A Radio Vehicle Position Reporting Technique that is Protected from Interception and Directional Fixes

Vehicles used to transport nuclear materials can determine their position with respect to an established grid by electronic means such as geosynchronous navigation satellite (GPS) or hyperbolic radio. It is frequently necessary to relay this position information via a radio link to a command center. This gives a potential adversary two means of determining the vehicle's location: first, by simply intercepting and reading out the transmitted position data, second, by making a radio directional fix on the mobile transmitter. If a modern data encryption technique is used, the usefulness of interception almost disappears. The DF threat can be countered by using a system that transmits at low power for very short intervals over a wide bandwidth. This is because the probability of a successful DF fix is related to the ability to lock a phase-locked loop in noise, and each of the above factors works against such locking. A robust system can be designed at reasonable cost to accomplish successful position data reporting with a very low probability of successful DF by an adversary because the DF problem is much more difficult technically then the data transmission problem, and a position reporting system does not need to spend much time actually transmitting. A hypothetical system is presented that is called burst mode frequency hopping (BMFH). The mobile transmitter sends very short modulated bursts that are hopped through a pseudo-random frequency map. Actual data are encrypted and then embedded in an interleaved error correcting code. A very low transmitter power is used to achieve from 10 to 20 dB signal to noise ratio at the base station during shallow fades. A DF adversary is forced to listen over a much wider bandwidth than the base station which causes him to have a lower signal to noise ratio. If he partitions his bandwidth, his cost increases exponentially. The combination of low power, wide bandwidth and short transmission times make DF very difficult. The additional presence of a good encryption scheme makes interception useless as far as position determination is concerned.