JED - February 2017 - 44

EW 101

Space EW - Part 9

Duration of Target Availability
By Dave Adamy

L
The Journal of Electronic Defense | February 2017

44

ast month, we talked about
the portion of the Earth that
can be seen by a satellite. Now,
we're going to talk about some
timing issues. This gets a little
complicated because the satellite is moving around the Earth while the
Earth is turning on its axis inside the
orbit. Back in the June 2016 "EW101" column, there is a table of the elements of
the orbit. Now, we're going to deal with
two more of these elements: the inclination of the orbit (the angle at which it
crosses the equator) and the ascending
node (the longitude at which the orbit
crosses the equator).
In Figure 1, you can see the satellite Earth trace with the ascending
node and the inclination. The satellite
is shown right above a threat location.
There is a spherical triangle defined by
the threat location, the ascending node
and the point at which the longitude
line of the threat intersects the equator. Figure 2 isolates that triangle and
labels it as we have been doing in earlier columns. Angle A is at the ascending
node. Angle B is at the threat location,
which is also the sub-vehicle point of
the satellite. Angle C is 90°. Side a is the
latitude of the threat. Side b is the longitude difference between the ascending node and the threat. Side c is the
geocentric angle between the ascending
node and the threat
Now we will look at the plane of the
satellite orbit in Figure 3. We are considering a circular orbit, so the height of the
orbit above the Earth remains constant,
and the angular velocity of the satellite
around the center of the Earth is also
constant. In this figure, the angle from
the ascending node (which is also side c
in Figure 2) can be calculated from the
expression: 360° x (time since the ascending node/satellite period).

Fig 1: The longitude of the ascending node, the inclination and the geocentric angle from the
ascending node determine the location of the satellite sub-vehicle point.

Fig 2: The spherical triangle formed by the threat, the ascending node and the intersection of the
threat's longitude line with the Equator allow calculation of the location of the threat and the
horizon distance.

First, we will stop the spin of the Earth
around its orbit to consider the geometry
over a non-turning Earth. Figure 4 shows
the satellite earth trace passing directly
over some point of interest (for example
a threat). With a non-rotating Earth, the
satellite would be able to see that threat

from the time its sub-vehicle point is at
point F to the time it is at Point G.

TIME AT WHICH SATELLITE
REACHES AN EARTH LOCATION
The time (after the ascending node
time) at which the satellite passes over



JED - February 2017

Table of Contents for the Digital Edition of JED - February 2017

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Asia-Pacific SIGINT Programs
Technology Survey: Analog-to-Digital Converters
Operator 101
EW 101
AOC News
Index of Advertisers
JED Quick LookThe
JED - February 2017 - cover1
JED - February 2017 - cover2
JED - February 2017 - 3
JED - February 2017 - 4
JED - February 2017 - 5
JED - February 2017 - The View From Here
JED - February 2017 - 7
JED - February 2017 - Conferences Calendar
JED - February 2017 - 9
JED - February 2017 - Courses Calendar
JED - February 2017 - 11
JED - February 2017 - From the President
JED - February 2017 - 13
JED - February 2017 - 14
JED - February 2017 - The Monitor
JED - February 2017 - insert1
JED - February 2017 - insert2
JED - February 2017 - 16
JED - February 2017 - 17
JED - February 2017 - 18
JED - February 2017 - 19
JED - February 2017 - 20
JED - February 2017 - 21
JED - February 2017 - World Report
JED - February 2017 - 23
JED - February 2017 - Asia-Pacific SIGINT Programs
JED - February 2017 - 25
JED - February 2017 - 26
JED - February 2017 - 27
JED - February 2017 - 28
JED - February 2017 - 29
JED - February 2017 - 30
JED - February 2017 - 31
JED - February 2017 - Technology Survey: Analog-to-Digital Converters
JED - February 2017 - 33
JED - February 2017 - 34
JED - February 2017 - 35
JED - February 2017 - 36
JED - February 2017 - 37
JED - February 2017 - 38
JED - February 2017 - 39
JED - February 2017 - 40
JED - February 2017 - Operator 101
JED - February 2017 - 42
JED - February 2017 - 43
JED - February 2017 - EW 101
JED - February 2017 - 45
JED - February 2017 - AOC News
JED - February 2017 - 47
JED - February 2017 - 48
JED - February 2017 - Index of Advertisers
JED - February 2017 - JED Quick LookThe
JED - February 2017 - cover3
JED - February 2017 - cover4
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