【正文】 GIS, meteorology, and geodynamics. The reason for this GPS wonder perhaps lies in the superior capability of GPS: it offers solutions to many problems that we could not or felt difficult to solve, and also enables us to do many things better than before. Navigation is one of these things, which has been greatly changed from the development of GPS. This paper will provide an overview of GPS as applied to navigation. It will first describe briefly the principles of GPS .The different GPS based positioning methods in navigation will then be discussed, followed by an review of GPS based systems for air, land and marine navigation. PRINCIPLES OF GPS POSITIONING GPS is a satellite based passive positioning system that was initially designed primarily for military use .It was developed and has been maintained by the United States Department of Defense (US DoD). The system is now used by both the military and civilian users to obtain high accuracy position, velocity and time information, 24 hours a day, under all weather conditions, and anywhere in the world. The system was 1993 and full operational capability (FOC) in April 1995. The Components of GPS One mon way to look at GPS is to resolve it into three segments: The space segment refers to GPS satellites that are orbiting at an altitude of about 20,200 km above the earth surface. The full operational capacity of GPS is achieved with 24 active satellites. There are currently 27 operational satellites, three of that are the active spares that can be used as replacements when the active satellites are out of services. The key ponents in satellite are the antennas sending and receiving signals, two large wings covered with solar cells to generate power for the satellite to consume, and atomic clocks that are accurate to about 1 second in 3,000,000 years. The control segment consists of 5 monitor stations, 3 ground antennas, and 1 master control station. The monitor stations passively track all satellites in view, accumulating ranging data. The tracked data are processed at the master control station to determine satellite orbits and to update each satellite’s Navigation Message. The updated information is transmitted to each satellite via the ground antennas. The user segment is anybody who has a GPS receiver. The surveyors, the navigators and the GIS data collectors are examples of the users. The signals that GPS satellites send out consist of two codes, the coarse acquisition (C/A) code and the precise (P) code, and a Navigation Message. The GPS codes are just like a series of 1’s and 0’s that are arranged into certain sequences, Figure 1. The C/A code is used for the standard positioning service (SPS) available to all users. The service offers a positional accuracy of about 100 m horizontally and 156 m vertically at the 95% probability level. The P code is used for the Precise Positioning Service (PPS) and can bi accessed only by authorized users such as the US military and its allies. The service provides a positional accuracy of about 15 m horizontally and 25 m vertically at the 95% probability level. The GPS Navigation Message contains such information as the orbital elements of the satellites, clock behavior, and an almanac that gives the approximate data for each active satellite. Two carrier frequencies on Lband, L1 and L2 are used to carry the signals described above. L1 has a wavelength of about 19 cm () and L2 a wavelength of about 24 cm (). Both L1 and L2 are microwave frequencies and can perate the atmosphere. L1 carries both the C/A and the p codes and L2 the p code only. The Navigation Message is carried on both of the two frequencies. To the more sophisticated users such as the surveyors, positioning using the code information cannot fulfill their accuracy requirements, say at the centimeter or millimeter level. In this case, the L1 or L2, or both L1 and L2 carrier phases are also observed and used for positioning. The Working Principles of GPS GPS measures positions by measuring distances. GPS satellites have known orbits and therefore known positions at any instant time. Therefore, if the distances to three or more GPS satellites can be measured from a point anywhere on near the earth surface, the threedimensional position of the point can bi calculated, Figure 2. The distances between the point and the satellites are determined either using the code or the carrier phase observations. The same GPS codes are generated at the same time by both the satellites and the GPS receiver. When the receiver receives the code information from the satellites, it correlates the signals it generates and those received from the satellites, Figure 3, which can determine the time that takes for the GPS signal to travel from a satellite to the receiver. The time can then be used to calculate the distance. Figure3. Time measurement by code correlation Figure4. Carrier waves and phase measurement As the clock of the receiver has usually a much lower accuracy than those on a GPS satellite, the clock time is in practice monly considered as an unknown parameter which is solved together with the position of the receiver point. In this case, signals from at least four GPS satellites are required, as there are four unknown values to be solves for. When carrier phase observations are used in GPS positioning, the distance between a receiver point and a satellite is determined using. Distance = ?? ??N (1) Where ? is the wavelength of the carrier wave。 N is the whole wave numbers counted from a satellite to the receiver and ?? is the length that is shorter than one wavelength, Figure4. ?? is determined directly from the phase measurements. N is the integer ambiguity and is usually solved for based