Choice of Receivers
GPS receivers are being manufactured by hundreds of different manufacturers. Some of the well known names are:- Trimble, Leica, Ashtech, Topcon, Sokkia, Magellan. The manufactures are coming out with different models. Receivers of the following types may be available :-

• S code (C/A code) receivers.
  
• S code + L1 Carrier phase.
  
• S code + L1 + L2
  
• S code + P code + L1 + L2
  
• L1 (not often used)
  
• L1 + L2 (not often used)
  

Another classification is given below :

• Multi channel receiver
  
• 6 channels
  
• 8 channels
  
• 9 channels
  
• 12 channel
  
• Sequential receiver: Not often used
  
• Multiplexing receiver :
  

Another classification is as per user community.

• Civilian Receiver
  
• Military Receiver
  
• Navigation Receiver
  
• Timing Receiver
  
• Geodetic Receiver
  

There is a further classification of receiver which is hand held receiver, pocket receiver and so on.

Choice of a particular receiver is a difficult one. One should first define his aim for which he wants to acquire a GPS receiver or a pair of GPS receivers. He should then see the funds available at his disposal. Receivers cost varies from Rs.15000/- to Rs.10 lacs for a single receiver. It is therefore essential to define the aim and then after due to deliberations decide on a particular type of receiver to be acquired. There are organizations who have simply given the specification for control survey that it should be provided by GPS without mentioning the accuracy desired for a particular survey. This is absurd and should be guarded against.

Achievable Accuracy
There is always a gap between accuracy achievable (claim) and accuracy actually achieved under various circumstances.

The following accuracies are possible.

Point Positioning	S-Code 10 ± 20 meters
	P-Code 3 ± 5 meters
Relative positioning	Distance 0.5 ± 1 part per million
Differential GPS	few centimeters to few meters.

The various manufacturers claim accuracies for their receivers which actually may not be possible in reality. One should check the claims on known stations/ known bases.

Maintenance
Service and maintenance of receivers is a problem as most of the receivers are manufactured by foreign companies, whose service/ maintenance centers are not in India. You have to wait for months for some times, before your receivers are repaired. The cost of the repair also may be very high. In case you are buying a number of receivers it is better to negotiate training of your own personnel in maintenance. There are organizations which have acquired GPS receivers which have not been used for months/years. One should therefore take care to see that proper maintenance support is available in India itself.

Training
Proper training in GPS is being imparted in a very few institutions. The best institution in India is Survey Training Institute which runs a number of courses in GPS and has a number of instruments. Expertise is also available. Normally the organizations buy the receivers and then try to get training whereas it should be other way round.

Conclusion
Some of the realities about GPS have been mentioned above. Knowledge of the above may benefit the prospective buyers and the whole GPS community. GPS is an outstanding surveying tool for all types of applications. One should acquire appropriate knowledge, evaluate his requirement and then acquire GPS receivers for their jobs.