Keywords: Image Simulation, Orientation Modeling, ROCSAT-2
Abstract The objective of this investigation is to propose a scheme to model the orientation parameters for ROCSAT-2 satellite. Due to the unavailability of the satellite imagery, a geometric simulation for ROCSAT-2 is also included. Four major components are included in the simulation procedure. The first is to perform the transformations between related coordinate systems. The second component establishes the satellite-flying model to describe the orbit and attitude variations. The third one is to form ROCSAT-2 images. Finally, we include systematic and random errors in the orbital and attitude data for simulating the ephemeris. In the orientation modeling, the ephemeris data with systematic and random errors are use to construct the approximate imaging geometry. Then ground control points are used to fine-tune the exterior orientation parameters. Experimental results indicate that the systematic errors may be mostly compensated while the random errors consistently influence the positioning accuracy.
1. Introduction
ROCSAT-2 is a sun synchronous remote sensing satellite, which is scheduled to launch in early 2003. It flies 890km in height on a plane with 98.99o inclination angle. The off-nadir look angle may reach 45o in the along-track and cross-track directions [NSPO, 2000]. The pushbroom imager on board has 12,000 pixels per line, for panchromatic band each with 2m-ground coverage. Thus, the swath is 24km nadir and the FOV is about 1.5 o. The multi-spectral imager includes 4 bands namely blue, green, red, and near IR. The ground resolution for multi-spectral imagery is 8m nadir. In this investigation, we only use panchromatic imagery to validate the geometric simulation and processing.
Since the trend is to integrate remotely sensed information and other data in geographic information systems, geometric correction for satellite imagery is required. In addition, ROCSAT-2 has a capability of stereoscopic observation by collecting along-track or cross-track stereopairs. Thus, generation of digital terrain models (DTMs) is possible. No matte what in the generation of DTMs or performing orthorectification, a modeling for precision orientation parameters is always the first step [Chen & Chang, 1998]. The objective of this investigation is to propose a scheme to model precision orientation parameters.
The imagery of ROCSAT-2 is not available at the moment. Thus, we need to simulate ones for validation of the geometric process. Since the point is for the geometric processing, the radiometric unsoundness is not considered. In addition, we ignore the building effect during imaging due to the unavailability of digital building models.
2. Image Simulation
The proposed procedures for image simulation are shown in fig.1. Given 6 elements for a satellite, namely right ascension, inclination (orbital plane with respect to equator), argument of perigee, semi-major axis and eccentricity of earth ellipsoid, and true anomaly, by assigning the initial time, we can determine the position of the satellite at a given instant [Tseng, 1983].
Figure 1. Proposed Scheme for Image Simulation
In order to simulate orientation parameters, transformations between following coordinate systems are considered:
(1) Attitude Reference System and Local Orbital Reference System
(2) Local Orbital Reference System and WGS84
(3) WGS84 and ECI (Earth Center Inertia)
(4) WGS84 and GRS67 (GRS67 is being replaced by WGS84 in Taiwan)
(5) GRS67 and Geographic System (Longitude, Latitude, Height)
(6) Map projection (Geographic System and 2 o Transverse Mercator)
When the satellite location is determined with respect to time, we may begin with image simulation for a target area. We need to calculate the attitude angle according to the target position with respect to satellite's. Then an image simulation may be performed by Top-Down approach as shown in fig. 2. Two major steps are included:
(1) Calculation of observation vector for each CCD detector.
(2) Determination of the grey value for each CCD detector by corresponding the object point and image pixel using Top-Down ray tracing in an iteration way.
Figure 2. Images Simulation by Ray Tracing
Considering the errors of ephemeris data for ROCSAT-2, we include systematic and random errors to complete the simulation. The scale of the systematic error may be selected according to the satellite specification. On the other hand, random error is not well known at the moment. We consider the following factors: (1) random error are fully random and independent, (2) each component corresponds to half-pixel coverage on ground, i.e., 1m.
