Aerospace & Defense Technology - August 2021 - 14

Space Electronics
Orbit
LEO
Applications
Communications, Earth Observation, Research,
Imagery, Manned Spaceflight (ISS),
Military, Space Observation, Spacecraft
Repair, Supply Transport (ISS) and Weather
MEO
GEO
Communications, Navigation
Communications, Earth Observation,
Military, Research, Space Exploration,
Space Observation, Weather
Deep Space Exploratory Rover, Manned Spaceflight,
Planet Exploration, Space Exploration
Launch
Vehicles
Various Programs
Table 1. Typical applications for earth-orbiting
satellites
Figure 1. The Increasingly Crowded Orbit " Spacescape "
Early commercial satellites were deployed
in geosynchronous (GEO) orbit.
A circular geosynchronous (geostationary)
orbit has a constant altitude of
35,786 km (22,236 mi). GEO satellites
remain in fixed position over the earth
allowing antennas to also remain fixed.
The GEO orbit height allows communication
over a wide area of the earth's
surface. An early example of a commercial
GEO satellite launched in 1964,
Syncom 3, was designed to allow PanPacific
coverage of the 1964 Summer
Olympics in Japan. Many commercial
applications today utilize GEO, particularly
systems employing large numbers
of fixed receivers (e.g., DirectTV).
Medium-earth orbit (MEO) satellites,
which occupy a wide altitude band
from 2,000 to 36,000 km above the
earth's surface, are not geostationary.
MEO satellites are only visible from any
receiver from two to eight hours. Telstar
is an early example.
The largest growth in the crowded
" Spacescape " is from the emergence of
low-earth orbit (LEO) satellites, orbiting
from 160 to 2,000 km above the earth's
surface. The low orbit results in a
smaller footprint for signal reception,
averaging 1,000 square kilometers.
Satellites in this " new space " orbit move
quickly, orbiting the earth every 90
14
Intro
Cov
minutes. LEO satellites are much less
expensive (per satellite) to launch, use
much less power, but need many satellites
(clusters) to effect continuous communications
with fixed antennas.
OneWeb is an early example.
Satellite Applications by Orbit
The range of applications for earthorbiting
satellites is quite extensive.
Table 1 provides a general view of the
types of satellite applications typically
found in each orbit.
Regardless of orbit, all of these applications
have in common the requirement
for ultra-high reliability electronic
components. Quartz crystal oscillators
are preferred in many of these applications
over alternative devices for timing
and frequency applications.
Performance Requirements Driven
by Orbit and/or Application
All satellites, regardless of orbit, require
space-qualified electronic components.
Reliability remains an essential
component. Hardware is expensive to
produce and to launch, so redundancies
are built into design. " Services calls " are
not possible, so parts must meet minimum
performance and reliability criteria
based on application, orbital location
and expected mission life.
www.aerodefensetech.com
ToC
+
-
Crystal oscillator packages are hybrid
devices containing the tuned crystal
and electronic circuitry. The types of devices
can fall into several categories:
* XO - Basic crystal oscillator
* TCXO - Temperature-compensated
crystal oscillator
* VCXO - Voltage-controlled crystal oscillator
*
MCXO - Microcomputer-compensated
crystal oscillator
* OCXO - Oven-controlled crystal oscillator
*
SO - Surface acoustic wave (SAW) oscillator
*
VCSO - Voltage-controlled SAW oscillator
The
remainder of this article will provide
a survey of the key operational features
and performance factors that must
be considered in the selection of quartz
crystal oscillators based on the application
and orbital location, along with
the four performance criteria shown in
Figure 2:
* Radiation Hardness
* Stability
* Phase Noise and Jitter
* Size, Weight and Power (SWaP)
Radiation Hardness - TID and SEE
Total Ionizing Dose (TID)
Solar ionizing radiation exposure increases
with the altitude of orbit and
the resulting reduction on the atmosphere's
ability to absorb/reflect its effects.
Ionizing radiation does not have
an intrinsic effect on a quartz crystal,
but as mentioned earlier, quartz crystal
oscillators are hybrid devices combining
Aerospace & Defense Technology, August 2021
A
µ
È
http://www.aerodefensetech.com http://www.abpi.net/ntbpdfclicks/l.php?202108TBNAV
Aerospace & Defense Technology - August 2021

Table of Contents for the Digital Edition of Aerospace & Defense Technology - August 2021
