7.7 GPS FIELD PROCEDURES The use of GPS for positioning to varying degrees of accuracy, in situations ranging from dynamic (navigation) to static (control networks), has resulted in a wide variety of different field procedures using one or other of the basic observables. Generally pseudo-range measurements are used for navigation, whilst the higher precision necessary in engineering surveys requires carrier frequency phase measurements. The basic measuring unit of the S-code (C/A) used in navigation is about 30 m, whilst the L1 carrier is 19 cm, with range measurement to millimetres. The basic point positioning method used in navigation gave the X, Y, Z position to an accuracy better than 30 m by observation to four satellites. However, the introduction of SA degraded this accuracy to 100 m or more and led to the development of the more accurate differential technique. In this technique the vector between two receivers (baseline) is obtained, i.e. the difference in coordinates (%X, %Y, %Z). If one of the receivers is set up over a fixed station whose coordinates are known, then comparison with the observed coordinates enables the differences to be transmitted as corrections to the second receiver (rover). In this way, all the various GPS errors are lumped together in a single correction. The corrections transmitted may be in a simple coordinate format, i.e. X, Y, Z, which are easy to apply. Alternatively, the difference in coordinate position of the fixed station may be used to derive corrections to the ranges to the various satellites used. The rover then applies those corrections to its own observations before computing its position. The fundamental assumption in Differential GPS (DGPS) is that the errors within the area of survey would be S1 S2 (Event: a) (Event: d) (Event: d) (Event: a) A % (Event: b) %B (Event: c) Fig. 7.15 Triple difference 324 Engineering Surveying identical. This assumption is acceptable for most engineering surveying where the areas involved are small compared with the distance to the satellites. Where the area of survey becomes extensive this argument may not hold and a slightly different approach is used called Wide Area Differential GPS. It can now be seen that, using differential GPS, the position of a roving receiver can be found relative to a fixed master or base station without the effect of errors in satellite and receiver clocks, ionospheric and tropospheric refraction and even ephemeris error. This idea has been expanded to the concept of having permanent base stations established throughout a wide area or even a whole country. As GPS is essentially a military product, the US Department of Defense has the facility to reduce the accuracy of the system by interfering with the satellite clocks and the ephemeris of the satellite. This is known as selective availability (SA) of the precise positioning service (PPS). There is also a possibility that the P-code could be altered to a Y-code, to prevent imitation of the PPS by hostile forces, and made unavailable to civilian users. This is known as anti-spoofing (AS). However, the carrier wave is not affected and differential methods should correct for most SA effects. Using the carrier phase observable in the differential mode produces accuracies of 1 ppm of the baseline length. The need to resolve for the integer ambiguity, however, results in post-processing. Fig. 7.16 Cycle slips Residuals Cycle slip Time (a) Single difference Residuals Time (c) Triple difference Cycle slip Residuals Time (b) Double difference Cycle slip Satellite positioning 325 Whilst this, depending on the software, can result in even greater accuracies than 1 ppm (up to 0.01 ppm), it precludes real-time positioning. However, the development of Kinematic GPS and ‘On-the-Fly’ ambiguity resolution has made real-time positioning possible and greatly reduced the observing times. The following methods are based on the use of carrier phase measurement for relative positioning using two receivers. 7.7.1 Static positioning This method is used to give high precision over long baselines such as are used in geodetic control surveys. One receiver is set up over a station of known X, Y, Z coordinates, preferably in the WGS 84 reference system, whilst a second receiver occupies the station whose coordinates are required. Observation times may vary from 45 min to several hours. This long observational time is necessary to allow a change in the relative receiver/satellite geometry in order to calculate the initial integer ambiguity terms. Accuracies in the order of 5 mm ± 1 ppm of the baseline are achievable as the majority of error in GPS, such as clock, orbital, atmospheric error and SA, are eliminated or substantially reduced by the differential process. The use of permanent active GPS networks established by a government agency or private company could result in a further increase in accuracy for static positioning. Apart from establishing high precision control networks, it is used in control densification using a leap-frog technique; measuring plate movement in crustal dynamics and oil rig monitoring. 7.7.2 Rapid static Rapid static surveying is ideal for many engineering surveys and is halfway between static and kinematic procedures. The ‘master’ receiver is set up on a reference point and continuously tracks all visible satellites throughout the duration of the survey. The ‘roving’ receiver visits each of the remaining points to be surveyed, but stays for just a few minutes, typically 2–15 min. Using double difference algorithms, the integer ambiguity terms are quickly resolved and position, relative to the reference point, obtained to sub-centimetre accuracy. Each point is treated independently and as it is not necessary to maintain lock on the satellites, the roving receiver may be switched off whilst travelling between stations. Apart from a saving in power, the necessity to maintain lock, which is very onerous in urban surveys, is removed. This method is accurate and economic where there are a great many points to be surveyed. It is ideally suited for short baselines where systematic errors such as atmospheric, orbital, etc., may be regarded as equal at all points and so differenced out. It can be used on large lines (> 10 km) but may require longer observing periods due to the erratic behaviour of the ionosphere, i.e. up to 30 min of dual frequency observation. These times can be halved if the observations are carried out at night when the ionosphere is more stable.
Posted on: Tue, 30 Jul 2013 09:19:41 +0000
Trending Topics
Recently Viewed Topics
© 2015