JED - September 2016 - 53

E W

Direction of
Earth's Rotation
North Pole
Max Latitude° =
Inclination of Orbit
Equator
Earth Trace for
Synchronous Orbit
with Inclination

101

ited to coverage between the equator and a maximum latitude. The maximum latitude is equal to
the inclination of the satellite's orbit relative to
the equator.
The Earth area over which a satellite can send
or receive signals to/from Earth-based stations
during each orbit depends on the altitude of the
satellite and the beam width and orientation of
antennas on the satellite.

POLAR ORBITS

South Pole
Figure 3: The Earth trace of a synchronous satellite is a single spot on the equator
unless the orbit has some inclination. Then it is a "figure eight" with maximum
latitude equal to the orbital inclination.

SYNCHRONOUS SATELLITES

A synchronous satellite has a SVP that stays
in one location on the Earth's surface. This requires that its orbital period be one sidereal day (i.e. 1,436
which way the Earth spins: The sun rises in the East once per day.
Figure 3: The Earth trace of a synchronous satellite is a single spot on the equator unless
minutes).
Therefore,
the
second
orbit
will
be
west
of
the
first
orbit
by
the
the orbit has some inclination. Then it is a "figure eight" with maximum latitude
equal toAnother requirement for a fixed SVP is that the orbit
have 0° inclination. Thus the SVP must be on the equaratio
of
the
orbital
period
divided
by
one
sidereal
day
multiplied
the orbital inclination.
tor. If the synchronous satellite orbit has an inclination, the
by 360 degrees. A sidereal day is 23 hours, 56 minutes and 4.09
Earth trace will be a "figure eight," as shown in Figure 3. This
seconds (i.e. 1,436 minutes or 86,164 seconds). For example, the
means that an Earth station will see a similar figure-eight
Earth trace of a satellite with a 90-minute orbital period will move
variation in the location of the satellite and must, therefore,
west by 22.56 longitude degrees for each subsequent orbit.
track the satellite if a narrow-beam ground antenna is used
(90 / 1436) x 360° = 22.56°
for transmission or reception.
By Kepler's third law, the altitude of a synchronous satellite
Figure 2 shows an equatorial view of the Earth trace. In
must have a semi-major axis of 42,166 km. In a circular orbit,
both of these figures. you can see that the satellite is limthe height of the satellite will be 35,795 km. The maximum
range from an Earth surface station to a synchronous satellite
with a circular orbit can be determined as shown in Figure 4.
Synchronous
This diagram is a plane triangle in the plane containing the
Satellite
satellite, the center of the earth and an Earth surface station. The Earth station in this figure sees the satellite at its
local horizon - that is zero degrees elevation. The minimum
and maximum range values for this satellite-to-ground link are
35,795 km and 41,682 km respectively. The shorter range would
apply if the satellite were to be positioned directly overhead,
35,795 km
and the maximum range is for the satellite on the horizon, as
41,682 km
shown. This means that the link loss for a 2-GHz signal would
be 189.5 dB to 190.9 dB.
Later in this series, we will deal with ranges and look
8.69°
angles for ground and airborne "Earth stations" for practical
EW problems.
You will also note that the angle from the SVP to the signal
path tangent to the Earth's surface is 8.69°. If the satellite has
an antenna with beam-width twice this angle (i.e. 17.38°), its
90°
beam would cover all of the Earth visible to the satellite. This
Local
Center
makes it an "Earth-coverage antenna."
Horizon

of Earth

6,371 km

Figure 4: The range to a synchronous satellite that is on the
horizon is 41,682 km.

WHAT'S NEXT
For the next two months, we will continue our discussion of
space EW with the calculation of the elements of the total path
loss to or from a satellite. For your comments and suggestions,
Dave Adamy can be reached at dave@lynxpub.com. a

The Journal of Electronic Defense | September 2016

If a satellite is placed in a polar orbit, its orbit
has 90° inclination, and will, therefore, eventually provide complete coverage of the surface of
the Earth.

53



JED - September 2016

Table of Contents for the Digital Edition of JED - September 2016

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Nothing to See (Or Hear) Here – Concealing Tactical Communications
Technology Survey: Manpack COMINT and DF Systems
The Heat is On
EW 101
AOC News
Index of Advertisers
JED Quick Look
JED - September 2016 - cover1
JED - September 2016 - cover2
JED - September 2016 - 3
JED - September 2016 - 4
JED - September 2016 - 5
JED - September 2016 - The View From Here
JED - September 2016 - 7
JED - September 2016 - insert1
JED - September 2016 - insert2
JED - September 2016 - Conferences Calendar
JED - September 2016 - 9
JED - September 2016 - Courses Calendar
JED - September 2016 - 11
JED - September 2016 - From the President
JED - September 2016 - 13
JED - September 2016 - 14
JED - September 2016 - The Monitor
JED - September 2016 - 16
JED - September 2016 - 17
JED - September 2016 - 18
JED - September 2016 - 19
JED - September 2016 - 20
JED - September 2016 - 21
JED - September 2016 - 22
JED - September 2016 - 23
JED - September 2016 - 24
JED - September 2016 - 25
JED - September 2016 - 26
JED - September 2016 - 27
JED - September 2016 - World Report
JED - September 2016 - 29
JED - September 2016 - 30
JED - September 2016 - 31
JED - September 2016 - 32
JED - September 2016 - 33
JED - September 2016 - Nothing to See (Or Hear) Here – Concealing Tactical Communications
JED - September 2016 - 35
JED - September 2016 - 36
JED - September 2016 - 37
JED - September 2016 - 38
JED - September 2016 - 39
JED - September 2016 - 40
JED - September 2016 - 41
JED - September 2016 - 42
JED - September 2016 - Technology Survey: Manpack COMINT and DF Systems
JED - September 2016 - 44
JED - September 2016 - 45
JED - September 2016 - 46
JED - September 2016 - 47
JED - September 2016 - The Heat is On
JED - September 2016 - 49
JED - September 2016 - 50
JED - September 2016 - 51
JED - September 2016 - EW 101
JED - September 2016 - 53
JED - September 2016 - AOC News
JED - September 2016 - 55
JED - September 2016 - 56
JED - September 2016 - Index of Advertisers
JED - September 2016 - JED Quick Look
JED - September 2016 - cover3
JED - September 2016 - cover4
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