mhz output of the vco 96 this count down is partially accomplished by the frequency divider 94 divide by 20 in cooperation with the frequency divider 98 divide by 2 in mode b operation the phase modulation of the approximately 400 mhz signal is the same as in mode a however in mode b operation the frequency stability is better than in mode a and accordingly suitable for measuring the doppler reconstruction circuitry not shown and the frequency stability of the 5 mhz reference oscillator not shown of the associated an srn 19 v 2 the primary purpose of the digital portion 84 of the satellite simulator 80 of fig3 is to generate the sequence of digital data signals a b and c for driving the phase modulator device 102 the digital data are similar to that from a system satellite in terms of having a repeating message data pattern as generated by the test message generator 106 and a two minute synchronization data pattern as generated by the sync word generator 108 the test message generator 106 is configured to generate a signal message data corresponding to a repeating sequence of two binary 34 ones 34 followed by two binary 34 zeros 34 the test message generator 106 continually outputs this repeating pattern via the output control device 110 as the drive signals a b and c the drive signal for the test message generator 106 and the count down divider 112 is a 2 5 mhz signal from the output.