JED - September 2016 - 52

EW 101

Space EW - Part 4

Earth Traces
By Dave Adamy

I

n this and the next couple of columns, we will discuss
some general geometrical issues with Earth satellite
orbits. During this discussion, we will be using terms
that were defined in the June 2016 "EW 101" column.
Hopefully, you will forgive me for rounding the numbers
during this discussion, because we
are EW folks rather than orbiteers, who
must get the numbers exactly right to
many decimal places.

The Journal of Electronic Defense | September 2016

52

North Pole

Greenwich
Meridian

SATELLITE EARTH TRACE
The Earth trace is the locus of latitude and longitude of the sub-vehicle point (SVP) as the satellite moves
through its orbit. For low-Earth orbits,
this determines the moment-to-moment
area of the Earth that is "seen" by the
satellite. It also allows us to calculate
the "look angle" and range to the satellite from a specified point on (or above)
the Earth at any specified time.
Figure 1 is a polar view of the Earth
trace of a low-Earth satellite as seen
from above the North Pole. Note that
the satellite crosses the equator going
north at the ascending node and reaches a maximum latitude equal to the inclination of the orbit. This view only
traces the northern half of the orbit.
The other half would be seen in a view
from above the South pole. From the six
elements of the ephemeris (defined in
the June 2016 column) you can calculate the exact location of the satellite at
any time. We will be going through that
process as we work EW problems later in
this article series.
The figure shows the Earth traces of
two orbits of the satellite. The first orbit
Earth trace is shown as a solid line, and
the next orbit is shown as a dashed line.
The orbit is actually affected by mountains and other features and anomalies
on the Earth's surface. However, we will
assume a perfect spherical Earth in this

discussion, so the orbit is assumed to follow a constant path
relative to the center of the Earth, as the Earth turns inside
of the orbit.
It is easy to become confused by the three-dimensional geometry when dealing with orbits. There is a simple way to remember

Direction of
Earth's Rotation

Satellite
Earth Trace
for One Orbit

Longitude of
Ascending
Node

Equator
Earth Trace
for the Next Orbit

Ascending
Node

Figure 1: The Earth trace of a satellite is the path of the sub-vehicle point over the Earth's surface.
This is a polar view.

Figure 1: The Earth trace of a satellite is the path of the sub-vehicle point
over the Earth's surface. This is a
polar view.
Direction
of

Earth's Rotation

North Pole

Satellite
Earth Trace
Ascending
Node
Earth
Trace
for
One Orbit

Equator
Earth
Trace
for
Next Orbit
South Pole

Figure 2: The Earth trace of a satellite is the path of the sub-vehicle point over the Earth's surface.
This is an equatorial view.

Figure 2: The Earth trace of a satellite is the path of the sub-vehicle point



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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