What is GPS in Surveying? How Does It Work?

The Global Positioning System (GPS) is a satellite-based navigation and surveying system used for the determination of precise position and time of a point using radio signals from the satellites. It works based on the principle of trilateration. Hence, it is primarily a navigation system for real-time positioning.

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Fig.1. GPS Surveying with Base Station Receiver and an Unknown Station

Conventional surveying techniques make use of ground-to ground survey measurements, that invites several errors and limitations. But GPS make use of ground-to-space measurements, which helps to overcomes these limitations arising due to terrestrial surveying methods.

GPS was originally developed for military use by US Army and employed for civilian use since 1990s. In addition to application of GPS in mobiles and car navigation system, surveying was the first commercial adaptations of GPS technology. GPS is used for more accurate boundary and construction surveys. 

Principle of GPS in Surveying

The working principle of GPS can be better understood by understanding the components of the whole system. A GPS system is divided into:

1. The space segment
2. Control segment
3. User segment

Figure-2. The Three GPS Segment; Image Courtesy: The Aerospace Corporation

1. Space Segment

The space segment consists of satellites that are placed in almost six circular orbits with an inclination of 55 degrees. Set of satellites is called as constellation.

Each satellite contain four precise atomic clocks, out of which one is used at a time.

These satellites are the ones that transmit signals from the space, based on which time and position of a user is measured.

2. Control Segment

Control segment forms the vital link of a GPS. It is used to monitor and control the satellite system continuously. It is composed for ground based monitor stations, three antennas and a master control station.

The master control station predicts the satellite ephemeris and the behavior of each satellite clock. This data is forwarded to the antenna. The antenna transmit this data to each satellite at least once a day. 

3. User Segment

The GPS user segment consist of GPS receivers . These receivers track 4 or more GPS satellites and computes its own position.(A minimum of 4 satellites are required to determine the position X,Y,Z and time.)

GPS Land Surveying Methods

Other than the use of GPS in navigation, it can be rapidly used in surveying, where it give the position directly in terms of latitude, longitude and height. Hence, there is no need for measuring the angles, and distances between the intermediate points. The only requirement is to have a clear sky to receive signals from the GPS satellites. Here, the known positions are the location of the GPS satellites and their orbits. 

GPS surveying is conducted by using the following instruments.

1. Static GPS Baseline

In this method, the coordinates of the survey points are recorded simultaneously by GPS observation, over a known and unknown point for 20 minutes. The data is later processed to determine the coordinates to an accuracy of 5mm.

2. Real-Time Kinematic (RTK) Method and Post Processing Kinematic Method (PPK)

RTK and PPK are correction technologies that are used to define the correct position.  This method uses an additional rover station along with the usual arrangement. Rover station can move between different positions, while the base station is fixed. The position of the rover station is calculated using a radio signal, with respect to a known reference point. The corrections determined are applied real-time itself in the RTK method, while the corrections are applied to all the positions after all the survey work is complete, in the case of PPK. 

3. Continuously Operating Reference Stations (CORS)

Here, a survey-grade GPS receiver is permanently installed in a particular location at the starting point of any GPS measurement in the area. The GPS survey equipment collects the field data and combines with CORS to calculated positions.

GPS Surveying Instruments

The different types of GPS surveying instruments used to conduct GPS land surveying are:

GPS INSTRUMENT	FUNCTION
GPS Receiver	Receive signals from the GPS satellites to make calculations. It comes with Bluetooth, Wi-Fi, OLED displays
GPS Rover Rods	This is used in RTK method. It is used to extend the rover’s reach. They can be made from a variety of durable materials and come in different lengths.
GPS Poles	Used to mount GPS surveying instrument; lightweight , durable and come in different length
GPS Bipods/Tripods	For greater stability in mounting GPS equipment, bipods and tripods come in adjustable lengths and numerous sizes
GPS Antennae	This equipment makes it possible for GPS systems to receive signals from satellites. Many systems come with an internal antenna, but you can purchase external antennas to boost the signal.
Total Station	A combination of electronic theodolite, EDM and software running. Used to calculate the coordinates of survey pints using angles and distances. Better accuracy when used along with GPS.

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