GPS, what it is, and how it works

The most well-known GNSS system is the Global Positioning System (GPS), which was developed and is operated by the United States government.

However, there are now a number of other GNSS systems in operation or development around the world, including Russia’s GLONASS, China’s BeiDou, and Europe’s Galileo. Despite the widespread use of GNSS systems, many people still have questions about how they work and what factors can affect their accuracy.

In this blog article, we will explore the key components of GNSS systems, including satellites, control stations, and user receivers, and discuss the technology behind trilateration and time-of-flight measurements that make GNSS positioning possible.

The Global Positioning System (GPS) consists of three main components:

Space segment, A constellation of orbiting satellites

Space segment: The space segment is made up of a constellation of orbiting satellites that transmit radio signals to GPS receivers on the ground. The GPS constellation consists of at least 24 satellites in six orbital planes, with additional satellites in reserve. These satellites orbit the Earth twice a day at an altitude of approximately 20,000 km (12,500 miles).

The GPS satellites transmit two types of signals: L1 and L2. The L1 signal has a frequency of 1575.42 MHz and is transmitted in two polarizations: right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP). The L2 signal has a frequency of 1227.60 MHz and is transmitted only in RHCP. Both signals are transmitted in a spread-spectrum format, which makes them resistant to interference and jamming.

The L1 signal is used by all GPS receivers for basic positioning and timing, while the L2 signal is used for more advanced applications such as precise positioning and atmospheric research. The L2 signal also provides a signal quality monitoring function that can be used to detect and correct for errors in the GPS signals caused by atmospheric effects.

The GPS satellites also transmit a range of additional information, including ephemeris data (which provides information about the satellite’s orbit and clock), almanac data (which provides information about the location and health of all GPS satellites), and satellite health information (which indicates whether a satellite is functioning properly or experiencing problems).

In addition to the standard GPS signals, the GPS system also includes a range of augmentation systems (SBAS) that can improve the accuracy and reliability of GPS positioning. These include the Wide Area Augmentation System (WAAS) and the European Geostationary Navigation Overlay Service (EGNOS), which provide correction data to GPS receivers to improve the accuracy of their position estimates.

Control segment, A network of ground-based control stations

The control segment consists of a network of ground-based control stations that monitor the GPS satellites and make corrections to their orbits and clocks. The control segment also uploads data to the satellites, such as orbital parameters and clock corrections, to ensure that the satellite signals are accurate.

To maintain the accuracy of the GPS signals, the control segment makes a number of corrections to the signals transmitted by the GPS satellites. These corrections fall into two main categories: orbit corrections and clock corrections.

Read more about orbits here: A Guide to Satellite Orbits

Orbit corrections are made to account for the fact that the orbits of the GPS satellites are not perfectly circular and are subject to various perturbations. The control segment uses data from a network of ground-based tracking stations to determine the actual positions of the GPS satellites and then calculates the necessary corrections to bring the satellite orbits into alignment with their intended paths. The orbit corrections are transmitted to the GPS satellites via uplink stations, which then transmit the corrected signals to GPS receivers on the ground.

Clock corrections are made to account for the fact that the atomic clocks onboard the GPS satellites are subject to relativistic effects, which can cause them to drift out of sync with the clocks on the ground. The control segment uses data from ground-based monitoring stations to determine the precise time offset between the GPS satellite clocks and the clocks on the ground, and then transmits this information to the GPS satellites via uplink stations. The satellites then adjust their clocks to bring them back into sync with the clocks on the ground.

In addition to these corrections, the control segment also provides a range of other services to support the GPS system, including satellite health monitoring, data management, and system security. The control segment is operated by the United States Air Force, with support from various civilian contractors.

User segment, GPS receivers

The user segment consists of the GPS receivers that are used by individuals, vehicles, and other devices to determine their position on the Earth’s surface. GPS receivers receive signals from multiple GPS satellites and use trilateration to calculate their position based on the time it takes for the signals to travel from the satellites to the receiver.

To calculate its location from the GPS satellites, the GPS receiver needs to have a clear line of sight to at least four satellites (three for 2D positioning). The receiver uses an antenna to capture the signals transmitted by the satellites and then uses signal processing techniques to extract the timing and location information from the signals.

GPS receivers calculate distance, not angles!

Once the GPS receiver has received signals from multiple satellites, it uses trilateration to calculate its position. Trilateration is the process of determining the position of a point by measuring its distance from three or more known points. In the case of GPS positioning, the known points are the positions of multiple GPS satellites in space, and the point being determined is the position of the GPS receiver on the Earth’s surface.

The distance between the GPS receiver and each satellite is determined by measuring the time it takes for the signal to travel from the satellite to the receiver. This measurement is known as time-of-flight or propagation delay, and it is made possible by the highly accurate atomic clocks that are onboard both the satellites and the receiver.

By measuring the time-of-flight from multiple satellites, the receiver can determine its distance from each satellite and use this information to perform trilateration calculations. The receiver then calculates its position by finding the intersection of the spheres or circles that represent the possible locations of the receiver based on the measured distances.

GPS receivers can also use additional information, such as ephemeris data and almanac data, to improve the accuracy of their position estimates. This information is typically provided by the control segment of the GPS system and is transmitted to the receiver along with the GPS signals.

The accuracy of GPS positioning

The accuracy of GPS positioning can be affected by a variety of factors, including:

Number and geometry of satellites:

The accuracy of GPS positioning is typically better when more satellites are in view and when they are distributed in a way that provides good geometric coverage. In some cases, satellites may be blocked by buildings or other obstructions, which can reduce the number and quality of the signals available to the receiver.

Atmospheric conditions:

The GPS/GNSS signals can be affected by atmospheric conditions such as ionospheric and tropospheric delays, which can cause errors in the ranging measurements and reduce the accuracy of the position estimate.

Signal quality:

The quality of the GPS/GNSS signals can be affected by a range of factors, including interference from other radio signals, signal multipath (where the signal is reflected off of surfaces before reaching the receiver), and signal attenuation (where the signal is weakened by passing through obstructions such as buildings or trees).

Receiver quality:

The accuracy of GPS/GNSS positioning is also dependent on the quality of the receiver hardware and software. Higher-quality receivers typically have better sensitivity, better filtering, and more sophisticated algorithms that can provide more accurate position estimates.

Environmental factors:

Environmental factors such as topography, vegetation, and other obstructions can also affect GPS/GNSS accuracy by blocking or reflecting the GPS/GNSS signals or interfering with the receiver’s ability to track the signals.

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