Improving instruments for infrared remote sensing

Thesis



Remote sensing of the Earth's atmosphere, typically performed in the infrared region of the spectrum, plays an important role in scientific research. In the past the instruments used to perform these observations have been large, massive devices and correspondingly have only been able to be placed on large satellites. There is currently a trend toward smaller Earth observing platforms, so-called micro-satellites, and there is therefore a need for smaller, less massive instruments. Typically these instruments utilise a semiconductor device that responds to incoming infrared radiation in a known way. Such devices are subject to a number of noise sources that reduce their performance. By cooling them to temperatures around 80K it is possible to significantly reduce the amplitude of this noise compared to the incoming radiation of interest, thus increasing the signal-to-noise ratio (SNR). Typically this cooling is performed by a mechanical cooler, but currently many of them are too massive and require too much power to be suitable for use on a small remote sensing satellite.

By considering a typical application, a performance target for a miniature cooler was determined to be a heat lift of 200mW at a cold tip temperature of 80K. Hardware has been created to investigate the feasibility of achieving this aim with a hybrid cooler/radiator. The cooler is a miniature integral Stirling machine and uses flat spiral flexures with a newly designed linear motor to drive the compressor piston; the displacer is driven pneumatically.

The prototype initially underwent characterisation without first being pre-cooled by the radiator. Although significant cooling was observed (to below 170K), initial characterisation highlighted a low thermal resistance between the warm end of the cooler and the cold tip. With pre-cooling the cold tip was able to reach a minimum no-load cold tip temperature of 92K, and with 200mW applied to the cold tip a temperature of 122.4 K was sustainable. Attempts were made to increase the thermal resistance between the warm end and cold tip by introducing thermal breaks into the regenerator, and whilst these did increase the thermal resistance, the overall performance of the cooler decreased.

The concept of a hybrid miniature cooler/radiator has been shown to be feasible. To achieve the target performance of a heat lift of 200mW at 80K further work needs to be performed to characterise loss processes within the cooler and increase the thermal resistance between the warm end and cold tip.

Authors: Camilletti, A; Camilletti, Adam

• Publication date: 2006
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Remote sensing; Infrared spectroscopy; Infrared detectors; Atmosphere