  Accurate positional measurements of planets and satellites are used to
improve our knowledge of their dynamics and to infer the accuracy of planet and
satellite ephemerides. In the framework of the FP7 ESPaCE project, we provide
the positions of Mars, Phobos, and Deimos taken with the U.S. Naval Observatory
26-inch refractor during the 1971 opposition of the planet. These plates were
measured with the digitizer of the Royal Observatory of Belgium and reduced
through an optimal process that includes image, instrumental, and spherical
corrections to provide the most accurate data. We compared the observed
positions of the planet Mars and its satellites with the theoretical positions
from INPOP10 and DE430 planetary ephemerides, and from NOE and MAR097 satellite
ephemerides. The rms residuals in RA and Dec. of one position is less than 60
mas, or about 20 km at Mars. This accuracy is comparable to the most recent CCD
observations. Moreover, it shows that astrometric data derived from
photographic plates can compete with those of old spacecraft (Mariner 9, Viking
1 and 2).
