High-precision Onboard Orbit Determination for Small Satellites – the Gps-based Xns on X-sat

X-SAT is a mini-satellite developed by the Satellite Engineering Centre of the Nanyang Technological University at Singapore. The focus of the technologydriven mission is the high-resolution remote sensing of the Southeast Asian region for environmental monitoring. To achieve the ambitious mission objectives, the GPS-based X-SAT Navigation System (XNS) will provide high-precision onboard orbit determination solutions as well as orbit forecasts. With a targeted real-time position accuracy of about 1–2 m 3D r.m.s., the XNS provides an unprecedented accuracy performance and thus enables the support of any satellite mission which requires precise onboard position knowledge. 1. THE X-SAT MISSION X-SAT is a technology demonstration mission undertaken as collaboration between the Nanyang Technological University (NTU) and the Defense Science Organisation (DSO) of Singapore. 1.1 X-SAT Orbit and Mission Operations X-SAT is scheduled for a piggyback launch on a Polar Satellite Launch Vehicle (PSLV) from Shriharikota, South India, in 2006. The X-SAT target orbit is a nearcircular, sun-synchronous orbit of 685 km altitude and 98.13o inclination [1]. The mission operations will be conducted at NTU’s Satellite Engineering Centre (SEC). The ground segment comprises a 6 m S-Band antenna for TM/TC operations as well as a 13 m X-Band reflector for the downlink of payload data, both located in Singapore. While the S-Band communication link supports 4 kbps for the uplink and 1 Mbps for the downlink [2], the XBand transmitter provides a data rate of up to 100 Mbps. 1.2 Spacecraft Description and Payload System X-SAT is a platform with a total mass of 120 kg and a size of about 60x60x80 cm (WxDxH) which is made of a honeycomb panel structure (Fig. 1). Two selfdeployable solar arrays with a total area of about 0.9 m provide an average total power of 140 W [3] at end of life. The core of the onboard data handling system (OBDH) is the onboard computer (OBC) a space-proven radiation-hardened 32-bit SPARC embedded processor of Atmel (TSC695F, implementing the ERC32 architecture) performing up to 20 MIPs at a clock speed of 25 MHz [4]. Communications between the OBC and the bus sub-systems as well as the payload is realized by two separate and redundant controller area networks (CAN). The satellite carries three major payloads: the IRIS multispectral push-broom scanner, the Advanced Data Acquisition and Messaging (ADAM) instrument for communication with remote mobile terminals, and the Parallel Processing Unit (PPU) with multiple applications, e.g. for onboard image processing. Flying at a nominal altitude of 685 km, the IRIS main payload will provide images with 10 m spatial resolution in the green, red, and near-infrared band at a swath width of 50 km in the nadir-looking mode. 1.3 Attitude Determination and Control System The requirements on the Attitude Determination and Control System (ADCS) are most demanding for the roll and pitch angles with a pointing accuracy of 0.33o and an attitude knowledge of 0.06o [5]. Fig. 1 X-SAT launch configuration with folded solar panels. The two GPS antennas close to the launch adapter face the -z-axis (zenith during earth-pointing mode).