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• Technology

Radar Performance Parameters

Radar Performance parameters define what a radar does. Characteristics describe how a radar achieves its performance. Features are desirable characteristics. This tech note discusses the nature of the parameters that define radar performance.

Maximum Detection Range

False alarm detection plays a significant role in the effectiveness of a radar and its maximum detection range. A false alarm is an event that erroneously signals the presence of a radar target when there is no legitimate target.

The maximum detection range of a radar is the longest distance from the radar to a target at which the radar can reliably declare that the return signal from the target has exceeded a set threshold. The return signal is usually very low in amplitude and must be detected above the thermal noise level in the radar electronics. In well designed radars, the thermal noise is quite low and small signal returns can be detected reliably. This threshold is set based primarily on reducing false positives (false alarms) that would occur on noise energy generated internally to the radar. This noise is present in all radars and is called thermal noise because its amplitude is a function of the radar temperature – higher temperature causes more noise.

Many factors influence the ability of a radar to detect a target. The radar design itself, the type of target (person, vehicle), what characteristics of the target determine its measurability (size, speed), the distance from the radar to the target, the size of the target (radar cross section), the environment between the radar and the target (rain, fog) and the environment in the immediate vicinity of the target called clutter, like trees, grass, building – they all compete with the target.

Since the target must be detected in a background of noise and/or clutter, the likelihood of detecting a target on any given opportunity (look) is statistical, that is, it varies from look to look. That is because the noise amplitude varies in a random manner from look to look, either adding to the target signal or subtracting from it. The statistical nature of the target creates even more uncertainty. The Probability of Detection on any given look is a measure of the likelihood of detecting the target, or of having the signal from the target cross the previously mentioned detection threshold. The nature of the statistics of the noise, the target and the clutter are all different, but can be described mathematically by complex equations which predict detection range for various false alarm conditions, which are selected by the radar designer.

All of the above factors affect the maximum detection range of a radar for a particular target, and any testing for determining detection range, must account for the statistical nature of the process by doing many trials to establish this parameter.

False Alarm Rate

From the foregoing description, it seems that the detection range could be increased by lowering the detection threshold to "see" lower signal levels. While this is true, the noise environment described means that, on any given look, there is a finite probability that a noise spike could cross the threshold, causing a false alarm. Thus exists the classical battle between sensitivity and false alarms; that is, the desire to increase the Probability of Detection is offset by the resulting increase in the Probability of False Alarms. The latter is generally quantified in time by using the parameter False Alarm Rate (FAR), which expresses the false alarm probability as a function of time. Thus, all comprehensive radar specifications contain a FAR requirement, say 2 or 3 per day, so that a radar operator is not unduly distracted attending to an alarm that doesn’t really exist. This becomes very important in security systems which combine many radars for perimeter or border protection over long distances, because higher false alarm rates require more responders to chase down the cause of an alarm.

Revisit Time

Radar revisit time is the time it takes for the radar to complete its search for targets and return to begin another search interval. For example, in a radar that goes around 360 degrees, it is the time for one revolution to be completed. Since the target detection process is statistical, it follows that the more time the radar “looks” at an area where there is a target, the sooner that target will be detected. Of course, the longer the detection range, the longer period of time can be allocated to the detection process. Also, the slower target speeds can be allocated more detection time. For example, a crawling person may need many looks to establish detection, due to the very small target size – but the crawler doesn’t travel very far during the process, so a relatively long detection time is acceptable. A fast moving, vehicle should be detected quickly or it will travel a long distance prior to detection.

A short revisit time improves the detection process and also improves target tracking after detection. Since target speed and direction are not controllable, more looks in a given time will result in more accurate tracking of the target.

Radar Resolution

The ability of a radar to detect and track a target is affected by the radar’s resolution, that is, how small of a "space" does the radar look at. Radar space is defined in four dimensions – range, azimuth angle, elevation angle and speed.

Not all radars measure in all these dimensions. For example, a police radar uses the angle and speed dimensions, and doesn’t measure range to the vehicle. Generally, a maritime navigation radar uses the angle and range dimensions, and doesn’t measure speed because most returns from this radar are not moving.

Almost all radars limit the area they look at in angle, because they use an antenna to focus the energy on a suspected targeted area. This angle is two dimensional, horizontal (azimuth) and vertical (elevation), generally expressed in degrees. A “pencil” beam is symmetrical in both planes, but many radars will have a very narrow azimuth beam width, depending on the radar’s function. For example, a maritime navigation radar will have a very narrow azimuth beam width to very accurately trace a shoreline, but will have a very wide elevation beam so the boat can pitch and roll in the waves but the land will still be within the beam.

Resolution in range is important to accurately determine the range to a target and to eliminate clutter behind and in front of the target. Some radars measure the radial speed of the target directly using the Doppler principle, which states that the speed of a moving object will affect the frequency of the return signed. This “speed resolution” feature is useful to discriminate moving vs. stationary target. However, because wind-blown vegetation and rain can appear to be moving, much care must be taken not to generate false alarms for ground radars susceptible to windblown clutter motion.

Radars which have small resolution cells are called high resolution. High resolution radars provide more clutter background rejection, which radars discriminate better against competing returns from the ground, grass, trees, and rainfall. The drawback of high resolution is that it takes longer to search for a target because more resolution cells must be looked at to find the target.

Search Volume

Typically the more "space" a radar can search for targets, the more utility the radar provides to a surveillance system. A 360 degree radar may be more useful than one that merely scans a sector, unless it is known with certainty where potential targets exist. However, there is a trade off between available search volume and resolution. It takes longer to search a given volume with a high resolution radar than a radar with less resolution. Therefore, there is a constant battle between revisit time, resolution and search volume.

Summary

This technical note has presented the major performance parameters which describe how well a radar performs. The design of a radar is primarily one of performance tradeoffs, involving range resolution, search volume, clutter rejection, false alarm rate and revisit time. These issues must be balanced against target types, frequency allocation regulations, size, weight, cost, power and environmental considerations, such as rain, snow, operating temperatures, vibration and shock. A well designed radar represents a delicate balance of many seemingly incompatible factors.

• Radar Performance