The Global Positioning System (GPS) is a satellite-based navigation system that was developed by the U.S. Department of Defense (DoD) in the early 1970s. Initially, GPS was developed as a military system to fulfill U.S. military needs. However, it was later made available to civilians, and is now a dual-use system that can be accessed […]

GPS Details

Positioning, or finding the user’s location, with GPS requires some understanding of the GPS signal structure and how the measurements can be made. Likewise, as the GPS signal is received through a GPS receiver, understanding the capabilities and limitations of the various types of GPS receivers is essential. Furthermore, the GPS measurements, like all measurable […]

GPS Errors and Biases Part 1

GPS pseudorange and carrier-phase measurements are both affected by several types of random errors and biases (systematic errors). These errors may be classified as those originating at the satellites, those originating at the receiver, and those that are due to signal propagation (atmospheric refraction) [1]. Figure 3.1 shows the various errors and biases. The errors […]

GPS Errors and Biases Part 2

Receiver measurement noise The receiver measurement noise results from the limitations of the receiver’s electronics. A good GPS system should have a minimum noise level. Generally, a GPS receiver performs a self-test when the user turns it on. However, for high-cost precise GPS systems, it might be important for the user to perform the system […]

Datums, Coordinate Systems, and Map Projections (GPS) Part 1

The ability of GPS to determine the precise location of a user anywhere, under any weather conditions, attracted millions of users worldwide from various fields and backgrounds. With advances in GPS and computer technologies, GPS manufacturers were able to come up with very user-friendly systems. However, one common problem that many newcomers to the GPS […]

Datums, Coordinate Systems, and Map Projections (GPS) Part 2

Map projections Map projection is defined, from the geometrical point of view, as the transformation of the physical features on the curved Earth’s surface onto a flat surface called a map (see Figure 4.6). However, it is defined, from the mathematical point of view, as the transformation of geodetic coordinates (f, l) obtained from, for […]

GPS Positioning Modes Part 1

Positioning with GPS can be performed by either of two ways: point positioning or relative positioning. GPS point positioning employs one GPS receiver that measures the code pseudoranges to determine the user’s position instantaneously, as long as four or more satellites are visible at the receiver. The expected horizontal positioning accuracy from the civilian C/A-code […]

GPS Positioning Modes Part 2

RTK GPS RTK surveying is a carrier phase-based relative positioning technique that, like the previous methods, employs two (or more) receivers simultaneously tracking the same satellites (Figure 5.6). This method is suitable when: (1) the survey involves a large number of unknown points located in the vicinity (i.e., within up to about 10-15 km) of […]

Ambiguity-Resolution Techniques (GPS)

The previous topic showed that centimeter-level positioning accuracy could be achieved with the carrier-phase observables in the relative positioning mode. A prerequisite to this, however, is the successful determination of the initial integer ambiguity parameters (in fact, the integer double-difference ambiguity parameters). This process is commonly known as ambiguity resolution. Resolving the ambiguity parameters correctly […]

GPS Data and Correction Services

GPS users employ differential techniques to achieve the highest possible positional accuracy. A straightforward manner of doing this is to use two GPS receivers, a base and a rover.However, this may not be cost effective in many instances. An alternative, which could significantly reduce the survey cost without degrading the positional accuracy, is to use […]