In the last decade or so new GPS surveying methods have been developed with two liberating characteristics of (a) static antenna set up no longer having to be insisted upon (b) long observation sessions no longer essential in order to achieve survey level accuracy.
If the results could be obtained immediately after the measurements have been made, then GPS could be used for time critical missions such as engineering statement etc. If the length of the time required to collect phase data for a reliable solution could be shortened, then GPS survey productivity would improve and technology would be attractive for many more surveying applications. That is, if antennas could be moving during a GPS survey, then new application of GPS survey could be addressed. During the late 1980s, considerable attention was paid to these issues, as they were considered to be unnecessarily restrictive for precise GPS technology. The main weaknesses of such procedures are that the observation time is comparatively long, the results are obtained after the field survey and the field procedures are rigid. There are essentially two types of conventional static GPS surveying techniques. The DGPS & RTK techniques, because they are able to deliver results in real time, are very powerful GPS positioning technologies. Real – Time Kinematic (RTK): versatile high precision techniques that use carrier phase measurements in an instantaneous positioning mode. Differential GPS (DGPS): instantaneous low to moderate accuracy positioning and mapping technique based on pseudo range measurements. Static and Kinematic GPS surveying techniques: High precision techniques based on post processing of carrier phase measurements 2. The following classes of relative positioning techniques can therefore be identified.
Although Single Point Positioning (SPP) accuracy of 5-10m is now possible, it is assumed that for most geo spatial applications only relative positioning are of relevance. When these classes of hardware are used in the appropriate manner for relative positioning, the accuracy that is achieved ranges from a few metres in case of standard pseudo range based techniques, to the sub centimetre level in case of carrier phase based techniques.
Nevertheless, the most fundamental classification system for GPS technique is based on the type of observable that is tracked (a) Civilian Navigation / positioning receivers using C/A code and L1 frequency (b) Military navigation / positioning receivers using the satellite P(Y) – code on both L – band frequencies (c) Single frequency (L2) carrier phase tracking receivers (d) dual frequency carrier phase tracking receivers. There are wide variety of GPS applications, which is matched by a similar diversity of user equipment and techniques.