by 
ZHOU
This article describes the coordinate systems, terminology, and conversion methods that are common in GNSS electronic driving test systems.
The following documents are indispensable for the application of this document. For dated references, only the dated version applies to this document. For undated references, the latest edition (including all amendments) applies to this document.
GB/T 17159-2009 Geodetic terminology
GB/T 18314-2009 Global Positioning System (GPS) Measurement Specifications
GB/T 19391-2003 Global Positioning System (GPS) terms and definitions
GB/T 22021-2008 National Geodetic Basic Technical Regulations
GB/T 28588-2012 Technical specifications for the continuous operation of the base station network of the global navigation satellite system
The following terms and definitions apply to this document.
Approximate to the shape and size of the earth, and its surface is an ellipsoid of the equipotential surface.
The rotating earth ellipsoid with a certain size and positioning parameters is the most suitable for the geoid of a certain area.
In the space Cartesian coordinate system, the xy plane coincides with the Earth's equatorial plane, the x-axis points to the 0° longitude direction, the y-axis points to the east longitude 90° direction, and the z-axis points to the geographic north pole perpendicular to the equatorial plane, forming a right-handed coordinate system. The xyz axis rotates with the earth, and the fixed direction is no longer described in the inertial space.
The earth coordinate system with the center of the ellipsoid as the origin, the starting meridional plane and the equatorial plane as the reference plane. Due to the irregular shape of the Earth, a standard ellipsoid is used in the GPS/Beidou to approximate the description, which makes it easy to calculate the longitude, latitude and altitude of the receiver. The ellipsoid can be represented by two parameters, the long semi-axis and the flatness, and other parameters such as the short half-axis, the eccentricity, and the second eccentricity can be derived from these two parameters.
As shown in Figure 3-2, the orange curve is the reference ellipsoid, the green curve is the surface (or GPS antenna position), h is the ellipsoid height, and the GPS positioning solution (ECEF) can be converted to latitude and longitude under the WGS84 ellipsoid. The ellipsoid is high h. The blue curve is the geoid Geoid, which corresponds to the global mean sea level (Mean Sea Level) under the least squares, and the constant gravity potential plane. Due to the uneven distribution of the earth's density, the gravitational potential varies from place to place, so the geoid is not Regular surfaces (Figures 3–3). The height H relative to the geoid is called orthometric height, or mean sea level high (MSL). The height N of the geoid is the height of the geoid relative to the ellipsoid, also known as the elevation anomaly. The relationship between the ellipsoid height h, the altitude H and the elevation anomaly is h=H+N.
图3-2 椭球高/海拔高/大地水准面高
Figure 3-3 Earth gravity potential
The station center coordinate system is also called the northeast sky coordinate system ENU, which is used to indicate the motion law of other objects centered on the observer. Taking the center of the station as the origin of the coordinate system, the z-axis coincides with the ellipsoid normal, the upward is positive, y coincides with the short semi-axis of the ellipsoid (north direction), and the x-axis coincides with the long semi-axis of the ellipsoid (eastward).
Since the earth is a sphere, in order to represent on the map, the ellipsoid is divided along the meridian into a plurality of narrow zones of equal difference, and then the bands are respectively projected onto the plane. Generally, the coordinates of a certain longitude on the equator are used as the origin, and the coordinates after projection are calculated. In order to make the coordinates after projection a positive number, a constant of 500 km is usually added to the horizontal axis.
Figure 3-4 Gaussian projection
Contains two meanings of coordinate system transformation and ellipsoid reference transformation. In the measurement data processing process, the applicable transformation model and transformation method are adopted, and the spatial point is converted from the coordinates under one reference ellipsoid reference to the coordinates under another coordinate system, such as WGS84 to Beijing 54. The coordinate conversion process is the solution process of the conversion parameters.
Under the same ellipsoid reference, the different coordinate representations of the spatial points are transformed. It includes the mutual conversion of the geodetic coordinate system and the spatial rectangular coordinate system, the conversion between the space rectangular coordinate system and the station center coordinate system, and the Gaussian projection coordinate forward and backward calculation.
The coordinate transformation of spatial points between different reference ellipsoids.
When the two coordinate systems are converted, the coordinates of the origin of the new coordinate system in the original coordinate system.
When the two coordinate systems are converted, the coordinate axes in the original coordinate system are rotated to the left to the angles at which the axes of the coordinate system are sequentially rotated when the coordinate axes corresponding to the new coordinate system are coincident or parallel.
The Bestposa or GGA statement outputted in the receiver contains the latitude, longitude, altitude and elevation anomalies in the WGS84 coordinate system. The latitude and longitude of the output WGS84 coordinate system is a common format, which is compatible with all manufacturers, and the positioning results are consistent at the same point. It should be noted that the latitude and longitude in GGA is the degree, and the latitude and longitude units in Bestposa and HPD are degrees.
The Trimble receiver can set the coordinate system and projection parameters through Configuration Tools, Figure 4 1 is the ellipsoid and translation parameters set to the Beijing54 coordinate system. After changing to the Beijing54 coordinate system, the GGA coordinates and PJK coordinates of the Trimble receiver will be switched to the coordinates of Beijing54, which is different from the GGA under the original WGS84 coordinates.
Figure 4-1 Trimble coordinate system parameter settings
1. Query the BJ54 coordinate system parameters of the equipment used by the customer to calibrate the map.
2. Configure the coordinate system translation parameters and configure the data in the red box above to our device.
SET SHIFTDATUM -31.400 144.300 81.200
3. Select the BJ54 coordinate system
DATUM BJ54
4, save the relevant configuration
SAVECONFIG
The GGA coordinates of the receiver output of Sinan are always WGS84 coordinates, the PJK coordinate output is Gaussian projection coordinates, and the projection parameter setting/query is performed by set/log pjkpara. It includes six parameters: the long axis of the ellipsoid, the flatness, the central latitude, the central meridian, the vertical axis plus a constant, and the horizontal axis plus a constant. The center latitude and central meridian define the position of the coordinate origin.
1. Log pjkpara queries the PJK parameters of the Sinan receiver.
2. Configure the PJK parameters to our device.
SET PJKPARA 6378245 298.3 0 121 0 500000
3, save the relevant configuration
SAVECONFIG
The receiver output of the satellite network HPD protocol includes the GGA coordinates in the WGS84 coordinate system and the northeast sky coordinates relative to the base station. The actual management and the test only use the northeast sky coordinates. Therefore, the location of the base station cannot be changed when it is adapted, otherwise the map needs to be panned or redrawn.
When adapting, output the HPD statement through LOG GPHPD ONTIME 0.2 and then save the configuration with SAVECONFIG.
For on-site debugging, you need to prepare the receiver and BYNAV receiver that need to be adapted. Follow the steps below to step through.
1. First install the receiver that needs to be adapted. The vehicle stops at an open position, queries the coordinate conversion parameters, waits for the fixed solution, and configures it to simultaneously output the GGA/Bestposa/HPD statement (select the receiver type according to the needs). , to ensure that it simultaneously outputs the WGS84 geodetic coordinates), the serial debugging assistant records the positioning result of about 10s.
2. Other configurations are unchanged. Replace the receiver with our receiver, check the satellite's satellite status, and the differential data. After waiting for the fixed solution, first compare the WGS84 coordinates with the previous one. Other than the Trimble receiver may be inconsistent (GGA may not be WGS84 coordinates), other types of receivers should be consistent. Since the RTK positioning accuracy is 1~2cm, the latitude and longitude difference is the seventh decimal place, and the elevation difference is 1~2cm.
3. After the WGS84 coordinates are consistent, set the coordinate conversion parameters to our receiver, configure the output statement, then save the configuration, restart the receiver, and compare the output of the PJK statement (if any) with the first step record. The projected coordinate difference is considered to be consistent from 1 to 2 cm.
