Page 66 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 3: Search and Survey 45 1. The History of GPS
The history of the GPS is interesting. The first GPS satellite was launched in 1978. The first ten satellites were developmental satellites called Block I. From 1989 to 1994, 24 production satellites, called Block II, were launched. Each GPS satellite transmits data that indicate its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. Basically, the instrument does a trilateration. A simple three-dimensional trilateration is done by knowing the precise positions of the satellites and the distance from the satellites to the GPS. When the first satellites were launched nobody realized the significance of GPS. As a result, it quickly became obvious that there were defense issues involved. This meant that the system, in the wrong hands, could be used to guide all sorts of weapon systems in real time. Theoretically, one could construct a missile guided by a GPS that could be targeted within a few meters of any site. Because of this concern, timing errors were inserted into GPS trans- missions to limit the accuracy of the Block II so that nonmilitary GPS receivers were limited to an accuracy of about 100 m. This part of GPS oper- ations was called selective availability (SA). During the Gulf War, because of the lack of military GPS receivers, the military bought a large number of commercial instruments and turned the SA off. So for the period of that conflict, ordinary operators had the possibility of an accurate GPS.
2. Differential GPS
Probably because of the various ways around SA using differential GPS (DGPS) and the growing commercial market for GPS in a huge range of applications, on May 1, 2000, the U.S. Department of Defense turned off SA for the worldwide GPS system. This effectively increased the accuracy of the GPS system to less than ±5 m. GPS accuracy is a complex issue (see the next section). Previously, with SA on, the GPS system was given an intentional “dither” or noise resulting in a minimum position accuracy of around ±50 to 100 m. This standard deviation of position would be a radius of 50 m. The artificial dither applied to the signal could be overcome by using the GPS in a differential mode or DGPS, where a base station records a fixed position and a mobile unit is used to do the recording (see Figure 3.14b). By comparing the signals, either in real time or in post survey, it was possible to cancel the effect of dither (as the dither at any moment would