Clearer outlook

A UK-designed remote-sensing satellite instrument could play a key role in a space mission that will compare cloud distribution with information about solar radiation. The information will give a more detailed picture of how the altitude and density of clouds affects climate change.

The device, developed by Cardiff firm

, will form an integral part of a highly-specialised cloud profiling radar (CPR) used in a joint

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(JAXA) project called EarthCARE (Earth Clouds, Aerosols and Radiation Explorer).

The mission will create an Earth-orbiting radar system that will emit a pulsed signal to bounce off clouds and back into the instrument. The returning signal will provide information about the distribution of material such as ice in the clouds at the top levels of the atmosphere, as well as their height and density.

The project is based on the knowledge that clouds and other particles in the atmosphere can affect climate. For example, low clouds cool the climate by reflecting short wave solar radiation back into space, whereas high clouds warm the climate because they trap infrared radiation while allowing the sunn's energy to pass through them.

In the present climate, these two effects are large but mostly cancel each other out. Uncertainty about the effect of clouds is a major limiting factor on predictions of future climate change.

Keating's device, known as a quasi-optical feed multiplexer, has been designed so the signal is sent and received by the same main 2.5m satellite dish. This makes the equipment more practical for inclusion on a satellite, but it also poses some technical challenges because the transmitted and received signals need to be separated. If a signal from the transmitter hits the receiver before it hits the cloud, it can cause great disruption.

'We've designed the multiplexer so when the transmitter emits a pulse — at about a kilowatt of power — very little finds its way directly to the receiver,' said Richard Wylde, the company's managing director. 'That's important because if you're putting out kilowatts it's not difficult to fry the receiver.'

Wylde said with testing at the UK

(NPL) his researchers were able to prove that only one part in 10,000 of the signal leaks from transmitter to receiver.

The team was able to maintain this separation of signals by using circular polarised radiation. The multiplexer is used to convert linear polarised signals from the receiver into circular polarised signals.

The NPL provided the team with an anechoic chamber, lined with materials that are designed to absorb signals, rather than bounce them back. It was also possible to conduct vibration and temperature testing there.

Contained within the box are two gold-plated tungsten wire grids, 20 microns wide. These are used to convert the signal from linear to circular polarised and keep the signal from travelling erroneously to the receiver. Another grid sits at the opening of the box and allows the signal beam to pass through.

The box processes the signal like an interferometer. Inside, the beam will be split into two and one part will suffer a delay of a quarter wavelength before it is re-combined with the other. The two beams are orthogonal in polarisation — one vertically and one horizontally polarised — and their sum is a circular polarised beam.

After the beam leaves the box, it will hit the hyperboloid sub-reflector to be emitted to the target cloud. When the signal returns from the cloud, it hits the satellite's main dish and sub-reflector before travelling back into the box. The beam will then hit the initial grid, but this time it will be polarised at 90º so that it reflects up to the receiver.

Wylde said the method is simpler, more robust and has less mass than other possible signal conversion technology that could be used on Earth, including using gyromagnetic materials.

The actual date for lift-off is tentative because of various funding issues. Wylde said that much of this has to do with the weakness of the Japanese yen.

But when the satellite is launched, it will be a proud day for Wylde and many of those involved in the project.

'It's great working on a project like this because I grew up at the time of the moon landings,' said Wylde. 'I have to admit there is a thrill to making subsystems for spacecraft.'