First, the advantages of GPS measurement
GPS is the abbreviation of Global Positioning System. GPS survey is a new survey technology using satellite positioning. Compared with the traditional measurement technology, it has the following advantages:
1) has a wide range of uses. GPS signals can be used in navigation, vehicle guidance, missile guidance, precise positioning, engineering survey, dynamic observation and so on.
2) simple observation. When measuring, the surveyor only needs to place the antenna unit of GPS receiver on the measuring station, turn on the power supply and start the receiving unit; After the measurement, just measure the antenna height and turn off the power supply, and then complete the field data collection. In addition, GPS is an all-weather measurement system, which can complete field work with less manpower and material resources in a short time.
3) high precision. Using carrier phase measurement for relative positioning, the relative positioning accuracy can reach (5 mm+ 1× 10-6 d) (d is proportional error), and the observation time is less than1h. If the fast positioning method is adopted, the observation time is only about 2min, and the positioning accuracy of centimeter level can be achieved.
4) High economic benefit. GPS survey does not require interstation visibility, which can save the cost of standard making for conventional survey. Moreover, due to the high accuracy of GPS measurement and short working time, the economic benefit is very remarkable.
Second, the GPS system
The GPS system includes the following three parts.
1.GPS satellite constellation (space part)
The GPS system includes 24 satellites, which are evenly distributed in six approximately circular orbits, and the included angle between orbital planes is 60. There are four satellites in each orbit, and the orbit is about 20200km from the ground. The time for a satellite to orbit the earth is 12h, and signals from at least four satellites can be received anywhere on the earth at any time.
Each GPS satellite continuously emits two radio waves with different frequencies (L 1= 1575.42MHz, L2= 1227.60MHz). Many kinds of signals are modulated on the carrier, among which the most important ones are ranging code (P fine code, C/A coarse code) and navigation message. The ranging code is used to measure the distance from the satellite to the ground point receiver; Navigation information is used to calculate the orbit parameters of satellites.
2. Ground monitoring system (ground control part)
Whether all kinds of equipment on the GPS satellite work normally and whether the satellite runs along the predetermined orbit is monitored by the ground monitoring system. The ground monitoring system includes a main control station, three injection stations and five monitoring stations, which are distributed in US military bases in the United States and other parts of the world.
GPS satellite is a dynamic known point, which is calculated according to the ephemeris (parameters describing satellite motion and orbit) sent by the satellite. The ephemeris broadcast by each GPS satellite is provided by the ground monitoring system.
In addition, the ground monitoring system also monitors the time of each satellite, calculates their relevant correction values, and then sends them to users through navigation messages to ensure that all satellites are in the same GPS time system.
3.gps receiver
The main function of GPS receiver is to decode and separate navigation messages and measure phase and pseudorange. Structurally, the GPS receiver is mainly composed of five units: antenna and preamplifier; The signal processing unit is the core of the receiver; Control and display unit; A storage unit; Power supply unit.
The GPS receiver is mainly used in the following two aspects:
1) static positioning. The user antenna is fixed in the process of tracking GPS satellites. The receiver measures the propagation time of GPS signal with high precision, and together with the known position of GPS satellite in orbit, the three-dimensional coordinates of fixed user antenna can be calculated. The latter can be a fixed point or a GPS network composed of several points. Static positioning is characterized by large redundant observation, strong reliability and high positioning accuracy.
2) Dynamic positioning. User antenna on carrier (vehicle, ship, plane, etc.). In the process of tracking GPS satellites, the receiver uses GPS signals to measure the state parameters of the moving carrier in real time. Dynamic positioning is characterized by measuring the state parameters of the moving carrier point by point, with few redundant observations and low accuracy.
There are many kinds of GPS receivers, including one single-frequency receiver with one carrier frequency (L 1) and two dual-frequency receivers with two carrier frequencies (L 1L2). Single-frequency receiver is cheap, and dual-frequency receiver can eliminate some effects of atmospheric delay. For precise measurement with side length longer than 10km, it is best to use a dual-frequency receiver, while for general control measurement, a single-frequency receiver is enough.
Third, the basic principle of GPS positioning
There are two basic observations in GPS measurement: pseudorange and carrier phase. The GPS receiver measures the propagation time of satellite signal (ranging code) from the satellite to the receiver, and then multiplies it by the propagation speed of electromagnetic wave to get the pseudo-range from the satellite to the receiver. However, because the propagation time includes the synchronization error between the satellite clock and the receiver clock and the delay error of the ranging code propagating in the atmosphere, the obtained pseudo-range is not equal to the geometric distance between the satellite and the station. Carrier phase measurement is to mix the received satellite signal with the receiver's own signal, and then carry out phase measurement. The accuracy of pseudorange measurement is about 1% of the symbol length of ranging code, about 30cm for P code and 3m for C/A code. The wavelength of carrier is much shorter (19cm and 24cm respectively), so the measurement accuracy of carrier phase is generally1~ 2 mm. Because phase measurement can only measure the part of the carrier whose wavelength is less than one wavelength, the measured phase can be regarded as a pseudo-range of an unknown integer multiple wavelength.
In GPS positioning, the satellite is regarded as a dynamic known control point, and the position of the receiver can be obtained by using the measured distance to rendezvous in space.
GPS positioning includes single point positioning and relative positioning.
The method of independently determining the absolute position of the point to be fixed in WGS-84 world geodetic coordinate system is called single point positioning or absolute positioning. Its advantages are that only one receiver is needed for independent positioning; The organization and implementation of outdoor observation is relatively free and convenient, and the data processing is relatively simple. However, the results are significantly affected by satellite ephemeris error and atmospheric delay error in the process of satellite signal propagation, and the positioning accuracy is poor, generally tens of meters. Single point positioning has a wide application prospect in the fields of ship and aircraft navigation, geological and mineral exploration, reef positioning, marine fishing, national defense construction and low-precision measurement.
Relative positioning is a positioning method to determine the relative position (three-dimensional coordinate difference) between several receivers tracking the same GPS satellite signal synchronously. In relative positioning measurement, many errors have the same or almost the same influence on synchronous observation stations. Therefore, in the calculation, these errors can be offset or greatly weakened, so as to obtain a high-precision relative position, with a general accuracy of several millimeters to several centimeters. Compared with single-point positioning, the organization and implementation of field observation and data processing are more complicated. Relative positioning is widely used in precise positioning fields such as geodesy, engineering survey and crustal deformation monitoring.
Fourth, the main error sources of GPS relative positioning
1) clock error. The clock error on the satellite and the clock error on the receiver are the main errors in GPS measurement.
2) Satellite position error. The position of the GPS satellite is calculated according to the ephemeris sent by the satellite, and the average error is about 20 mm Let dr be the satellite position error, and its influence on the relative positioning can be approximately estimated by the following formula, namely
Architectural engineering survey
Where: d- the distance between two receivers;
DD- relative position error;
S- The distance from the receiver to the satellite is about 20,000 kilometers.
For example, if dr=20m, the influence on the two-point phase is 1× 10-6.
3) The influence of atmospheric delay. Satellite signals can only reach the receiver through the atmosphere, so the atmosphere has a delay effect on satellite signals (affecting their propagation speed). The atmosphere from the ground to a height of about 50km is called the troposphere, and the delay of the troposphere is a function of air temperature, air pressure and humidity, which can be corrected by meteorological elements measured by the station. The atmosphere above 50km is called ionosphere, and its influence is corrected by the measurement results of dual-frequency receiver.
4) Multipath error. The signal that reaches the receiver after being reflected by the surface of some objects and the signal directly from the satellite are superimposed on the receiver, which makes the measurement error. Its influence is related to the surrounding environment of the antenna. Therefore, choosing a suitable station is the main measure to reduce this error.
5) Observation error. The observation error is related to the signal wavelength used in the measurement. Using C/A code and P code for pseudo-range observation, the errors are 3m and 0.3 m respectively. The measurement error of carrier phase is 65438±0 ~ 2mm.
Generally speaking, the accuracy of GPS relative positioning can be expressed as
σ2=a2+b2 D2 (6-26)
Where: σ-relative positioning error;
A- fixed error part;
B- proportional error part;
D—— the distance between two stops.
Review exercises
1. What are the field work of theodolite wires?
2. What problems should be paid attention to when selecting traverse points?
3. What are the similarities and differences between the calculation of traverse and attached traverse?
4. According to the known data in Table 6- 1 1, calculate the coordinate values of each point of the closed traverse.
Table 6- 1 1 closed traverse coordinates