Spaceborne GNSS-Receiving System Performance Prediction and Validatio

Abstract

Spaceborne high-end geodetic-type dual-frequency Global Navigation Satellite System (GNSS) receivers have become an enabling technology for contemporary earth observation missions in Low-Earth Orbits (LEO). Such receivers deliver measurements for space vehicle Precise Orbit Determination (POD) with position accuracy of a few centimeters to geo-locate the measurements taken by other sensors. Roughly speaking, the more precise an earth-observation instrument, the more accurate its position must be determined in order to be able to produce accurate measurement maps. In addition, real-time positioning, accurate within tens of meters - a byproduct of POD-receivers - performed for internal purposes, readily supports the Attitude and Orbit Control Subsystem (AOCS) of the host Spacecraft (S/C). While the notion of recurrence and off-the-shelf availability is reflected in customer price and delivery schedule expectations, POD-receivers are typically procured according to mission specifications concerning performance under a broad range of boundary conditions. This is due to the fact that knowledge concerning important issues like antenna accommodation, maneuvers and on-board interference environment only gradually evolve during the course of the mission design and that the space environment is always good for a surprise. Consequently, accurate performance prediction techniques are of paramount importance to avoid disappointment on the one hand and over-specification on the other. In the present paper we place emphasis on the verification component of our spaceborne GNSS-receiver product platform which we routinely apply for product end-to-end testing, design validation and systems engineering purposes. The latter goes far beyond the level of usual feasibility studies in that in-orbit measurement data can be analyzed and fed back into the design process for optimizing both receivers and verification platform. After a brief introduction of our spaceborne GNSS-receivers and the ground test- and system-in-the loop simulation environment, we present results of the evaluation of initial in-orbit measurements provided by the three SWARM GPS receivers during the commissioning phase which widely overlapped with a period of increased solar activity, resembling a stress case for a high precision dual-frequency POD receiver. Our analyses concern carrier-, code-phase, signal-to-noise ratio, real-time positioning as well as POD and show that the receivers work very well within their specifications due to considerate design margins but they also reveal subtle differences between real space conditions and models typically used for system simulation and GNSS satellite simulator based ground testing.