rotor is geared to its mechanical input and one rotation of the input results in one revolution of the transmitter’s rotor. The speed of a synchro transmitter is tied to the gear  ratio  between  the  mechanical  input  to  the transmitter and the transmitter’s rotor; that is, 1:1,36:1, and  soon. In a 36-speed synchro system, the rotor of the synchro transmitter is geared to rotate 36 times for one revolution of the input shaft (36:1). Units transmitting data atone speed (1-speed, 36-speed, and so forth) are called single-speed synchros. The entire range of data to be transmitted is contained in the output of the single-synchro transmitter. It is quite common for shipboard synchro systems to transmit data using two different speed synchros with the  same  reference  or  supply  voltage.  For  example, ship’s  course  (ownship’s  heading)  information  is usually transmitted to other locations using a 1-speed synchro and a 36-speed synchro. A synchro system that transmits  data  using  two  different  speed  synchros  is called a dual-speed synchro system or a double-speed synchro system. COARSE  AND  FINE  DATA  TRANSMIS- SION.  —Dual-speed   synchro   transmissions   are combined  to  improve  the  accuracy  of  the  data transmitted. The use of two transmitting synchros allows for a coarse value and a fine value to be sent at the same time. The synchro with the lowest ratio (1:1) sends the coarse value. The synchro with the highest ratio (36:1) sends the fine value. The coarse and fine values transmitted can be likened to the hour and minute hands of a clock. The course value represents the time in hours. The fine value represents the time in minutes. The two values must be combined to give the time in hours and minutes. Let’s look at a coarse synchro and a fine synchro transmitting an angular position such as ship’s course (ownship’s heading), which can be from 0 degrees to 359 degrees true. The coarse synchro (1:1) indicates 360  degrees  of  ship’s  course  with  one  rotation. However, the accuracy of the data is limited to plus or minus 1 degree of heading. This degree of accuracy is not enough for most navigation systems to keep an accurate track of ship’s movement. The fine synchro (36:1) rotates 36 times for each rotation of the coarse synchro. This means the fine synchro rotates once each 10 degrees (360/36). Within its 10-degree segment, the fine synchro is 36 times as accurate as the coarse synchro. The use of dual-speed synchros requires two S/D conversions to take place; one to determine the position of the rotor in the coarse synchro transmitter and one to determine the position of the rotor in the fine synchro   transmitter. SYNCHRO SIGNALS. —A single-speed synchro transmitter outputs three waveforms that indicate the angular position of the rotor in the transmitting synchro, for example a control transmitter (CX). Waveforms are induced in the stator coils by the magnetic field of the rotor coil. The two rotor connections of the CX (R1 and R2) are fed from a 115-volt ac supply voltage (also called the reference voltage). This voltage is also fed to the synchro-to-digital (S/D) converter circuitry. The reference  voltage  is  important  in  the  conversion process. It provides a reference for the S/D converter to use when sampling the stator voltages. The amplitude of the voltage output between the stators (S1 to S2, S1 to S3, and S2 to S3) at any instant is dependent on the position of the rotor in the CX. The 115-volt supply voltage induces an ac voltage into the stator windings. The phase relationship of the signals induced  on  each  stator  winding  is  dependent  on  the angular position of the rotor within the CX. The rotor can normally be rotated 360 degrees within the synchro. The range of values being transmitted is tied to this 360 degree  rotation.  The  minimum  value  is  normally transmitted with the rotor at the 0-degree position and the maximum value is sent when the rotor is positioned to approximately 359 degrees. All three stator signals are ac voltages that have the same characteristics (frequency and amplitude). They have   a   120-degree   phase   difference   (phase displacement) from each other due to the 120-degree separation of the wye windings of the stator coils in the synchro transmitter. At any instant, a phase relationship exists  between  the  rotor  supply  (excitation)  voltage  and the three stator voltages. This phase relationship is the key to the S/D conversion process. Basically, the phase relationship of the individual stator voltages, across terminals S1, S2, and S3, varies with the rotor supply voltage (R1-R2) as the rotor is rotated within the synchro transmitter. Each position of the rotor has a unique stator voltage phase relationship to the supply (reference) voltage. At any instant, the amplitude and polarity of the stator signals when compared  to  the  supply  voltage  indicate  the  angular position of the rotor. For dual-speed synchro systems, two sets of stator voltages are transmitted, one set for the coarse synchro and one set for the fine synchro. A single supply voltage   (reference)   is   used   for   both   synchro 13-6