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GPS Fundamentals and Common Sense

GPS Fundamentals and Common Sense

The technical term
GPS—(Global Position System, Global Positioning System), the full name is NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Position System, Navigation Satellite Timing and Ranging Global Positioning System). GPS is a space-based all-weather navigation system developed by the US Department of Defense. It is used to meet the requirements of the military to obtain position, velocity and time information in a common reference frame on the ground or in near-earth space.

DGPS—(Differential Global Positioning System, Differential Global Positioning System), install a GPS monitoring receiver on a precise known location (reference station), and calculate the distance correction number between the reference station and GPS satellites.

SBAS—(Satellite-Based Augmentation System, satellite-based augmentation system), through geostationary orbit (GEO) satellites equipped with satellite navigation augmentation signal transponders, can broadcast to users various corrections such as ephemeris errors, satellite clock errors, and ionospheric delays Information, to achieve the improvement of the positioning accuracy of the original satellite navigation system, and thus become a means for the development of the space powers. (Including WAAS, SDCM, EGNOS, MSAS and GAGAN)

AGPS—(Assisted Global Positioning System, Assisted Global Positioning System), is a GPS operation mode. It can use the information of mobile phone base stations and cooperate with traditional GPS satellites to make the positioning faster.

C/A code—a pseudo-random code sent by GPS satellites, used for rough ranging and capturing GPS satellites, is actually a Gold code, which is generated by a G code composed of two 10-level feedback shift registers. The accuracy is within 14 meters.

P code—a pseudo-random code sent by GPS satellites, which is a precision code corresponding to C/A code, with a code rate of 10.23MHZ. It is obtained by the product of 2 pseudo-random codes PN1(t) and PN2(t). It can be controlled within an error of 3 meters, but it is only for the military.
GPS development history
In October 1957, the first artificial earth satellite, Sputnik Ⅰ, was successfully launched, and space-based navigation and positioning began; in
1958, the design of NNSS-TRANSIT, the Meridian satellite system, was started; in
1964, the system was officially put into operation; in 1967, the
system was declassified for civilian use
In 1973, the U.S. Department of Defense approved the development of GPS; in
the Gulf War in 1991, GPS was first used in actual combat on a large scale; in
1994, GPS was fully completed and put into
use; an artificial restriction strategy).
GPS service strategy
There are two types of GPS services:
SPS--Standard Positioning Service, for civilian use, with an accuracy of about 100M;
PPS--Precision Positioning Service, for military and authorized civilian users, with an accuracy of up to 10M.
Two measures to limit the accuracy of civilian positioning (to ensure that national interests are not infringed):
SA--choose availability, thinking that the measurement accuracy of ordinary users will be reduced, and the horizontal positioning accuracy is limited to 100M, vertical 157M (cancelled on May 1, 2005) ;
AS - Anti-spoofing.
GPS system composition
GPS system = space part + control part + user part

1. Space part
The GPS space part is mainly composed of 24 GPS satellites, including 21 working satellites and 3 spare satellites. 24 satellites operate on 6 orbital planes with a period of 12 hours. It is guaranteed that more than 4 satellites can be observed at any time and any place with an elevation angle of more than 15 degrees.
Main function: to send satellite signals for navigation and positioning.
Composition: 24 satellites = 21 working satellites + 3 spare satellites

2. Control part
The GPS control part consists of 1 main control station, 5 detection stations and 3 injection stations.
Composition: GPS control part = master control station (1) + monitoring station (5) + injection station (3)
Function: monitor and control satellite operation, compile satellite ephemeris (navigation message), keep system time.
Master control station: collect satellite data from various monitoring stations, calculate satellite ephemeris and clock correction parameters, etc., and inject satellites through injection stations; issue instructions to satellites, control satellites, and dispatch spare satellites when satellites fail.
Monitoring station: Receive satellite signals, detect satellite operation status, collect weather data, and transmit this information to the main control station.
Injection station: Inject the satellite ephemeris and clock correction parameters calculated by the master control station into the satellite.

Distribution:
Master Control Station: Located at Falcon Air Force Base in Colorado (Calorado), USA.
Injection stations: Ascendion, Atlantic Ocean; Diego Garcia, Indian Ocean; Kwajalein, Eastern Pacific.
Monitoring stations: 1 with the main control station; 3 with the injection station; the other one is in Hawaii (Hawaii), the western Pacific.

3. User part
GPS user equipment part includes GPS receiver and related equipment. A GPS receiver is mainly composed of a GPS chip.
For example, vehicle-mounted and ship-mounted GPS navigators, mobile devices with built-in GPS functions, and GPS surveying and mapping equipment are all GPS user equipment.
Composition: Mainly GPS receiver
Function: Equipment for receiving, tracking, transforming and measuring GPS signals, consumer of GPS system.
Principles of GPS Positioning

24 GPS satellites orbit the earth at an altitude of 12,000 kilometers above the ground in a period of 12 hours, so that at any time, more than 4 satellites can be observed at any point on the ground at the same time.
Since the position of the satellite is known accurately, in GPS observation, the distance from the satellite to the receiver, using the distance formula in the three-dimensional coordinates, using 3 satellites, can form 3 equations to solve the position of the observation point (X, Y, Z). Considering the error between the satellite's clock and the receiver's clock, there are actually 4 unknowns, X, Y, Z and the clock difference, so it is necessary to introduce the 4th satellite to form 4 equations to solve, so as to obtain the latitude and longitude.
In fact, the receiver can often lock more than 4 satellites. At this time, the receiver can be divided into several groups according to the constellation distribution of the satellites, each group has 4 satellites, and then the group with the smallest error is selected by the algorithm for positioning. Improve accuracy.
Due to errors in satellite orbits and satellite clocks, and the influence of the atmospheric troposphere and ionosphere on signals, the positioning accuracy of civilian GPS is only 10 meters. In order to improve the positioning accuracy, differential GPS (DGPS) technology is commonly used to establish a reference station (difference station) for GPS observation, and use the known precise coordinates of the reference station to compare with the observed value to obtain a correction number and release it to the outside world . After receiving the correction number, the receiver compares it with its own observation value, eliminates most of the errors, and obtains a more accurate position. Experiments show that using differential GPS, the positioning accuracy can be increased to 5 meters.

There are many ways to use GPS for positioning.
According to the position of the reference point, the positioning methods can be divided into:
(1) Absolute positioning. That is, in the protocol earth coordinate system, a receiver is used to determine the position of the point relative to the protocol earth barycenter, which is also called single point positioning. Here it can be considered that the reference point coincides with the center of mass of the protocol earth. The protocol earth coordinate system adopted by GPS positioning is the WGS-84 coordinate system. Therefore, the coordinates of absolute positioning are initially produced as WGS-84 coordinates.
(2) Relative positioning. That is, in the protocol earth coordinate system, more than two receivers are used to measure the relative position between the observation point and a certain ground reference point (known point). That is to measure the coordinate increment from the ground reference point to the unknown point. Since the ephemeris error is correlated with the atmospheric refraction error, these errors can be eliminated by taking the difference of observations, so the accuracy of relative positioning is much higher than that of absolute positioning.
According to the different motion states of the user receiver during operation, the positioning methods can be divided into:
(1) Static positioning. That is, during the positioning process, the receiver is placed on the station point and fixed. Strictly speaking, this static state is only relative, and usually means that the receiver does not change relative to its surrounding points.
(2) Dynamic positioning. That is, during the positioning process, the receiver is in motion.
GPS absolute positioning and relative positioning both include static and dynamic methods. That is, dynamic absolute positioning, static absolute positioning, dynamic relative positioning, and static relative positioning. According to the different principles of distance measurement, it can be divided into code measurement pseudo-range positioning, phase measurement pseudo-range positioning, differential positioning, etc.
The GPS activation method
first popularizes two concepts: Ephemeris and Almanac.
In order to shorten the satellite locking time, the GPS receiver needs to use the almanac and the time of the local location to predict the satellite operating status.
Both almanac and ephemeris are parameters that represent the operation of satellites. The almanac includes the approximate position of all satellites, which is used for satellite forecasting; the ephemeris is only the precise position of the satellite observed by the current receiver, which is used for positioning.
Generally, the cold start time is much longer than the hot start time. Taking the signal is good enough as an example, the hot start time of SirFIII is within 15 seconds, and the cold start time is within 42 seconds; in the case of weak signal, it takes longer.
1) cold (cold start): no previous location information, no ephemeris, no time estimation.
Cold start refers to the start-up process of starting the GPS in an unfamiliar environment until the GPS contacts the surrounding satellites and calculates the coordinates. The following situations are all cold starts: 1. When using for the first time; 2. When the battery is exhausted and the ephemeris information is lost; 3. When the receiver is turned off, the receiver is moved more than 1000 kilometers. That is to say, cold start is a forced start through hardware, because the internal positioning information has been cleared from the last GPS operation, the GPS receiver loses satellite parameters, or the existing parameters are too different from the actual received satellite parameters. , causing the navigator to fail to work, and the coordinate data provided by the satellite must be obtained again. Therefore, starting the navigation from the basement is 100% cold start, which is why it takes a long time to search for satellites from the basement.

2) warm (warm start): with almanac information, approximate location and time, but no ephemeris information.
Warm start refers to the start that is more than 2 hours away from the last positioning time, and the positioning time of Search Satellite is between cold start and hot start. If you have used GPS positioning the day before, then the first startup of the next day is a warm startup, and the last location information will be displayed after startup. Because the latitude, longitude and altitude before the last shutdown are known, but due to the long shutdown time, the ephemeris has changed, and the previous satellites cannot be received. Several satellites in the parameters have lost contact with the GPS receiver, and it is necessary to continue to search Satellites supplement location information, so the time to search for satellites is longer than that of hot start and shorter than that of cold start.

3) hot (hot start): With ephemeris information, the approximate time and position can be known, usually more accurate than the time and position information of warm start.
Hot start means to start the GPS without too much movement at the place where the last shutdown was done, but the time from the last positioning must be less than 2 hours. Through the software, start after some preparations such as saving and shutting down before starting.
GPS frequency band
1, L1 band - 1.57542GHz.
2. L2 band - 1.22760GHz.
3. L3 band - 1.38105GHz.
4. L4 band - 1.84140GHz (expected to be used in 2017).
GPS signal structure and accuracy
1. Signal structure
Generally, civilian GPS uses the L1 carrier of the GPS system with a frequency of 1575.42 MHz. On this carrier frequency, two different pseudo-random noise codes are loaded with phase modulation: C/A code and P code. The C/A code is a ranging code used for civilian use. The code length is 1023 code elements, that is, 1023 jumps from digital zero to digital 1. These 1023 code elements repeat 1000 times per second, that is, 1.023MHz, or every Beats once in a millionth of a second. The P code is a military code, the code length is very long, and the code speed is 10.23MHz, that is, it beats once every ten millionth of a second.
Since the GPS receiver calculates the time from the satellite to the receiver by comparing the jitter of the symbol, and then converts it into a distance, it is obvious that the time accuracy of the P code is 10 times higher, and the distance accuracy is also 10 times higher: modern signal processing The time accuracy of technical calculation code element beating is 1% of the code width, and the converted distance of one millionth of a second is 300 meters, and its one hundredth is 3 meters. The accuracy of the P code is one-tenth of this value, that is, 0.3 meters. In other words, when calculating the actual distance of a certain satellite from the receiver, the theoretical accuracy of the C/A code is 3 meters.
The receiver "knows" the distance between itself and the satellite, but cannot calculate its own position, because it does not know the position of the satellite when it transmits radio waves, so a 50Hz navigation message is loaded on the satellite carrier, this navigation message Including: satellite orbit parameters, clock parameters, orbit correction parameters, correction values ​​of atmospheric refraction of GPS signals, etc. The GPS receiver calculates the position of a certain satellite in space at a certain moment through these parameters, and then determines the distance between itself and the satellite, and then calculates its actual position. The total length of the navigation message is 1500 bits. In the case of 50Hz transmission, each cycle is 30 seconds.
If you compare the technical parameters of each model of GPS receivers of GARMIN and MAGELLAN, you will find that the cold start time of all models of GPS receivers (that is, GPS knows nothing about satellites and their location) is about 45 seconds. It's not that the manufacturer is lazy and doesn't want to improve the GPS performance, but it's already approaching the technical limit: 30 seconds out of 45 seconds are used to receive navigation messages, and the remaining 15 seconds are used to calculate the receiver's position.
When the GPS receiver calculates the forward speed, it uses the Doppler effect: after deducting the Doppler effect of the satellite moving relative to the receiver, the extra part is the moving part of the receiver, and this part is calculated , the forward velocity of the receiver can be calculated. This calculation method is more accurate than directly calculating the forward speed per second, and the accuracy is 0.5 kilometers per hour. You can do an experiment yourself: take some snaking routes or figure-eight routes that confuse the GPS while walking or driving, and see if the GPS speed display is continuous and accurate. Generally, the GPS direction display will be slightly delayed in this case. Because it is only calculated once every second, and the calculation of the direction is to carry out a weighted average with the direction of the previous few seconds.
2. Accuracy
There are many factors that will affect the accuracy of GPS:
The following is a brief table of GPS error introduction:
Satellite clock error: 0-1.5 meters
Satellite orbit error: 1-5 meters
Error introduced by the ionosphere: 0-30
The error introduced by the atmosphere: 0-30 meters The
noise of the receiver itself: 0-10 meters
Multiple reflections: 0-1 meters
The total positioning error: about 28 meters
It can be seen that the main GPS positioning errors come from the ionosphere and the atmosphere. It is because the gas molecules in the ionosphere and the water vapor molecules in the atmosphere will refract the GPS microwave signal, making it slightly curved in the route from the satellite to the receiver, causing the receiver to treat the curved path as a straight path , thus introducing errors. This phenomenon is more pronounced when microwave signals cross the ionosphere and atmosphere obliquely, because the time and distance traveled by microwave signals are longer.
Features of GPS system
(1) All-weather;
(2) Global coverage;
(3) Three-dimensional constant speed and high precision;
(4) Fast, time-saving and high efficiency;
(5) Wide application and multi-function.
Other satellite navigation systems
GLONASS (Global Orbiting Navigation Satellite System, Glona, ​​Russia)
— Composed of 24 satellites, with an accuracy of about 10 meters, it is used for both military and civilian purposes. It is designed to expand its service scope to the whole world by the end of 2009.

Galileo-ENSS (European Navigation Satellite System, European Galileo Project)
— Composed of 30 satellites, the positioning error does not exceed 1 meter, mainly for civilian use. The first test satellite was successfully launched in 2005. The positioning service has been opened in 2008.

COMPASS (Beidou Navigation System, China)
— is composed of 5 geostationary orbit satellites and 30 non-geostationary orbit satellites. China plans that the "Beidou" system will cover the Asia-Pacific region around 2012 and the whole world around 2020. It has successfully launched ten Six Beidou navigation satellites.

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