Pulse radar

The most common type of radar signal consists of a repetitive train of short-duration pulses.

A typical pulse waveform transmitted by radar.

This is a simple representation of a sine-wave pulse that might be generated by the transmitter of a medium-range radar designed for aircraft detection. The sine wave in the figure represents the variation with time of the output voltage of the transmitter. The numbers given in brackets in the figure are only meant to be illustrative and are not necessarily those of any particular radar. They are, however, similar to what might be expected for a ground-based radar system with a range of about 50 to 60 nautical miles (or 90 to 110 kilometres), such as the kind used for air traffic control at airports. The pulse width is given in the figure as one millionth of a second (one microsecond). It should be noted that the pulse is shown as containing only a few cycles of the sine wave; however, in a radar system having the values indicated, there would be 1,000 cycles within the pulse. In Figure 2 the time between successive pulses is given as one thousandth of a second (one millisecond), which corresponds to a pulse repetition frequency of 1,000 hertz (Hz; cycles per second). The power of the pulse, called the peak power, is taken here to be 1,000,000 watts (1 megawatt). Since pulse radar does not radiate continually, the average power is much less than the peak power. In this example, the average power is 1,000 watts (1 kilowatt). The average power, rather than the peak power, is the measure of the capability of a radar system. Radars have average powers from a few milliwatts to as much as one or more megawatts, depending on the application.
 

A weak echo signal from a target might be as low as one trillionth of a watt (10-12 watt). In short, the power levels in a radar system can be very large (at the transmitter) and very small (at the receiver).
 

Another example of the extremes encountered in a radar system is the timing. An air-surveillance radar (one that is used to search for aircraft) might scan its antenna 360 degrees in azimuth in a few seconds, but the pulse width might be about one microsecond in duration. (Some radar pulse widths are 1,000 times smaller--i.e., of nanosecond duration.)
 

The range to a target is determined by measuring the time that a radar signal takes to travel out to the target and back. Radar waves travel at the same speed as light--roughly 300,000,000 metres per second (or 186,000 miles per second). The range to the target is equal to cT/2, where c = velocity of propagation of radar energy, and T = round-trip time as measured by the radar. From this expression, the round-trip travel of the radar signal is at a rate of 150 metres per microsecond. For example, if the time that it takes the signal to travel out to the target and back were measured by the radar to be 600 microseconds (0.0006 second), then the range of the target would be 90 kilometers.