Radar'd Out

April 1, 2006 By: Stanley F. Williams

GPS Vulnerable to High-Power Microwaves

Commercial GPS units are quickly proliferating on both civilian and military vessels plying the world's waterways and can be found in an increasingly wide variety of places including remote navigational systems, automatic piloting systems, automatic control and marshalling systems, public transportation, and numerous National Defense systems to name a few. As a result, our dependency on commercial GPS technology is also proliferating, increasing the possibility of Electromagnetic Interference (EMI) or damage to these units.

The potential effects of EMI on commercial GPS units that are integrated into critical or automated systems could result in catastrophic consequences. Although it is a well-known fact that GPS is susceptible to in-band frequency emissions or jamming, commercial GPS susceptibility to out-of-band emissions, which can result in temporary failure or permanent damage, remains unclear.

Damage Reports. In May 2000, Naval Sea Systems Command (NAVSEA) launched an investigation into GPS susceptibility after receiving United States Navy (USN) message traffic indicating a United States Naval Ship (USNS) had experienced commercial GPS damage during a routine boarding operation training exercise. During this type of exercise ships come within close proximity to each other (typically 1500 to 3000 feet) for the purpose of an official boarding and inspection. In this particular exercise one of two ships had active high power microwave radars operating while the other ship had multiple commercial GPS antennas installed on board that employed active electronics within the antenna radome. Although both ships followed standard Navy operating procedures the exercise still resulted in permanent commercial GPS antenna failure. Further investigation revealed that this was not an isolated event and that permanent GPS antenna failure is not an uncommon occurrence in the USN.

In 2001, the United States Coast Guard (USCG) Maritime Liaison Office (MARLO) in Bahrain reported that commercial maritime operators were experiencing GPS failure after coming in close proximity to international naval security vessels operating in the area.

In 2002, a shipment of newly procured radio sets, using a commercial GPS unit to identify exact location, could not acquire GPS satellite signals after delivery to a USN aircraft carrier at sea. The helicopter delivering the units flew through the carrier's radar emissions. An analysis by the manufacturer attributed the failure to the degradation of the field effect transistor (FET) characteristics. The manufacturer believes this type of failure can only be attributed to a high electric field, electrostatic discharge, or lightning strike. A high electric field could be explained by the high power microwave energy from the carrier's radars which could have been received through the radio antenna creating excessive voltage at the GPS amplifier FET gates, breaking down the semiconductor substrate.

Other examples of commercial GPS antenna failure were documented within the USN including commercial GPS antennas integrated into both the naval satellite communication systems and ballistic range detection buoy system used for determining naval gunfire accuracy. In both examples, commercial GPS antennas were being exposed to high power radar emissions resulting in permanent antenna failure. Because satellite communication equipment requires a GPS position fix to assist in locating the exact location of the communication satellite, a GPS antenna failure results in virtually total loss of performance.

The USN owns and operates its own militarized version of GPS, which was introduced into the fleet in the mid 1980's. The official nomenclature for the USN militarized GPS is the AN/WRN-6 which utilizes a fixed reception pattern array (FRPA) antenna called AS-3819. In 2005, the USN introduced the replacement antenna for the AS-3819 called the GPS Antenna System (GAS-1). The GAS-1 is the USN attempt at minimizing the potential for GPS jamming. During the development of both these antennas the harsh electromagnetic environment of USN surface ships was a primary concern. As a result, a military standard that addresses this environment was invoked. MIL-STD 464 "Electromagnetic Effects Requirement for Systems" addresses the electromagnetic environment in which a GPS antenna must survive and MIL-STD 461 'Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment' addresses the levels that a system must be tested to.