GPS Spoofing Countermeasures

Civilian Global Positioning System (GPS) receivers are vulnerable to a number of different attacks such as blocking, jamming, and spoofing. The goal of such attacks is either to prevent a position lock (blocking and jamming), or to feed the receiver false information so that it computes an erroneous time or location (spoofing). GPS receivers are generally aware of when blocking or jamming is occurring because they have a loss of signal. Spoofing, however, is a surreptitious attack. Currently, no countermeasures are in use for detecting spoofing attacks. We believe, however, that it is possible to implement simple, low-cost countermeasures that can be retrofitted onto existing GPS receivers. This would, at the very least, greatly complicate spoofing attacks.  The civilian Global Positioning System (GPS) is widely used by both government and private industry for many important applications. Some of these applications include public safety services such as police, fire, rescue and ambulance. The cargo industry, buses, taxis, railcars, delivery vehicles, agricultural harvesters, private automobiles, spacecraft, marine and airborne traffic also use GPS systems for navigation. In fact, the Federal Aviation Administration (FAA) is in the process of drafting an instruction requiring that all radio navigation systems aboard aircraft use GPS . Additional uses include hiking and surveying, as well as being used in robotics, cell phones, animal tracking and even GPS wristwatches. Utility companies and telecommunication companies use GPS timing signals to regulate the base frequency of their distribution grids. GPS timing signals are also used by the financial industry, the broadcast industry, mobile telecommunication providers, the international financial industry, banking (for money transfers and time locks), and other distributed computer network applications [2,3]. In short, anyone who wants to know their exact location, velocity, or time might find GPS useful. Unfortunately, the civilian GPS signals are not secure . Only the military GPS signals are encrypted (authenticated), but these are generally unavailable to civilians, foreign governments, and most of the U.S. government, including most of the Department of Defense (DoD). Plans are underway to upgrade the existing GPS system, but they apparently do not include adding encryption or authentication to the civilian GPS signal . The GPS signal strength measured at the surface of the Earth is about –160dBw (1x10-16 Watts), which is roughly equivalent to viewing a 25-Watt light bulb from a distance of 10,000 miles. This weak signal can be easily blocked by destroying or shielding the GPS receiver’s antenna. The GPS signal can also be effectively jammed by a signal of a similar frequency, but greater strength. Blocking and jamming, however, are not the greatest security risk, because the GPS receiver will be fully aware it is not receiving the GPS signals needed to determine position and time. A more pernicious attack involves feeding the GPS receiver fake GPS signals so that it believes it is located somewhere in space and time that it is not. This “spoofing” attack is more elegant than jamming because it is surreptitious. The Vulnerability Assessment Team (VAT) at Los Alamos National Laboratory (LANL) has recently demonstrated the ease with which civilian GPS spoofing attacks can be implemented . This spoofing is most easily accomplished by using a GPS satellite simulator. Such GPS satellite simulators are uncontrolled, and widely available. To conduct the spoofing attack, an adversary broadcasts a fake GPS signal with a higher signal strength than the true GPS signal. The GPS receiver believes that the fake signal is actually the true GPS signal from space, and ignores the true GPS signal. The receiver then proceeds to calculate erroneous position or time information based on this false signal.  In this manner, each satellite transmits a unique identification number that allows the GPS receiver to know which satellites it is receiving signals from. The Nav/System data provides the GPS receiver with information about the position of all the satellites in the constellation as well as precise timing data from the atomic clocks aboard the satellites. L1 Carrier 1575.2 MHz