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Communication Subsystem

The communications subsystem consists of four S-band patch antennas,two receive and two transmit, a coupler, a receiver/demodulator unit, a transmitter/baseband unit, a band reject filter and an RF switch. Shown above is a simple block diagram of the system.

The SNOE communications subsystem was supplied by Cincinnati Electronics, except for the antennas, which were designed and built by LASP. The system is based on a 5 W NASA-compatible S-band transceiver that receives at 2092.9621 MHz and transmits at 2272.9 MHz. The primary ground station is the NASA Transportable Orbital Tracking Station (TOTS) at Poker Flat, Alaska, but global communication coverage is availble using the Wallops Island tracking station and Deep Space Network stations at Goldstone, Canberra, and Madrid during the early orbit campaign and in case of emergency.

One receive and one transmit antenna is mounted on each end of the spacecraft. Both receive antennas stay on at all times, so that omnidirectional coverage is obtained. The RF coaxial switch allows for selection of the transmit antenna. Depending on the orientation of the satellite orbit with respect to the ground station, the appropriate transmit antenna selection is made for each ground contact. Before the switch is a band reject filter. This filter suppresses noise generated by the transmitter at the received frequency, thus decreasing interference between the two signals.

As is indicated in the diagram, there are two channels in the communications system. Nominally, both real time and playback data will be downlinked at each ground pass. This system does, however, have the capability to send only playback, or only real time data if the need should arise. The data rates are indicated in the diagram for both the real time and playback data. For the 512 bps Bi-Phase-L real time data, the standard PCM/PSK/PM modulation will be used. The downlink will have a NASA standard 1.024 MHz subcarrier and will use standard PCM/PM modulation for the 128 kbps Bi-Phase-L playback data. The commands uplink will also use NASA standard PCM/PSK/PM modulation and will have a 16 kHz subcarrier. The uplink uses a data rate of 2.0 kbps. Link margin analyses have been performed and ample margin is available for all ground stations. For the worst case, downlink has a margin of 7.9 db and uplink has a margin of 9.36 db.

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