Reference: http://www.radartutorial.eu/01.basics/Distance-determination.en.html
Distance/Range
Range is the distance from the radar site to the target measured along the line of sight.
Direction-determination
The angular determination of the target is determined by the directivity of the antenna. Directivity, sometimes known as the directive gain, is the ability of the antenna to concentrate the transmitted energy in a particular direction. An antenna with high directivity is also called a directive antenna. By measuring the direction in which the antenna is pointing when the echo is received, both the azimuth and elevation angles from the radar to the object or target can be determined. The accuracy of angular measurement is determined by the directivity, which is a function of the size of the antenna.
In actual practice, search radar antennas move continuously; the point of maximum echo, determined by the detection circuitry or visually by the operator, is when the beam points direct at the target. Weapons-control and guidance radar systems are positioned to the point of maximum signal return and maintained at that position either manually or by automatic tracking circuits.
Maximum Unambiguous Range
- Definition: The maximum unambiguous range (Rmax)is the longest range to which a transmitted pulse can travel out to and back again between consecutive transmitted pulses.
Rmax is the maximum distance radar energy can travel round trip between pulses and still produce reliable information.
The relationship between the PRF or their reciprocal value interpulse period T (PRT) and Rmax determines the unambiguous range of the radar. Suppose the radar emits a pulse that strikes a target and returns to the radar in round trip time t:
If t < T then the return signal arrives before the next pulse has been emitted.
If t = T then the return signal arrives exactly when the next pulse has been emitted.
If t > T then the return signal arrives after the next pulse has been emitted and there is an ambiguity, ie the radar
The reflection of a target of the first pulse is received after the second pulse has been transmitted. This causes some confusion since the radar, without any additional information, cannot determine whether the received signal is a reflection of the first pulse or of the second pulse. This leads to an ambiguity in determining the range, this received echo signal be mistaken as a short-range echo of the next cycle.
The pulse repetition time (PRT) of the radar is important when determining the maximum range because target return-times that exceed the PRT of the radar system appear at incorrect locations (ranges) on the radar screen. Returns that appear at these incorrect ranges are referred to as ambiguous returns, second-sweep echoes or second time around echoes.
Exemptions
More modern 3D- radar sets with a phased array antenna (like the RRP-117) don't have this problem with an ambiguous range. The system computer steers the transmitted beams so that ambiguous returns from the previous pulses are not received while the antenna beam points in another direction.
If the radar uses intrapulse modulation and uses a different waveform in each transmit pulse, the maximum unambiguous measuring distance is of no significance for the radar. Each received echo signal can be assigned to exactly its origin (the individually transmitted pulse) and thus the runtime over several pulse periods can be measured.
Radar sets in satellites for the remote sensing of the earth can, however, also represent targets at the correct distance, if they are actually ambiguous. The general height of the orbit is known, so only a distance can be measured that differs by a few kilometers from the height of the orbit. Compared to Fig. 1, this means that at an altitude of 400 km only the measurement result received in the second pulse period can be valid.
Minimal Measuring Range
Monostatic pulse radar sets use the same antenna for transmitting and receiving. During the transmitting time, the radar cannot receive: the radar receiver is switched off using an electronic switch, called duplexer. The minimal measuring range Rmin (“blind range”) is the minimum distance which the target must have to be detected. Therein, it is necessary that the transmitting pulse leaves the antenna completely and the radar unit must switch on the receiver. The transmitting time τ and the recovery time recovery should are as short as possible if targets shall be detected in the local area.
Targets at a range equivalent to the pulse width from the radar are not detected. A typical value of 1 µs pulse width of a short-range radar corresponds to a minimum range of about 150 m, which is generally acceptable. However, radars with longer waveforms suffer a relatively large minimum range, notably pulse compression radars, which can use pulse lengths of the order of tens or even hundreds of microseconds. Targets at ranges closer than this minimum are said to be eclipsed.