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150 MEMS and Microstructures in Aerospace Applications

8.1 INTRODUCTION

The communications subsystem is responsible for reception and demodulation of
signals sent up from the ground station (uplink) as well as transmission of signals
back to the ground station (downlink). The system is also responsible for any commu-
nication with other satellites. The uplink signal consists of commands and range
tones, which are signals first transmitted by the ground station, and then received
andretransmittedbythesatellite.Thedelayisusedtodeterminethesatellite’sdistance
from the station. In addition to range tones, the downlink signal includes telemetry for
spacecraft status and any payload data. The downlink signal is usually coherent in
phase with the uplink signal, which allows for Doppler shift detection of spacecraft
velocity.
The signal frequency range for ground to satellite communications is from 0.2 to
50 GHz, depending on the application. Intersatellite links sometimes use 60 GHz
signals.Uplinkandtelemetry downlinkdataratesaretypicallylessthan1kbit/sec,and
1
are transmitted using low-bandwidth, widebeam antennas. When payload data re-
quires a higher transmission rate, high-gain, directional antennas are used. These
antennas need to be steered either mechanically or electrically. Mechanical steering
places additional demandsontheattitude determinationand control subsystem,which
must balance the reaction forces caused by antenna movement. For more detailed
information on the communications subsystem, interested readers should consult
2
Morgan and Gordon. Applications for MEMS in spacecraft communications systems
include routing switches, phase shifters, electrically steerable antenna, higher per-
formance filters for transmitter or receiver circuits, and scanning mirrors for inter-
satellite optical communications.
Optical communication links offer many advantages over microwave links. In
particular, free space laser systems can provide narrow beam widths and high gains
with much smaller hardware. High gains allow for much higher data rates, on the
order of Gbps for sufficiently close link ranges, for example, near terrestrial space.
Because of the significant attenuation of optical frequencies by the atmosphere,
optical links are most easily employed for intersatellite communications, which is
particularly attractive for crosslinks within satellite constellations. 1
Optical communication hardware is well suited to small satellites. The flight
mass of an optical communications subsystem is typically 55 to 65% of that of a
3
conventional microwave subsystem. This derives from the use of low-mass de-
tectors and semiconductor laser diodes, and fiber amplifier or lasers, many of which
4
were developed for the terrestrial fiber optics communications market. However,
macroscale electromechanical beam steering subsystems make up a significant
fraction of the mass of these systems. This is where MEMS offer a solution in
optical communications for many aerospace applications. 5

8.2 MEMS RF SWITCHES FOR SPACECRAFT
COMMUNICATION SYSTEMS

Microwave and RF MEMS are especially applicable to commercial communication
satellites, where communication systemsmake upthepayload aswell as are part of the

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