Lander position determination on Mars using Phobos transits: application to Beagle 2

Apart from the obvious significance of the martian moon Phobos in the general context of planetary science research, the shadow that it casts on the surface of Mars may be used to determine location co-ordinates of a lander for scientific as well as engineering purposes. A model is presented to predict the occurrence and characteristics of such “transits” of Phobos across the solar disk as seen from an observing site on the martian surface. Tests are carried out using Viking data obtained in 1977 to validate the model. It is concluded that a surface accuracy of 20 km in longitude and 5 km in latitude is attainable. The model is applied in the case of the Beagle 2 lander scheduled to reach Mars in late 2003. Specific transit events suitable for position determination of that lander are identified. An observational strategy is proposed which should maximize the possibility of obtaining the required dataset with due regard to the operational constraints of that mission.