Despite its marvelous success over the last three decades, the Global Positioning System (GPS)has two Achilles heels: its signals are weak and|in the case of civil GPS|unauthenticated. At UTAustin's new Radionavigation Laboratory, directed by Assistant Professor Todd Humphreys, we areworking to address both these vulnerabilities.
Weak GPS Signals: Just how weak are the signals transmitted by GPS satellites? Consider thiscomparison: a hand-held GPS device recovers less power from a GPS satellite than it would from a30-Watt light bulb held 4000 km away. For typical GPS users this means that, unlike cell-phone andtelevision signals, GPS signals do not penetrate well indoors. Weak signals also make GPS an easytarget for jamming: A single well-placed handheld 1-Watt jammer built with the-shelf parts couldknock out GPS across the entire Manhattan borough of New York City. The deepening dependence ofthe civil infrastructure on GPS - especially for timing synchronization - and the potential for financialgain or high-profile mischief makes civil GPS jamming a gathering threat. Finally, weak signals makeGPS vulnerable to natural interference events like solar radio bursts and ionosphere-induced powerfluctuations called scintillation.
At the Radionavigation Laboratory, we are developing techniques to harden GPS against man-madeand natural interference and to push GPS use deeper inside buildings. This work involves innovative signal tracking techniques, re-purposing signals of opportunity for navigation (e.g., from Iridiumsatellites, cell phone towers, etc.), and collaborative navigation. Lab members are also developing aninterferometric network-based system to detect and locate sources of radio interference. The system,called INTERLOC for INTERference LOCator, addresses the growing concern about GPS interferencewithin the Department of Homeland Security.
Unauthenticated GPS Signals: GPS signals come in two flavors, military and civil. MilitaryGPS signals are strongly encrypted to prevent counterfeiting, but civil GPS signals support no suchauthentication; they are broadcast in the clear," which means they are vulnerable to spoofing.To better assess and defend against the spoofing threat, Dr. Humphreys and collaborators at Cornell University and Coherent Navigation, Inc. recently built a fully functional portable civil GPSspoofer. They demonstrated that a malefactor in possession of such a device could dupe any the-shelf civil GPS receiver into reporting erroneous position and time data, and they warned thatsuch spoofing could threaten cell telephone systems, power grids, and financial transactions, which alldepend on GPS-based synchronization, as well as location-based regulatory practices such as fishingregulation and road-user charging schemes. To address this threat, investigators at the Radionavigation Laboratory are busy developing effective and practical defenses against civil GPS spoofing.
Todd E. Humphreys is an assistant professor in the department of Aerospace Engineering and Engineering Mechanics at the University of Texas at Austin. He received a B.S. and M.S. in Electrical andComputer Engineering from Utah State University and a Ph.D. in Aerospace Engineering from Cornell University. His research interests are in estimation and filtering, GNSS technology, GNSS-basedstudy of the ionosphere and neutral atmosphere, and GNSS security and integrity.