The GPS (Global Positioning System) is a complex technology which together with a dedicated receiver for receiving and calculate position, velocity of an object; it is controlled by the US Department of Defense (DoD).
The system was initially designed for military purposes and consists of 24 satellites that orbit the earth in 12 hours; in reality there are more, because they are launched of new ones to replace older, for a total of 32.
Since the duration of the sidereal day does not reflect the duration of the UTC day, on a day the satellites perform two completely whole orbits: so this means that, every day and secured a point, will not always be visible the same satellites.
The system has become fully operational in 1994 and consists of 6 orbital planes each occupied by satellites 4 (3 + 1) equally spaced and inclined by 55 ° with respect to the equatorial plane; This constellation provides the user with a visibility of between 5 and 8 satellites in each point of the earth.
Each satellite carries on board:
- An atomic clock of extreme precision (that is the basis of the system).
- A control computer.
- A radio transmission system.
- An attitude control system.
Since the GPS is a system that bases its operation on the measurement of time, each receiver must have:
- An antenna small in size.
- A quartz watch.
- A digital signal processing software (ENS)
- A radio receiver.
The entire system is controlled by a series of stations located in different parts of the world, (mainly on the equator) but the earth station with the role of “Master” is located in Colorado, to Schriever Air Force Basewhich monitor the signals generated the satellite equipment.
The main task of each GPS receiver is to convert the satellite signal in terms of position, velocity and time estimate: to do this are necessary the data of 4 satellites from which are derived the 4 spatial coordinates and velocity (x, y, z , v).
A second use of GPS consists of the distribution of a precise time signal used for example in telecommunications networks, in the formulation of standard laboratory and astronomical observations such as the measurement of atmospheric parameters of our planet.
There are two types of satellite identification:
- SPS: Standard Positioning Service
- PPS: Precise Positioning Service
The first type of identification (SPS) is used for civil and does not have unrestricted use. It has an accuracy of about 100 m in the horizontal and 150 m in the vertical: the accuracy of the timing signal is 340 ns (UTC). The second type of identification (PPS) is used by military forces and government agencies with special receivers; It has an accuracy of 22 m horizontally and 27.7 m in the vertical: the accuracy of the timing signal is 220 ns (UTC).
Each satellite transmits on two frequencies, the carrier is higher than GHz (for through the clouds):
- L1 (1575.42 MHz) used by SPS.
- L2 (1227.60 MHz) used by PPS.
There are then two different codes that modulate the carrier L1 or L2:
- the C / A code (Coarse Acquisition) modulates the L1 carrier. (Civil Use)
- The P-Code Code (Precise) (military use)
The C / A code is a pseudo random number generated at a frequency 1 MHz: there is a different code for each satellite which serves to identify it. The “spread” code band in a range of 1 MHz (CDMA) protocol. The C / A code modulates the Navigation Message is a 50 Hz signal which contains the satellite ephemeris.
The P-Code (Precise) instead modulates both frequencies and is much longer; since the code is also encrypted, each receiver must be equipped with a module affixed to decode.
The navigation message (GPS Navigation Message – NM) begins with a series of bits that identify the beginning of each frame. In general frame consists of 1500 bits divided into 5 parts. A set of 25 frames (25 x 5 sub frame) make up the Navigation Message that is sent every 12.5 minutes.
The meaning of the fields of the frame is the following:
- Clock: timing signal on board the satellite (clock).
- Satellite ephemeris: describe the satellite orbits for small celestial sections. Usually the receiver collects data every hour although it can also use them for 4 hours. The ephemeris are used in the algorithm that calculates the position within the orbit period described in orbital parameters.
The set of ephemeris of each satellite is called almanac and a dozen almanacs can describe the orbits of each satellite for months.
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