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Figure 12-23.A microwave distance-measuring device (Model 99).
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Engineering Aid 3 - Beginning Structural engineering guide book
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Heavy  steel  tapes  are  repaired  in  a  similar manner, using the tape repair kit shown in chapter 11, figure 11-55. MEASURING  BY  THE  ELECTRONIC DISTANCE-MEASURING   SYSTEM The  electronic  distance-measuring  system  is now  incorporated  in  various  present-day  surveying practices,  including  traverse  and  triangulation network.   In   traverse   measurements,   accurate distances  are  directly  measured  in  a  straight line  and  with  minimum  instrument  setups.  In triangulation, the system is used to conduct base line  measurements  that  are  precise  enough  to maintain  the  accuracy  of  the  survey. In the electronic distance-measuring system, the  length  of  a  linear  interval  is  determined by  the  use  of  equipment  that  (1)  sends  out an  electronic  impulse  of  some  sort,  such  as a  radar  microwave  or  a  modulated  light  wave, and  (2)  measures  the  time  required  for  the impulse   to   travel   the   length   of   the   interval. The  velocity  or  rate  of  travel  of  the  impulse is   known.   Therefore,   once   the   time   is   also known,  the  length  of  the  linear  interval  can  be determined by applying the well-known equation “distance  =  rate  x  time.” Two  types  of  electronic  distance-measuring devices   (also   called   EDMs)   commonly   used today are the MICROWAVE DEVICES and the LIGHT  WAVE  DEVICES. Figure 12-24.-Setting a microwave distance-measuring unit. Measuring by Microwave Devices The   microwave   distance-measuring   device (fig.  12-23)  is  an  electronic  instrument  that transmits  precisely  controlled  RADIO  WAVES between two units. The waves are compared and electronically  changed  into  a  visually  readable form from which the distance between the units can  be  computed. As  shown  in  figure  12-24,  the  unit  that originates  and  transmits  the  modulated  radio waves is called the master. The unit at the opposite end of the line from the master is known as the remote. The two are identical instruments, each being adaptable to use as either master or remote. At  the  remote  unit,  the  original  transmission is   received,   interpreted,   and   put   on   a   new carrier.  This  new  modulation  is  amplified  and retransmitted  to  the  master.  The  master  analyzes the new transmission and translates it into a trace on a cathode ray tube that can be read visually. The  trace  information  is  converted  into  a  distance based on the velocity of the radio waves. Because this  velocity  is  affected  by  atmospheric  conditions, corrections   for   temperature   and   barometric pressure  are  applied  according  to  instructions. Each  instrument  is  equipped  with  a  shortwave telephone set. By this means, the person at each instrument  can  maintain  communication  with  the other.  Details of the method of operating  the system must be learned from the manufacturer’s instructions. 12-25







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