A radar (radio detection
and ranging) transmits radio-frequency pulses
which, when reflected by the object being monitored (a passing
ship, say), are received back again. The time elapsed between sending and
receiving is a measure for the distance of the object. The shift in frequency
due by the Doppler effect, is a measure for the velocity of the object.
By sending a number of pulses which vary in
frequency and time, a greater precision can be achieved than by sending just one pulse. Thus, instead
of one pulse with a fixed frequency, we send a package of n
pulses with varying frequencies on n consecutive time intervals.
We still have a choice which frequency to send first, which one
next, etc.
In this way we come to a
detection pattern that can be represented in a 2-dimensional fashion:
the transmitted signals correspond to the black squares on an
n × n grid; the horizontal direction is the time axis,
the vertical direction is the frequency axis.
If, for example, a radar of a boat sends out a detection pattern, then this pattern is reflected by the ships in the neighborhood. The distance and speed of these ships can be determined with the help of the reflected pattern. However, the reflected patterns may have some overlap. The frequencies in the overlap interfer, which may cause some errors. If we choose a pattern with minimal overlap, then the number of errors will be minimized.
