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APPENDIX C 166 The difficulty with using carrier phase tracking is the necessity to solve for an unknown quantity termed the integer or cycle ambiguity. Reliable techniques for using carrier phase data in static surveying applications have existed, however, since the mid 1980s. More recently, ambiguity resolution techniques adapted to dynamic applications such as aircraft and ship navigation have also been developed. The success of these new algorithms hinges on the ambiguity resolution technique. One very effective technique, known as wide-laning, relies on carrier phase measurements from both the L1 and L2 frequencies.36 Multi-channel GPS receivers have recently been developed that take advantage of L1 and L2 wide-laning to resolve carrier phase cycle ambiguity by squaring the L2 signal or cross correlating L1 and L2 within a single receiver. The term "codeless" has been associated with these receivers because, as with earlier carrier phase techniques using two receivers, knowledge of the Y-code itself is not required. 37 Pseudolites A "pseudolite" or pseudo-satellite is a land-based GPS transmitter capable of generating a signal similar to that of an actual GPS satellite. This signal can be received by a user's GPS receiver without the need for additional frequency reception capability. Pseudolites can improve accuracy, integrity, availability, and continuity of service by simply increasing the number of satellite signals available to the receiver. Adding a differential correction to the broadcast signal makes pseudolites even more effective. Like GPS satellites, however, a pseudolite is only effective if it is within the line of sight of a GPS receiver. The signal power of a pseudolite must also be carefully adjusted to avoid interfering with actual GPS signals. Receiver Autonomous Integrity Monitoring (RAIM) Receiver Autonomous Integrity Monitoring (RAIM), as the name implies, is a method to enhance the integrity of a GPS receiver without requiring any external 36 Wide-lane ambiguity resolution (wide-laning) is a processing technique developed by civilian DGPS users to process carrier phase data after using codeless techniques to track or "acquire" the carrier phase. With wide-laning, the two carrier frequencies, which are obtained through codeless techniques, are mixed to provide a difference frequency of longer wavelength. Using L2 and L1, the wavelength of the difference frequency is about 4.5 times that of L1, improving the speed and reliability of cycle ambiguity resolution. The wide-laning technique is available to cross-correlation types of receivers today, but at a serious loss in effective carrier-to-noise ratio as compared to a true dual-frequency code tracking receiver, such as a military PPS receiver using the Y-code on both L1 and L2. 37 For more information on the operation of "codeless" receivers, and GPS receivers in general, see: A. J. Van Dierendonck, "Understanding GPS Receiver Terminology. A Tutorial," GPS World, January 1995, pp. 34-44.
