3. EQUIPMENT AND INSTRUMENTS 



63 



system is based on the measurement of a 

 phase difference in two radio signals, one 

 emanating from a transmitter aboard ship 

 and the other from a transmitter at one of 

 two fixed stations ashore. In order to obtain 

 the distances to both ground stations it is 

 necessary to measure the phase difference 

 at three places — aboard ship and at each 

 ground station. 



Four radio transmitters and frequencies 

 are required to operate the system — the 

 two distance-measuring frequencies men- 

 tioned above and two others to transmit in- 

 formation to the ship concerning the phase 

 relationship at each ground station. The 

 latter frequencies do not enter into the 

 determination of distance, however, inter- 

 ference on any of the four frequencies will 

 affect operation of the lane counters in the 

 phase meter. The ground station where the 

 distance-measuring transmitter is located 

 is referred to as the Rj or "Red" station, and 

 the other the Rj or "Green" station. 



Because it is difficult to measure phase 

 directly at the radio frequencies used, a 

 technique known as heterodyning is em- 

 ployed. If two radio signals are fed into a 

 detector, the issuing signals will have a fre- 

 quency that is equal to the difference between 

 those of the entering signals and will por- 

 tray the phase changes that take place 

 between them. In Raydist the entering sig- 

 nals are separated by 400 cycles, and after 

 detection this frequency is fed into a phase- 

 meter. The heterodyne technique permits 

 the measurement of high frequency signals 

 to be made at a low frequency without sacri- 

 fice of accuracy. 



3-50 Distance measurements. — The dis- 

 tance from the ship to the Red station is 

 measured by comparing the phase of the 

 transmitter on the ship with the phase of 

 the transmitter at the Red station. The dis- 

 tance is measured in terms of half-wave 

 lengths of frequency between the two sta- 

 tions. These half-wave lengths are called 

 "lanes", and for presently assigned frequen- 

 cies, the lanes are about 45 meters or 150 

 feet wide. The width of a lane is a function 

 of the frequency of the transmitter at the 



mobile station and is expressed by the for- 

 mula L^=V/2F in which L= lane width; 

 F=velocity of propagation of radio wave 

 or 983,167,315 feet per second, and F= 

 frequency of the transmitter. When drawing 

 distance circles on the survey sheet the value 

 of L is assumed to be the same for both 

 the Red and Green stations. However, the 

 distances to the Green stations are subject 

 to correction because of the elliptical system 

 employed. Computation of this correction 

 is described in 3-56. 



3-51 Installation aboard ship. — Three 

 whip antennas are required aboard ship, one 

 for transmitting distance measuring signals, 

 one for receiving the distance measuring 

 signals, and one for receiving information 

 from the ground stations. The equipment 

 aboard ship includes : 



(a) Transmitter — A 100-watt unit for 

 transmitting the distance-measuring signal. 



(b) Navigator — Containing three re- 

 ceivers. 



(c) Monitor — ^Used to measure the fre- 

 quency of the 400 cycle signals and to check 

 the audio tones received from various points 

 in the system. 



(d) Phasemeter — Registers the lane count 

 on dials to the nearest 0.01 lane. 



(e) Brush Recorder — Records in ink on a 

 special tape the lane changes as shown on 

 the phasemeter dials. 



(f) Power Supply— Converts the main 

 power supply to proper voltage for use in 

 the equipment. 



The transmitter and power supply may 

 be located in any convenient place, but the 

 other units should be mounted near the plot- 

 ting table. The equipment required at the 

 mobile station, except the antennas, is shown 

 in Figure 15. 



It is difficult for one person to read both 

 phasemeter dials simultaneously, especially 

 when both distances are changing rapidly. 

 For this and other reasons, it is desirable 

 to attach a recorder which will print phase- 

 meter dial readings on a tape at any instant. 

 (Fig. 16) The printer is not a part of the 

 Raydist equipment. 



A partially transistorized 10-watt Raydist 



