The large antenna of the 3 GHz weather radar gives it an extremely narrow beam, only 0.25 degrees wide, resulting in an increased resolution (at 100km from the dish, the resolution of an 0.25 degree beam is 0.4km). It has dual-polarisation capability, which means that it can transmit and receive both horizontally and vertically polarised pulses.This makes it possible to determine the shape and orientation of cloud and precipitation particles in the atmosphere. It also has full 'Doppler' capability, which means it can map the radial component of a wind field.
CAMRa acts as a vital resource for a range of projects requiring meteorological radar data. Chilbolton has provided key event-based data for many recent Chilbolton Group programmes:
- Cloud and precipitation measurements for several NERC-funded experiments using the FAAM aircraft, such as APPRAISE and WINTEX.
- Modelling the effects of rain and weather conditions on short terrestral links, typically used for personal communications networks systems at 22 and 38 GHz and MVDS systems at 42 GHz, in the Swindon area.
- CAMRa provided precipitation data to support measurements of the variations in received signals from beacons on the GBS satellite.
The CAMRa Radar System
Over 20 years ago, an S-Band ex-air surveillance radar was first installed on the 25 metre antenna at Chilbolton for studying precipitation and clear-air phenomena. Since then, the original radar has been improved many times to measure new parameters. CAMRa operates at 3 GHz, where propagation effects are generally so small they can be neglected. At this frequency, using a large antenna provides the good sensitivity and high resolution needed for long range observations of precipitation. The majority of funding for the radar has come through the former Radiocommunications Agency of the DTI, now OfCom, and is used for developing and testing propagation models for terrestrial and satellite communications systems design.
Rain can cause millimetre waves to fade significantly. Dry ice, however, does not have such a severe effect. A key requirement for the radar is therefore the ability to distinguish ice from rain. In the 1970s, dish engineers installed fast polarisation switching, allowing researchers to measure differential reflectivity. This is the ratio of the horizontally and vertically polarised returns, which in turn depends on the different dielectric properties of ice and water.
Dry ice particles show less difference between vertically and horizontally polarised signals than raindrops of a similar shape. Thus many forms of ice, such as needles, snowflakes and hail have only a small differential signature, until they begin to melt. On melting, they show the combined effect of asymmetrical shapes and increased dielectric constant. This results in an increase in differential reflectivity which means that radar meteorologists can infer whether the hydrometeors are made of ice or liquid water. An example of these effects can be found in the research section of this site.
In the 1980s, the CAMRa system was dramatically overhauled. Engineers added equipment for measuring cross-polar returns. The cross-polar return is the ratio of the signal received in the horizontal polarisation channel from a vertically polarised transmission, to the co-polar reflectivity in the horizontal channel. This parameter is sensitive to particle shape, dielectric constant and orientation with respect to the plane of radar polarisation. It thus responds to canting of the particles in the radar pulse volume and is useful for identifying melting snow and ice. Another important development was the implementation of a pulse-to-pulse recording system by a group at UMIST (now at Reading University), which allowed studies of co-polar correlation techniques. A further enhancement to CAMRa took place in 1992/3.
A dual-channel receiver allows the co- and cross-polar parameters to be recorded simultaneously, and a novel technique has been implemented to enable Doppler and differential phase to be measured. PC-based data acquisition systems have been added, with data storage on CD-ROM. All real-time software for the PC and Signal Processors was written in-house, providing a flexible and easily modified system. Funding for this came from the former Radiocommunications Agency, now OfCom, with an important contribution to the Doppler implementation coming from NERC funding of the Reading University group.
Technical specifications of CAMRa radar
Technical specifications of 25m antenna