AP 3302 Pt. 3
Section 1
CHAPTER 1
PULSE-MODULATED RADAR
Range = (Speed of sound waves x Time interval)/2
where range is in feet, speed is in feet per second, time interval is in seconds.
A pulse-modulated radar system uses radio waves in a manner similar to that above to measure the range of a target. Let us assume that an aircraft is exactly one mile from the radar installation, as in Fig 7, and that we wish to find the time interval between transmission and reception of the pulse.
The
speed of e.m. waves is known to be 186,000 miles per second (or 300 x lO
6 metres per second). Therefore the time taken to travel from the transmitter
to the aircraft and back is: 1/186,000 + 1/186,000 = 1/93,000 second.
Expressed in microseconds (uS) this is: 1/93,000 x 1,000,000 = 1000/93 = 10.75 uS
This figure is known as the time of one radar mile. For each mile of range the time interval is l0.75 microseconds. Thus if an aircraft is at 20 miles range the time interval is 20 x 10.75 = 215 microseconds.
In practice, of course, we work the other way round. The time interval is measured and from this we find the range. This can be found from the relationship mentioned earlier:
Range = (Speed of e.m. waves x Time interval)/2
The units used depend upon whether the range is required in miles or in metres. If the range is required in miles, speed is given in miles per second (186,000) and time in seconds; if the range is in metres, speed is in metres per second (300 x lO 6) and time in seconds.
Examples:
a. Time interval between transmission of pulse and reception of echo is 320 microseconds.
Range (in miles)= (186000 x 320 x 186000 x 320 x lO 6 )/2 = 29.76 miles
