IEEE Aerospace and Electronic Systems Magazine - July 2020 - 14
Improving Small Satellite Communications in Deep Space
Figure 3.
Emergency mode communications; assumptions.
on the chart using different colors to distinguish them. For
CubeSats, the link margin becomes zero at 200 million km;
therefore, the link will not close for about one half of Mars'
orbit. For MicroSats, the distance increases to 700 million
km, covering Mars' orbit but not the orbit of Jupiter. The
takeaway from this chart is that with the values provided in
Table 1, emergency mode becomes a limiting factor for
SmallSats. To remedy this situation, a fan-beam antenna
that requires pointing in only one direction may be used to
increase antenna gain, however, this solution would not
work if spacecraft is tumbling or lost position control. A
study of emergency mode options needs to be performed to
investigate options such as use of directional antennas,
diversity antennas, lower data rates, etc.
RANGE AND DOPPLER
The study also looked at range and Doppler data for
SmallSats. We concluded that SmallSats are not limited in
this area because of the very powerful ground antennas
used for these measurements. For example, CubeSat missions can collect radiometrics data well beyond Mars distance both at X- and Ka-bands frequencies.
OPTICAL COMMUNICATIONS
Radio frequency (RF) waves have been used for communications from deep space since the dawn of space exploration. The technology for RF communications is mature
and reliable because of decades of use in Earth/space communication systems, not to mention more than a century of
14
Table 7.
Assumptions for Figure 3
Spacecraft
Large
SmallSat
($100 kg)
Small
SmallSat
(6U)
Spacecraft transmitter
power
4W
30 W
Spacecraft antenna
(single patch array)
8 dBi
8 dBi
Elevation
20
20
DSN station (31-m
antenna, 20 kW
amplifier)
DSS34
DSS34
Telemetry modulation
index
46
46
Telemetry data rate
10 b/s
10 b/s
Frequency X
8415 MHz
8415 MHz
Modulation
BPSK
BPSK
Coding
Turbo
(1784, 1/6)
Turbo
(1784, 1/6)
Weather
90%
90%
Margin (dB)
3
3
Ground
Link
IEEE A&E SYSTEMS MAGAZINE
JULY 2020
IEEE Aerospace and Electronic Systems Magazine - July 2020
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