Precise Range and Range-rate Equipment (Prare) is included for the accurate determination of the satellite's position and orbit characteristics, and for precise position determination (geodetic fixing). It is a satellite tracking system which provides two-way microwave range and range-rate measurements to ground-based transponder stations.

The Prare system was developed by the Institut für Navigation (INS) at the University of Stuttgart, Kayser-Threde GmbH, Munich and the Deutsches Geodätisches Forschungsinstitut, Munich, as a German national experiment.

Function

The system was designed to:

• provide, in all weather conditions, precise satellite-to-ground or satellite-to-satellite range and range-rate information,
• very reliable measurements through cross-checks and calibration procedures,
• ensure highly effective operation of the ground segment through data collection and dissemination via the satellite itself, and control of the global network via one central ground station,
• allow rapid generation of products at an archiving, processing and distribution centre.

Principle

The Prare measurement principle involves two signals sent from the sensor on board the satellite, one signal in the S-band (2.2 GHz) and the other in the X-band (8.5 GHz). Both signals are modulated with a PN code (pseudo-random noise). The time delay in the reception of the two simultaneously-emitted signals is measured at the ground station with great accuracy (<1 ns) and retransmitted to the on-board memory for ionospheric data correction. Collection of meteorological data by the ground station allows corrections to be applied for tropospheric refraction.

Further information on Prare instrument (Esa website)