LINE-OF-
SIGHT MODEL
Image orthorectification is a key function of Spacemetric's software’s. High-accuracy image geolocation is possible using rigorous photogrammetric methods using physical sensor models and advanced statistical treatment of geometric reference data.
The line-of-sight model (LOS) describes in an analytical way how a pixel in a satellite image is projected onto the ground using several distinct sub-models that can be independently modified or replaced. It defines the relations between the sub-models and the flow of transformation this involves, see figure 1.
Figure 1: Generic LOS model for optical sensors
Sensor model
The sensor model takes a pixel in the satellite image and computes its look vector uS in the sensor coordinate system. It also computes the time for the instance of this look. The sensor model is initialised with parameters for the sensor design that are updated through a post-launch calibration activity. The sensor model can be a generic model for a push-broom scanner with parameters such as focal length, detector positions in the focal plane and scan-line time interval. Alternatively, it can be a highly specialised model for a more complicated sensor. The sensor model is the sub-model that is most often modified when implementing a new satellite system.
Body model
The body model is used to rotate the look vector from the sensor coordinate system to the satellite body coordinate system. It is used to model either the intended off-nadir sensor mounts or the small misalignments in the nominal mount.
Attitude model
The attitude model is used to rotate the look vector from the body coordinate system to the flight coordinate system. The rotations between these systems are due to deviations in satellite attitude. This is a time-dependent variation, usually measured by devices such as earth horizon detectors, gyroscopes or star trackers. The attitude model is initiated by a set of time-coded attitude and/or attitude rate measurements. The time of the look is used to calculate the transformation to be used.
Flight model
The flight model is used to rotate the look vector from the flight coordinate system to the Earth Centered Inertial (ECI) coordinate system. It is also used to calculate the position of the satellite. It employs orbital mechanics and is initiated by sets of parameters such as one or several ephemeris, several time-tagged position vectors or two-line elements. The time of the look is used to calculate the transformation and position to be used.
Astronomical model
The astronomical model is used to transform the position and look vectors from the ECI system to the Earth Centered Rotating (ECR) coordinate system. It is primarily a rotation of the x-axis from the vernal equinox to the Greenwich meridian. The transformation is time-dependent and the time of the look is used to calculate the transformation to be used.
Intersection model
The intersection model calculates the intersection point between the look vector and an ellipsoidal Earth that is centered in the ECR system. The ellipsoidal height is also input to provide a unique position. An atmospheric model is applied to correct for the deviation caused by atmospheric refraction.
Geodesy model
The geodesy model transforms the Earth intersection point, expressed in ECR coordinates, to a geographic coordinate (longitude, latitude, orthometric height). It uses a geoid model to account for the irregularities in the Earth's zero potential surface. The result is a coordinate in the WGS84 system.