Radar Terminology

Radar video is derived as a sequence of returns, sometimes called spokes, which contain a set of samples of video intensity as a function of range for a specific azimuth. The radar may rotate or scan, and in so-doing creates a sequence of returns. In the situation of a search radar, for example, the rotating radar creates returns at a rate defined by the pulse repetition frequency (prf) and the rotation of the radar increases the azimuth from 0 to 360 degrees.

ACP/ARP: The azimuth of the radar for each return may be provided using a set of pulses called the ACP and ARP. The ACP (Azimuth Change Pulse) is a pulse train where the interval between pulses is a fixed angle. Commonly there are 2048, 4096 or 8192 pulses per 360 degrees. The ARP (Azimuth Reset Pulse) is a reset pulse that occurs at a reference point. The angle of the radar is therefore derived by counting the number of ACP pulses since the last ARP pulse.
Fast Time Constant (FTC): A FTC operation can be applied to a radar video signal to remove low frequency components, for example due to weather effects. The FTC processing will filter these low frequency components, so that only pulses that rise and fall quickly will be displayed.
Scan Rate/period: The rotation rate/period of a search radar, for example as the time between two North crossings.
Pulse Repetition Frequency (PRF): The PRF is the rate at which the radar generates new returns. In a pulse radar it is the frequency of transmission of new pulses.
Range resolution: The ability of the radar to discriminate two targets that are closely spaced in range. For example, a range resolution of 10 metres means that two targets that are on the same azimuth and 10 metres apart in range can be resolved.
Range sampling: Radar video provided as an analogue signal is sampled at discrete intervals. The frequency of sampling is limited by the capture hardware and is ideally chosen to ensure that the full bandwidth of the radar video is captured. For example, a radar video signal that is bandwidth limited to B Hz can be fully reproduced by sampling at 2B Hz.
Sector Blanking: Sector blanking is the process by which a sequence of azimuth values are blanked, meaning that the video is forced to value 0.
Range Blanking: Range blanking is the process by which a sequence of range samples are blanked, meaning that the video is forced to value 0.
PPI: The Plan Position Indicator is the view of radar video that shows the radar as it would appear on a plan view, that is a polar coordinate display of the area surrounding the radar platform. The radar position is represented as the origin of the sweep, which is normally located in the center of the scope, but may be offset from the center.
A-Scan: A-Scan view of radar video is traditionally associated with an oscilloscope display. It shows signal intensity on the vertical axis and range (time) along the horizontal axis. The trace is updated for each azimuth at the pulse rate of the radar.
B-Scan: B-Scan is a view of radar video that shows azimuth along the horizontal axis and range along the vertical axis. It is commonly seen in fire control radars.
Azimuth Correlation: If there are more input returns than required for display or processing, or if some degree of processing is required for filtering, then azimuth correlation may be employed. The process combines a number of input returns for each output using either a peak-picking, averaging or other method of combination.
Range Correlation: If there are more samples per return than required for display or processing, then range correlation may be used to affect a reduction. A number of range samples are combined using either a peak-picking, averaging or other method of combination.
Sparse azimuths: If the number of returns being generated by the radar is less than a pre-defined store dimension, then the radar can be said to be generating sparse azimuths. For example, if a store dimension is set of 200 but the radar only generates 500 returns per scan, then only 1 in 4 of the azimuths in the store is populated. Special processing can be invoked to fill-in the sparse azimuths.
Random scan: If the radar can switch azimuths, for example because it is electronically rather than mechanically steered, then it can generate azimuth numbers that are not necessarily increasing steadily over time. In general, the radar can produce any arbitrary azimuth sequence and the display is expected to show this effect. In this situation the radar is said to be working in random scan mode.
Sector scan: In the situation where the radar is scanning between a start and stop azimuth, rather than a continuous 360 degree scan, the radar is said to be operating in sector-scan mode.