|
|
|
GPS - Reality bites
N.K.Agrawal
Directore (Retd.) STI, SOI, Advisor & Consultant
Geodesy & GPS Services, 202, KNR Apartments
Street No. 8, Habsiguda, Hyderabad-500007
nande@rediffmail.com
GPS – Reality Bites
Global Positioning System probably is the most effective general purpose tool created to date for determining “Time, Place, Motion”. These three are first degree relationships for everyone in this world. It is however important for the users to know what are its real capabilities and limitations that bite. Some of these are detailed in the following
paras.
Datum
GPS provides coordinates in WGS84 System whereas all mapping in India is on Indian Geodetic System (Everest Spheroid). The coordinates in two systems may differ by a few metres to tens of metres at different places. One should therefore know transformation parameters (3 translation, 3 rotation and 1 scale) in order to use the same in India. Reasonably accurate transformation parameters have not been calculated. Some agencies are however engaged in finding the same for various regions.
Remedy lies in using GPS as a surveying tool to get vectors (distances and directions) and then find out coordinates of various points in the desired system. There is however a debate going on regarding unrestricted availability of maps and data for civil use. Recently it has been agreed by the Govt. of India and its Ministry of Defence that a new series of maps on WGS84 datum will be prepared and made available to everyone for unrestricted civilian use. This initiative may help users to obtain data pertaining to any part of the country without any restriction. To understand the implications of this decision let us know the difference between the two systems.
WGS84 and Everest Spheroid
Certain assumptions are made to define the datums which are identical for all, but the realization differs. Centre of the ellipsoid of rotation is assumed to be at C.G. of the earth. On realization it has however been found that center of Indian datum (Everest Spheroid) is estimated to be nearly one km away from c.g. of the earth whereas WGS84 is estimated to be only + 2 metres away. Similar is the case for the three axes. WGS84 can therefore be termed as Geocentric world system which is accurate and close to the Nature (reality), where as Indian system is approximate, not geocentric (a local datum) and away from reality.
U.S.A. is a super power and it needed WGS84 and GPS to exert its supreme authority over the world. In all modern systems like missiles and other weapons are based an accurate positions provided by GPS throughout the world at any moment of time all through the year. Why then are we deciding to open the new series of maps and data for unrestricted civil use? The reasons for restriction of data in Indian system are not publicly known. Many term it as arbitrary. Some restriction is definitely necessary in National interest as even USA has restricted, availability of P code for civil use. The point here is, as to what extent it is necessary? It is not understood if good reasons exist for restricting data in Indian system then why the same reasons do not exist for WGS84? To my mind derestriction of WGS84 may not be in our interest as it is an accurate system, which will be useful if launching pad and target of intercontinental missile are in WGS84, whereas if the coordinates are in local system. e.g Everest Spheroid the target may not be reached.
We should therefore consider derestricting maps/data in Indian system for civil use for development whereas data in WGS84 may have some restrictions, and may be used for defence and scientific purposes only. Contents of maps in Indian system may be revised by the authorities depending upon various considerations.
Intentional Limitations of GPS Accuracy
With selective availability, SA put off by a US presidential order of May 2, 2000 only Anti spooting is operative. Under this only S code (C/A code) is available for civilian use wherever P code is reserved for US, their friends and allies. The codes we receive are S code and Y code instead of S code and P code. P code is encrypted by mixing it with another frequency so that we receive Y code only is available which can not be deciphered.
With antispooting operative, coordinates in point positioning mode (single receiver) can be obtained to an accuracy to 10 to 20 metres in position which may not be sufficient for most mapping purposes. The remedy lies in relative positioning and or Differential GPS(DGPS) in which two or more receivers are required.
Further the US reserves the right to degrade the quality of signals without notice. We can therefore say that integrity of signals is doubtful. In times of war this is particularly done. One should therefore verify the integrity of signals before the same is used for surveying and mapping.
Area jamming is another counter measure US is likely to take in times of war in their interest so that the enemy does not get the signals at all. One has therefore to see whether he can be over dependent on GPS system?
Receiver Independent Exchange Format (RINEX)
Due to different formats of receivers of various makes, data of two receivers of different make can not be processed simultaneously to get vectors. While purchasing receivers, we should ask the vendors to give translator software which can correct data into RINEX format so that data can be processed. RINEX was developed for this purpose and published in a symposium in 1989.
|
|
|