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  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 govemment 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 Raging ) 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 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 meter 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 meters 

The GPS navigation system relies on satellites that continuously broadcast their own positive 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 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 

The practice although the satellite clocks are almost perfectly synchronized to the GPS time system the user clock will have an error or offset so the user is not directly able to measure the range to a particular satellite but only the pseudo range i - e the actual range with an unknown but instantaneously fixed offset a four parameter solution therefore requires simultaneous observations to four satellites at least four satellites must be visible at all times to any observer wherever he / she may be on or above the surface of the Earth not only must at least four satellites be visible but also they or the best four if there are more must be in a good geometric arrangement with respect to the user 

Now the GPS is fully operational relative positioning to several millimetres with short observation accurate than EDM traversing therefore GPS has a wide application in engineering surveying than that of introduction of GPS has had an even greater impact on practice in engineering surveying than that of EDM a part 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 system 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 saving 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 the following difficulties may arise 

(1) A good electronic view of the sky is required so that the satellites may be seen and tracked there should not be obstructions that block the line of sight from the receiver to the satellite this is usually not a problem for the land surveyor but may become one for the engineering surveyor as a construction rises from the ground satellite surveying cannot take place indoors nor can it take place underground 

(2) The equipment concerned is expensive a pair of GPS receivers costs about the same as three or four total stations though this will very from manufacturer to manufacturer like total stations however prices are falling while capabilities are increasing 

(3) Because satellites orbit the whole Earth the coordinate system that describe the positions of satellites are global rather than local thus if coordinates are required in a projection then the relationship between the local projection and datum and the coordinate system of the satellite must also be known 

(4) The value of height determined by satellite is not that which the engineering surveyor would immediately recognize since the coordinate system of GPS is Earth mass centred then any height of a point on the Earth’s surface will be relative to some arbitrarily defined datum such as the surface of an ellipsoid if height above the geoid or mean sea level is required then the separation between the geoid and the chosen ellipsoid will also be required some GPS receivers may have a model in their software to solve this problem however the model be coarse