Some articles about GPS, A-GPS, here to make a record!

GPS (global positioning system), Global Positioning System:
GPS consists of three parts:
1. Space:
The spatial part of GPS is composed of 24 working satellites. It is located over 20 200km of the earth's surface, evenly distributed on 6 orbital planes (4 in each orbital plane), and the orbital angle is 55 °. In addition, three active backup satellites are running in orbit. The distribution of satellites allows more than four satellites to be observed anywhere and at any time in the world, and the navigation information stored in the satellites can be pre-stored. Due to atmospheric friction and other problems, GPS satellites gradually reduce the accuracy of navigation over time.

2. Ground Control:
The ground control system is composed of a monitoring station, a master monitoring station, and a ground antenna, the main control station is located in colorado spring, Colorado ). The ground control station is responsible for collecting information sent from satellites and calculating satellite calendars, relative distances, atmospheric correction, and other data.

3. user devices:
The user's device is the GPS signal receiver. Its main function is to capture the satellites to be tested selected according to a certain satellite cutoff angle and track the operation of these satellites. After the receiver captures the tracked satellite signal, it can measure the pseudo distance and distance change rate between the receiving antenna and the satellite, and call out satellite orbit parameters and other data. Based on the data, the micro-processing computer in the receiver can locate and calculate based on the positioning solution, and calculate the longitude and latitude, height, speed, and time of the user's location. The receiver hardware, internal software, and post-processing software package of GPS data constitute a complete GPS User device.
The structure of the GPS receiver is divided into two parts: the antenna unit and the receiving unit. Generally, the receiver uses two types of DC power supply: In-device and out-of-device. The purpose of setting the internal power supply is to observe the power without interruption when the external power supply is replaced. The battery is automatically charged when the external power supply is used. After the instance is shut down, the battery in the instance powers the RAM memory to prevent data loss. At present, the size of various types of receiver machines is getting smaller and lighter, making it easy for field observation. Second, it is the user receiver. There are two types of single frequency and dual frequency. However, due to the price, most of the users buy single frequency receivers.

GPS principle:
The basic principle of the GPS navigation system is to measure the distance between a satellite at a known position and a user's receiver. Then, the specific position of the receiver can be known based on the data of multiple satellites. To achieve this goal, the satellite location can be found in the satellite calendar based on the time recorded by the satellite clock. The distance from a user to a satellite is measured by recording the time the satellite signal was transmitted to the user, and then multiplied by the speed of light (due to the interference of the atmosphere in the electron, this distance is not the real distance between the user and the satellite, but a pseudo distance (PR): When the GPS satellite works normally, the system constantly uses a pseudo-random code (pseudo code) consisting of 1 and 0 binary code elements to send navigation messages. There are two types of pseudo-code used by the GPS system: Civil c/a code and military P (y) code. The C/A code frequency is 1.023 MHz, and the repetition period is one millisecond. The Code interval is 1 microsecond, equivalent to 300 m. The P code frequency is 10.23 MHz, the repetition period is 266.4 days, and the code interval is 0.1 microseconds, equivalent to 30 m. The Y-Code is based on the p-code, which provides better confidentiality. Navigation messages include satellite calendars, working conditions, clock correction, temporal Delay Correction, atmospheric refraction correction, and other information. It is decoded from satellite signals and transmitted at a carrier frequency of 50b/s. Each main frame of a navigation message contains five sub-frames with a length of 6 s. Each of the first three frames has 10 character codes, which are repeated every thirty seconds and updated every hour. The last two frames are 15000b in total. The content of a navigation message mainly includes three data blocks: telemetry code, conversion code, 1st, 2, and 3. The most important one is the calendar data. When a user receives a navigation email, the distance between the satellite and the user can be obtained by extracting the satellite time and comparing it with his/her own clock, then, the satellite calendar data in the navigation message is used to calculate the position of the satellite when the satellite sends the electronic message. The user can know the location and speed in the WGS-84 coordinate system.
It can be seen that the function of GPS navigation system satellites is to continuously launch navigation messages. However, since the clock used by the user's receiver cannot always be synchronized with the satellite clock, except for the user's 3D coordinates x, y, and z, we also need to introduce a delta T, that is, the time difference between the satellite and the receiver, as the unknown, and then use the four equations to resolve the four unknowns. Therefore, if you want to know where the receiver is, you must be able to receive signals from at least four satellites.

The GPS receiver can receive accurate to Nanosecond-level time information that can be used for timing. It is used to forecast the star calendar of the approximate position of the satellite in the next few months; it is used to calculate the broadcast calendars of the Satellite coordinates required for positioning. The accuracy ranges from several meters to dozens of meters (different satellites may change at any time) and GPS system information, such as satellite conditions.
The distance from the satellite to the receiver can be obtained through the GPS receiver's code measurement. Due to the error of the satellite clock containing the receiver and the atmospheric transmission error, it is called a pseudo distance. The pseudo-distance measured for the 0a code is called the UA code pseudo-distance, and the accuracy is about 20 meters. The pseudo-distance measured for the P code is called the P code pseudo-distance, and the accuracy is about 2 meters.
The GPS receiver decodes the received satellite signals or uses other technologies. After removing the information modulated on the carrier, the carrier can be restored. Strictly speaking, the carrier phase should be called the carrier frequency phase, which is the difference between the carrier phase of the satellite signal received by the Doppler frequency shift and the signal phase generated by the local oscillator of the receiver. Generally, the measurement is based on the time period determined by the receiver clock. When the satellite signal is tracked, the change value of the phase can be recorded, however, the initial phase values of the receiver and the satellite oscillator at the start of the observation are unknown, and the phase Integers of the starting period are unknown. That is, the integer of the starting period can only be used as parameters in data processing. The accuracy of the phase observations is higher than that of the millimeter, but the premise is to solve the integer ambiguity. Therefore, the phase observations can be used only when the relative positioning and continuous observations exist, to achieve positioning accuracy better than meters, only phase observations can be used.
Based on the positioning method, GPS positioning is divided into single point positioning and relative positioning (differential positioning ). Single point positioning is to determine the receiving position based on the observation data of a receiver. It can only use the pseudo-distance observation volume and can be used for rough navigation and positioning of vehicles and ships. Relative positioning (differential positioning) is a method to determine the relative position between observation points based on the observation data of more than two receivers. It can use both pseudo-distance observation volume and phase observation volume, phase observations should be used for relative positioning in geomeasurement or engineering measurement.
GPS observations contain satellite and receiver clock difference, atmospheric transmission delay, multi-path effect, and other errors, which are also affected by the Satellite Broadcast calendar error during positioning and computation, the positioning accuracy is greatly improved because most common errors are offset or weakened during relative positioning. The dual-band receiver can offset the main part of the atmospheric Fluctuation Based on the observed amount of the two frequencies, when the precision requirement is high and the distance between receivers is relatively long (there is a significant difference in the atmosphere), dual-band receivers should be used.

GPS classification:
Classification by receiver carrier frequency
Single-Frequency Receiver
A single-frequency receiver can only receive L1 carrier signals and locate carrier phase observations. Due to the inability to effectively eliminate the influence of the elliptical delay, the single-frequency receiver is only applicable to precision positioning of short baselines (<15 km.
Dual-band Receiver
Dual-band receivers can receive both L1 and L2 carrier signals. The dual-band delay can be used to eliminate the delay of the electromagnetic wave signal. Therefore, the dual-band receiver can be used for precise positioning up to thousands of kilometers.

By number of receiver Channels
The GPS receiver can receive signals from multiple GPS satellites at the same time. In order to separate signals from different satellites, the GPS receiver can track, process, and measure satellite signals, devices with such functions are called antenna signal channels. According to the receiver's channel types, it can be divided:
Multi-Channel Receiver
Sequential Channel Receiver
Multi-Channel Receiver

Difference between GPS function A-GPS (assist-GPS:
A-GPS (GPS) and GPS scheme, also need to add GPS receiver module in the mobile phone, and modify the mobile phone antenna, but the mobile phone itself does not calculate the location information, the GPS location information data is transmitted to the mobile communication network, and the location is calculated by the Network Locating server. Meanwhile, the mobile network generates auxiliary data according to the GPS Reference Network, for example, the differential correction data and satellite operation status are transmitted to the mobile phone, and the approximate location of the mobile phone and the location information of the residential area are found in the database to the mobile phone. Then, the mobile phone can quickly capture the GPS signal, the first capture time will be greatly reduced, generally only a few seconds. Does not need to be like the first GPS capture time may be 2? 3 minutes. The precision is only a few meters, which is higher than that of GPS. Qualcomm's GpsOne uses the A-GPS solution.

There are two ways to use mobile phone assistance and mobile phone autonomy: (1) mobile phone-assisted GPS positioning. This solution transfers most of the functionality of a traditional GPS receiver to a network processor. This method requires antennas, RF units, data processors, and other devices. The GSM network sends a series of extremely short auxiliary information to the mobile phone, including the time, the visible satellite list, the satellite signal Doppler parameters and the Code Phase search window. These parameters help the built-in GPS module reduce the acquisition time of GPS signals. The auxiliary data comes from the pseudo-distance data generated after processing by the mobile phone GPS module, and can last for several minutes. After receiving the pseudo distance data, the corresponding network processor or locating server can roughly estimate the location of the mobile phone. After necessary corrections are added to the GSM network, the positioning accuracy can be improved. (2) mobile phone GPS Positioning method. This type of mobile phone contains a full-featured GPS receiver with all functions of the mobile phone in the (1) mode, plus the satellite location and mobile phone location computing functions. At the beginning of the calculation, more data is required than the mobile phone assistance method. The data can be updated for more than four hours or as needed, it usually includes time, reference location, satellite calendar, and time verification parameters. If some applications require higher precision, they must send a differential GPS (DGPS) signal to their mobile phones at an interval of about 30 s. The DGPS signal is valid in a very wide region and can serve a wide region with a reference receiver as the center. The final location information is calculated by the mobile phone itself. If needed, this location information can be sent to any other application.

 

Some useful links:

Basic Principles of GPS Positioning

Http://mol365.net/post/2010/04/15/GPSe5ae9ae4bd8de59fbae69cace58e9fe79086.aspx

Basic Principle of GSM cell base station positioning

Http://mol365.net/post/GSMe89c82e7aa9de59fbae7ab99e5ae9ae4bd8de59fbae69cace58e9fe79086e6b585e69e90.aspx

Analysis on the basic principles of agps Positioning

Http://mol365.net/post/AGPSe5ae9ae4bd8de59fbae69cace58e9fe79086e6b585e69e90.aspx