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Mobile Sensing: GPS Location

February 23, 2023 • César Daniel Barreto

The Global Positioning System (GPS)

The Global Positioning System (GPS) is a location system developed by the United States Department of Defense for military purposes. It provides accurate estimates of position, velocity, and time by using a computer network and a constellation of 24 satellites to determine the altitude, longitude, and latitude of any object on the Earth’s surface through triangulation. The system has been operational since 1995.

Civil Use of GPS

In the civil sphere, due to security reasons, only a degraded subset of GPS signals is allowed. However, the civil community has found alternatives to obtain excellent location precision through differential techniques. These techniques have led to tremendous growth in civil applications, and there are currently over 70 manufacturers of GPS receivers.

Principles of Operation

The purpose of the GPS system is to calculate the position of any point in a space of coordinates (x,y,z), starting from the calculation of the distances of the point to a minimum of three satellites whose location is known. The distance between the user (GPS receiver) and a satellite is measured by multiplying the flight time of the signal emitted from the satellite by its propagation speed.

To figure out how long the radio signal took to travel, the satellites and receivers’ clocks must be set to the same time. This is because they both have to send out the same code at the same time. But while the clocks on the satellites are very accurate, the clocks on the receivers are cheap quartz oscillators that are not accurate. Distances with errors due to synchronism are called pseudoranges. The difference in the clocks of the receivers is another unknown that makes it necessary to use at least four satellites to figure out where the receivers are.

Calculation of Pseudoranges

In calculating the pseudoranges, it is taken into account that GPS signals are weak and are immersed in the background noise inherent to the planet in the radio band. This natural noise is made up of random pulses, which leads to the generation of an artificial pseudo-random code by GPS receivers as a pattern of fluctuations. At each instant, a satellite transmits a signal with the same pattern as the pseudo-random series generated by the receiver. Based on this synchronization, the receiver calculates the distance by moving its pseudo-random code in time until it coincides with the received code; this displacement corresponds to the time of flight of the signal. This process is carried out automatically, continuously, and instantly in each receiver.

Ground Stations and Atomic Clocks

Although the speed of the satellites is high (4 km/s), their instantaneous position can be estimated with an error of less than several meters based on a prediction of previous positions in a period of 24 to 48 hours. The ground stations periodically review the atomic clocks of the satellites, two of cesium and two of rubidium, sending the ephemeris and the corrections of the clocks, since the precision of the clocks and the stability of the trajectory of the satellites are key in the operation of the GPS system.

Sources of Error in GPS

There are several sources of error that can affect GPS measurements. Some of the major sources are:

Ionospheric disturbance: The ionosphere is made up of a layer of electrically charged particles that can affect the speed of radio signals passing through it.

Meteorological phenomena: Water vapor in the troposphere, the layer closest to the Earth’s surface, can slow down electromagnetic signals and cause errors in GPS measurements that are difficult to correct.

Imprecision in clocks: Even with careful adjustment and control, both GPS satellite atomic clocks and receiver clocks can show slight deviations.

Unforeseen electrical interference: Electrical interference can cause errors in the correlation of pseudo-random codes or in orbit calculation, leading to mismatches of up to one meter.

Multipath error: GPS signals can be reflected off surfaces before reaching the receiver, leading to errors in the measured distance. It’s challenging to minimize this error as it depends on the GPS antenna’s environment.

Selective Availability (S/A): The military intentionally introduces this source of error, making it the greatest one.

Receiver-satellite topology: The spatial arrangement of visible satellites used in the calculation of distances can affect the precision of GPS measurements. Advanced receivers can modify the distance measurement error.

These sources of error can be categorized into those that depend on the satellites’ geometry and those that don’t. Selective availability and clock errors are not affected by the shape of the satellite, but ionospheric and tropospheric delays and multipath errors are very affected by the shape of the satellite. Each GPS position measurement has a value called “uncertainty” that is based on all the different sources of error.

GPS applications

GPS applications have multiple fields of use, including navigation aid systems, atmospheric and terrestrial space modeling, and high-precision time measurement. Here are some examples of GPS systems used in civil fields:

Study of atmospheric phenomena: GPS signals are useful in developing weather prediction models by analyzing the changes in speed caused by water vapor in the troposphere.

Location and navigation in inhospitable regions: GPS is used in research expeditions in difficult-to-access areas or areas without landmarks. This helps to deepen knowledge of polar or desert regions.

Geological and topographic models: GPS is used by geologists to study the movement of tectonic plates and for earthquake prediction. It is also a basic tool in topography for land surveys and forest and agricultural inventories.

Civil engineering: GPS is used to monitor in real time the deformations of large structures subjected to loads, such as metallic or concrete structures.

Automatic alarm systems: GPS is used in alarm systems connected to sensors that monitor the transport of high-risk polluting and perishable goods. The alarm allows for quick assistance to the vehicle.

Synchronization of signals: The electrical industry uses GPS to synchronize the clocks of its monitoring stations to locate possible failures in the electrical service.

Guidance for the physically handicapped: GPS systems are being developed to help blind people navigate through the city. It is also being studied for use in tourism to optimize routes between different places on a route.

Navigation and control of vehicle fleets: GPS is used for trajectory planning and vehicle fleet control by organizations such as the police, emergency services, taxi stations, courier services, and delivery companies.

Civil aviation systems: GPS is used in civil aviation to help with navigation and landing operations. Its importance has led to the development of systems aimed at improving GPS accuracy.

Unassisted vehicle navigation: GPS is incorporated in DGPS systems for precise maneuvering in areas of intense traffic, in autonomous land vehicles, surveillance in hostile environments, and cargo transport.

With the high precision of GPS measurements, important advances have been made in space in low orbits. Robots can now do dangerous jobs on their own, like inspecting, fixing, and putting together artificial satellites.

The future of GPS

In 1996, regulations governing GPS systems determined that selective availability would be suppressed in 2006 and one more frequency would be incorporated for civil use. This means that within a few years, GPS satellites will send civil code on both the L2 and L1 frequencies. This will create a redundancy that will make it possible to estimate ionospheric errors and give an absolute mode precision similar to what can be achieved with differential techniques. The signal on the L1 frequency will stay the same, so current receivers will still be able to work.

The control segment will be improved with the start-up of a new control system, currently in the design phase, for the expert station. This will include up to a total of twenty monitoring stations, resulting in more precise control of the ephemeris and satellite clocks.

However, current GPS, GLONASS, and GPS/GLONASS navigation systems are unable to meet the rigorous safety standards required by some civil applications, such as air navigation. The notification of errors to the user regarding the operation of the system can take from one second, when the error occurs in the satellite, to several hours, in cases where the control segment detects the failure.

To solve these drawbacks, Europe is developing EGNOS (European Geostationary Navigation Overlay Service), which will be operational in 2003. This system will meet safety standards for air navigation by installing a network of 34 fixed receiving antennas (RIMS) on land to receive GPS signals and reduce positioning errors. These signals will be sent to a control center where the satellite information will be calibrated, measuring the possible error to correct it, and sent again to 10 ground stations. Additionally, these signals will be sent to two new geostationary INMARSAT satellites located at an altitude of 35,000 km, which will act as repeaters and send the signals to the users. Both the US (WAAS: Wide Area Augmentation System) and Japan (MTSAS: MTSAT Satellite-Based Augmentation System) are working on similar services.

Europe will also launch GNSS-1, which stands for “Global Navigation Satellite System 1.” This system will combine the services of GPS, GLONASS, EGNOS, WAAS, and MTSAS. This will be the first step toward a European positioning system (GNSS-2 or Galileo) that will use a constellation of European satellites.

Finally, it should be noted that in the next five years, GPS and GPS/GLONASS will be the only positioning systems based on satellites that will be operational.

In conclusion

Reaching high levels of accuracy requires an awareness of the origins of GPS measurement error. The NOAA GPS Toolbox is an essential tool for researchers and engineers working with GPS technology since it offers useful software and resources to help reduce these inaccuracies.

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César Daniel Barreto