The year of decision for GPS vulnerability

April 1, 1998
Threats to vital Global Positioning System signals are growing every year, and defense officials must begin improvements today so they can field jam-resistant systems over the next six years

Threats to vital Global Positioning System signals are growing every year, and defense officials must begin improvements today so they can field jam-resistant systems over the next six years

By John Rhea

Concerns about the vulnerability of the ubiquitous Global Positioning System (GPS), heart of the U.S. military`s navigation capabilities, have been steadily rising to where top military officials now believe 1998 is the year to take corrective action.

Defense officials are focusing their efforts on the ground portion of the system for now, and will formulate a firm operational requirement by August, says U.S. Air Force Maj. Joe Lortie, head of navigation warfare - better known as "navwar" - in the GPS Joint Program Office at the Air Force Space and Missile Systems Center in El Segundo, Calif. Lortie is to submit this requirement up the line to the Joint Requirements Oversight Committee (JROC) and then to the Defense Acquisition Board (DAB).

"We take our role as stewards of the GPS utility very seriously and we are concerned about the emerging capability to jam the GPS signal," proclaims Col. Jim Armor, director of the GPS Joint Program Office. "We have a rich set of technology studies and demonstrations now under way, and will bring a comprehensive recommendation to the GPS modernization/navwar program decision milestone in August."

Defense experts have recognized the vulnerability of GPS to "spoofing," "shading," and even active electronic countermeasures ever since GPS became operational in the 1970s. Lortie says Defense Department leaders have spent $14 billion on navwar since 1973 on a wide variety of measures aimed at protecting U.S. navigation assets while denying adversaries the use of similar assets.

One of the fears, which officials never expressed publicly, was the threat that Iraqi jammers might have rendered U.S. GPS equipment inoperative if the recent impasse over United Nations arms inspectors in Iraq had led to military action.

Real worries

Driving these fears home was the appearance of an ominous device at an air show in Moscow last September: a 4-watt portable jammer capable of interfering with civil and military signals from the GPS and Russian Global Navigation Satellite System (GLONASS) satellites to distances of 120 miles.

Developers did not demonstrate the jammer at the show, sources say because it might have interfered with aircraft operating in the area. Its builders, engineers from the Russian firm Aviaconversia, consider it to be effective against military P code receivers. The unit was battery operated and weighed between 18 and 26 pounds. A second-generation model is reported to be in development at a weight of between nine and 13 pounds.

There have been unconfirmed reports of sales to Middle East countries, including Iraq, Libya, and Syria. "We must assume they have or have access to this technology," comments one industry source. "It`s in the open literature."

The current U.S. efforts to protect GPS signals predate this disclosure, but it adds an extra sense of urgency for researchers at the GPS Joint Program Office.

Their first order of business is coming up with a quick fix to safeguard U.S. hand-held GPS receivers. Yet researchers must also consider incremental upgrades to existing GPS satellites. Air Force experts are negotiating with leaders of The Boeing Co. in Seattle for the next-generation Block 2F satellites. Placing these new satellites in orbit will take longer than simply replacing the ground units.

U.S. officials must make a decision on GPS satellites this year as part of what Lortie considers to be the overall system architecture of navwar. For the past three years a constellation of 24 GPS satellites has been in operation, plus three on-orbit spares. To launch new satellites around 2003 or 2004, officials must lock in any changes now.

The Rockwell Collins Avionics and Communications Division in Cedar Rapids, Iowa, dominates the ground segment of the market with the Precision GPS Receiver (PLGR, pronounced "plugger"). Lortie estimates some 120,000 hand-held units of various types are currently fielded with U.S. Forces. An additional 16,000 airborne platforms in the DOD inventory can use GPS in varying degrees, and designers are increasingly incorporating the technology in "smart munitions."

Nonetheless, military use of GPS is minuscule compared to the booming automotive, cellular communications, and other consumer markets. DOD, in fact, represents less than 5 percent of the total market, according to the Washington-based trade association, the U.S. GPS Industry Council.

That means that users, not industry, will end up paying for any military-unique enhancements. It also means that U.S. adversaries inevitably will acquire their own GPS capabilities, use the Russian GLONASS, or both. Given the widespread availability of GPS receivers from outlets such as Radio Shack, the U.S. military must develop countermeasures rather than attempt the hopeless task of choking off the source of supply as they deal with the proliferation of GPS receivers.

The activity that is forcing the issue on GPS vulnerability is a small advanced concept technology demonstration launched by the Joint Program Office in 1995 and due to be completed by next year at a total cost of $55 million. The ground rules specify that only mature technologies are candidates - not improvements to GPS receiver antennas. Lortie reports that tests to date have been successful for a next-generation PLGR to be known as the Defense Advanced GPS Receiver (DAGR, pronounced "dagger").

Rockwell Collins engineers delivered about 75 prototype hand-held units operating in the L1 and L2 frequencies over 12 channels. Designers put these units through a series of multi- service tests. The original PLGRs, themselves were an improvement over the commercial hand-held GPS receivers that soldiers used during the Persian Gulf War of 1991, which they had to frantically buy from commercial sources.

For the demonstration, company engineers configured the hand-held units using a dual-frequency capability derived from the advanced Special Operations Lightweight GPS Receiver and an equivalent ordered for British military forces. These units are light, rugged, hand-held receivers that Special Operations units are testing, and which officials expect to field later this year.

The results of the tests on the upgraded units are in, and Lortie says he is sufficiently satisfied to start buying units next year if members of the JROC and DAB agree.

Military leaders would need at least 100,000 DAGRs to replace today`s PLGRs for U.S. and allied forces, and the costs of the new units would be more than the $800 PLGR. Although Air Force officials will not comment officially, industry sources say Joint Program Office leaders have established a $2,000 ceiling price.

This increase represents the increased capability of two frequencies and 12 channels, but the biggest cost driver is a new component known as the Selective Availability Anti-Spoof Module, which is not in any of the current hand-held units. Designers will package this device as the GPS Receiver Applications Module (GRAM) in an attempt to build an open-system architecture. GRAMs are to be embedded in a variety of GPS systems, using such standard form factors as SEM-E and VME. Other countermeasures, such as adaptive filters and nulling antennas, are beyond the scope of the demonstration.

The demonstration involves more than hardware. It also involves training in the protection of assets, and the generation of new user operating requirements.

That is still not good enough, maintains Stanley Alterman, a consultant to the Defense Science Board based in Jupiter, Fla., who has persistently criticized the services for not taking sufficiently vigorous action to protect the GPS. He wants a three-year, $150 million research program concentrating on two areas: giving GPS receivers greater discrimination against jamming signals, and increasing signal power to the satellites.

Like the Joint Program Office, Alterman chose the receivers first because results can be achieved sooner. Based on a successful development effort, he maintains that all GPS users could have an electronic counter countermeasures (ECCM) capability within five years at a total cost of $1 billion t o $2 billion. After that he would move on to an integrated system of receivers and advanced antennas.

Moreover, Alterman`s plan would tailor the degree of countermeasures to the value of the weapon system. Guided munitions and hand-held units would be at the low end, and engineers could upgrade them for $500 to $1,000 each, he estimates. This is comparable to the Joint Program Office`s reported ceiling price. At the high end, Alterman estimates that designers could upgrade the integrated navigation system in an aircraft such as the U.S. Navy F/A-18 fighter-bomber for $25,000 to provide anti-jam capabilities.

Looking beyond the immediate threat to military GPS applications, Alterman points out that commercial aviation is vulnerable to terrorists armed with small, cheap jammers, which they can buy on the international arms market or make themselves. As the aviation industry increasingly depends on GPS, airline leaders put their aircraft and passengers in jeopardy from terrorists, he maintains. "The fundamental issue is that GPS is a wonderful system," he says. "It`s seductive because it`s so cheap."

GPS, of course, is not the totality of military and commercial navigation; it complements inertial navigation systems (INS) such as ring-laser, fiber-optic, and spinning-mass gyros. Lortie stresses that the two must be closely coupled. Although GPS was particularly effective in the flat desert during the Gulf War, the same is not true in heavy foliage, urban areas with tall buildings, or even in mountainous regions.

The two technologies fit well because GPS can update the INS, which drifts over time. Thus systems integrators can mix and match the accuracy of GPS and jamming invulnerability of INS.

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The Position Navigation Unit dynamically tuned gyro assembly from Smiths Industries can complement GPS receivers by providing navigational information when GPS signals are masked or jammed.

GPS vulnerability: a primer

The Global Positioning System (GPS) is vulnerable to jamming because its satellite signal level is 130 dBm when it reaches the Earth. This is only about 30 dB below the noise threshold. Substantial GPS receiver degradation occurs from interfering signals higher than -80 dBm, which is only 0.01 nanowatt, according to a briefing prepared by Mayflower Communications in Billerica, Mass.

Using a 1-watt jammer as the baseline, Mayflower analysts project this vulnerability for civil signals and military receivers. The commercial receiver has 24 dB of anti-jam capability that can withstand an input jammer signal of less than -106 dBm (-130+24=-106), and thus the 1-watt jammer is effective to a distance of about 50 miles. A military receiver has 54 dB of anti-jam and can withstand an input jammer signal of less than -76 dBm (-130+54=-76). The same 1-watt jammer is only effective against a military GPS to about 1.6 miles. Increasing the jammer output power by a factor of 10 or 100 increases the effective range accordingly.

What the GPS needs, observers agree, is another 30 dB of discrimination against jamming signals, which is the equivalent of a thousandfold improvement. Engineers can achieve this by increasing the radiated power from the satellites. This approach, however, is expensive and requires long lead times to meet launch schedules.

Mayflower analysts are stressing another solution at the receivers: filtering. To counter narrowband jammers at a relatively low cost per receiver, Mayflower experts are proposing adaptive temporal filters (ATFs). The basic principle here is that jamming signals are correlated and are therefore predictable over time.

First, the jammer characteristics are measured at time X, and then these characteristics are subtracted at time X+Y. This technique uses digital signal processing and a multi-tap delay line, and is applicable to civil and military receivers.

Spatial filtering is the only way to counter wideband jamming, explains Jim Pellien, vice president for government operations at Mayflower. The signals of Wideband jammers are not time-correlated and cannot be filtered with an ATF. In a spatial filter a four-element antenna array forms deep nulls in the direction of as many as three jammers.

Given the availability and costs of jammer technology, Mayflower experts say the biggest threat from terrorists involves low-power narrowband jammers. Developing countries, meanwhile, can deploy medium- to high-power narrowband jammers and the more technologically sophisticated nations can use a combination of narrowband and wideband jammers at various power levels.

Airlines could respond to the terrorist threat with temporal filters. Mayflower officials are selling an ATF on a chip for $200 per GPS receiver. Military GPS systems could use a combination of temporal filters to defeat the narrowband threat and the addition of spatial filters for selected high-value units to defeat narrowband and wideband jamming. -J.R.

Air Force to test anti-jam munitions

U.S. Air Force officials are developing anti-jam capabilities for their Joint Direct Attack Munition (JDAM) weapons under a program known as the Anti-Jam GPS Technology Flight Test (AGTFT).

Program officials expect this project to include at least six flight tests of a tightly coupled Global Positioning System (GPS) receiver and inertial measurement unit (IMU).

Conducting the program are experts at the Navigation & Control Branch of the Air Force Research Laboratory at Eglin Air Force Base, Fla. The tests will subject the munitions to the kinds of jamming a weapon would experience as it approaches targets. GPS alone cannot provide attitude information at an adequate rate for stable weapon control, so it must link with IMU.

Air Force scientists earlier developed an adaptive GPS filter/antenna that simultaneously accomplished jammer discrimination in the spatial and temporal modes under the Tactical GPS Anti-jam Technology program. - J.R.

Simulation for navwar

Simulation, a design tool that the aviation industry is rapidly adopting, is also becoming applicable to navigation warfare (navwar), says B. J. Singh, vice president for marketing at Simulation designer Emultek Inc. of Herndon, Va. Engineers can use Emultek`s Simulation for evaluating threats and then writing the code to build the hardware, Singh says.

Company officials earlier supplied their Rapid software package to US Airways for computer-based flight training - all done on a Windows-based personal computer - and Singh contends the same techniques can be applied to navwar.

For example, developers can simulate accuracy and fault tolerance for precision-guided munitions based on a functional description from the users. Experts can introduce other factors, such as costs, responses to false signals, ability to deactivate a munition during its trajectory, and various tradeoffs between relatively low cost precision munitions and the more costly cruise missiles. - J.R.

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The Rapid software package from Emultek Inc., depicted above, can help provide low-cost flight simulations on Windows workstations and PCs.

Commercial GPS use expands

The military Global Positioning System (GPS) business perches on top of a huge commercial industry, thus achieving economies of scale. Two of the leaders in this market sector are expanding applications and driving down costs.

Leaders of Trimble Navigation Ltd. in Sunnyvale, Calif., last year began applying their GPS technology to the U.S. Air Force Wind Corrected Munitions Dispenser program. As a subcontractor to Lockheed Martin of Bethesda, Md., Trimble is providing the GPS components to be integrated with inertial navigation systems (INS) in guidance kits to convert old unguided bombs into new "smart" weapons.

The aircraft-dropped aerodynamic munition dispensers that release clusters of submunitions over armored columns and other ground targets. By combining GPS and INS, the guidance kits correct the weapon`s free fall trajectory to compensate for launch transients and wind. These kits also permit release from high altitudes to reduce the risks to pilots and crews.

Leaders of Magellan Systems Corp. of San Dimas, Calif., meanwhile, are expanding into applications with potential military use. Magellan officials introduced a $99.99 hand-held unit, the Pioneer, for outdoorsmen. At the high end Magellan has installed 10,000 units of its $1,995 in-vehicle navigation system, the PathMaster, to provide directions to motorists. Eight thousand of those are on Hertz rental autos.

Developers also can establish levels of security in various threat scenarios. -J.R.

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The NAV 6000 hand-held GPS receiver from Magellan Systems Corp., is part of the dynamically growing civil GPS market that is spinning off from military applications. This unit has a high-resolution graphics display with built-in global background map.

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