Triangulation is the process of determining the location of a point by measuring the angles to it from two known points. The precise satellite locations are included in the transmission and the time-of-flight of the signal is used to calculate the distance to each satellite.
The receiver then does some math and calculates its location on the Earth. The more satellites the receiver can track, the more accurate the location calculation.
The receiver calculates 4 parameters; latitude, longitude, altitude and time. As a result, the receiver generally needs to see at least 4 satellites to calculate the 4 unknowns.
It can give estimates for the values with fewer satellites, but the potential error increases. The basic triangulation math is not that complicated, but the fact that the known points, the satellites, are moving very fast and the fact that the Earth is a curved surface adds quite a bit of complexity.
In addition, the Earth is not a perfect sphere and is not uniformly shaped or curved. This adds some error depending on how far off the average curvature a specific location is. For this reason, local augmentation systems are used. The receiver can also use regional data sets that better describe the local geography and ultimately give a more accurate position.
In the early days of the U. This error decreased the accuracy of the system so that it was not as effective for those outside the United States military. Commercial organizations began to use terrestrial beacons on the Earth to augment the system and account for the error. These beacons were built along the coast and waterways by the United States Coast Guard and similar organizations in other countries to help ships navigate local coastlines and waterways.
This required a separate receiver, which increased the cost and really prevented the systems from becoming commercially viable. Lower orbits would require more satellites to maintain the same coverage while higher orbits would reduce coverage extent. In addition an extensive ground infrastructure distributed worldwide is required to uplink the navigation signals, keep the different clocks of the constellation synchronised and correct any onboard timing or positioning deviation.
The satellite navigation signals are very faint, equivalent to car headlights shone from one end of Europe to another. The signals are based around pseudo-random number codes that identify each satellite in a constellation.
The receiver has records of each of these complex codes, so a full-power replica can be generated within the receiver from the faint signal received and used for the calculations deriving the final navigational data displayed to the user. You have already liked this page, you can only like it once! As these signals travel at the speed of light, the journey times are tiny fractions of a second.
The time marks are controlled by a highly accurate atomic clock on board each satellite. These clocks, however, are too expensive to incorporate into standard receivers, which have to make do with small quartz oscillators like those found in a wristwatch. Quartz oscillators are very accurate when measuring times of less than a few seconds, but rather inaccurate over longer periods. For this system of measurement to work, all satellites need to be synchronised so that they can start transmitting their signals at precisely the same time.
This is achieved by continuously synchronising all on-board atomic clocks with a master clock on the ground. These super-accurate clocks have an accuracy equivalent to one second in three million years. It is only possible to determine a location on Earth if you know the location of the navigational satellites very precisely.
This is achieved by placing the satellites in highly stable Medium Earth Orbits MEOs at an altitude of about 22 kilometres. Despite the predictability of the MEOs, it is still necessary to monitor the precise location of each satellite constantly to achieve high positioning accuracy. This is done from a global network of reference stations on the ground, whose positions are known to within centimetres. The ground system sends data derived from the reference station measurements to each satellite.
This precise location information, called ephemeris data, is then relayed by the satellite to the user receivers with the navigation signal. Numerous errors can degrade the accuracy of a position measurement. For example, errors in satellite to receiver distances can creep in if conditions within the ionosphere, the electrically charged outer layer of the atmosphere, slow down the signal. Conditions within the ionosphere are influenced by the level of activity on the surface of the Sun.
Inaccurate distance measurements will also occur if the signal takes an abnormally long path because it is reflected off tall buildings or other surfaces before reaching the receiver. There are various ways of overcoming such inaccuracies.
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