Development of Techniques to
Identify and Locate Global Positioning
System Interfering Signals, 16-9215
Principal Investigator
Thomas H. Jaeckle
Inclusive Dates: 08/29/00 - Current
Background - Global positioning system (GPS) receivers are commonplace today and are used in applications ranging from recreational use to computer network timing. The military alone has purchased more than 200,000 GPS devices for use on aircraft, vehicles, and dismounted troops and in the guidance of weapons. To function, the GPS receivers receive and use a very weak signal transmitted from satellites 20,200 kilometers in space. These weak signals can be overwhelmed by interfering signals from Earth, generated either deliberately or inadvertently. In either case, the user is left without the services of the GPS equipment and without any indication of why it is not functioning.
Approach - The objectives of this internal research project are to understand and characterize how different types of interfering signals affect GPS receivers and to develop a means to detect interference signals. To determine the presence of an interfering signal, a receiver was selected for which the operational microcode was available. With this microcode, the research team can modify the software, which implements the receiver, so that the internal components of the state vector of the receiver can be observed. Normally, these internal state variable components are not available to the user. These components contain extensive information about the status of each receiver tracking channel. Some of this information is available only when the receiver is navigating. An important goal of this research is to determine which receiver parameters will provide information even if the receiver has been jammed and, therefore, is not navigating. Research efforts under this program will examine the internal processing of GPS receivers in an attempt to correlate the abnormal conditions within the receiver to externally applied jamming and interference signals. Ideally, this work will result in the discovery of a unique signature for each type of interference. Detection of the signature will allow a properly modified receiver to alert the user to the presence of interference.
Accomplishments - A GPS receiver test bed facility has been set up in which interfering signals may be applied to the GPS receiver under test without contaminating the radio-frequency environment for other local GPS receivers which are not part of this test. The research team obtained a GPS receiver and its applicable microcode. The code was modified to write the state vector observable to a file for subsequent analysis. During this analysis of the receiver hardware, a point was found in the radio-frequency portion of the receiver that provides a voltage output proportional to the total power, incident on the receiver's input. Because the GPS satellite signals are below the thermal noise contained in the bandwidth over which these signals are spread, any energy above the thermal noise represents power that cannot be accounted for by legitimate signals and is therefore a potential source of interference. Fortuitously, this receiver observable permits detection of potentially interfering signals at levels below those required to cause the receiver to cease navigating or to prevent it from initial navigation. This observable, combined with the individual receiver channel observables that deal with correlation lock and carrier phase lock, provide a powerful insight into the operating environment of the receiver.
