Pulse-repetition frequency
Since the radio signal has to travel out to the target and back again, the required inter-pulse quiet period is a function of the radar's desired range.
Conversely, higher PRFs produce shorter maximum ranges, but broadcast more pulses, and thus radio energy, in a given time.
This gives each radar system a characteristic PRF, which can be used in electronic warfare to identify the type or class of a particular platform such as a ship or aircraft, or in some cases, a particular unit.
Modern radar systems are generally able to smoothly change their PRF, pulse width and carrier frequency, making identification much more difficult.
[1] Conversely, a high PRR/PRF can enhance target discrimination of nearer objects, such as a periscope or fast moving missile.
Many dual-purpose and navigation radars—especially naval designs with variable PRRs—allow a skilled operator to adjust PRR to enhance and clarify the radar picture—for example in bad sea states where wave action generates false returns, and in general for less clutter, or perhaps a better return signal off a prominent landscape feature (e.g., a cliff).
The PRF for the strobe light is adjusted upward from a low value until the rotating object appears to stand still.
Other types of measurements involve distance using the delay time for reflected echo pulses from light, microwaves, and sound transmissions.
For example, an L-Band radar with 500 Hz pulse rate produces ambiguous velocity above 75 m/s (170 mile/hour), while detecting true range up to 300 km.
Low PRF radar have reduced sensitivity in the presence of low-velocity clutter that interfere with aircraft detection near terrain.
Moving target indicator is generally required for acceptable performance near terrain, but this introduces radar scalloping issues that complicate the receiver.
Low PRF radar intended for aircraft and spacecraft detection are heavily degraded by weather phenomenon, which cannot be compensated using moving target indicator.
For example, an L band radar system using a PRF of 10 kHz with a duty cycle of 3.3% can identify true range to a distance of 450 km (30 * C / 10,000 km/s).
Systems using PRF above 30 kHz function better known as interrupted continuous-wave (ICW) radar because direct velocity can be measured up to 4.5 km/s at L band, but range resolution becomes more difficult.
High PRF is limited to systems that require close-in performance, like proximity fuses and law enforcement radar.
[2] Sonar systems operate much like radar, except that the medium is liquid or air, and the frequency of the signal is either audio or ultra-sonic.
Like radar, lower frequencies propagate relatively higher energies longer distances with less resolving ability.
Signals propagate at the speed of sound in the medium (almost always water), and maximum PRF depends upon the size of the object being examined.
