A recent article in IEEE Spectrum revealed and detailed a very alarming threat to airline safety posed by GPS tests conducted by the U.S. military. According to the article, military tests that jam GPS signals to test military defenses against such jamming are an accident waiting to happen. The tests, for example, have forced pilots to land without vertical guidance and would have caused an aircraft to crash into a mountainside had it not been for an alert air traffic controller who noticed its path and instructed the pilot in time to change heading. Based on databases of aviation safety reports kept by NASA and the Federal Aviation Administration, the article’s author concludes that “[i]n all likelihood, there are many hundreds, possibly thousands, of such incidents each year nationwide, each one a potential accident.” Even worse than losing GPS signals completely, he notes, is “when accurate GPS data is quietly replaced by misleading information,” (spoofing) as was the case in many of the reported incidents, in which the pilots had no indication that they had inaccurate GPS data and were off course.
The U.S. military has already tried to understand and mitigate threats to GPS using simulators. For example, they use CAST simulators to stimulate the CRPA antenna’s electronic sub-systems with GPS and jamming to simulate a hostile environment to understand how the antennas and the GPS receivers are influenced and perform in a “less than optimum” type of environment. CAST simulators can also simulate spoofing, which is the intentional copying and re-transmission of satellites, making them produce signals that can deceive receivers as to their true location. Those simulations are conducted via machine-to-machine cable connections, without broadcasting a signal that could be harmful in real life. However, the U.S. military is now finding it necessary to conduct live tests, which do include those broadcasts.
A simulator can enable equipment manufacturers, airlines, and pilots to plan mitigation efforts by simulating the kinds of situations encountered by GPS receivers during these military tests. A simulator can model what would happen if a GPS receiver were knocked out or tricked into providing erroneous information, for example by reducing or jamming the signals from some or all the satellites or by transmitting erroneous navigation data. Simulators can also model multiple simultaneous adverse events, enabling users to obtain navigation information to understand how these types of events can impact performance. How would they affect the aircraft? What indications, if any, would the pilots have of a malfunction? What could they do to prevent an accident?
Following are a few additional examples of problems that can occur with military receivers that can be modeled and analyzed using a CAST simulator:
- If a receiver is returned to a platform for service halfway around the world after being sent back to the manufacturer for repair, when it is returned to service it may still think it is on the other side of the planet. It requires much more than the normal time to acquire the satellites because it first must re-acquire a current almanac, making it appear as though it is malfunctioning even though it is not. A simulator can be used to reveal that condition.
- Years ago, some F-22s Raptors that were flying from the United States to Japan experienced a malfunction of their GPS receivers when they crossed the international date line due to the change in the local date and time. This was a scenario that had never occurred before. The aircraft were able to land safely in Japan, but the incident was brought to the attention of the manufacturers of the navigation equipment. By using a CAST Navigation simulator, they were able to recreate the problem, track it down to a coding error, fix it, and verify that it had been fully resolved.
- When anti-spoof software was first installed on military GPS receivers it sometimes caused strange malfunctions, such as dropping good satellite signals because it erroneously deemed them to be spoofed. A simulator made it possible to recreate and solve the problem.
- The encrypted Chimera signal being tested on GPS satellites to help prevent spoofing will challenge GNSS receivers. A simulator can be used to recreate that signal and verify that a receiver is handling it properly.
