Mixed Signals: Using Civil GPS Receivers in Combat

Due to budgetary and manufacturing constraints, the DoD handheld Global Positioning System (GPS) receiver supply has fallen short of perceived demand. Less expensive, smaller, lighter, and easier to use civil receivers are more widely available than the military models. Why shouldn't acquisition officials fill the gap with commercial SPS receivers? Isn't a civil GPS receiver better than no GPS receiver for our deployed warriors?

Many in our military believe that the relatively inexpensive and widely available civil GPS receivers provide the same positioning, navigation, and timing capabilities as the DoD models. This belief, along with insufficient numbers supplied through official channels, has led American troops to buy handheld civil GPS receivers and use them in combat. In its 2003 and 2004 Annual Report, Garmin International, a civil GPS receiver manufacturer, published letters from Marines, sailors, and soldiers describing how they used Garmin receivers in Iraq and Afghanistan. Many cite a perceived shortage of DoD handheld receivers, and offer the rationale that "something (a civil GPS receiver) is better than nothing (a map and a compass)."

Others prefer the civil receivers because all but the newest handheld military receivers require cryptography which makes the devices "classified when keyed" and a major security headache if lost or stolen. A final group believes that the widespread availability and relative small cost of civil receivers make it a requirement to have one. This "If everyone can get one, then everyone should have one" philosophy fuels the argument that DoD is not supporting our military's GPS receiver needs.

To determine if civil GPS receivers perform as well as military receivers in a combat environment, I compared the performance of both using Navigation Tool Kit, a software application that analyzes GPS performance in complex environments. My analysis shows that the belief that the two systems offer similar capabilities depends on the user's equipment and the electronic environment in which he is operating. To understand why this is so, we need to understand the difference between civil and military GPS services.

Two GPS Services

Central to NAVSTAR GPS is a constellation of 24 orbiting satellites each broadcasting two navigation signals, L1 at 1575.42 MHz and L2 at 1227.6 MHz. L1 carries the Coarse Acquisition Code (C/A Code) and the Precise Code (P Code) and L2 carries only the P Code. A GPS receiver acquires and correlates the appropriate signals from four satellites to compute a position.

The C/A Code on L1 provides the "civilian" GPS service known as Standard Positioning Service (SPS). The receivers purchased for personal navigation from civilian vendors are SPS receivers. The SPS architecture is wide open to everyone for exploration and exploitation. The civil GPS service can be very accurate in benign environments but because of its "openness" and signal structure it is vulnerable to electronic interference and electronic attack.

Military GPS service, called Precise Positioning Service (PPS), is designed to be very accurate and resistant to electronic interference and electronic attack. DoD encrypts the P Code to control access to this service. The encryption "closes" the PPS architecture making it exclusive to users whose receivers have the keys to decrypt and read the PPS signal. To access the military GPS service, a military receiver first acquires the C/A Code then switches to the P Code. Once acquired, PPS is more resistant to interference than the civil SPS signal.

My anaysis used the industry standard algorithms and precise satellite data in the analysis software to compare the performance of the PPS Defense Advanced GPS Receiver (DAGR) and a multi-channel civil SPS receiver during three scenarios near Baghdad. Scenario 1 featured a benign environment with no GPS interference. During Scenario 2, I introduced unintentional friendly radio interference on L1. Scenario 3 involved a denial jamming array. The three scenarios used a High Mobility Multi-wheeled Vehicle (HMMWV) carrying both a PPS DAGR and a multi-channel civil SPS receiver as it traveled along a 46 minute route into Baghdad. The results highlight the shortcomings of civil SPS receivers in environments where GPS interference is present and the potential danger of using civil receivers in combat.

Scenario 1: Benign Environment

In this scenario, I compared the position accuracy of the PPS DAGR and the civil SPS receiver along the test route in an interference-free environment.

Both receivers consistently computed positions. The PPS DAGR position error plot was tighter and more consistently accurate than the wider spread civil SPS receiver plot.

The mean position error for the PPS DAGR was 2.02 meters compared to the civil SPS receiver mean position error of 4.03 meters. In the interference-free environment, my analysis showed both the PPS DAGR and the civil SPS receiver to be sufficiently accurate for most combat operations, with the PPS DAGR being the more consistently accurate.

Scenario 2: Signal Interference on L1 (1575 MHz)

The second scenario used the same route and the same PPS DAGR and civil SPS receiver. However, in this scenario, I introduced signal interference on L1 from a series of U.S. military AN/GRC226 UHF radio communication retransmission sites northeast of the test route. The friendly radio transmitters emitted 10 watts each.

Jam-to-signal interference contours ranged from 20db (least interference) to 60dB (greatest interference). The test route in the southeast transited high jamto-signal areas. In these areas, the civil SPS receiver produced position errors as great as 1 kilometer. Over this portion of the test route, the PPS DAGR warned it was being jammed and refused to calculate erroneous positions.

Comparing the position errors produced by the PPS DAGR and the civil SPS receiver illustrated three interesting points. First, the U.S. military can adversely affect its own GPS performance by emitting 10 watt radio signals on or near the GPS navigation signal frequencies.

Second, the PPS receiver's mean position accuracy rate was eight times better than the civil SPS receiver during this scenario. Recall from the benign environment in Scenario 1 that the PPS DAGR was only slightly more accurate than the civil SPS receiver. In this scenario, Navigation Tool Kit analysis showed both receivers suffered degraded accuracy but the civil SPS receiver produced exponentially larger position errors when friendly GPS signal interference was introduced.

The third point concerns how the PPS DAGR and the civil SPS receiver responded to high jam-to-signal areas along the southeast portion of the test route. During this period, the DAGR detected a critical level of signal interference. In such a situation, the DAGR would display, "Warning! Jamming Environment Detected" on the receiver screen as it attempted to navigate in the jammed environment. However, during the same period, the civil SPS receiver calculated erroneous positions with no indication of signal interference to the user other than lost satellite tracking. In this experiment, civil SPS position errors during periods of high jam-to-signal interference ranged from 100 meters to more than 1 kilometer.

Scenario 3: Denial Jammer Array

The third scenario represented a GPS countermeasure threat that the U.S. could encounter. In this scenario, I made the GPS environment challenging by placing six 20-watt GPS jammers in a circular pattern around a portion of Baghdad. The intent of this jammer array was to deny accurate GPS solutions within the confines of the jammer circle. The orange and red jam-tosignal contours, which indicate the highest levels of interference, overlapped toward the center of the jammer array, showing degraded GPS positioning accuracy in that area.

The PPS DAGR and civil SPS receiver position error scale differed between the DAGR (0-100 meters) and the civil SPS receiver (O-1000 meters) because the civil SPS receiver produced position errors far greater than the PPS DAGR.

The Scenario 3 position error plots showed the DAGR and civil SPS receivers responded to interference similar to the response in Scenario 2. Both receivers were affected by the signal interference, but the PPS DAGR generally refused to calculate a position if it was being jammed. In the cases that the DAGR could calculate a position, the worst error was less than 60 meters.

As it did in Scenario 2, the civil SPS receiver calculated erroneous positions during half the Scenario 3 test route travel time with no indication of interference to the user. Civil SPS receiver position errors were often more than 500 meters, with excursions exceeding 1 kilometer in several instances.

The Bottom Line

Since Operation Desert Storm, GPS has become the core positioning, navigation, and timing technology for the U.S. military and our allies. Our system-wide reliance on GPS, combined with the fact that GPS signals traveling 12,400 miles through space to your receiver are vulnerable to interference, makes GPS a prime electronic warfare target in our enemies' eyes. During Operation Iraqi Freedom, enemy forces repeatedly attempted to jam GPS signals. Threats to GPS are very real in the modern battlespace.

The analysis presented in this article illustrates possible consequences of using civil SPS receivers in combat. First, the civil SPS receiver provided acceptable accuracy in an interference-free environment but was not as consistently accurate as the PPS DAGR. Second, electronic interference from friendly UHF radio transmitters and GPS denial jammers adversely affected both receivers. However, in the scenarios where GPS interference was present, the civil SPS receiver position error was consistently eight to sixteen times that of the PPS DAGR, with multiple position error excursions of more than 1,000 meters. Finally, when the jam-to-signal ratio became too high during the test scenarios, the PPS DAGR provided interference warning and refused to calculate an erroneous position. When exposed to the same interference, the civil SPS receiver produced gross position errors while providing no jam warning to the user.

The Navigation Tool Kit analysis shows that the civil SPS receiver user simply cannot be sure the position he reads from his receiver screen is valid in the modern battlespace. Today's combat environments are rife with friendly and enemy induced electronic interference that can accidentally or purposely spoof or jam the unprotected civil GPS receiver, forcing it to produce a positioning error with no warning to the user. Such an error could prove deadly under fire.