Medium Wave Infra Red Telescope
Introduction
On 7 June 2000, the DERA Medium Wave
Infra red (MWIR) telescope was launched
on a 12 month mission. MWIR’s mission is
to detect aircraft in flight using space based
infra-red technology and characterise the
infra red background.
The Theory
The medium wave infra red part of the electromagnetic spectrum (3.0 to
6.0 m) has often been overlooked in terms of detection technologies.
Work at DERA during the early 90’s indicated that non-afterburning
aircraft in flight could be detected in these wavebands by virtue of their
thermal contrast with the background. At high altitude, the cold air,
moving over the surface of the aircraft, cools the wings and body to a
lower temperature than the background. At low altitudes, the fast moving
heats the wings and fuselage to a higher temperature than the
background.
The images would have the target aircraft and the background in view.
Depending on the nature of the background (sea, urban, rural etc)
considerable clutter may be present. To overcome this, a frame differencing concept was used. Two
images, taken a short time apart, could be frame differenced to reduce background clutter.
The Design and Build Phase
The MWIR was designed and built by DERA with
support from UK industry (most notably Sira Ltd
and GEC-Marconi Infra Red Limited). The MWIR
optics were assembled at Sira, with the whole
telescope and electronics integrated and tested at
DERA’s world class facilities in Farnborough, UK.
Its total mass is 23kg and it uses less power than
a 60W light bulb. Cooling of the focal plane is
achieved by a Stirling Cycle Cryocooler which
pumps heat away from the detector and radiates
it to space. A further significant feature is that the
sensor housing is manufactured from a carbon
fibre composite material.
This not only
contributes to its low mass, but with the carbon fibres carefully arranged, ensures that the telescope
remains in focus despite the temperature changes experienced in space.
Within the body of the telescope a filter wheel allows any one of
six filters to be selected for an image in conjunction with either
of the two calibration sources. The filters allow different regions
infra-red spectrum to be viewed. The DERA designed and built
electronics consist of a 32Mbyte direct access memory together
with control and measurement electronics to plan and execute
the image. The resulting image is approximately 16 x 120km.
Graphic courtesy of JPL
MWIR
DERA/KIS/SPACE/PUB001039/B
The MWIR telescope and controlling electronics were
integrated into the Space Technology Research Vehicle 2
(STRV-2) Module at the Jet Propulsion Laboratory (JPL),
California. Finally, the STRV-2 Module was mated to the host
bus (TSX-5) at Orbital Sciences (Germantown USA) and
shipped to the launch site.
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The Main Spacecraft
The MWIR telescope is the one of the
principal experiments on the STRV-2
Experiment Module, the primary payload on
the American Tri-Service Experiment 5
satellite (TSX-5). STRV-2 also carries a
number
of
US
experiments;
laser
communications, vibration isolation, radiation
monitoring, and electronics testing.
Photo courtesy of Orbital Sciences Inc
The launch
TSX-5 was launched on a Pegasus
rocket launched from Vandenberg (see
http://mocc.vafb.af.mil/launchsched.asp
for additional details). The three-stage
Pegasus launch vehicle is carried aloft by
the L-1011 “Stargazer” aircraft to a point
approximately 40,000 feet over open
ocean areas, where it is released and
then free-falls in a horizontal position for
five seconds before igniting its first stage
rocket motor. With the aerodynamic lift
generated by its delta wing, the small
rocket achieves orbit hundreds of miles
above the Earth in approximately ten
minutes.
Graphic courtesy of Orbital Sciences Inc
The orbit achieved was 1706 x 408 km at
an inclination of 69.
Inclination = 69
Perigee
408km
Apogee
1706k
m
The Mission
The selected orbit is a compromise between
the varied and disparate requirements for all
the experiments. However, from the MWIR
perspective, this leads to the usable perigee
sector of the orbit (below 500km) moving
north after launch to arrive over the UK 6
weeks after launch, with a local imaging time
of around 19:00. During the summer the
perigee sector of the orbit continued to move
north and local imaging time became earlier
each day, reaching around 08:00 in early
September.
The mission initial plan was to ensure the
instrument had survived launch and start
gathering background data.
Once the
perigee sector of the orbit was over the UK, carefully planned experiments were conducted with
dedicated aircraft flown by DERA-Boscombe Down. Image passes over known air lanes were
compared with data provided by the UK National Air Traffic Service (NATS) to prove that aircraft were
detected.
DERA/KIS/SPACE/PUB001039/B
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Results
The MWIR has operated flawlessly. Since launch over 50 images have been taken. A selection of
these images is presented below.
Big Bear Lake,
nr Los Angeles
Gibraltar
Newbury, England
London
MWIR has achieved its primary mission goal, by detecting this 747 aircraft over the Thames estuary
(UK). The left hand image shows the context of the detection, with the aircraft position aligned to
NATS data. Clouds, Margate town and another aircraft “con-trail” are also visible. The central image
pair are taken from the MWIR’s twin detectors. These image the same area approximately 0.7
seconds apart, showing how the small cold (dark) image of the 747 has moved. The 2 images are
“frame differenced” in the 3rd image to illustrate a “detection dipole”. A “slice” through that dipole is
shown in the graph, clearly illustrating the peak and trough.
Detection of Boeing 747 over Thames Estuary
Track from radar
Pixel Intensity Profile through the Dipole
Dipole Bright Spot
Background Variation
Contrail from
another
aircraft.
Dipole Dark Spot
First Image
Clouds
Second Image
Dipole
Boeing 747
Margate
DERA/KIS/SPACE/PUB001039/B
Differenced Image
(after processing)
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The Future
MWIR has met its main mission goals. The characterisation of the infra-red background in the
medium wave is dependent on the number of images obtained. Therefore, MWIR plans to continue
operations well into the year 2001. The results obtained will enable future spacecraft to benefit from
this low cost technology demonstrator. DERA has recently been awarded a contract by BNSC (in
conjunction with MoD) to develop the Topsat spacecraft. Topsat will build on the lessons of MWIR to
provide World Class support to MoD and further develop the UK’s small satellite industry.
DERA/KIS/SPACE/PUB001039/B
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