LEOSAR Satellite Payloads

Space Segment

The space segment consists of three basic units:

a platform moving in low Earth polar orbit as a mounting for the other units,
a 121.5 MHz repeater unit on Cospas satellites and a 121.5, 243 and 406 MHz repeater unit on Sarsat satellites designed for retransmission of distress signals in the local coverage mode (the SARR instrument); and
a receiver-processor and memory unit (the SARP instrument) on Cospas and Sarsat satellites designed to receive, process and store signals received on 406 MHz for retransmission in the local and the global coverage mode.

The repeater unit and the receiver-processor and memory unit are described below.

Repeater Unit (SARR)

The repeater unit receives either 121.5 MHz signals (Cospas), or 121.5, 243 and 406 MHz signals (Sarsat) transmitted by activated distress beacons. After amplification and frequency conversion, the signals are retransmitted on the 1544.5 MHz downlink. Automatic level control (ALC) is provided to maintain a constant output level.

The 1544.5 MHz transmitter on the repeater unit:

accepts input from the uplink receivers;
adjusts the relative power level in accordance with ground command;
phase modulates a low frequency carrier with the composite signal;
multiplies the frequency to produce 1544.5 MHz;
amplifies the power level; and
transmits the composite baseband signal via the spacecraft downlink antenna.

Receiver-Processor and Memory Unit (SARP)

The 406 MHz receiver-processor:

demodulates the digital messages received from beacons;
measures the received frequency; and
time taggs the measurement.

All these data are included in the output signal frame for transmission to LUTs on the 1544.5 MHz downlink.

The frame is transmitted at 2400 bits per second in the processed data mode, and simultaneously stored in memory.

The on-board memory is dumped in the same format and at the same bit rate as local mode data. LUTs thus receive the stored beacon messages acquired during previous orbits. If a beacon signal is received during the stored memory dump, the dump is interrupted so that the signal can be processed and the resultant message interleaved with the stored data.