Introduction
The Global Positioning System (GPS) has rapidly become an integral part of the U.S. warfighting architecture. However, unless prudent measures are taken to protect this asset, GPS may very well emerge as an area of vulnerability rather than the key component of combat capability it has become. Precise absolute worldwide navigation is essential for ships, aircraft, land vehicles, and even for individual soldiers to carry out maneuvers in warfare missions.
Precise location information enables the warfare commander to effectively control forces in the "fog of war." Weapons employing precision navigation have dramatically increased our military effectiveness, reducing the number of missions required to accomplish objectives and reducing unintended collateral damage. Efforts are underway to expand this precision navigation system to safety-of-life missions, such as aircraft all-weather approach and landing.
Civilian applications have exceeded all expectations – transcending even expanded military usage. Commercial applications have expanded far beyond vehicle navigation to include geodynamics measurements for earthquake and volcano studies; vehicle tracking and fleet management; precision agriculture and precise timing to facilitate/synchronize communications for applications such as cellular telephones and financial transactions. Efforts are well underway to expand this precision navigation capability to safety-of-life missions such as commercial aircraft all-weather approach and landing and ship navigation through channels and harbors in restricted visibility.
GPS, the core of these advances, is a signal in space with a received power similar to the illumination received in San Francisco from a 100-watt bulb in Tokyo! The GPS transmitted signal, by design, is a very low power spread spectrum waveform. Decreases by free space and losses/gains from the antenna to the receiver result in a signal literally "in the noise." Although some interference resistance is inherent in the receiver signal processing, a relatively low power interference source can still disrupt GPS signals.
CWO4 Todd Conley wrote an excellent article on Electromagnetic Interference (EMI) in the CHIPS Fall 2001 issue. Here, GPS interference will be broadly categorized as intentional or unintentional. The threat of intentional GPS interference, or jamming, is increasing as is the availability and capability of commercially available GPS jammers.
The highly sensitive GPS receivers are also vulnerable to unintentional interference from sources such as spectrum encroachment, harmonics from out-of-band emitters, and improperly designed or maintained systems. For example, GPS interference at Moss Landing harbor, near Monterey, California, was traced to a Radio Shack (Tandy Electronics) television antenna with an internal preamplifier. Although this may be an isolated instance, it does illustrate the potential for GPS disruption that would be unacceptable for safety-of-life applications. Whether from intentional or unintentional sources, GPS interference must be mitigated in order to protect GPS navigation accuracy.
There are presently two complementary approaches to GPS interference mitigation. The first is to detect, identify, locate, and avoid or eliminate the GPS interference source. The second is to improve the GPS system resistance to interference. Both options are under aggressive investigation as part of a comprehensive synergistic mitigation capability.
GPS Detection/Location Efforts
GPS Interference Situational Awareness (GISA) represents time-sensitive knowledge of GPS signal quality across the area of interest. This is a concept in its infancy. Although the GPS transmitted signal is closely monitored and information is readily available, currently there is no viable means to provide timely GPS interference information to GPS users. SPAWAR PMW-132 (Space and Naval Warfare Systems Command Navigation Systems Program Office) and SSC SD 231 (Space and Naval Warfare Systems Center San Diego GPS/Navigation Systems Division) successfully investigated a proposed strategy, during Federated Battle Laboratory (FBL) GISA, conducted in June 2000. This experiment involved three technologies in an operational scenario and demonstrated that GPS jamming information could be gathered, formatted, and inserted into the Global Command and Control System (GCCS).
Participating technologies included the LOCation Of GPS Interferers (LOCO GPSI), Guardrail Common Sensor, and GPS Anomalies And Monitoring Equipment System (GAMES). This experiment was conducted in a virtual environment isolated from real-world C4I systems, but used actual field interference data as condition to the simulation. This experiment used existing communications systems, such as the Information Broadcast System (IBS), to disseminate GPS interference information from the theater of operations to a central national GISA authority that would then provide global warnings of GPS jamming as appropriate.
Warfighter involvement in GISA operational issues is essential. Fortunately, increased participation from the operational community facilitates full exploration of the concept and establishment of realistic rules of engagement. SSC SD 2313 (Navigation Systems Research and Development) has participated in a variety of warfighter forums exploring GISA. This includes the North American Air Surveillance Council Spectrum Working Group, a North American Aerospace Defense Command (NORAD) activity, and the U.S. Air Force Air Combat Command Integrated Product Team, which is developing the GPS Operational Requirements Document Electronic Support Annex.
SSC SD 2313 is also working directly with operational warfighters at the unit, major command, and headquarters levels to both educate and obtain new insight on GPS dependence, utility, vulnerability, and protection. Civil interest in GISA is on the rise as well. Discussions have been held with the FAA (Federal Aviation Administration) and Volpe Center (John A. Volpe National Transportation Systems Center) regarding civil GPS interference issues.
Existence of the infrastructure required to disseminate GPS interference information is necessary, but is currently not sufficient to ensure needed force-wide GPSI situational awareness. Before the dissemination architecture can be of value, GPSI information of sufficient quality must be obtained. This is no small task; given that even relatively low power jammers/interference sources can still influence unprotected GPS receivers at operationally significant ranges.
The GPS Joint Program Office (JPO) has investigated several existing systems for their potential to detect and locate GPS interference sources. Some of these systems, such as the Army's Guardrail Common Sensor, have demonstrated credible capability for this mission. However, they were not designed for GPS interference detection and this is not their primary mission. The Volpe Center is also developing prototype ground and airborne GPS interference detection systems.
Most promising of the emerging technologies is LOCO GPSI, developed under a congressionally directed effort by FALON Incorporated, and managed for the government by SSC SD 2313 for SPAWAR PMW/A-156. LOCO GPSI is based on Frequency Extension (FX) technology. FX was designed to passively detect, identify, characterize, and precisely determine direction location of emitters across A through D Radiated Frequency (RF) bands. FX was intended to provide the aircrew real-time electronic environment situational awareness during combat missions. LOCO GPSI modified this proven technology to focus on the GPS L-band frequencies.
LOCO GPSI uses an extremely sensitive receiver, three-element short-baseline antenna array, and sophisticated processing to detect and characterize GPS interference sources beyond their effective range, it also performs precise direction finding on the emitter. As LOCO GPSI moves in time, successive lines of bearing are used to determine emitter location.
The current prototype system, housed in an AN/ALE-41 pod, [used to disperse chaff, which confuses heat seeking weapons, causing the weapons to miss their targets--usually aircraft], successfully completed a ground and flight demonstration program. This was coordinated with support from SSC SD 273 (Information Warfare Technology) and included laboratory, anechoic chamber, outdoor range calibration and dynamic range verification tests. This culminated in flight tests conducted at the Naval Strike Aircraft Warfare Center, Fallon, Nevada, against GPS-specific and other L-band emitters in May 2001.
LOCO GPSI was able to detect, characterize, and locate the jammers while transmitting this information in real-time to a ground display. Even though current volume of the LOCO GPSI system and support equipment, 1.46 cubic feet, is less than 14 percent of ALE-41 pod volume, component miniaturization will be the major focus of future activities. The objective is to make LOCO GPSI compatible with a variety of applications including smaller, more operationally realistic pods, and to provide direct integration on manned and unmanned air vehicles. Future efforts include incorporating design enhancements (determined as a result of the tests) into both the current prototype system and the new miniaturized system.
High power jammers, especially airborne jammers will probably be targeted and quickly destroyed in combat. However, it will likely be impractical or impossible to eliminate all jammers distributed across the battlefield. In civilian safety-of-life GPS applications, users may not have the luxury to cease operations while interference is dealt with. Also, absent a continuously maintained GPS interference sensor system, delays in eliminating civilian GPS interference are inevitable. Thus, for both military and civilian safety-of-life GPS systems, protection against GPS interference is required.
The GPS JPO has a wide range of initiatives designed to enhance the GPS resistance to interference including new signal architecture, selective increased signal power, and user equipment improvements. SSC SD 231 is deeply involved in several of these initiatives, providing technical and project management support to the GPS JPO and PMW/A-156. Development efforts are underway for improvements to the GPS Central Engineering Laboratory that will provide flexible, reprogrammable systems capable of generating and processing existing and future GPS signal architectures to test not only legacy and future user equipment, but also the signals themselves before satellites are launched in space.
GPS Protection Efforts
GPS anti-jam antennas currently represent the most promising technology for GPS receiver protection against interference. These systems are able to detect GPS interference, and modify the antenna gain pattern to either reduce gain toward the interference (nulling) or increase gain away from the nulling (beam steering). SSC SD 231 has made great advances in anti-jam antenna systems test and evaluation on both ships and aircraft.
The GPS Antenna System–1 (GAS-1) is a seven-element nulling antenna system in operational use by the U.S. Air Force on large aircraft. (The number of elements is significant because it defines the number of interference/jamming sources that can be nulled. An N-element antenna can null N-1 interferers/jammers. Ships operating in the littorals, particularly amphibious assault vessels and ships supporting amphibious missions, will also require protection from GPS jamming. The GAS-1 is the first application of a GPS anti-jam antenna for ships.
While aircraft antennas can use the aircraft skin as a groundplane, mast-mounted ship antenna systems require a groundplane for the antenna elements to properly establish and maintain the desired gain patterns. To date, SSC SD 231 has led the first integration, installation, at-sea test and validation of the GAS-1 onboard the Navy's Landing Craft Air Cushion (LCAC), and completed integration, installation and threat/mission representative Developmental Testing on the Avenger-class Mine Countermeasure (MCM) ship.
In order to streamline the acquisition process and save program resources, it is intended that test results from a particular class of ships (such as Avenger-class MCM) be used to justify procurement and installation on similar ship classes, such as the Osprey-class MHC. [The Avenger-class MCM represents the Mine Countermeasures Ships force--hunter-killers, capable of destroying moored and bottom mines. The Osprey-class MHC represents the Coastal Mine Hunters force designed to clear mines from vital waterways.]
While the GAS-1 provides protection against GPS interference and jamming, its size makes it impractical for smaller tactical aircraft. SSC SD 231 obtained authorization and funding under the Foreign Comparative Test (FCT) Program to evaluate a four-element version of the GAS-1, called the GAS-1N, to see if a four-element anti-jam antenna could provide sufficient protection for tactical strike aircraft.
The Integrated Product Team (IPT) led by SSC SD 2315, for the GAS-1N FCT Program, included members/resources from SSC SD's GPS Central Engineering Laboratory; Naval Air Warfare Center (NAWC) Aircraft Division, Patuxent River; NAWC Weapons Division, China Lake; 746th Test Squadron, Holloman AFB; Air Force Research Laboratory and industry. The combination of live test data from laboratory, anechoic chamber, ground, flight tests with extensive modeling and simulation, together with the metrics devised for the evaluation, literally redefined state-of-the-art for GPS anti-jam antenna testing.
The final assessment was actually done using a virtual flight test capability comparing results from strike missions using the GAS-1N, GAS-1, and a legacy unprotected GPS antenna. The GAS-1N FCT was named the Navy's FCT Program of the Year by the Department of Defense for 2000; the GAS-1N FCT team received the SPAWAR Lightning Bolt Award in 2001. This award-winning project proved that four-element antenna systems could provide significant interference/jamming resistance for tactical aircraft scenarios. PMW/A-156 is presently investigating the GAS-1N for the A/V-8B Harrier, and the Army is investigating a variant of the GAS-1N for the Comanche helicopter.
In addition to our involvement with the GAS-1N system, we lead the research and development of small GPS anti-jam antenna systems for space limited platforms, such as submarines and some tactical aircraft. This includes a Mini-Array Program sponsored by the Office of Naval Research (ONR) and a Small Controlled Reception Pattern Antenna (S-CRPA) sponsored by the GPS JPO. Code 231 is the Program Manager in support of the GPS JPO for the development of the Digital Antenna Electronics (DAE), which provides advanced anti-jam capabilities to meet emerging threats using the latest Digital Signal Processing techniques.
Code 231 also initiated the Link 16/GPS Navigation Enhancement program with ONR, GPS JPO, and SPAWAR to investigate ways to take advantage of the robust Link 16 communications capability, and the precision GPS time and position accuracy capabilities in a synergistic fashion. This provides an overall more robust Navy platform navigation capability and improves situational awareness. Analysis, which started in FY00, led to laboratory and field demonstrations with SSC SD 232 and 245 participation in FY01 and continuing in FY02.
Helping to Preserve GPS
The future brings many challenges and opportunities to the GPS community. Increasing capability and availability of GPS jammers and higher demand for scarce RF spectrum bands suggest that GPS interference will be more rather than less likely. New GPS signal architecture and improvements in user equipment including digital antenna electronics and smaller beam-steering anti-jam antennas closely coupled with all-in-view receivers and inertial measurement units are emerging to counter the threat.
Integrating GPS with the Joint Tactical Information Distribution System (JTIDS) and other communication or situational awareness systems may also help mitigate GPS jamming threats. SSC SD is poised to ensure all relevant, cost-effective technologies are successfully developed and fielded to the warfighter, thereby preserving GPS as a battlefield asset -- not the weakest link.
Kenneth S. Simonsen is the Branch Supervisor for the Navigation Systems Research and Development Branch at the Space and Naval Warfare SSC SD. He has also been the Project Manager for the LOCO GPSI Program since its inception. Dean Nathans is the Branch Supervisor for the GPS and Navigation Systems Product Development Team, D215,SSC SD. Mark L. Suycutt is a Program Manager and Principal Systems Engineer with Science Applications International Corporation, Navigation Systems Division. John Wohlfiel is the President of FALON Incorporated. Arch Turner is a former Senior Manager for Precision Strike Systems, Whitney, Bradley, and Brown, Inc., Vienna Va. Bob Crumplar is Manager of Electronic Warfare Systems/Requirements, Whitney, Bradley, and Brown, Inc., Vienna Va.