# JED - December 2016 - 86

```EW 101

Space EW - Part 7

Location of an EW Threat

S
The Journal of Electronic Defense | December 2016

86

ince we are starting to talk about spacebased Electronic Warfare systems, we need
to consider the geometry between the satellite and hostile threat locations. This
month, we will consider threats on the surface of the Earth. We will just call a hostile
transmitter or receiver a "threat." An EW system on the
satellite will either intercept signals from a threat transmitter or transmit jamming signals to a threat receiver
at the considered location.
As shown in figure 1, the location of the threat from
the satellite will be defined in terms of the azimuth and
elevation of a vector from the satellite that would point
at the threat location, and the range between the satellite and the threat. You could think of the vector as
the pointing information for a satellite antenna aimed
at the threat.

Horizontal
Plane

N

Azimuth
To threat

CALCULATING THE LOOK ANGLES
The azimuth is the angle in a plane perpendicular to the vector
from the sub-vehicle point (SVP) from true North to the threat location. The elevation is the angle between the SVP and the threat. For
the azimuth calculation, we need to consider the spherical triangle in
figure 2 defined by the North pole, the SVP and the threat location.
For convenience, we define the parts of the triangle in the way
we have in earlier discussion in this series: The capital letters
are for "angles" (i.e. the intersection of two great-circle planes
through the center of the Earth). The lower case letters are for
"sides" (i.e. the angle between points located on one of the great
circle planes that form the spherical triangle). Remember that, in a
spherical triangle, both the sides and the angles are angles. In this
case A is at the North Pole, B is at the SVP, and C is at the threat
location. Side a (opposite angle A) is the path along the Earth's
surface from the SVP to the threat. Side b is 90° less the latitude of
the threat location. The latitude is, of course, the geocentric angle
from the equator up to some point on
the Earth's surface. Side c is 90° less
the latitude of the SVP.

North
Pole

CALCULATING THE AZIMUTH
TO THE THREAT

THREAT
Center of
Earth
Elevation
To threat
Sub-vehicle
Point
Fig 1: The azimuth and elevation angle from nadir define the direction to a threat from
the satellite.

The azimuth to the threat is angle
B. (This is what we want to calculate.)
Since we start the problem by entering the locations of the SVP and the
threat, we know the sides b and c and
the angle A. Angle A is the longitude difference between the SVP and the threat.
(∆ longitude). Before diving into these
trigonometric equations, you may want
to review the July EW101 column which
defines all of the plane and spherical
trigonometric formulas we will be using
over the next few columns.
First, we calculate side a from the
(spherical) law of cosines for sides:

```

# JED - December 2016

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
Interview: Col Jeffrey N. Aldridge, USAF
Interview: US Rep. Joe Pitts
The Future of Airborne Electronic Attack
Regaining the Advantage – Cognitive Electronic Warfare
Technology Survey: Flightline and Portable EW Simulators
AOC EW Singapore Preview
New Products
Report from the 2016 EW GCC Conference
Crows in the Community
The Heat is On
EW 101
Report from the 2016 AOC CEMA Conference
2016 AOC Award Winners
AOC News
JED Quick Look
JED - December 2016 - cover1
JED - December 2016 - cover2
JED - December 2016 - 3
JED - December 2016 - 4
JED - December 2016 - 5
JED - December 2016 - The View From Here
JED - December 2016 - 7
JED - December 2016 - Conferences Calendar
JED - December 2016 - 9
JED - December 2016 - Courses Calendar
JED - December 2016 - 11
JED - December 2016 - From the President
JED - December 2016 - 13
JED - December 2016 - 14
JED - December 2016 - The Monitor
JED - December 2016 - insert1
JED - December 2016 - insert2
JED - December 2016 - 16
JED - December 2016 - 17
JED - December 2016 - 18
JED - December 2016 - 19
JED - December 2016 - 20
JED - December 2016 - 21
JED - December 2016 - 22
JED - December 2016 - 23
JED - December 2016 - 24
JED - December 2016 - 25
JED - December 2016 - Washington Report
JED - December 2016 - 27
JED - December 2016 - World Report
JED - December 2016 - 29
JED - December 2016 - Interview: Col Jeffrey N. Aldridge, USAF
JED - December 2016 - 31
JED - December 2016 - 32
JED - December 2016 - 33
JED - December 2016 - Interview: US Rep. Joe Pitts
JED - December 2016 - 35
JED - December 2016 - 36
JED - December 2016 - 37
JED - December 2016 - The Future of Airborne Electronic Attack
JED - December 2016 - 39
JED - December 2016 - 40
JED - December 2016 - 41
JED - December 2016 - 42
JED - December 2016 - 43
JED - December 2016 - 44
JED - December 2016 - 45
JED - December 2016 - 46
JED - December 2016 - 47
JED - December 2016 - 48
JED - December 2016 - 49
JED - December 2016 - 50
JED - December 2016 - 51
JED - December 2016 - 52
JED - December 2016 - 53
JED - December 2016 - 54
JED - December 2016 - 55
JED - December 2016 - Regaining the Advantage – Cognitive Electronic Warfare
JED - December 2016 - 57
JED - December 2016 - 58
JED - December 2016 - 59
JED - December 2016 - 60
JED - December 2016 - 61
JED - December 2016 - 62
JED - December 2016 - Technology Survey: Flightline and Portable EW Simulators
JED - December 2016 - 64
JED - December 2016 - 65
JED - December 2016 - 66
JED - December 2016 - 67
JED - December 2016 - 68
JED - December 2016 - 69
JED - December 2016 - 70
JED - December 2016 - AOC EW Singapore Preview
JED - December 2016 - 72
JED - December 2016 - 73
JED - December 2016 - New Products
JED - December 2016 - 75
JED - December 2016 - Report from the 2016 EW GCC Conference
JED - December 2016 - 77
JED - December 2016 - Crows in the Community
JED - December 2016 - 79
JED - December 2016 - The Heat is On
JED - December 2016 - 81
JED - December 2016 - 82
JED - December 2016 - 83
JED - December 2016 - 84
JED - December 2016 - 85
JED - December 2016 - EW 101
JED - December 2016 - 87
JED - December 2016 - 88
JED - December 2016 - 89
JED - December 2016 - Report from the 2016 AOC CEMA Conference
JED - December 2016 - 91
JED - December 2016 - 92
JED - December 2016 - 2016 AOC Award Winners
JED - December 2016 - 94
JED - December 2016 - 95
JED - December 2016 - 96
JED - December 2016 - 97
JED - December 2016 - 98
JED - December 2016 - 99
JED - December 2016 - AOC News
JED - December 2016 - 101
JED - December 2016 - 102
JED - December 2016 - 103
JED - December 2016 - 104
JED - December 2016 - Index of Advertisers
JED - December 2016 - JED Quick Look
JED - December 2016 - cover3
JED - December 2016 - cover4
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