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.