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Accurate positional measurements of planets and satellites are used toimprove our knowledge of their dynamics and to infer the accuracy of planet andsatellite ephemerides. In the framework of the FP7 ESPaCE project, we providethe positions of Mars, Phobos, and Deimos taken with the U.S. Naval Observatory26-inch refractor during the 1971 opposition of the planet. These plates weremeasured with the digitizer of the Royal Observatory of Belgium and reducedthrough an optimal process that includes image, instrumental, and sphericalcorrections to provide the most accurate data. We compared the observedpositions of the planet Mars and its satellites with the theoretical positionsfrom INPOP10 and DE430 planetary ephemerides, and from NOE and MAR097 satelliteephemerides. The rms residuals in RA and Dec. of one position is less than 60mas, or about 20 km at Mars. This accuracy is comparable to the most recent CCDobservations. Moreover, it shows that astrometric data derived fromphotographic plates can compete with those of old spacecraft (Mariner 9, Viking1 and 2).

Astrometric observations of Phobos and Deimos during the 1971 opposition of Mars