JED - October 2014 - 69

TECHNOLOGY SURVEY
A SAmpling of Anti-JAm gpS SyStemS
By Ollie Holt

by making the antenna pattern in the direction of
the jammer very poor, thereby reducing the jammer
signal power to less than the GPS signal power. A
Controlled Reception Pattern Antenna (CRPA), also
known as a Controlled Radiation Pattern Antenna,
basically does this. This is an antenna array and a
processing unit that basically detects a jamming
signal, determines its direction of arrival and then
makes nulls in the direction of the interfering (jamming) signal. These CRPA systems usually have five
or more antenna elements. As a rule of thumb the
number of antenna elements typically defines the
maximum number of jammers that can be defeated
at any single moment (i.e., the number of nulls it
can create).
The other anti-jam method is to process the
jamming signal out in the receiver signal processing chain. Using better filtering in the RF chain
to remove interfering out of band signals is a first
step in improving GPS receiver performance. Using
analog-to-digital converters with more effective bits
can help by providing better Spurious Free Dynamic
Range (SFDR) and lower noise figure to remove unwanted signals. Also, a reconfigurable filter can be
dynamically modified via software or firmware control to remove interfering signals. Of course, a combination of both a well designed CRPA and a good GPS
receiver that combines all of the above processing
traits provides the best results. This survey covers
anti-jam antennas, anti-jam receivers and complete
(antenna and receiver) anti-jam systems.
Another item not discussed and not specifically
covered by the survey is the Selective Availability
Anti-Spoofing Module (SAASM). This is a module
that can be added to a GPS receiver to decode the
encrypted P(Y)  code. The survey was for GPS antijam systems, and not just a module, so some of the
GPS systems in the survey may include a SAASM but
stand-alone SAASMs were not included.
Our next survey will cover portable and flightline threat simulators and will appear in the
November issue.

The Journal of Electronic Defense | October 2014

T

his month's JED survey covers antijam GPS receiver systems. GPS has now
become a utility that many military and
commercial systems have become dependent upon. This makes the GPS signal an
attractive target for jamming.
GPS signals are broadcast on two frequencies,
1.57542  GHz (L1 signal) and 1.2276  GHz (L2  signal). GPS satellites transmit their signal in a couple
of different codes; the C/A  code, for civilian use,
transmits data at 1.023  million chips per second,
whereas the P  code, for US military use, transmits
at 10.23  million chips per second. The L1 signal is
modulated by both the C/A and P code, while the L2
is only modulated with the P code. The P code can
be encrypted as P(Y) code that is only available to
military equipment with a decryption key. Both the
C/A and P(Y)  codes impart the precise time-of-day
to the user.
The problem is the signal strength of GPS signals,
which are transmitted from satellites using a 25 to
30-Watt transmitter located approximately 12,500
miles (20,000 km) above the earth, results in about
a -120 to -133 dBm signal on the ground and even
less if the GPS receiver's antenna is masked by the
platform or is inside something making them vulnerable to jamming. With that signal strength at the
receiver, a simple 1-Watt jammer could effectively
jam a radius of 100 km.
GPS receivers are designed to detect these -120
dBm signals, but in the presence of jamming it takes
very little to disrupt the GPS operation. There are
two types of GPS jamming: denial and deceptive. Denial GPS jamming can be wideband or narrow band
noise, a simple CW tone or pulse. Theses types of
signals typically raise the noise floor of the receiver
above that of the GPS signal. Deceptive-type GPS
jamming tries to replicate the GPS signal to deceive
the GPS receiver into tracking the wrong signals.
There are basically two ways of defending against
GPS jamming: 1) prevent the GPS receiver from seeing the jamming signal or 2) process out the jamming. Beam forming and beam nulling are methods
of reducing the received power from the GPS jammer

69



JED - October 2014

Table of Contents for the Digital Edition of JED - October 2014

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
Maneuver Warfare in the Electromagnetic Battlespace
The Race to Keep Pace: Countermeasures Against Radar-Guided Anti-Ship Missiles
CONVERGING INTERESTS - Is CEMA the Right Solution for the US Army’s EMS Mission?
Technology Survey: Anti-Jam GPS Receivers and Antennas
New Products
Book Reviews
EW 101
AOC 2014 Award Winners
AOC News
Index of Advertisers
JED Quick Look
JED - October 2014 - cover1
JED - October 2014 - cover2
JED - October 2014 - 3
JED - October 2014 - 4
JED - October 2014 - 5
JED - October 2014 - The View From Here
JED - October 2014 - 7
JED - October 2014 - Conferences Calendar
JED - October 2014 - 9
JED - October 2014 - Courses Calendar
JED - October 2014 - 11
JED - October 2014 - From the President
JED - October 2014 - 13
JED - October 2014 - 14
JED - October 2014 - The Monitor
JED - October 2014 - 16
JED - October 2014 - 17
JED - October 2014 - 18
JED - October 2014 - 19
JED - October 2014 - 20
JED - October 2014 - 21
JED - October 2014 - 22
JED - October 2014 - 23
JED - October 2014 - 24
JED - October 2014 - 25
JED - October 2014 - Washington Report
JED - October 2014 - 27
JED - October 2014 - World Report
JED - October 2014 - 29
JED - October 2014 - Maneuver Warfare in the Electromagnetic Battlespace
JED - October 2014 - 31
JED - October 2014 - 32
JED - October 2014 - 33
JED - October 2014 - 34
JED - October 2014 - 35
JED - October 2014 - 36
JED - October 2014 - 37
JED - October 2014 - 38
JED - October 2014 - 39
JED - October 2014 - 40
JED - October 2014 - 41
JED - October 2014 - 42
JED - October 2014 - 43
JED - October 2014 - 44
JED - October 2014 - 45
JED - October 2014 - The Race to Keep Pace: Countermeasures Against Radar-Guided Anti-Ship Missiles
JED - October 2014 - 47
JED - October 2014 - 48
JED - October 2014 - 49
JED - October 2014 - 50
JED - October 2014 - 51
JED - October 2014 - 52
JED - October 2014 - 53
JED - October 2014 - 54
JED - October 2014 - 55
JED - October 2014 - 56
JED - October 2014 - 57
JED - October 2014 - 58
JED - October 2014 - 59
JED - October 2014 - 60
JED - October 2014 - 61
JED - October 2014 - 62
JED - October 2014 - 63
JED - October 2014 - 64
JED - October 2014 - CONVERGING INTERESTS - Is CEMA the Right Solution for the US Army’s EMS Mission?
JED - October 2014 - 66
JED - October 2014 - 68
JED - October 2014 - 67
JED - October 2014 - Technology Survey: Anti-Jam GPS Receivers and Antennas
JED - October 2014 - 70
JED - October 2014 - 71
JED - October 2014 - 72
JED - October 2014 - 73
JED - October 2014 - 74
JED - October 2014 - 75
JED - October 2014 - 76
JED - October 2014 - 77
JED - October 2014 - New Products
JED - October 2014 - 79
JED - October 2014 - Book Reviews
JED - October 2014 - 81
JED - October 2014 - EW 101
JED - October 2014 - 83
JED - October 2014 - 84
JED - October 2014 - 85
JED - October 2014 - AOC 2014 Award Winners
JED - October 2014 - 87
JED - October 2014 - 88
JED - October 2014 - 89
JED - October 2014 - 90
JED - October 2014 - 91
JED - October 2014 - 92
JED - October 2014 - 93
JED - October 2014 - AOC News
JED - October 2014 - 95
JED - October 2014 - Index of Advertisers
JED - October 2014 - 97
JED - October 2014 - JED Quick Look
JED - October 2014 - cover3
JED - October 2014 - cover4
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