The ``range and range rate'' (ri + Â¿i) system in its very simplest form is described. In particular, the errors in position and velocity are treated using pessimistic values of the measured quantities ri and Â¿i. Thus, a realistic evaluation of tracking qualities can be made for different orbits over certain tracking stations. The range and range rate system briefly described in this paper is a high-precision tracking system. Knowledge of the uncertainty in position Â¿xi is. important, but knowledge of the uncertainty of the velocity vector Â¿xÂ¿i is of the utmost importance. Thus the use of coherent Doppler measurements to determine the velocity has a great advantage over any pulsed system and, in addition, permits extremely narrow frequency bandwidths (in the order of 10 to 100 cps) to be employed, reducing the power requirements considerably. The basis for using range ri and range rate Â¿i only is the fact that ri and Â¿i can be measured to very high precision, thus furnishing r and Â¿ with low errors. The nature of these errors is discussed.