Attitude Determination by Exploiting Geometric Distortions in Stereo Images of DMC Camera

A description is presented of a neoteric approach to determine the orientation of the spacecraft for any arbitrary attitude by exploiting the offsets in the Earth observational imagery from a pair of canted pushbroom sensors. Due to angular separation one of the imager projections is ahead of the other, which causes a delay in the imaging of the same ground features by the sensors. Thus, the angular separation creates perspective and temporal distortions in the imagery, which give rise to inter-image offsets in the overlapping region. A mathematical model is developed using the sensor configuration of the UK-DMC (disaster monitoring constellation) satellite to demonstrate how the offsets are introduced in the imagery and quantify that model with the aid of synthetic images. To determine the attitude from the image offsets, the attitude model is inverted. The accuracy of attitude estimates is highly dependent on the image texture, angular separation between the sensors, spacecraft attitude, and image registration method. With small along-track separation between the sensors, the registration time is very short, thus, changes in the inter-image offsets are not very rapid at small arbitrary rotations. However, with the increase in along-track separation between the sensors, the registration time increases, which allows changes in inter-image offsets to be discerned due to attitude variation and increases the sensitivity of the technique. But with large along-track separation, the attitude estimates are averaged over the registration time period. At small arbitrary rotations with half-a-degree angular separation between the sensors, this technique gives attitude accuracies of ±0.03°. As attitude estimates come directly from the payload rather than a dedicated sensor, this corresponds to significant savings in cost, mass, and volume, which are critical for small satellites.