Abstract: The downlink ranging modulation index was variable while the signal to noise ratio (S/N) changes in the uplink channel or the telecommand and ranging operates simultaneously or respectively,which was due to the multi-subcarrier modulation and the structure of the transponder.The expressions were formulated and the computed data and test results were compared.And the test data is in accord with the principle analysis result.In addition,the expressions are different due to the different structure model of transponders.The expressions deduct by the paper,for example,are different form that recommended by CCSDS standards.

Key words: Downlink ranging modulation index, Signal to noise ratio, Uplink channel, Deep space detection, Principle analysis