Precision Approach Radar (PAR) : Tech Question

[CoffmanStarter]

Back in analog radar days (the 70's and earlier) Air Traffickers would announce the need to "put the Rain Plates in" when high levels of precipitation were such that clutter on both the azimuth and glide slope plots made it difficult to resolve an aircraft trace during a GCA.

My question concerns the technical nature of said Rain Plates. I assume they were some kind of primitive filter or attenuation technology ... but exactly what ... I can't recall. Anyone got a clue ?

In today's world a nifty bit of digital signal processing would easily be able to remove such noise ... both precipitation and ground clutter.

A bit of an anorak question I know

Best ...

Coff.

[Rallyepilot]

PAR

If my memory serves me correctly the PAR had/s plates within the slotted wave-guide arrays (antennas) which, when selected by the operator, change the polarization of the radar emissions / reception from linear polarization (horizontal or vertical) to circular polarization. The use of circular polarization improves the radar performance in adverse weather conditions such as rain, thick cloud and fog etc.

[gayford]

Here is a quote from AP 3357 Operating Manual of Ground Controlled Approach Aids dated 1960:-

SLA 3-C (Precision Approach Radar)

CIRCULAR POLARIZATION

Circular polarization is a means of making an antenna less sensitive to responses from rain, while retaining sensitivity to other responses.
On this equipment each antenna (Azimuth and Elevation) is fitted with a "rain filter" (often called quarter-wave or circular polarization plates.) For operational flexibility the rain filters are hinged and provided with an electric drive motor so that they can be independently switched "in" or "out" of operation either locally of from the tower.

On the previous equipment, the SLA 3-B, the plates had to be manually inserted, which took circa 30 minutes. The SLA 3-C plates took less than a minute to motor in or out.

[MAINJAFAD]

When transmitted radio energy is circular polarised, any photon transmitted that is reflected off a water droplet is always phase shifted 180 degrees, therefore with a quiet simple bit of signal processing that reject signals 180 degrees out of phase with transmitted signal results in most of the rain clutter being rejected. It does however result in a loss of overall range performance. Hence the use of normal polarisation when rain clutter is not a problem. Modern PAR like RPAR still has a rain mode which results a reduction of range, so I would be surprised if it doesn't have a switchable polariser of some description within the antenna system. (All the software does is replace a shed load of hard-wired circuits within the old style signal processing system which processes the received signal after the analog receiver system has amplified it and done things like phase detection.)

[CoffmanStarter]

Thanks chaps all makes perfect sense now