read-out after landing. The available storage volume of 900 kByte is sufficient to hold 2 Hz samples of position and velocity as well as raw data (pseudoranges, pseudorange rates) and status information at a reduced data rate. Thus a dynamical post mission adjustment of the reentry trajectory is even possible in case of limited tracking conditions with less than 4 satellites in lock.
The receiver and interface board measure 95 x 50 mm each and are stacked on top of each other inside the housing shown in Fig. 2. The power consumption of the complete GPS unit amounts to roughly 3W. The standard Mitel firmware has received numerous changes to improve the tracking performance under highly dynamical conditions and to allow a fast acquisition of the receiver. This include updates of various receiver parameters (operational limits, filter parameters) as well as fixes of the Doppler computation and the kinematic navigation solution for fast moving host vehicles. Also, a small time offset that would otherwise introduce a measurable along track error in space applications has been corrected in DLR's receiver software.
A major modification concerns the use of a position-velocity aiding concept, which makes use of a piece-wise polynomial approximation of the nominal flight. Based on this, the reference position and velocity of the vehicle in the WGS84 reference frame are computed approximately once per second. The result is then used to obtain the line-of-sight velocity and Doppler frequency shift for each visible satellite, which in turn serve as initial values for the steering of the delay and frequency locked loops. For use on IRDT-2, the polynomials are referred to the instant of separation from the launcher, which coincides with the boot time of the receiver.
Operations Concept
Prior to the final integration the receiver will be briefly activated and connected to an outside antenna. This allows the receiver to synchronize itself to the current time and to receive a recent almanac of the GPS constellation Following the subsequent power-down the correlator's internal real-time clock is kept alive by an backup battery. Likewise, relevant auxiliary data like the almanac and the IRDT reference trajectory are stored in a non-volatile part of the memory. Using the above information, the absolute time is available to the receiver at start-up with an accuracy of a few seconds, which in turn allows the prediction of the GPS satellite constellation. Likewise the time since boot (i.e. the time since separation) is available within the receiver, which is required to read-out the nominal trajectory. In this way the receiver is both able to predict its approximate position and velocity as well as the position and velocity of the GPS satellites. Using these data the channel allocation and the Doppler offset for the signal acquisition are determined. This allows a full warm start of the receiver irrespective of the actual launch date and time of the mission. Based on corresponding signal simulator tests, it is expected that position and velocity measurements are available within a minute after activation, provided that the tumbling of the capsule after separation does not impose major restrictions of the GPS satellite visibility.Flight Results
Actual flight data will be presented here after successful performance of the IRDT-2 mission. The launch has repeatedly been postponed and is now scheduled for mid July 2002.
Further Reading
Inflatable Re-Entry Shield Now Tested In Space
Solar-Sail project of the Planetary Society (Cosmos-1)
Montenbruck O., Markgraf M., Leung S.;
Ein GPS Empfänger für Raumfahrtanwendungen ;
Deutscher Luft- und Raumfahrtkongress 2001 der DGLR, Hamburg 17.-20. Sept. (2001).
Montenbruck O., Markgraf M., Leung S., Gill E.;
A GPS Receiver for Space Applications;
B1-Ax; ION GPS 2001 Conference, Salt Lake City, 12-14 Sept. 2001 (2001).
