THE SPACE TECHNOLOGY RESEARCH VEHICLE 2 MEDIUM WAVE INFRA RED IMAGER

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1 THE SPACE TECHNOLOGY RESEARCH VEHICLE 2 MEDIUM WAVE INFRA RED IMAGER S J Cawley, S Murphy, A Willig and P S Godfree Space Department The Defence Evaluation and Research Agency Farnborough United Kingdom 1 ABSTRACT The Space Technology Research Vehicle (STRV-2), launched on June 7 th 2000 on a Pegasus XL carries a Medium Wave Infrared (MWIR) imager. STRV-2 is the primary payload of the USAF TSX-5 spacecraft. The instrument is designed to detect aircraft from space and to collect imagery enabling the characterisation of various IR background types. The objectives and scope of the mission and the sensor design are addressed. The focus of the paper however is on the results of the mission. Data presented demonstrates the detection of aircraft from space and the quality of imagery obtained in the Medium Wave Infra Red from a small satellite. The telescope has also provided a demonstration and test of Commercial Off the Shelf (COTS) technologies through the incorporation of a Stirling cycle cryocooler and the flawless performance of this device is described. 2 INTRODUCTION The Space Technology Research Vehicle 2 (STRV-2) forms the payload module of the USAF Tri Service Experiments 5 (TSX-5) satellite, a technology demonstration mission, launched on 7th June 2000 aboard a Pegasus XL. On board the spacecraft is the Medium Wave Infra Red telescope (MWIR), an instrument designed and built by DERA in partnership with UK industry. The objectives of the MWIR are: 1. Demonstrate the use of infrared sensors to detect aircraft form space 2. Collect terrestrial background imagery over a range of terrain types and conditions 3. Assessment of MWIR capabilities against other targets (particularly ships) The MWIR instrument weighs 23 kg and consumes less than 60 watts of power. The telescope was designed, integrated and tested in the world class facilities at DERA, Farnborough. The main features of the telescope are: a) Dall-Kirkham optics comprising, a 200 mm diameter primary mirror, a secondary mirror and refractive transfer optics. A filter wheel containing 6 filters is used to select the required waveband. This contains 2 relative calibration sources. These are imaged pre and post image to give flat field calibration across the focal plane b) A focal plane assembly of 2, 2x512 staggered arrays of Cadmium Mercury Telluride (CMT) detectors separated in the along track direction. Thus the sensor operates as a dual push-broom sensor, obtaining 2 near simultaneous images.

2 c) A Texas Instruments 1 Watt Stirling cycle cryocooler to cool and maintain the focal plane to its operating temperature of 80K. This is a COTS design and this mission represents the first use of it on a space based imaging system. d) Carbon fibre composite telescope structure. This was designed and constructed at DERA so as to give a low CTE allowing the telescope to retain its focus, over a range of operating temperatures. e) Control electronics, pre-amplifier and memory designed and built at DERA. Figure 1 illustrates the main components of the instrument. Filter Wheel Cryocooler Radiator Focal Plane Primary Mirror Figure 1 Schematic diagram of MWIR instrument 3 CONCEPT OF OPERATIONS Aircraft movement through the air results in aerodynamic heating of the aircraft surface, which in turn results in a thermal contrast between the aircraft and the surrounding background. If the aircraft is at low level where the ambient air temperature is warm, the aircraft will appear hotter than the background. Therefore the aircraft will be in positive thermal contrast. Conversely, at high altitude, this aerodynamic heating is not sufficient to offset the low ambient air temperature and will appear colder than the ground underneath it (negative thermal contrast). At intermediate altitudes, aircraft and background radiances are similar and so a region of zero contrast exists. The detector arrays produces a ground resolution of 35 metres at perigee, which is an order of magnitude improvement over previous commercial instruments operating in the Medium wave IR (4-6 µm). However, at this resolution most aircraft will still be sub-pixel in size. This means that the thermal contrast may not be visible amongst the natural variation in background radiance or clutter. In order to reduce this clutter, the two near simultaneous images are co-registered and subtracted from each other. The resultant frame differenced image removes the background features. However objects that are moving or have changed between images, will show up as a pair of bright and dark spots or dipole corresponding to the object location in the two images. Aircraft detections at various speeds and altitudes, as well as the properties of different background types, will allow us to validate the assumptions made in the model.

3 The MWIR instrument is run by a small operations team based at DERA Farnborough. The instrument has successfully met its mission objectives within its 12 month operational life and had achieved 90 images in its first 6 months of operation. 4 RESULTS 4.1 Aircraft Detection and Background Data Collection Initial images were designed to check the correct functioning of the DERA ground segment, characterisation of the instrument alignment and the optimisation of various instrument settings. The first image was taken on the 14 th June 2000 over Cape Borda on the North West coast of Australia. Once the imager was successfully checked out, the operation of the MWIR was focussed on achieving the primary objective of aircraft detection. To do this images were taken during passes over busy air traffic routes in the UK and US, observing aircraft targets of opportunity. In addition to this, to ensure a small number of guaranteed detections, aircraft were chartered by DERA to fly under the spacecraft during passes over the UK. Air traffic control data from the UK and USA was procured to enable validation of the aircraft detection.

4 Radar Track First Image Second Image Contrail s Boeing 747 Clouds Dipole Margate Figure 2 Detection of Boeing 747 in MWIR imagery Figure 2 illustrates the detection of a Boeing 747 using frame differencing. The aircraft was detected flying at ft over a sea background off the northern coast of Kent. Also visible in the image is another aircraft contrail. The dipole can easily be seen in the section from the frame difference image. The aircraft detection phase of the mission was concluded in December As the spacecraft orbit makes the UK and USA less accessible, attention has now turned to achieving the other mission objectives. This includes imaging of various different targets and background types in order to better understand their spatial and temporal characteristics. Key measures of background distributions including mean and standard deviations in radiance and clutter lengths for different land cover types will be extracted. This in turn will allow validation of some of the assumptions made in the models used to support the design of MWIR and to allow refinement of these models for such systems in the future. Detailed comparison of the aircraft detection results and model predictions will be carried out after the operations phase is completed. However these models were used to provide guidance as to which aircraft sizes, altitudes and speeds would be detectable and was used to organize the DERA chartered flights and led to their

5 successful detection. Therefore we are confident that broad principles behind the model are correct. 4.2 Cryocooler Operation A Low Mass cryocooler was flown on the STRV-1b satellite built by DERA and launched in The objective of this payload was to establish whether such a COTS device, developed for terrestrial applications (it was designed as a cooler for a thermal imager on a tank) could perform satisfactorily in the harsh environment of space. This device achieved over 3000 operational cycles, and was still functional when the mission was terminated after 4.5 years. The STRV 1a/b mission served as a useful risk reduction opportunity. The successful use of a cryocooler on STRV-1b gave sufficient confidence that such a device could be used on STRV-2. Throughout operations the performance of the cryocooler has been flawless. Cool-down of the focal plane assembly to 80K is consistently achieved in 12 minutes. The power savings resulting from this better than expected performance, were sufficient to allow an increase in the frequency of image takes. 5 CONCLUSIONS To date the MWIR instrument has functioned well and has achieved all of its mission objectives within its 12 month operational life. It has been demonstrated that: High quality, moderate resolution imagery from a small satellite is achievable The principle of aircraft detection from space, using the aerodynamic heating of aircraft surfaces is possible. Frame differencing provides a useful method of reducing background clutter and aiding the detection of aircraft. The method does however have limitations, particularly since it relies on accurate co-registration of images, which may not be possible over cloudy backgrounds. A cryocooler built for terrestrial applications has been operated flawlessly in the harsh space environment of space both on STRV-2 and in previous DERA microsatellite missions demonstrating the utility of COTS components in small satellite, earth observation missions.