GPS Positioning Overview

GPS Positioning

Determining our position on Earth means providing the latitude and longitude of a certain point on the Earth’s surface. Therefore, most receivers provide the values ​​of these coordinates in units of degrees (°) and minutes (‘). Both latitude and longitude are angles and therefore must be measured with respect to a well-defined 0 ° reference.

Latitude: Northern and Southern Hemispheres

Latitude is measured relative to the Equator (latitude 0 °). If a certain point is in the northern hemisphere or southern hemisphere, its latitude coordinate will be accompanied by the letter N or S in each case. Another type of nomenclature refers to northern latitudes with positive numbers and southern latitudes with negative numbers.

Length: East, West

For historical reasons, longitude is measured relative to the Greenwich meridian. If we measure an angle to the east or west of the Greenwich meridian we write the letter E or W depending on the case accompanying the number that gives the longitude. Negative numbers are sometimes used. For example, the following longitude values ​​are equivalent: W 90 °; E 270 °; and -90 °.

Global Positioning System

GPS (or Global Positioning System according to abbreviationfinder) is a constellation of 24 artificial satellites uniformly distributed in a total of 6 orbits, so that there are 4 satellites per orbit. This configuration ensures that at least 8 satellites can always be “seen” from almost anywhere on the earth’s surface. GPS satellites orbit the Earth at an altitude of about 20,000 km and travel two full orbits every day. They describe a type of orbit such that they “rise” and “set” twice a day. Each satellite transmits radio signals to Earth with information about your position and when the signal is emitted. We can receive this information with GPS receivers (GPS receivers), which decode the signals sent by several satellites simultaneously and combine their information to calculate their own position on Earth, that is, their latitude and longitude coordinates with an accuracy of about 10 meters. There are more sophisticated receivers that can determine position with an accuracy of a few millimeters.

Function of a GPS receiver

As we have said before, the GPS receivers receive the precise information of the time and the position of the satellite. Exactly, it receives two types of data, the Almanac data, which consists of a series of general parameters on the location and operation of each satellite in relation to the rest of the satellites in the network, this information can be received from any satellite, and once the GPS receiver has the information from the last Almanac received and the precise time, knows where to look for satellites in space; the other series of data, also known as Ephemeris, refers to the precise data, only, of the satellite that is being captured by the GPS receiver, they are exclusive orbital parameters of that satellite and are used to calculate the exact distance from the receiver to the satellite. When the receiver has captured the signal from at least three satellites, it calculates its own position on Earth using trilateration [2] of the position of the captured satellites, and they present us with the calculated Longitude, Latitude and Altitude data. GPS receivers can and usually do receive signals from more than three satellites to calculate their position. In principle, the more signals they receive, the more accurate the position calculation. Taking into account that the initial conception of this system was to make military use of it, it should be noted that the receivers found on the market are for civilian use, and that these are subject to a precision degradation that ranges from 15 at 100 meters RMS or 2DRMS depending on the geostrategic circumstances of the moment, as interpreted by the US Department of Defense, who manages and provides this service. This degradation is regulated by the Selective Availability Program of the US Department of Defense or SA (Selective Availability) and, as we have indicated before, introduces an error in the transmission of the position for the receivers of civil use. This is, naturally, to maintain a strategic advantage during military operations that require it. It follows from all this that GPS receivers usually have a nominal error in the calculation of the position of approximately 15 m. RMS that can increase up to 100 m. RMS when the Government of the USA. deems it appropriate. If the use given to the GPS receiver requires even more precision, such as topographic work, cartographic surveys, orientation races, location of beacons, etc., almost all firms have optional antennas with DGPS devices for some of their receivers that correct by differential calculation this error, reducing it to a margin of 1 to 3 meters RMS.

Uses of a GPS receiver

You can use a GPS receiver for whatever you think it might be useful for. However, it must be taken into account that they are exclusively data receivers that calculate the exact position and that they do not work with any analog data (temperatures, pressure, humidity…). They are extremely useful devices for any navigation task, route tracking, point storage for later studies,… but in no case can we expect to deduce atmospheric data from them. However, it should also be appreciated that even the “smaller” models that GPS manufacturers make available for personal navigation are an evolution of aeronautical navigation systems.and maritime that have been perfected daily for years. This supposes a series of important advantages for the users of GPS’s for the terrestrial personal navigation. First of all, a matter of scale. It is clear that the dimensions of aeronautical and maritime navigation compared to the dimensions of land navigation, even with motorized vehicles, are much greater. This means that the “small” receivers also have the navigation resources and the accuracy of the large ones, only the former have less sophisticated functions than the latter for the navigation itself. The displays and graphical functions required by the pilot of a boat incorporated into his GPS receiver must be much more and more sophisticated than those required to orient himself in smaller dimensions. But the reception system, and the calculation of the position is the same in one case as in another. A GPS receiver provides many more features for land navigation than is needed for orientation. Course deviation monitoring, route monitoring, Electronic compasses, etc., are functions that can be found in a “small” GPS’s.

GPS receiver

  • The situation of the satellites can be determined in advance by the receiver with the information of the so-called almanac (a set of values ​​with 24 orbital elements), parameters that are transmitted by the satellites themselves. The collection of almanacs for the entire constellation is completed every 5-20 minutes and stored in the GPS receiver.
  • Information that is useful to the GPS receiver in determining your position is called ephemeris. In this case, each satellite emits its own ephemeris, which includes the health of the satellite (whether or not it should be considered for taking the position), its position in space, its atomic time, Doppler information, etc.
  • The GPS receiver uses the information sent by the satellites (time at which they emitted the signals, their location) and tries to synchronize its internal clock with the atomic clock that the satellites have. Synchronization is a trial and error process that occurs once every second on a portable receiver. Once the clock is synchronized, you can determine your distance to the satellites, and use that information to calculate your position on earth.
  • Each satellite indicates that the receiver is at a point on the surface of the sphere, centered on the satellite itself and the total distance from the receiver radius.
  • Obtaining information from two satellites indicates that the receiver is on the circumference that results when the two spheres intersect.
  • If we acquire the same information from a third satellite we notice that the new sphere only cuts the previous circumference in two points. One of them can be ruled out because it offers an absurd position. In this way we would already have the position in 3D. However, since the clock in the GPS receivers is not synchronized with the atomic clocks of the GPS satellites, the two points determined are not precise.
  • By having information from a fourth satellite, we eliminate the inconvenience of the lack of synchronization between the clocks of the GPS receivers and the clocks of the satellites. And it is at this time that the GPS receiver can determine an exact 3D position (latitude, longitude and altitude). As the clocks between the receiver and the satellites are not synchronized, the intersection of the four spheres centered on these satellites is a small volume instead of being a point. The correction consists of adjusting the time of the receiver in such a way that this volume becomes a point.

DGPS or differential GPS

Field team conducting seismic data surveying using a Navcom SF-2040G StarFire GPS receiver mounted on a mast.
DGPS (Differential GPS), or differential GPS, is a system that provides GPS receivers with corrections of data received from GPS satellites, in order to provide greater precision in the calculated position. It was conceived primarily due to the introduction of selective availability (SA).
The rationale lies in the fact that the errors produced by the GPS system affect the receivers located close to each other equally (or very similarly). Errors are strongly correlated in neighboring receivers.
A GPS receiver fixed on the ground (reference) that knows its position exactly based on other techniques, receives the position given by the GPS system, and can calculate the errors produced by the GPS system, comparing it with its own, known in advance. This receiver transmits the error correction to the receivers close to it, and thus these can, in turn, also correct the errors produced by the system within the signal transmission coverage area of ​​the reference GPS equipment.
In short, the DGPS structure would be as follows:

  • Monitored station (reference), which knows its position with very high precision.

GPS Positioning