Dynamic positioning technology and data processing for airborne 3D remote sensing You Hongjian, Li Shukai (Institute of Remote Sensing Application, Chinese Academy of Sciences, Beijing 100101) Images and DEM, this article is an original airborne 3D remote sensing image developed by China itself The characteristics and requirements of the dynamic GPS positioning technology in the mapping system are analyzed, and the GPS positioning data processing and algorithm flow applied to 3D remote sensing are discussed.
1 Introduction Remote sensing, as an important technical means to obtain information on the earth's surface, has been widely used and developed at home and abroad. Since the 1990s, people have put forward higher requirements for the positioning of remote sensing, because traditional remote sensing depends on ground control and three-dimensional observation or background maps to obtain positioning information. It has a long period and low efficiency, making it difficult to meet the requirements. With the global positioning system GPS entering full operation stage (FOC) and the application of high repetition frequency laser ranging technology, GPS positioning technology, inertial navigation technology (INS) and laser ranging technology are integrated to obtain an airborne scanning laser terrain system It has become one of the research hotspots in the remote sensing community at home and abroad, and provides positioning information for remote sensing images obtained on the same machine or synchronously. Remote sensing operation efficiency. For example, AIMS, FLIMAP, and Jiatong in the United States generally use GPS, INS, and laser ranging to form an air-ground positioning system, which can quickly obtain a high-precision digital elevation model (DEM). Geo-encoding video. In the early 1990s, Li Shukai innovatively proposed that laser ranging and scanning imagers be strictly matched in hardware to form a combined scanning ranging imaging remote sensor, and then integrated with GPS and INS to form a 3D remote sensing image mapping system. Plan information acquisition and With the support of the processing technology theme (308 theme), the development of the principle prototype has been completed in 1996, and it has entered the development of a practical system since 1998. The biggest feature of the 3D remote sensing image mapping system is that it can quickly process DEM and geo-encoded images in real time without ground control.
At present, almost all the airborne scanning laser terrain (or remote sensing) systems at home and abroad use GPS as the technical means of air positioning. This is because GPS has the characteristics of high positioning accuracy, stability and reliability, and all-weather availability. In fact, as early as the late 1980s, people began to study the application of GPS in photogrammetry and achieved success, and now GPS-assisted aerial triangulation has been used in production operations. In the airborne 3D remote sensing image mapping system, the GPS positioning system is a very important remote sensor to provide positioning information. Unlike other GPS applications, GPS has some special requirements for the airborne 3D remote sensing mapping system, such as synchronous data collection, GPS The position of the antenna and the optical center of the remote sensor do not coincide, the data requirements under airborne flight conditions, etc. In addition, GPS data processing also has its particularity. The following is a detailed description of the dynamic GPS positioning technology and data processing in the airborne 3D remote sensing mapping system.
2 GPS positioning technology suitable for airborne three-dimensional remote sensing and its characteristics 2.1 Development of GPS positioning technology GPS positioning system is the second largest space investment in the United States following the Apollo moon landing plan. GPS has entered remote sensing in March 1995 The journal is fully operational, so it can provide users around the world with seamless positioning 24 hours a day. For military purposes, the United States implements the SA policy on GPS, so that single-machine positioning can only reach 100 m (only for low-precision navigation and positioning), and high-precision positioning and navigation must use differential technology to improve accuracy. It generally uses two One or more receivers form a differential system. Because the GPS system has two measurement values ​​of pseudorange and carrier phase (the measurement accuracy is in the decimeter and millimeter level respectively), the difference also includes the pseudorange difference (the positioning accuracy can reach the meter level or even the submeter level) and the phase difference Positioning accuracy in centimeters). In actual operation, a data communication chain should be set up to form a real-time differential system. Generally, UHF and VHF digital radio stations can be selected. At present, the pseudo-range real-time differential has been widely used. Since 1994, the real-time carrier phase differential RTK (Realtime Kinematic) system has also begun to enter the commercial application stage, and its real-time positioning accuracy has reached several centimeters. Therefore, the positioning accuracy of differential GPS technology can fully meet the accuracy requirements of remote sensing applications.
2.2 Requirements and characteristics of airborne 3D remote sensing for GPS positioning Airborne 3D remote sensing is a dynamic flight operation, and it is required to obtain high-precision remote sensor 3D coordinates in the air as much as possible. Therefore, GPS positioning in airborne 3D remote sensing should have the following characteristics and performance: High-precision differential GPS positioning for airborne three-dimensional remote sensing requires a differential GPS system. To this end, a high-performance GPS receiver (equipped with a geodetic GPS static antenna) must be set up as a reference station at a known point on the coordinates of the survey area. Set up a data transmitting station to send the necessary data to the receiving station on the operating aircraft.
In order to ensure the accuracy of real-time GPS positioning, the reference station should generally be set up at the center height of the survey area, and away from high-voltage lines, water surface and large structures to prevent interference sources that may generate multi-path signals to the GPS.
Synchronous connection of GPS data and remote sensing data In order to apply GPS data to the positioning of airborne three-dimensional remote sensing, it is necessary to directly connect GPS data and remote sensing data. Generally, a GPS receiver is used to record the pulse time of the remote sensor in real time, so onboard The GPS receiver should have an Event Mark function that records the time of an external pulse signal in real time (currently most measurement GPSs can be equipped with this function option). In this way, when the scanning range remote sensor scans to a certain pixel on the ground, it will generate a synchronous pulse signal. The GPS can immediately receive this signal and find the time in the GPS time system at this time (the accuracy can reach 1 μs). The GPS receiver can Store this time in the memory (such as Trimbel, Ashetech and other measuring GPS receivers). Remote sensing applications are eager to extract the positioning results from the GPS receiver in real time. At present, some receivers already have this function (such as NovAtel receivers), but they must also know precisely when the receiver receives the satellite signal to give the positioning. The time delay of the result moment, because the delay of 10 ms has little effect on static users, but for remote sensing users whose flight speed is tens of meters to hundreds of meters per second, it means an error of nearly half a meter to a few meters, so When reading positioning data in real time, this time delay must be calculated strictly. The functional relationship of GPS in the airborne 3D remote sensing image mapping system is shown in Figure 1.
Adapt to the requirements of airborne dynamic flight operations. The airborne 3D remote sensing image mapping system is a dynamic flight operation environment. Therefore, the dynamic performance of the GPS receiver is better, and it must generally be suitable for the high-speed flight of tens of meters to hundreds of meters per second of the remote sensing platform. Certain acceleration performance, to meet certain anti-vibration and anti-vibration standard GPS antennas must also be highly dynamic (equipped with highly sensitive preamplifiers), which can quickly capture GPS signals in high-speed movement and try to keep the lock as possible. Because GPS can only calculate the position of the GPS antenna, it cannot be coincident with the optical center position of the scanning range remote sensor, so there is a vector relationship, that is, an eccentric vector, which must be accurately determined in the field. Close-range photogrammetry and theodolite can be used. With the total station measurement method, flat glass direct projection measurement method and other mature methods for high-precision data processing, the GPS receiver data sampling interval is as small as possible. At present, many GPSs already have a high-speed sampling function with a 20Hz sampling rate. That is, 20 positioning results per second can be given, which can fully meet the application requirements of aviation remote sensing. 3 GPS data processing of airborne three-dimensional remote sensing. GPS data processing is to convert GPS data into the position of the optical projection center of the remote sensor in line with the actual application of remote sensing. The processing process is shown in Figure 3.1. GPS differential processing In order to obtain high-precision positioning results, it is generally required to use differential technology to achieve a three-dimensional position of meters or even centimeters. If real-time differential technology is used, it must be equipped with a digital transmission radio station, and at the same time a microcomputer must be used to record the positioning results in real time. The original measurement data received by the ground reference GPS receiver and the airborne GPS receiver can also be processed with high accuracy afterwards, which can be used as a data backup, and You Hongjian, etc .: suitable for dynamic GPS positioning of airborne 3D remote sensing The post-processing accuracy of technology and its data processing is often higher than real-time positioning. The post-difference processing generally uses GPS carrier phase data for dynamic resolution, and some mature algorithms such as search algorithm and Kalman filter algorithm can be used for carrier phase ambiguity calculation. At present, the processing speed is also very fast, and the accuracy can be better than a few centimeters.
3.2 Resolution of GPS antenna synchronous position at the time of scanning pulse Because GPS positioning can only perform high-precision measurement and solution of GPS antenna position at a fixed sampling rate (such as 1 time per second or up to 20 times per second), while scanning The pulse time from the imaging remote sensor to a certain pixel is arbitrary, and it cannot be strictly synchronized in time with the measurement results given by GPS, so it must be based on the GPS positioning result sequence at equal intervals according to the scanning pulse time t Interpolate out. Generally, a polynomial least squares fitting algorithm can be used to accurately interpolate the three-dimensional position (X, Y, Z) at the time of the pulse. After many experiments and tests, the GPS positioning results of 5 s before and after the pulse time are used to perform 3 curve fitting processes, which can fully achieve the centimeter-level fitting accuracy. Sequential algorithm, the calculation time is short, 4 h flight data can be completed in only 5 min, and the accuracy is guaranteed. The interpolation model is: the coefficient a is inversely calculated using the least square method based on the 5 s data before and after time t.
3.3 Coordinate reference conversion All GPS observations and calculations are based on the WGS84 (World Geodetic System 1984) ellipsoid as the coordinate reference reference, while China ’s remote sensing and cartography generally use the Beijing 54 ellipsoid as the coordinate reference. For this reason, it must be converted according to BursaWolf The formula of the model undergoes coordinate reference transformation: where ΔX, ΔY, and ΔZ are the three origin coordinate translation parameters, three coordinate rotation parameters, and Δk is the scale ratio parameter.
These 7 conversion parameters are generally available from the national surveying and mapping department, or can be reversed by using several common coordinate known points evenly distributed in the two coordinate reference systems. For a small area (within 50 km), ε can be regarded as a conversion formula of 3 parameters.
3.4 Coordinate projection transformation The geodetic coordinates (φ, l) after the reference transformation must also be transformed into Gaussian plane coordinates: where: the conversion accuracy using this formula is 0.001 ml longitude of the central meridian.
Journal of Remote Sensing 3.5 Correction of eccentric vectors Because the phase center of the GPS antenna cannot be consistent with the optical center of the scanning imaging ranging remote sensor, there is a distance between the two, that is, there is an eccentric vector. The eccentric vector (u, v, w) At the same time, the 3-axis attitude parameters at this time can be obtained from the attitude measurement device, so that the position of the optical center can be obtained according to the coordinate transformation formula: where: Among the three attitude parameters: pitch ψ roll ω heading κ by Attitude measurement device (inertial navigation system INS) to synchronize measurements.
After the above processing, the three-dimensional position of the optical center of the scanning range imaging remote sensor at the time of scanning can be obtained.
4 Test flight and data processing From May to June 1997, the 3D remote sensing mapping system supported by the China 863 Program was developed in Beijing ’s Jiuli Mountain and Dongsheng Coalfield, Tokoto, Guyang, and Hohhot in Beijing Waiting for the actual flight in 5 regions. The flight altitude is about 600 m. The dynamic GPS positioning mainly uses the Ashtech MD XⅡ measurement (single frequency) receiver and the NovAtel 1 R differential GPS receiver (single frequency). The GPS receiver and other devices perform synchronous data collection. Among them, NovAtel 1 R and UHF data stations constitute a real-time differential GPS system. In fact, the positioning results are stored and the positioning results are provided to the navigation display system for flight navigation. Ashtech MD XⅡ is used to receive the synchronous pulse signal of the scanning ranging imaging remote sensor. Complete data was obtained in 5 flights. After using the above-mentioned processing flow and algorithm to process the GPS data in the flight area, and obtain the three-dimensional position of the optical center at the time of scanning, then use the three-dimensional position and attitude data, The laser distance measurement data calculates the three-dimensional position of the ground sampling point, thereby generating the ground DEM. At the same time, the three-dimensional position of the ground sample point and the remote sensing spectral data obtained simultaneously are used to generate the geocoded image of the survey area. The result is satisfactory .
In order to check the accuracy of the final DEM, the Beijing Geological Research Institute of the Ministry of Nuclear Industry of China tested the Jiuli Mountain survey area in Beijing, using GPS to measure the obvious features in the survey area, and compared the measurement results with the results obtained by airborne 3D remote sensing , Statistics show that the accuracy of airborne three-dimensional remote sensing is 6-8 m. Because the accuracy of attitude measurement is about 2 ′, the accuracy of laser ranging is about 1 m, and the accuracy of positioning results given by GPS is about 1 m. The assembly errors on the hardware, the existence of these errors leads to the final DEM accuracy of about 6-8 m. From these processing results and accuracy, GPS positioning technology and data processing are reasonable. In order to further improve the accuracy of the airborne 3D remote sensing system, a dual-frequency GPS receiver must be used, so as to ensure that the positioning accuracy reaches the centimeter level, while also improving Algorithm accuracy of GPS data processing.
5 Conclusion As a brand-new remote sensing system, the 3D remote sensing image mapping system must use GPS to accurately solve the three-dimensional air position of the remote sensor. Therefore, some new requirements are placed on the GPS positioning technology, and the GPS data processing is also different from the general. GPS data processing. Suitable for three-dimensional remote sensing GPS positioning system generally requires real-time dynamic positioning performance, high-speed sampling, and must be able to synchronize with the data of the remote sensor, and the positioning accuracy must meet the requirements of remote sensing applications. GPS data applied to three-dimensional remote sensing must be processed by differential processing, interpolation of synchronous data, coordinate reference conversion, projection transformation, eccentric vector correction, etc. to obtain the precise position of the remote sensor optical center that meets the practical application of remote sensing. The GPS positioning technology and data processing described in this article can also be used as a reference for positioning in general remote sensing systems. With the improvement of the application level of remote sensing, especially the requirements for positioning, GPS positioning technology will surely Data processing plays an increasingly important role.
The latest progress of scanning ranging / imaging mapping system [J]. Chinese Image Graphics You Hongjian et al: Dynamic GPS positioning technology and data processing remote sensing image mapping system suitable for airborne 3D remote sensing [A]. Statement Peng, Tong Qingxi, Guo Huadong. Research on the mechanism of remote sensing information [C] Beijing: Science Press, 1998.] Used in photogrammetry and remote sensing [M]. Beijing: Surveying and Mapping Press, 1996.] Using geodetic coordinate system and its transformation [M]. PLA Press, 1990.] Principle and application of bit system [M]. Beijing: Surveying and Mapping Press, 1993.] Journal of Remote Sensing
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