Mars Express' HRSC has five "panchromatic" detectors that each look down from the spacecraft at a different angle: there is one nadir (straight-down) channel, two "stereo" channels S1 and S2 that are each angled forward and backward along the spacecraft's path at 18.9°, and two "photometric" channels P1 and P2 that are at slightly smaller angles. As a result of these different look angles, Mars Express saw Phobos from five different perspectives.

ESA / DLR / FU Berlin (G. Neukum)