GLOBAL Positioning System, referred to as GPS, that is, global satellite
positioning system, has been widely used in recent years, and has generated
considerable demand for GPS products. And with the improvement of the level of
science and technology and the continuous development of application directions,
GPS will undoubtedly penetrate into people's daily life quickly.
The GPS positioning system we often mention is designed and controlled by
the US military. In addition, my country's Beidou dual-star positioning system
is silently contributing to my country's modernization; Russia's GLONASS system
has also had a glorious history; the Galileo satellite positioning system
designed by the European Union is compatible with the currently widely used GPS
system, In a few years time will add a brighter page to the global positioning
system.
The GPS system consists of three parts: the space part, the control part
and the user part.
The space part is a general term for GPS satellites. The average height of
the artificial satellite is about 20200Km, the orbit is an ellipse, and the
earth is located at a focus of the ellipse; the operation period is about 12
hours. Nearly 30 navigation satellites are unevenly distributed on the six
orbital planes with an inclination of about 55°, some of which are spare
satellites. The US military can adjust the number of working satellites through
the ground control part. In the GPS system, the GPS satellite is a dynamic known
point, and all the navigation and positioning information of the user terminal
is calculated based on the "ephemeris" sent by this dynamic known point. GPS
ephemeris is actually a series of real-time state parameters describing the
movement and orbit of GPS satellites. The broadcast ephemeris received by the
civil GPS module is the forecast ephemeris directly broadcast to the user by the
GPS satellite through the navigation message in the spread spectrum
communication mode for real-time data processing, and broadcasts the civil
pseudo ephemeris at different rates on different carriers. Random code C/A code
ephemeris and military P code ephemeris.
For the whole GPS system, the ground control part is actually the core of
the whole system. The ephemerides used for navigation and positioning broadcast
by all GPS satellites are provided by 5 monitoring stations distributed on the
ground. The ground system is responsible for monitoring GPS signals, collecting
data, calculating and injecting navigation messages, status diagnosis, orbit
correction, etc. It is the massive data processing of the ground monitoring
system that enables the GPS system to operate accurately.
The GPS positioning module we often say is called the user part. It
receives and demodulates the broadcast C/A code signal of the satellite like a
"radio", and the intermediate frequency is 1575.42MHz. The GPS module does not
broadcast signals and belongs to passive positioning. By calculating the
pseudo-distance with each satellite, the four parameters of longitude, latitude,
altitude and time correction of the receiver are obtained by using the distance
rendezvous method. The module for initial positioning requires at least 4
satellites to participate in the calculation, which is called 3D positioning. 3
satellites can achieve 2D positioning, but the accuracy is not good. The GPS
module continuously outputs the positioning information and auxiliary
information in NMEA format through the serial communication port for the
receiver to choose the application.
The most expensive indicators of GPS module performance mainly include
receiving sensitivity, positioning time, position accuracy, power consumption,
time accuracy, etc. The module startup positioning time is very different in
different startup modes. Generally speaking, the cold start time refers to the
situation that the module does not save any data that is helpful for
positioning, including ephemeris, time, etc., generally nominally within 1
minute; the warm start time means that there are newer satellites inside the
module Ephemeris (generally no more than 2 hours), but the time deviation is
very large, generally within 45 seconds; the hot start time refers to the case
where the shutdown does not exceed 20 minutes, and the RTC time error is very
small. Typically nominally within 10 seconds; the recapture time is like a car
going through a tunnel and recapturing a satellite as it exits the tunnel.
Generally nominal within 4 seconds.
If the module is placed for a long time after positioning, or the module is
transported to a place hundreds of kilometers away after positioning, there is
an ephemeris inside the module, but this ephemeris is wrong or has no reference
significance. In these cases, the positioning time can take several minutes or
more. Therefore, the general GPS module should clear the ephemeris and other
data inside the module when it leaves the factory, so that the customer can
quickly locate the module by cold start after getting the module.
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