Introduction of Satellite positioning
Before commencing this chapter, the reader should have studied Chapter 8 and acquired a knowledge of geoid models, ellipsoids, transformations and heights, i.e. Sections The subject of satellite positioning is changing fast. Throughout this chapter a number of websites are referred to for further information and data. The websites are mostly government or academic and are considered to be likely to be maintained during the life of this edition of this book, although of course that cannot be guaranteed.
The concept of satellite position fixing commenced with the launch of the first Sputnik satellite by the USSR in October 1957. This was rapidly followed by the development of the Navy Navigation Satellite System (NNSS) by the US Navy. This system, commonly referred to as the Transit system, was created to provide a worldwide navigation capability for the US Polaris submarine fleet. The Transit system was made available for civilian use in 1967 but ceased operation in 1996. However, as the determination of position required very long observation periods and relative positions determined over short distances were of low accuracy, its application was limited to geodetic and low dynamic navigation uses.
In 1973, the US Department of Defense (DoD) commenced the development of NAVSTAR (Navigation System with Time and Ranging) Global Positioning System (GPS), and the first satellites were launched in 1978.
The system is funded and controlled by the DoD but is partially available for civilian and foreign users. The accuracies that may be obtained from the system depend on the degree of access available to the user, the sophistication of his/her receiver hardware and data processing software, and degree of mobility during signal reception.
In very broad terms, the geodetic user in a static location may obtain ‘absolute’ accuracy (with respect to the mass centre of the Earth within the satellite datum) to better than ±1 metre and position relative to another known point, to a few centimetres over a range of tens of kilometres, with data post-processing. At the other end of the scale, a technically unsophisticated, low dynamic (ship or land vehicle) user, with limited access to the system, might achieve real time ‘absolute’ accuracy of 10–20 metres.
The GPS navigation system relies on satellites that continuously broadcast their own position in space and in this the satellites may be thought of as no more than control stations in space. Theoretically, a user who has a clock, perfectly synchronized to the GPS time system, is able to observe the time delay of a GPS signal from its own time of transmission at the satellite, to its time of detection at the user’s equipment. The time delay, multiplied by the mean speed of light, along the path of the transmission from the satellite to the user equipment, will give the range from the satellite at its known position, to the user. If three such ranges are observed simultaneously, there is sufficient information to compute the user’s position in three-dimensional space, rather in the manner of a three-dimensional trilateration. The false assumption in all this is that the user’s receiver clock is perfectly synchronized with the satellite clocks.
Now that GPS is fully operational, relative positioning to several millimetres, with short observation periods of a few minutes, have been achieved. For distances in excess of 5 km GPS is generally more accurate than EDM traversing. Therefore GPS has a wide application in engineering surveying. The introduction of GPS has had an even greater impact on practice in engineering surveying than that of EDM. Apart from the high accuracies attainable, GPS offers the following significant advantages:
(1) The results from the measurement of a single line, usually referred to as a baseline, will yield not only the distance between the stations at the end of the line but their component parts in the X/Y/Z or Eastings/Northings/Height or latitude/longitude/height directions.
(2) No line of sight is required. Unlike all other conventional surveying systems a line of sight between the stations in the survey is not required. Each station, however, must have a clear view of the sky so that it can ‘see’ the relevant satellites. The advantage here, apart from losing the requirement for intervisibility, is that control no longer needs to be placed on high ground and can be in the same location as the engineering works concerned.
(3) Most satellite surveying equipment is suitably weatherproof and so observations, with current systems, may be taken in any weather, by day or by night. A thick fog will not hamper survey operations.
(4) Satellite surveying can be a one-person operation with significant savings in time and labour.
(5) Operators do not need high levels of skill.
(6) Position may be fixed on land, at sea or in the air.
(7) Base lines of hundreds of kilometres may be observed thereby removing the need for extensive geodetic networks of conventional observations.
(8) Continuous measurement may be carried out resulting in greatly improved deformation monitoring. However, GPS is not the answer to every survey problem.