BIRD Onboard Navigation System

Overview

BIRD is a small satellite for technology demonstration and remote sensing applications, developed by the German Aerospace Center (DLR). BIRD was launched on October 22, 2001 by an Indian PSLV rocket into a near-circular sun-synchronous orbit at 568 km altitude with a local equator crossing time of 10:30.

The BIRD major mission objectives comprise the test of a new generation of infrared array sensors as well as the detection and scientific investigation of hot spots, like forest fires, or volcanic activities.

To that end, BIRD carries a total of four imaging sensors operating at visible and infrared wavelengths. Among these, the Medium Wave Infrared Sensor (MWIR, 3.5-4.3 mm) and the Long Wave Infrared Sensor (LWIR, 8.5-9.3 mm) provide frame images with a ground sample distance of 185 m at a swath width of 190 km. WAOSS (Wide Angle Optoelectronic Stereo Scanner) is a 3-line CCD stereo camera, which maps the Earth at a pixel size of 185 m and a swath width of 533 km. In addition, the panchromatic array camera HORUS has been added to the payload segment to provide a 6 m ground pixel size at a ground footprint size of 4.5 km.

ONS Objectives

As a technology and scientific satellite, BIRD is equipped with a GPS-based autonomous Onboard Navigation System (ONS) for precise real-time orbit determination and prediction. The ONS objectives arise both from requirements to support the BIRD Attitude Control System (ACS) as well as from its role as a technology demonstrator and comprise the

Provision of the ACS with orbit information, that is required for the Earth-pointing of the spacecraft
Provision of Earth-fixed position information for the geocoding of the BIRD image data
Support of the spacecraft clock synchronization using the GPS One-Pulse-Per-Second

ONS Conception

To realize these objectives and to satisfy the accuracy requirement of 90 m over a one hour orbit prediction arc, the ONS key functionality consists of a precise orbit determination using GPS data. The GPS position fixes are treated by the ONS as statistically independent pseudo-measurements which are processed within an extended Kalman filter.

The ONS 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 Hermite interpolation of the spacecraft position allows for an efficient provision of dense position output, that is required for the high-frequency geocoding of the payload images. The ONS force model applies the JGM-3 coefficients to model the Earth's gravity field, that is completely taken into account up to order and degree of 10.

GPS Receiver and Flight Computer

The BIRD Onboard Navigation System makes use of a GPS Embedded Module (GEM-S) by Rockwell Collins to obtain GPS position fixes for real-time orbit determination. GEM-S is a five channel L1 SPS C/A- and P-code receiver, that has earlier been flown on several Space Shuttle missions. In addition to the GPS data interface, a One-Pulse-Per-Second signal is issued by the receiver, that is used for synchronization of the BIRD onboard clock.

The ONS executes on the BIRD flight computer, which features an industrial Power PC 823e processor operated at 48 MHz clock rate with a performance of 66 MIPS. The real-time preemptive and multitasking operating system BOSS separates the kernel run-time system and a hardware dependant layer, which allows emulation on standard Linux workstations as well as an easy adaptation to different processors. Processes are executed as separate threads, which are controlled by a central scheduler based on pre-assigned priorities and timers. In this way, short and high-priority activities (e.g. commanding) can well be separated from computation intensive tasks with long duty cycles (e.g. orbit determination).

ONS Flight Results

All GEM-S initializations conducted so far were successful and required Time-To-First-Fixes of 45-290 s, in accordance with typical values found in the Hardware-in-the-Loop (HWIL) tests. Based on a precise reference trajectory, the GEM-S navigation solutions exhibit a position accuracy of 10 m (3D rms) together with a velocity accuracy of 0.4 m/s (3D rms).

Upon filter convergence, the obtained orbit determination accuracy for the position is 5.3 m (3D rms). Thus the dynamical model applied within the Kalman filtering improves the kinematic GEM-S position accuracy by a factor of about two. Even of higher importance than the position error is the velocity accuracy, resulting from a dynamical smoothing of kinematic GEM-S position fixes, which yields 6.3 mm/s (3D rms). The major improvement from a dynamical smoothing of the instantaneous GEM-S solutions is thus gained for the velocity errors which are improved by more than a factor of 50.

When current GPS data are unavailable, e.g. due to power or thermal constraints, it has been shown that the ONS still provides position data with an accuracy of about 110 m within a prediction arc of 110 mins.

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