Type 921 S-Band Satellite Transceiver

Type 921 S-Band Satellite Transceiver

The type 921 TM/TC Unit provides the full duplex telemetry and telecommand interface between the satellite and it's ground station. It has been designed specifically as a compact, low cost unit for use with low-earth-orbit satellites and operates in S-Band, is fully redundant, and is compliant with all relevant CCSDS Standards.

The Type 921 TM/TC Unit comprises two Transceiver and an RFDU enclosed on five sides by 5 mm aluminium plates. Overall weight is 6 kg. The unit is finished in a flat black polyurethane coating (Z306).   RF connectors are nonmagnetic, Type SMA (MIL-C-39012), Supply and Data/Monitor connectors are non-magnetic, sub-miniature Type D (NASA GSFC-S-311-P-4).

The build standard of the Type 921 TM/TC Unit is fully compliant with the requirements of MIL-STD-975 for Category 2 missions. With either transmitter available for use with either receiver the predicted failure rate for the Type 921 TM/TC Unit in low-earth orbit is 0.25% per annum.

Each Transceiver houses a receiver and transmitter packaged together to save size and weight. The receiver and transmitter are separately supplied via DC/DC-converters, fed via EMI filters to reduce reflected ripple current to within the limits set by MIL-STD-461C, Method CE03 and have isolated inputs.

The RFDU comprises a diplexing filter to enable the TM/TC Unit to transmit and receive simultaneously using one antenna, a coaxial switch to connect one of the two transmitters to the diplexer, a 3 dB power divider to connect the diplexer to the two receivers, and a coupler connecting the nadir and zenith antenna ports to the diplexer.

The receiver accepts an up-link signal in the frequency range 2,025 to 2,110 MHz. The up-link carrier is phase modulated by a sinusoidal sub-carrier at 8 or 16 KHz. Carrier modulation index is restricted to less than 1.5 to retain sufficient residual carrier for doppler tracking. The sub-carrier is BPSK modulated with the telecommand data with the data clock being a binary sub-multiple of the sub carrier. The up-link bit rate for a low-earth-orbit satellite is normally in the range 2 to 4 kbps.

The low-earth-orbit is characterised by short pass-times lasting only a few minutes, Fast and reliable acquisition of the up-link is therefore of paramount importance. To ensure this end the Type 921 Receiver incorporates a relatively wide-band carrier-tracking- loop and a compensated 'long-loop' vcxo to minimise variation in receiver-loop-rest-frequency with temperature.The receiver comprises downconverter, carrier-tracking-loop and sub-carrier/data regenerator pcbs.

The transmitter provides a 1 Watt down-link carrier in the frequency band 2200-2290 MHz phase-modulated by the telemetry data in either SP-L or NRZ-L format. SP-L data is transmitted at low-bit rates with the carrier modulation index restricted to less than 1.5 to retain sufficient residual carrier for doppler tracking at the ground station. At high down-link data rates the ITU regulations governing PFD at the earth's surface mean that BPSK modulation must be used to suppress the carrier. Occupied bandwidth is an important parameter at high down-link bit rates. A 6-pole Bessel pre-modulation filter is used to restrict this bandwidth.

Technical Data

Transmit Frequency in range 2200-2290MHz
Frequency Stability better than 10 ppm
Spurious Emission/Harmonics less than -60 dBc
Transmit Power at Antenna Port up to 5W
Modulation SP-L to 100kbps, BPSK to 2 Mbps or QPSK to 6 Mbps
Power Consumption per Transmitter 7W @ 1W o/p power
Receive Frequency in range 2025-2110 MHz
Frequency Stability better than 10 ppm
Carrier Tracking Range not less then 100 kHz
Carrier Acquisition at 50kHz/sec certain at -120dBm
Receive Bit Error Rate less than 1E-5 @ 4kbps/-116dBm
Power Consumption per Receiver 3W


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