Radar Basics

Surveillance Radar vs. Weather Radar

This page introduces a comparative View of the characteristics of PSR and weather radar systems in tabular form.

Frequency

PSR operates in L, S-band (mainly L-band). Weather radar operates normally in S, C, X-band (also L-band if there is a weather Channel implemented in the PSR system). As discussed earlier. the L-band for PSR is the most suitable frequency for tracking moving targets. The shorter wavelength in S, C and X-band make these bands more suitable for weather tracking.

Doppler

Although Doppler processing was not introduced to the weather radar field until comparatively recently, all of today's commercially available Weather Radar Systems have Doppler processing installed. There are still some non-Doppler weather radars in operation but as a rule, when these reach the end of their operational service life, any replacement is normally Doppler capable.

Antenna Scanning

PSR with a Cosecant² Antenna scans normally azimuth only. (There are often specially Hightfinders in comparison.)

Weather radar operate in azimuth and Elevation with help of a “Pencil Beam Antenna” (variable elevations in horizontal scans is most common configuration).

One of the main limitations of PSR in comparison with weather radar it scans constantly with a wide elevation, and single elevation range. This is necessary as there is an operational need for a target picture during every scan. this is why, using PSR with a “weather channel” mostly provides low quality weather pictures.

Weather Radar uses more advanced antenna technology to allow it some variance of its elevation in the vertical plane as well as the horizontal azimuth. The most common operational configuration is to modify the scanning elevation on each scan (or couple of scans to compile a volume picture of the weather system. This elevation variance is possible due the fact that the weather picture is only output every few minutes at most (weather is more predictable than aircraft and the safety aspect is not as significant).

Processing

The processing necessary to compile the target picture from a PSR is supplemented by a significant number of functions and filters. The processing of PSR pictures can be defined as „complex”. Weather Radar performs a similar function, with processes and filters applied to discard unwanted replies. However, the complexity of the processing task increases for weather Radars as they have to compile a volume scan of the weather, using the multiple elevation scans performed by the antenna.

Polarisation

Linear and Circular polarisation techniques are used to eliminate weather clutter in L- band PSR radar systems. Weather Radar because they operate on multiple elevations) are able to compare horizontal and vertical linear polarisation to help estimate different weather types. (more...)

Peak Power

The power level of each type of system will vary, depending on the microwave source and the technical characteristics which each frequency requires. However, every system varies and the general charachteristics of both systems could foresee peak power levels between 200 MkW and about 1500 MW

Processing

Both PSR and Weather Radar use standard I & Q processing techniques.

“Picture” Update

PSR systems provide an updated MTI picture on every scan (this is commonly between 6 and 12 seconds). This is necessary to meet operational requirements. For weather Radar, the “weather picture” comprises data from multiple scans which may be over a period of several minutes.

Clutter processing

Both systems use clutter processing techniques extensively. The number and exact type of functions built into the system is largely implementation dependent (although all are normally Doppler based). The only main difference is that for PSR, weather is clutter and for weather radar, moving targets are a cause of clutter.

Antenna Sizes

The physical requirement for antenna size depends to a large extent on the frequency (and the wavelength) being used. In general, longer wavelength (e.g. L-band) will require considerably larger antenna technology to support them than shorter wavelengths (e.g. X- band - which can fit into an aircraft nose).