Figure 1. The PRARE-2 instrument during integration
The Precise Range and Range-Rate Experiment (PRARE) instrument is designed both to give the ERS-2 mission a geodetic and geodynamics capability and to support the satellite's Radar Altimeter instrument. It is a two-way microwave ranging system operating at S- and X-band, providing state-of-the-art microwave positioning (Fig. 1).
Figure 1. The PRARE-2 instrument during integration
The PRARE instrument's operation is based on onboard measurement of the propagation delay between signal transmission and reception at X-band to provide range information. The round-trip X-band carrier phase is also measured to derive range-rate data. The purpose of the instrument is to provide high-precision measurements of the satellite's position using a network of dedicated ground stations.
The PRARE on ERS-1 was an 'Announcement of Opportunity' instrument funded and procured by the German Bundesministerium für Forschung und Technologie (BMFT) and the Deutsche Agentur für Raumfahrt-angelegenheiten (DARA). It was developed by the Technische Universit t Berlin and the Institut für Navigation in Stuttgart. ESA's involvement was limited to management of the satellite interfaces and to the instrument accommodation aspects. The PRARE-2 instrument for ERS-2 was procured directly by ESA.
PRARE-1 was launched aboard ERS-1 on 17 July 1991 and was switched on for the first time seven days later. After almost five days of operating nominally, the instrument auto-matically switched itself off and could not subsequently be recovered. Prior to the failure, all instrument telemetry was nominal, except for a high number of errors appearing in the main processor memory.
Failure occurred whilst the spacecraft was in the so-called 'South Atlantic Anomaly', which is a region of the Earth's magnetosphere notorious for its high levels of proton radiation. Unfortunately, the spacecraft was not in contact with a PRARE ground station at the time the anomaly occurred, which meant there was very little telemetry data available for the subsequent analysis conducted by the ESA Failure Review Board (FRB).
This Board's final report, issued in December 1991, indicated that the failure could have been caused by a proton-radiation-induced destructive latch-up of a Random Access Memory (RAM) device in PRARE's main processor. This conclusion was supported by a radiation test with a particle accelerator exposing the RAM device to proton radiation, which resulted in its failure. Further tests showed that only this component was radiation-sensitive. It transpired that this first PRARE contained no radiation-hardened components, mainly to keep the instrument's cost to a minimum. The Failure Review Board also identified a number of other more minor design and manufacturing deficiencies that could have given rise to problems later in the mission. All of these areas were addressed in the subsequent design and development of the PRARE-2 instrument for ERS-2.
The fact that PRARE-1 had consciously been designed to have an absolute minimum number of telecommand and telemetry interfaces between it and the spacecraft, with a view to providing instrument compatibility with several different spacecraft, turned out to be a major drawback after the failure. It imposed severe limitations on those investigating the in-orbit failure.
As already mentioned, it had been agreed prior to ERS-1's launch that the PRARE flight segment for ERS-2 would be directly procured by ESA. The PRARE-2 contract had therefore been signed in 1990 assuming that just a simple rebuild of PRARE-1 would be needed.
Following the PRARE-1 in-orbit failure in July 1991, and the first findings of the Failure Review Board, all PRARE-2 manufacturing activities were put on hold. Once the FRB's final report was published, the ERS-2 Project Team decided that the PRARE-1 design would have to be substantially modified prior to the re-flight on ERS-2.
The design changes were related primarily to the suspected cause of the in-orbit failure and the other potentially problematic areas, i.e.
These changes were first implemented and tested using the PRARE-1 engineering model, which ultimately successfully survived a rigorous proton-radiation test.
Changes were also made in the PRARE operating concept, largely as a result on the findings of the Failure Review Board, although not directly related to the in-orbit failure of PRARE-1:
The PRARE-1 in-orbit failure and the short time then remaining for the above re-design effort meant that extensive support from the ERS-2 Project Team was needed to fulfil the Agency's responsibilities in the procurement of the PRARE-2 instrument. This support covered all aspects of product assurance, electrical and software engineering, electrical ground-support equipment, assembly integration and testing, as well as management support.
Only through the combined efforts of the Contractor and the ESA Team was it possible to implement all of the above-mentioned modifications successfully and still deliver both PRARE units in time for the overall satellite integration and test activities.
As part of the German/Russian space cooperation, it was agreed to fly a PRARE precursor mission on a Russian Meteor satellite. Apart from the satellite interface, this instrument, launched in January 1994, is identical to PRARE-2. Since it was switched on in February 1994, the PRARE on Meteor has performed flawlessly, delivering excellent results. This Meteor experience served to demonstrate the validity of the modifications implemented following the problem on ERS-1.
PRARE-2 on ERS-2 was switched on immediately after the satellite launch and early orbit phase, on 26 April 1995. It has now been operational for several weeks and all of its functions have been verified. The range (Fig. 2) and range-rate (Fig. 3) data are already providing good results. The instrument has successfully demonstrated its ability to detect the total electron content in the signal-propagation path by comparing the group delay and phase delay in the round-trip signal (Fig. 4).
The uncalibrated instrument stability over the first five weeks of operation was of the order of 1.9 cm. Each ranging session is further enhanced by closing the instrument's internal calibration loop after the ground-station contact, leading to a final measurement uncertainty of less than 1 cm. Consequently, confidence is high that all PRARE-2 scientific and mission objectives will be met.
The PRARE-2 instrument will enter its routine operational phase in August, once the current in-orbit commissioning activities have been completed.
Figure 2. Typical range variation during ground-station contact
Figure 3. Typical Doppler variation during ground-station contact
Figure 4. Difference between group delay and phase delay as a measure of total electron content
The history of the PRARE instrument has allowed two important lessons to be learnt:
The early results from PRARE-2 are demonstrating the high precision of the instrument. This, together with PRARE's unique capabilities, will hopefully stimulate further research into microwave tracking systems and their applications.
ESA Bulletin Nr. 83.