A wide variety of GPS receivers are commercially available today. Depending upon the type of application, accuracy requirements and cost, the users can select the type of GNSS receiver which best meets the requirements. These receivers cover a wide range from the high-precision receivers with built-in atomic clock, to the hand-held navigation receivers, which can give the precise position to few-metres. Even wrist-watches with built-in GNSS receivers are now commercially available. Three broad categories of GNSS are explained below (Garg, 2021).

(a) Navigation receivers:

Navigation in three dimensions is the primary function of GNSS. Navigational receivers are made for aircraft, ships, ground vehicles, and for hand carried by individuals. These are used for navigation, positioning, time dissemination, measuring atmospheric parameters, surveying, geodetic control, and plate tectonic studies. These receivers are normally single-frequency, C/A code, hand-held light weight receivers, which can give the position with a few metres to few tens of metres accuracy. These receivers are very much portable, weighing only few hundred grams, and are fairly cheap. Single channel receivers, which can track 4 or more satellites, are now being replaced by two or five channel receivers. The accuracies in positioning obtained by these type of receivers are in the range of few tens of metres in absolute positioning (in the absence of SA), and few tens of cm in relative positioning, over short baselines of few km.

(b) Surveying receivers:

The surveying type of receivers are single frequency, multi-channel receivers, which are useful for most surveying applications, including cadastral mapping applications, providing tertiary survey control, engineering surveys, etc. They are more expensive than the navigational receivers, but more versatile. The data from many of these receivers can be directly imported to most commonly used GIS software packages. Most of these receivers can also be used in DGNSS mode.

(c) Geodetic receivers:

The geodetic receivers are multi-channel, dual-frequency receivers, generally with the capability of receiving and decoding the P-code. They are heavier and more expensive than the navigation and surveying receivers. They are capable of giving accuracies of few cm in absolute positioning with precise post-processed satellite orbit information and of few mm in relative positioning. These receivers are usable for applications related to geodetic, geodynamic, detailed GIS and topographic engineering survey, etc. A modern geodetic receiver should be able to measure accurately and reliably anywhere under any condition.

The GNSS receivers can be classified into two basic types: (i) Code phase receivers, and (ii) Carrier phase receivers (Seeber, 2003; Dhunta, 2001).

(i) Code phase receivers

These receivers are also called code correlating receivers as they access the satellite navigational P- or C/A-code signal for their operation. They have a unique capability to begin calculations without having an approximate location and time. Code phase receivers provide real-time navigation data using almanac data from satellite message for operation and signal processing. These receivers have anywhere-fix capability as they can synchronize themselves with GNSS time at a point with unknown coordinates. For this purpose, we need to lock the signals of four satellites to start the survey with a quicker start-up time. In code based receivers, phase position of the received code sequence is compared with the phase of an identical code replica, generated by the receiver (using the same algorithm as used for the code from the satellites) via a correlation technique. Hence, the observable is also called the code phase. These receivers can be used for the rapid calculation of baselines where high accuracy is not required, for example, in exploration or offshore work. The two code sequences are shifted stepwise in phase until a maximum correlation is obtained. These receivers have a complete code dependent correlation channel which produces: code phase, carrier phase, change of carrier phase (Doppler frequency), and satellite message.

(ii) Carrier phase receivers

These receivers utilize the actual GNSS signals to calculate a position. Two common types of such receivers are; (i) single frequency, and (ii) double frequency, which are compared together, as given in Table 3.4. The single frequency receivers track L1 frequency signal only, and are cheaper than dual frequency receivers. They can be effectively used in relative positioning mode for accurate baselines of less than 50 km or where ionosphere effects can normally be ignored. The double frequency receivers track both L1 and L2 frequency signals, and are more expensive than the single frequency receivers. They can effectively be used to measure longer baselines of more than 50 km where ionosphere effects have a larger impact, as ionosphere effects are eliminated by combining L1 and L2 observations.

Following factors should be kept in mind while selecting a receiver (Seeber, 2003, Spirent, 2011, https://www.sparkfun.com/GPS_Guide):

(i) Capability of tracking all the signals from each visible satellite at any time (as GPS+GLONASS system needs 20 dual frequency channels)
(ii) Have low phase and code noise
(iii) Contains high data rate (> 10 Hz) for kinematic applications with high memory capacity
(iv) Low power consumption, low weight and small size
(v) Full operational capability under Anti-Spoofing (AS).
(vi) Capability to track weak signals (under foliage, and difficult environmental conditions)
(vii) Able to do multipath mitigation, interference suppression, stable antenna phase centre
(viii) DGNSS and RTK capability
(ix) 1 pps timing output
(x) Ability to accept external frequencies
(xi) Few or no cable connection
(xii) Availability of radio modem
(xiii) Can operate over difficult meteorological conditions
(xiv) Ease of interfacing with other GNSS systems.
(xv) Flexible set up (tripod, pole, pillar, vehicle)