Mars Global Surveyor
Mars Orbiter Camera

Why is the image of Cydonia only 1024 pixels wide?

This question has been asked a great deal over the past few days. The field-of-view of the MOC is 0.44°, and at the range of the Cydonia image, this translated to about 3.1 km (1.9 miles). The viewing angle was about 45°, so the projected width of the field was about 4.4 km (3 mi). The narrow angle detector is 2048 pixels across, so the intrinsic resolution was about 2.1 m/pixel. The maximum image size (in bytes) is 9.8 MBytes, or 2048 X 4800 pixels. At 2.1 m/pixel, this permitted a down-track field of view of 11 km. Considering the uncertainties in pointing of the spacecraft, this 11 km downtrack dimension was considered insufficient to insure a reasonable chance of hitting the target.

There are two ways of extending the downtrack dimension. First, we can use "lossless" compression to extend the distance by a factor of about 1.8. We do this with most of the MOC images. However, losses within the communication system occasionally create black bands (accounting for the loss of about 7-15% of the data) through the images. The MGS Project felt such losses would not be tolerable in the Cydonia images. The second way to increase downtrack dimension is to decrease resolution. If you sum the pixels two by two (that is, replace four pixels--two across by two down--with the average of the four), you can extend the image downtrack by a factor of four. This, then, changes the dimensions of the image from 2048 X 4800 to 1024 X 9600, and the images covers the same 4.8 km width but extends somewhat over 44 km downtrack. The resolution is lower (4.3 m/pixel vs. 2.1 m/pixel), but the image covers much more ground.

Since this resolution was still substantially better than the best Viking image (by about a factor of ten), the Project decided this was the best compromise in coverage and protection against data loss.