Richard B. Langley

Dept. of Geodesy and Geomatics Engineering

University of New Brunswick

<lang@unb.ca>

13 March 2003

1. If a war starts with Iraq, will GPS be turned off or will there be a global

reduction in available accuracy?

Prior to 2 May 2000, the accuracy afforded users of the GPS Standard Positioning

Service (SPS) was purposefully degraded through a policy and technique known as

Selective Availability (SA). The use of SA gave military users of GPS a position

accuracy advantage ? one it did not wish to share with potential adversaries. SA was

effected by manipulating or dithering the output of each GPS satellite?s active atomic

clock. This clock controls the generation of all of the satellite?s signals and hence the

measurements made by a GPS receiver. SA was imposed at a level which would yield a

stated SPS horizontal (latitude and longitude) accuracy of 100 metres or better 95 percent

of the time for any point in the world during a measurement interval of one day. On 2

May 2000, by presidential decree, the level of SA was set to zero. SPS users immediately

saw a quantum jump in positioning accuracy with factors of 5 to 10 improvements. Even

a simple handheld receiver can now often yield horizontal position accuracies of 5

metres.

When SA was set to zero, the United States Government stated that it had no intent to

ever use SA again. According to the Interagency GPS Executive Board, the U.S.

government agency that manages GPS, there has been no change in this policy. The

decision to effectively switch off SA came four years after the adoption of a government

policy document which stated, in part, that it was the government?s intention ?to

discontinue the use of GPS Selective Availability (SA) within a decade in a manner that

allows adequate time and resources for our military forces to prepare fully for operations

without SA.? Basically, this meant that the military would develop measures to prevent

the hostile use of GPS to ensure that the U.S. retains a military advantage. But it was to

do this in a manner which would not unduly disrupt or degrade civil use of GPS. The

number of civil GPS users far exceeds the number of military users and is estimated to be

in the tens of millions worldwide. Civil GPS uses include the navigation of commercial

aircraft, oil tankers, trucks, and space vehicles. Many emergency service providers now

rely on GPS to more quickly respond to accidents.

The technique the U.S. military developed which allowed them to switch off SA is

selective in-theatre jamming of the SPS signal.

So, if there is a war with Iraq, it is highly unlikely that SA would be re-imposed as it

would affect all civil GPS users worldwide. It is also highly improbable that GPS signals

would be simply switched off.

2. Will the U.S. jam GPS in Iraq?

The U.S. military certainly has the capability to jam the GPS SPS signal using either

ground-based jammers or jamming transmitters onboard aircraft. Extensive jamming tests

have taken place on military reservations in the U.S. over the past decade. But would the

U.S. actually use jammers in Iraq?

To answer this question, we must know something about the GPS signals and how they

are used by civil and military receivers. The SPS uses the C/A-code component of the

GPS L1 signal which is transmitted on 1575.42 MHz. The C/A-code, which stands for

coarse/acquisition-code, is a pseudorandom noise code which the GPS receiver uses to

determine the distance to a satellite. The distance is determined by aligning the received

code with a replica of the code generated in the receiver. By measuring at least four such

distances to different satellites simultaneously and knowing where the satellites are from

the navigation messages they transmit, the receiver can figure out where it is. The C/Acode

is a relatively short code which repeats every millisecond and a GPS receiver can

easily lock onto or acquire it.

The military?s GPS capability is known as the Precise Positioning Service (PPS). It relies

on a much longer code called the P-code (for precise or precision) which is transmitted

on both the L1 frequency and the L2 frequency at 1227.60 MHz. The P-code is encrypted

(and it?s then called the Y-code) so that it cannot be accessed by unauthorized users.

Encryption also prevents a military GPS receiver from being fooled or spoofed by a fake

GPS signal transmitted by an enemy. The encryption process is known as Anti-Spoofing.

Military GPS receivers have decryption capabilities which permit them to recover the Pcode.

Each satellite?s unique P-code segment is one week long. In order to determine the

distance to a satellite using the P-code, the receiver must align a replica of the code it

generates with the arriving code. With such a long code, it was formerly difficult for the

signal processors in P-code receivers to quickly find the correct alignment point in the

code without help. It got this help from the C/A-code. So even though military GPS

receivers determine their position (and velocity and time) from the P(Y)-code, they

generally have acquired the C/A-code first and then using information from that signal

have zeroed in on the P-code. Most of the military-grade GPS receivers now in existence

work on this principle.

Advances in receiver technology now permit direct acquisition of the P-code without

relying on the C/A-code. This new technology has been combined with improved

security procedures to prevent the use of military GPS receivers by unauthorized

personnel. GPS receivers with this capability include a module known as SAASM

(Selective Availability Anti-Spoofing Module). All new receivers acquired by the

military after 30 September 2002 are to include the SAASM. According to its Web site,

the GPS Joint Program Office had authorized over 2,050 procurements of SAASMs and

SAASM-equipped devices as of 22 October 2002. Although some older receivers can be

retrofitted with the SAASM technology, the Defense Department has not mandated such

action.

So, to get back to the question: ?Will the U.S. jam GPS in Iraq? ? they certainly have the

technology to do so. The C/A-code can be selectively jammed without significantly

affecting reception of the P(Y)-code. SPS receivers would then become useless.

However, as we discussed, most military GPS receivers now in the field use the C/Acode

before switching to the P-code. Operation of these receivers would be made difficult

in the presence of C/A-code jammers. It might be possible to initialize a receiver outside

the range of a jammer so that it is already operating on the P(Y)-code once it gets inside

the jammer?s coverage zone. However, should it temporarily lose lock on the satellite

signals, the receiver might have difficulty in reacquiring them.

Some military GPS receivers come equipped with anti-jam technology, such as nullsteering

antennas, and these receivers could continue to operate in the presence of a

jamming signal.

So, the U.S. military may or may not jam GPS in Iraq. The decision to do so will depend

on whether or not they think the Iraqis are using GPS (would you really want to rely on

your enemy?s system?) and whether or not its use would compromise their own

operations.

3. Will the Iraqi military jam GPS?

It is relatively easy to build a GPS jammer. One can be built from a surplus direct-tohome

satellite TV receiver or from scratch using plans which can be found on the Web. A

company in Russia actually markets GPS jammers and it has been reported that the Iraqis

have purchased some of them. So, the Iraqi military might try to use GPS jammers in an

attempt to foil GPS-equipped weapons such as the Joint Direct Attack Munition (JDAM).

But,

4. Where can I learn more about GPS and its military uses and jamming and antijamming

technologies?

The Precision Revolution: GPS and the Future of Aerial Warfare by Michael Russell Rip

and James M. Hasik, published by the Naval Institute Press, Annapolis, Maryland in

2002.

?Satellite-Guided Munitions,? by Michael Puttré in Scientific American, Vol. 288, No. 2,

February 2003, pp. 66-73.

?Jamming GPS: Susceptibility of Some Civil GPS Receivers,? by Börje Forssell and

Trond Birger Olsen in GPS World, Vol. 14, No. 1, January 2003, pp. 54-58;

< link

Global Positioning System: Signals, Measurements, and Performance by Pratap Misra