Encyclopedia Astronautica
1993.09.26 - Posat 1


Portugal's first satellite achieved through a technology transfer programme with Surrey Satellite Technology Ltd. Carried store and forward, DSP communications, GPS and Earth observation payloads. Still operational in 2000.

PoSat-1 carried the following experimental payloads:

Earth Imaging System (EIS) - consisting of two charge-coupled device (CCD) imagers, two lenses, and a Transputer Data Processing Experiment to provide on-board image processing and data compression. One imager had a wide field of view with 2km ground resolution; the second narrow field imager provided 200m ground resolution.

Star Sensor - The star sensor was based on the same technology as the EIS but was optimised for imaging the faint light from stars for use as part of the spacecraft's attitude determination system. The star-field image was analysed by the Transputer Data Processing Experiment and the resulting measurement data returned to the On-Board Computer.

GPS Navigation Experiment - PoSAT-1 carried a Global Positioning by Satellite (GPS) receiver based on the Trimble Trans-II receiver. The received data was decoded and filtered by the Transputer Data Processing Experiment to provide the satellites' position and velocity as well as an accurate on-board time reference. This enabled the satellite to generate its own orbital element set, provide scheduling and synchronisation to other computers, and allowed groundstations equipped with a GPS receiver to experiment with applications for real-time differential GPS data.

Cosmic Ray Experiment (CRE) - The CRE monitored the space radiation environment experienced in orbit by the satellite and enabled analysis of its effect on spacecraft semiconductor electronics. The CRE contained a PIN diode and multi-channel analyser capable of detecting energetic particles with a wide range of Linear Energy Transfer to build up a spectrum of observed energies of particles within the spacecraft. Special RADFET's were also incorporated to monitor the accumulated ionising dose. The larger memory devices in the on-board computers were regularly 'washed' to detect and log Single Event Upset (SEU) information.

Digital Signal Processing Experiment (DSPE) - The DSPE consisted of two Texas Instruments processors from the TMS320 series, the C25 and C30. The DSPE could be used as a programmable communications modem to modulate downlink data from or demodulate uplink data for the OBC thus enabling experiments with new modulation techniques optimised for Low Earth Orbit satellite mobile communications.

Store-&-Forward Communications - The main spacecraft On-Board Computer (OBC), based on an 80C186 8MHz processor with 16 MB of SRAM, also supported digital store-&-forward communications using AX25 packet protocols and communications links optimised for communications using very low cost, simple and portable groundstations. This design took advantage of the latest research developments undertaken at UoSAT/SSTL - including high data rate modems at 38.4 kbps data rate and a new synthesised transmitter design.

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