Topic: Differential GPS and Spring term UROP Aim: Discuss the implementation issues associated with differential GPS and start a discussion of the Spring semester UROP. Discussion Differential GPS Differential GPS involves uses measurements from a receiver at nominally a known location to effectively estimate the satellite clock errors and using those corrections to correct measurements from other GPS receivers. Differential methods are the most effective way to use carrier phase measurements because the unknown number of cycles at the beginning of tracking a satellite reduces to the difference in the number of cycles between the two receivers. One method of processing the data is called 8364;œdouble differencing 8364; where measurements between satellites at one station are differenced (single difference) thus removing effectively the unknown receiver clock and phase offset, and then single differences between from another station are differenced (hence the term double differencing). In the differencing processing, the initial values of the phase of the oscillators in the GPS receivers cancel (provided the measurements are made at the same time) and the unknown starting values for the phase reduces to the integer number of cycles between the two receivers, If the two receivers are close enough, then this number of cycles can be resolved to integer values. This process is called 8364;œambiguity resolution 8364; or 8364;œbias fixing 8364; When the ambiguities can be resolved to integer values, GPS is capable of making measurements in real-time or post processing to better than 10 mm. For real-time applications, a radio link is established between two receivers (could be more since the fixed station only needs to transmit and the receiving/moving site does all the calculations). This is called RTK (real-time kinematic GPS). Differential GPS corrections can be transmitted locally over VHF radio (or even higher frequencies now e.g. FreeWave 900Mhz and 2.4 Ghz radio modems) or from satellites (normally just pseudo range corrections). JPL is testing a system called RTG (Real-time-GYPSIE (name of JPL GPS processing software) that transmitted phase correction information globally in near real-time (about 1-3 second latency).