GPS

Definition:

GPS, Global Positioning System) refers to a satellite positioning system operated by the United States of America (a system for measuring the current position on the earth).

Overview

Among the approximately 30 GPS satellites launched by the United States for military use, the GPS receiver receives signals from several satellites in the sky, and the receiver knows the current position of the satellite.

 President Reagan announced that after the Korean aircraft shooting down incident, it will be opened so that it can be used for non-military use (civil use) for the safe navigation of civilian aircraft was done. After that, the "Initial Operation Declaration" that satisfied the accuracy of civilian operation was made in 1993, and the "Complete Operation Declaration" that satisfied the accuracy of military operation was made in 1995.

The signal from the GPS satellite includes time data from an atomic clock mounted on the satellite, information about the ephemeris (orbit) of the satellite, and the like. The distance from the satellite can be found by receiving the radio wave from the GPS satellite, measuring the transmission time, and multiplying the time difference between the transmission and reception by the propagation speed (light speed) of the radio wave.

GPS satellites are quasi-synchronous satellites that move at an altitude of about 20,000 km in about 12 hours (not geostationary satellites). About 30 satellite constellations launched in several orbits can cover the entire area of ​​the earth. In addition, because it is in the Earth's orbit, it is advantageous as a signal transmission power, and since the arrangement changes momentarily from a certain area, it is possible to average the error on the whole earth.

Principle

The principle of GPS positioning is that the speed of light c is constant in the local inertial system.

{\ displaystyle c} c = 2.99792458 × 108 m / s

If both the GPS satellite and the receiver have a clock that can be considered accurate, the distance can be found by multiplying the difference between the transmission time (measured value) {\ displaystyle T} T and the reception time {\ displaystyle t} t by c.

The position of the GPS satellite {\ displaystyle i} i is the coordinates ({\ displaystyle X_ {i}, Y_ {i}, Z_ {i} X_ {i}, Y_ {i}, Z_ {i}) of the receiver. If the position is ({\ displaystyle x, y, z} x, y, z), to obtain the position of the GPS satellite, the navigation message signal superimposed on the received data is demodulated and combined with the transmission time.

The time of reception {\ displaystyle t} t is the value of the clock of the GPS receiver, and if it is accurate, at least three to get the three variables (unknown) x, y, z that are the position of the receiver. However, the GPS receiver clock is not so accurate, and the reception time {\ displaystyle t} t needs to be an unknown number. Therefore, these four unknowns can be obtained by receiving from four or more satellites.

Distance measurement by SS modulation

A transmission signal is generated by multiplying transmission data by a pseudo noise sequence that is an artificially created code (which cannot be distinguished from a true random number).Therefore, compared to FM and AM modulation, it can be transmitted with a wide bandwidth and low power, and confidentiality (data cannot be demodulated unless the pseudo-noise sequence is known) and confidentiality (because it is indistinguishable from white noise, transmission is not possible) and the same band can be used multiple times with different pseudo-noise sequences.

One of the features is that if the time of the start position of the pseudo-noise sequence is set, the sending time can be known accurately during demodulation, which is the basis of distance measurement.

GPS takes advantage of these features to simultaneously perform positioning and data (including ephemeris (orbit) information).

There are two types of signals for the L1 radio waves (1.57542GHz) from GPS satellites, one that uses the publicly announced C / A code for the pseudo-noise series and one that does not publish the P (Y) code. The P (Y) code is intended for military purposes, and the order of the generator polynomial of the sequence is large (it takes a long time for the pseudo-noise sequence to complete one cycle), so the accuracy is very high (about 16 cm), and missiles and guidance Used to guide bombs.

Using unencrypted C / A code data, which is allowed for private use, the accuracy is about 95% or more, and the coordinates within 10 m from the correct latitude / longitude can be obtained. This is accuracy in a short time, and precise measurement is possible by continuing to receive for a long time.

         

Positioning method

The positioning method of GPS is divided into a method (code positioning method) based on a code (modulation of carrier wave) and a method (carrier positioning method) based on the phase of a carrier wave. Code positioning is generally used, but carrier positioning is used for precision positioning.

DGPS (Differential GPS)

Differential GPS (relative positioning method). The mobile station that is the target of positioning, a base station whose position is known also receives GPS radio waves to eliminate errors. If the correction information generated by the base station is transmitted and received by the mobile station, the correction processing of DGPS can be performed in real time. Error number m. Correction information was transmitted by the Japan Coast Guard's medium-wave beacon, but "improved GPS accuracy", "operation of another correction system using satellites, etc.", "April 2019 " It was abolished at noon on March 1, 2019 because the reliability cannot be guaranteed due to the rollover that occurs on the 7th.

 Method Interferometric positioning.

Similar to DGPS, the phase difference of the electric wave received from the electronic reference point is measured and the positioning is calculated. Positioning time less than 1 minute, error of several cm is possible. At the survey point, a standard receiver is installed at a reference reference point (known), and positioning is performed by (multiple) mobile receivers.

Error

If the error of the clock installed in the GPS receiver is one millionth of a second, the error of the distance will reach 300m. Therefore, many receivers frequently calibrate the current time by receiving radio waves from four or more GPS satellites, and calculate accurate reception time and receiver coordinates (points in three-dimensional space) by positioning calculation.