JED - June 2016 - 46

EW 101

Space EW - Part 1

Orbits
By Dave Adamy

T

his column starts a new EW101 series about
Electronic Warfare (EW) involving space. For thoroughness, it will include a little about orbits as
they apply to EW applications. Other material will
concern radio propagation to and from space, the
vulnerability of satellite links, and intercept and
jamming from space platforms.

ORBITAL RELATIONSHIPS

The Journal of Electronic Defense | June 2016

46

The altitude of a satellite in a circular orbit is constant: the
radius of the orbit less the radius of the Earth. e is the ec-

SATELLITE

SEMI-MAJOR AXIS
APOGEE

PERIGEE
We will consider geosynchronous orbits (sometimes referred
to as GSOs) and low Earth orbits (LEOs). First, we'll cover some
FOCUS OF
basic relationships in orbits. As in all of the subjects covered
ELLIPSE AT
CENTER OF
in EW101 columns, we will skip many of the niceties of derivaEARTH
tions and minor details. We will go directly to the information
required for us to work practical, EW related problems
The path of an Earth satellite is affected by the gravitaFigure 1: An
Earth1:
satellite
an elliptical
path with the center of
Table
Earthfollows
Satellite
Ephemeris
tional pull of every other object in space, but most of those
the Earth at one focus of the ellipse.
Figure 1: An Earth satellite follows an elliptical path with the
objects are far away and thus have only secondary
center of the Earth at one focus of the ellipse.
effects. The main elements determining the orbit are
Ephemeris Value
Significance
the gravitational pull of the Earth and the velocity
a Semi-major axis
Size of the orbit
of the Earth satellite. By ignoring other factors, we
e Eccentricity
Shape of the orbit
have what is called the "two body problem." For our
i Inclination
Tilt of orbit relative to equatorial plane
purposes at this time, we assume there is nothing
n Right ascension of the Longitude at which the satellite crosses the
in space except the Earth and one satellite moving
ascending node
equator going north
around the Earth. Working with the mechanics of the
w Argument of perigee
Angle between ascending node and perigee
orbit, and dealing with angles and distances between
v True anomaly
Angle between perigee and the satellite
satellites and the transmitters and receivers involved
location in the orbit
in EW operations, will require the use of spherical
trigonometry, so we will also cover that (rather genTable 1: Earth Satellite Ephemeris
tly) in this series.
The shape of an Earth satellite orbit is an ellipse with the
North
Earth at one focus as shown in Figure 1. We will treat the
Pole
Earth as a perfect sphere, which it is not. As you probably
know, the Earth is flatter at the poles by about 25 miles, but
Perigee
with a radius of 6,371 km, that still makes it a more perfect
Satellite
sphere than the best ball bearing you can buy.

ELEMENTS OF AN ELLIPTICAL ORBIT
The location of an Earth satellite is defined by a series of six
numbers, which are called the Keplerian ephemeris, because
they are credited to the work of Johannes Kepler, a 17th Century German mathematician and astronomer. Table 1 shows
these six numbers and defines them.
a is the semi-major axis as shown in Figure 1. This is half
of the long dimension of the satellite's elliptical orbit. It is
also the average distance of the satellite from the center of
the Earth. For a circular orbit, it is the radius of the orbit.

v

w
n
Vernal
Equinox
Direction

Orbit

i

r

ato

Equ

Ascending
Node

Figure 2: The Ephemeris defines the location of the satellite with six
factors.
Figure 2: The Ephemeris defines the location of the
satellite with six factors.



JED - June 2016

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
Advances in COMINT/ DF Systems
Technology Survey: Communications and IED Jammers
SIGINT History
EW 101
Report from the 45th Annual Collaborative EW Symposium
AOC News
2016 AOC International Election Guide
Index of Advertisers
JED Quick Look
JED - June 2016 - cover1
JED - June 2016 - cover2
JED - June 2016 - 3
JED - June 2016 - 4
JED - June 2016 - 5
JED - June 2016 - The View From Here
JED - June 2016 - 7
JED - June 2016 - Conferences Calendar
JED - June 2016 - 9
JED - June 2016 - Courses Calendar
JED - June 2016 - 11
JED - June 2016 - From the President
JED - June 2016 - 13
JED - June 2016 - 14
JED - June 2016 - The Monitor
JED - June 2016 - 16
JED - June 2016 - 17
JED - June 2016 - 18
JED - June 2016 - 19
JED - June 2016 - 20
JED - June 2016 - 21
JED - June 2016 - Washington Report
JED - June 2016 - 23
JED - June 2016 - World Report
JED - June 2016 - 25
JED - June 2016 - Advances in COMINT/ DF Systems
JED - June 2016 - 27
JED - June 2016 - 28
JED - June 2016 - 29
JED - June 2016 - 30
JED - June 2016 - 31
JED - June 2016 - 32
JED - June 2016 - Technology Survey: Communications and IED Jammers
JED - June 2016 - 34
JED - June 2016 - 35
JED - June 2016 - 36
JED - June 2016 - 37
JED - June 2016 - 38
JED - June 2016 - 39
JED - June 2016 - 40
JED - June 2016 - 41
JED - June 2016 - 42
JED - June 2016 - SIGINT History
JED - June 2016 - 44
JED - June 2016 - 45
JED - June 2016 - EW 101
JED - June 2016 - 47
JED - June 2016 - Report from the 45th Annual Collaborative EW Symposium
JED - June 2016 - 49
JED - June 2016 - AOC News
JED - June 2016 - 2016 AOC International Election Guide
JED - June 2016 - 52
JED - June 2016 - 53
JED - June 2016 - 54
JED - June 2016 - 55
JED - June 2016 - 56
JED - June 2016 - Index of Advertisers
JED - June 2016 - JED Quick Look
JED - June 2016 - cover3
JED - June 2016 - cover4
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