Keywords: laser range-finder, multi-spectral imaging, integrated technology system.

Abstract
A new airborne remote sensing system, airborne 3D imager supported by Chinese High-Tech Plan is presented in this paper, and this 3D imager is developed by institute of Remote Sensing Applications, Chinese Academy of Sciences and Shanghai Institute of Technology Physics, Chinese Academy of Sciences. Optical-mechanical scanning laser range-finder, multi-spectral scanning imager, Global Positioning System (GPS), INS/GPS combined attitude measurement unit and special synchronization match technology are used to design this airborne remote sensing integrated technology system based on aerial ranging principle. Quasi 2D laser ranging points matched image pixels which are called point couples are densely distributed in remote sensing image and those point couples can be directly used to calculated their geographical coordinates (X, Y,Z) according to aerial ranging principle. Then remoter sensing image can be processed into geo-referenced image matched with DEM through simply fitting and interpolating. The result of some fights tests show that it is a high efficient airborne remote sensing integration system which can generate above results without ground control point (GCP) or with only a few GCPs. This result improves efficiency up to 10 ~100 times higher than traditional technology, and it becomes an important remote sensing technology of the future.

The principle and technology structure of this technology system are briefly introduced in this paper, and the result of some flight tests and its characters are also given. The development and application fields of this technology system are analyzed in the end.

1. Introduction

Environment, resources and population have been the focus problems of body. It is now being well known that our earth should be regarded as a united "earth system", the earth environment where people live should be researched in global scale, regional scale or national scale based on the earth system science. Digital earth, digital Asia and digital Asia and digital China should be set up. All kinds of change, models and comprehensive models which influences the living environment should also be set up in order to guide the human being to adjust their activities to the nature, and then earth environment can be protected and develop. Earth observation technology, an important to understand the nature, should meet the requirement of society and it should be also develop and innovate continuously. Meeting the requirement of the society is the development source of earth observation technology.

There are many remote sensors and observation modes in present earth observation systems because there are many different application fields and different technology conditions and some others factors, and they are still being developed. Stereo observation system, laser ranging & multi-spectral imaging coupled system, interfere SAR are the main earth observation systems which can be get the spectral information of the targets and 3D terrain information simultaneously.

Stereo observation system has being developed and improved for nearly 100 years. Now it is experiencing an innovated change from analog image to digital image and from ground-to-air positioning principle to air-to-ground positioning principle. Today stereo observation system is a universal advanced mapping technology is every country of the world, and some satellite-borne and airborne earth observation systems still uses stereo observation mode. Laser ranging & multi-spectral imager coupled system (Airborne 3D imager) was proposed eight years ago and its prototype was developed three years ago. Conic scanning airborne system was finished recently, and its accuracy has reached the 1:10000 scale mapping criteria. Similar satellite borne system is to be developed. Interfere SAR (InSAR) is an all-weather earth observation system belonging to microwave. Both satellite-borne InSAR and airborne InSAR have the ability to acquire 3D terrain information. But it will take some time to be developed into a commercial technology system.

The first systems are optical remote sensing while the latter one is microwave remote sensing. They are three different technology systems with different principle, different advantage, different spectrum and different technology. Many technologies are integrated into each technology system, especially the Airborne 3D Imager and InSAR integrate many high technologies. The system that is to be get multispectral information and 3D terrain information simultaneously is the development trend of the future.

2. Principle and System Structure

2.1 Principle

2..1.1 Basic equation

assuming there is a vector OP, when the coordination of the start point O (Xo, Yo, Zo), the slope range of the vector S and the attitude of the vector (f, w, k) are known, then the geographical coordinate of terminal point can determined precisely according to following formula:

Where (X_o, Y_o, Z_o) is the center of the remote sensing determined by global Positioning System (GPS) receiver, (f, w, k) are three attitude parameters provided by high accurate attitude measurement unit (AMU) and slope S is measured by high rate laser range finder without reflector.

2.1.2 Laser point matching image pixel
Optical mechanical scanning multi-spectral imager uses a rotate mirror controlled by coder to generate image. in this system, scanning laser rangefield and scanner use the same optical system. Both the laser and scanner are controlled by the coder. The coder drives the scanner continuously to acquire the spectral information while the coder drive the laser rangefield to measure the slope range per fixed several pixel. Because of using the same optical system, the laser point matches pixel every several pixel in the image. Figure 1 shows how the laser points are distributed in the image.

2.1.3 Time synchronization of the system
The synchronization of the different component is realized through coder. When the scanner scans to the nadir, the code will generate a pulse and this pulse then drive the GPS receiver to generate a time mark. The position of the remote sensing (Xo, Yo, Zo) is Interpolated according to this time mark from the regulated GPS positions. At the same time this pulse will also drive the AMU to provide the instantaneous attitude of the scanner. That means (f, w, k) is know.

2.14 Rigid connection of the system
The AMU, scanner and laser rangefinder are all mounted in a rigid plate. There is a fixed vector between the antenna of GPS and the center of the scanner and this vector can be surveyed precisely through feasible method.

2.1.5 Two scanning modes
According to different application, either line scanning mode or conic scanning mode can be used. These two modes have been finished. Line scanning mode can be applied in remote sensing thematic mapping while conic scanning mode can be applied in complex terrain and acquiring 3D urban building and some other fields.

Figure 1. Distribution of Laser Points and Pixels



Figure 2 Illustration of Line Scanning Mode and Conic Scanning Mode.