Basically, a receiver obtains pseudo-range or carrier phase data to at least four satellites. As GPS receiver technology is developing so rapidly, it is only possible to deal with some of the basic operational characteristics. The type of receiver used will depend largely upon the requirements of the user. For instance, if GPS is to be used for absolute as well as relative positioning, then it is necessary to use pseudo-ranges. If high-accuracy relative positioning were the requirement, then the carrier phase would be the observable involved.
For real-time pseudo-range positioning, the user’s receiver needs access to the navigation message (Broadcast Ephemerides). If carrier phase observations are to be used, the data may be post-processed and an external precise ephemeris may also be used. Most modern receivers are ‘all in view’, that is they have enough channels to track all visible satellites simultaneously.
Achannel consists of the hardware and software necessary to track a satellite’s code and/or carrier phase measurement continuously. When using the carrier phase observable, it is necessary to remove the modulations. Modern geodetic receivers may work in a code correlation or codeless way. Code correlation uses a delay lock loop to maintain alignment with the incoming, satellite-generated signal. The incoming signal is multiplied by its equivalent part of the generated signal, which has the effect of removing the codes. It does still retain the navigation message and can therefore utilize the Broadcast Ephemeris. In the codeless mode a receiver uses signal squaring to multiply the received signal by itself, thereby doubling the frequency and removing the code modulation. This process, whilst reducing the signal-to-noise ratio, loses the navigation message.
Handheld GPS receiver
Code correlation needs access to the P code if tracking on L2 frequency.As the P code may be changed to the Y code and made unavailable to civilian users, L2 tracking would be impossible. However, with code correlation receivers are able to track satellites at lower elevations. Some receivers used for navigation purposes generally track all available satellites obtaining L1 pseudorange data and for entering the majority of harbours need to be able to accept differential corrections (DGPS) from an on-shore reference receiver.
Geodetic receivers used in engineering surveying may be single or dual frequency, with from 12 to 24 channels in order to track all the satellites available. Some geodetic receivers also have channels available for GLONASS, the Russian system equivalent to GPS.
All modern receivers can acquire the L1 pseudo-range observable using a code correlation process illustrated later. When the pseudo-range is computed using the C/A code it can be removed from the signal in order to access the L1 carrier phase and the navigation message. These two observations could be classified as civilian data. Dual frequency receivers also use code correlation to access the P code pseudo-range data and the L2 carrier phase. However, this is only possible with the ‘permission’ of the US military who can prevent access to the P code. This process is called Anti-Spoofing (AS). When AS is operative a signal squaring technique may be used to access the L2 carrier.