X-SAT is a mini-satellite for technology demonstration and remote sensing applications, developed by the Satellite Engineering Centre of the Nanyang Technological University (NTU), Singapore. The focus of the technology-driven mission is the high-resolution remote sensing of the Southeast Asian region for environmental monitoring. To achieve the ambitious mission objectives, X-SAT will carry a GPS-based Navigation System (XNS) for high-precision, real-time, and onboard orbit determination and prediction. With a targeted position accuracy of about 1-2 m 3D rms, the XNS provides an unprecedented accuracy and thus enables the support of any satellite mission which requires precise onboard position knowledge.
X-SAT is a small platform with a total mass of less than 120 kg and a size of about 60 cm x 60 cm x 80 cm. The satellite carries three major payloads which comprise the IRIS multispectral sensor, the advanced data acquisition and messaging (ADAM) instrument for communication with remote mobile terminals and a parallel processing unit (PPU), e.g. for onboard image processing. Targeted for a launch in 2006 by an Indian PSLV rocket, a near-circular sun-synchronous orbit at a nominal altitude of 685 km is the current mission baseline. From this altitude, the IRIS main payload will provide a 10 m spatial resolution in the green, red, and near-infrared band at a swath width of 50 km.
As a technology and scientific satellite, X-SAT is equipped with a GPS-based Onboard Navigation System for precise and real-time orbit determination and prediction. The XNS objectives arise both from mission support requirements of NTU, such as the
as well as from its role as a technology demonstrator with the following requirements
The X-SAT Navigation System comprises GPS receiver hardware as well as GPS tracking and navigation software which constitute a highly-integrated navigation system. As a consequence, minimum interfaces to X-SAT are required and the XNS development, testing, and integration is greatly facilitated.
The XNS software comprises a GPS tracking software, comprising
which is well separated from the real-time navigation software system, comprising
A convenient exchange of data between the two parts is enabled through a common data pool, which holds e.g. the GPS measurements and ephemerides.
Core of the XNS is DLR/GSOC's Phoenix low-cost GPS receiver, development based on a Zarlink GP4020 chip which provides L1 code and carrier tracking in twelve channels. The GPS tracking software has been designed for fast acquisition of GPS signals in LEO with a typical time-to-first-fix (TTFF) of about 30 s based on the availability of Twoline elements. The receiver has successfully undergone total dose radiation tests of up to 15 krad, an important prerequisite for space applications. The small form factor of 75 mm x 50 mm of the GPS board, a total mass of less than 250 g (including interface board and housing), as well as a power consumption below 1 W renders this receiver of particular interest for use on small satellites.
The XNS employs an advanced numerical integration scheme (RK4R), that extends the common Runge-Kutta 4th order algorithm (RK4) by a Richardson extrapolation and a Hermite interpolation. The algorithm can be shown to be effectively of 5th order. The XNS force model applies the GGM01 model of the Earth's gravity field, that is completely taken into account up to order and degree of 15. Furthermore, the accelerations due to solar radiation pressure, atmospheric density as well as the third body forces from the Sun and Moon are taken into account. To cope with unmodeled accelerations acting on the spacecraft, a set of three empirical acceleration components in the orbital reference frame is furthermore determined as part of the estimated state vector.
While previous onboard navigation systems have largely been based on the filtering of kinematic position fixes, a more advanced and complex approach applies raw GPS measurements itself. The XNS basic measurement type is a linear combination of GPS L1 code and carrier phase. Since both data types are affected by systematic ionospheric errors with the same magnitude but opposite signs, the resulting so-called GRAPHIC data type is free of ionospheric errors, thus removing the dominant systematic error source of raw GPS data. As a matter of fact, the GRAPHIC data provide a low-noise biased range. For the Phoenix receiver with a pseudo-range and carrier phase accuracy of 0.4 m and 0.5 mm, respectively, the GRAPHIC data type accuracy is about 0.2 m. A drawback of using the GRAPHIC data type stems from the fact, that range bias values for each of the twelve receiver channels have to be estimated, which significantly complicates the estimation process.
Based on a Kalman filtering of GPS GRAPHIC data, the estimated state vector accounts for 22 components, which comprise the orbit position (3) and velocity (3), the receiver clock bias (1), the empirical accelerations (3) as well as the range bias values (12). A particular advantage of the complex orbit determination scheme is a dynamically filtered orbit solution based on ionosphere-free GPS data. As demonstrated in hardware-in-the-loop tests, the concept allows to provide a position accuracy of 1-2 m. The major remaining single error source in the overall position error budget is the ephemeris error of the GPS orbits.
Due to the integrated system design of the XNS as well as its low mass and volume characteristics, the accommodation of the XNS on any small satellite is easily achievable. The low power consumption furthermore enables a continuous GPS receiver operation throughout the orbit which facilitates the operations of the receiver. A high-precision performance of the XNS at the meter level together with a Pulse-per-Second signal and a flexible output message structure renders the XNS a powerful navigation system for a wide range of applications on small satellites.
Gill E., Montenbruck O., Arichandran K., Tan S. H., Bretschneider T.;High-Precision Onboard Orbit Determination for Small Satellites - The GPS-based XNS on X-SAT;Abstract submitted to the 6th Symposium on Small Satellites Systems and Services, Sept. 20-24, La Rochelle, France (2004).
Bretschneider T.;Singapore’s satellite mission X-Sat;Proceedings of the International Academy of Astronautics Symposium on Small Satellites for Earth Observation, IAA-B4-0506P, pp. 105-108 (2003).