A Calibration Facility for Satellite Borne Remote Sensing Instruments


G. S. Mand and G. V. Bailak

Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7

Abstract

This paper briefly describes the University of Toronto Instrument Characterisation Facility (ICF), located in the Physics Department. It will describes some of the calibration equipment which may be used to characterise an instrument. The ICF will initially be used to calibrate the Measurements Of Pollution In The Troposphere (MOPITT) instrument. It is due to be launched on NASA's Earth Observing Satellite, EOS AM-1 platform in mid 1998. In the future it is hoped that the ICF will be used to calibrate other satellite instruments.

Introduction

Rigorous pre-flight calibration and characteristion of space instruments is vital in understanding on-orbit instrument behaviour over various timescales, to end of life. Pre-flight calibration data sets can be incorporated into the retreival algorithm producing more accurate final data sets.

The University of Toronto, Instrument Characterisation Facility (ICF) is located in the Physics Department. This facility will first be used to calibrate and characterise the MOPITT engineering and flight models.

MOPITT will measure global concentrations of carbon monoxide (CO) and methane (CH4) in the lower atmosphere. It will measure CO profiles and columns and CH4 columns using the correlation spectroscopy technique. MOPITT is a cross track scanning, nadir viewing eight channel infra-red (IR) radiometer. Figure 1 shows the external layout with covers open. The instrument has four input scan mirrors and two identical "mirror imaged" optical tables with internal calibration sources, scan mirrors, choppers, modulators and cold dewar assemblies containing narrow band filters and the detector package. The coldplate located underneath the base plate is the main thermal sink for all modules except the main power supply which is thermally isolated from the baseplate and radiatively cooled. The instrument has a 7.2° x 1.8° nadir field of view and improves its coverage by cross track of scanning ±14 fields. The scan mirror can also rotate through 90° for a space view and through a further 90° to view the internal radiometric targets. The instrument itself is discussed in more detail in another paper in this conference (Drummond, 1994).

Instrument Characterisation Facility

The ICF, shown in fig. 2, consists of a clean room, preparation room, pump room, general purpose room and the control room. The vacuum chamber used for all tests is located in the clean room.

Clean Room

The clean room is a "room within a room". It consists of vinyl coated modular walls with return air ducts. The ceiling is a vinyl coated 10ft suspended tile ceiling. The floor is painted with a two part epoxy floor paint. The clean room is class 10000, designed and tested, and consists of pre-filters for the building air and presently seven HFM2000 HEPA filter fan modules located in the suspended ceiling. The HEPA filter units can be easily replaced, their locations changed and additional units can be can be added if required. Air is drawn down from the plenum volume above the suspended ceiling through the HEPA filters, which entrap any particulates, and into the clean room volume. Air and particulates are swept towards the room perimeter and returned to the plenum volume via the return ducts located around the perimeter. A portion of the plenum volume is constantly changed through the building air supply system.

Amenities within the room consist of compressed air lines as well as 208v three phase and 110v electrical outlets. Wire equipment and tool racks are also available as well as a 3x5ft "laminar flow" bench consisting of one HEPA filter.

Preparation Room

All personnel and equipment enter the clean room through a preparation room using approved clean room entry procedures. This room is an extension of the clean room having the same walls and ceiling system with one HEPA filter unit. The preparation room has a gowning area and a clean table located directly below the HEPA filter unit.

Pump Room

The pump room, located behind the clean room, holds the mechanical backing pump and the two cryo-pump compressors. In future it is planned that re-circulating chillers used for the MOPITT coldplate and calibration blackbody source may also be stored in this area. This equipment has been isolated in order to minimise the thermal dump, vibration and microphonics into the clean room area. An isolated exhaust air duct is located in this area. Filtered building water is available and is used to cool the mechanical pump. Five 208v three phase inputs are available, two of which are currently dedicated to the cyro-pump compressors, furthermore, there is one 600v dedicated input. 110v electrical outlets are also available. Access to the pump room is through the general purpose and control rooms only.

General Purpose Room

The general purpose room will primarily hold the 500l liquid nitrogen dewars which will feed the direct line to the vacuum chamber in the clean room. This line consists of an inner 1/2" liquid nitrogen line, with an outer return gas line and an outermost vacuum jacket sleeve. The line runs up into the plenum volume and into the clean room area to the vacuum chamber. Work benches are also provided in this area.

Control Room

All communications and data handling will be conducted through the control room. All instrument, vacuum chamber and ancillary cabling is ducted to this area. This area consists of the vacuum chamber control console, a Sun Sparc station 10 with further PC's and network options. The control room has work areas for four people. There is also visual access and a voice link into the clean room.

Vacuum Chamber

The aluminum vacuum chamber, shown in fig. 3 is 6m long with a 2.2m inner diameter. It has four sections, a fixed base sections that holds the instrument and the three movable sections that hold test equipment and can be brought up to the fixed base section.

The fixed base section has one 8", four 12" and two 20" flanges. The two 20" and 8" flanges are dedicated to the pumping system. One 12" flange is dedicated to the vacuum chamber instrumentation and the other three are intended for the instrument. The other central section has a similar flange configuration. The larger flanges allow for upgrading the pumping system and the five smaller flanges will be used for test equipment, electrical feedthroughs and plumbing. The two end sections have liquid nitrogen feedthroughs and a 12" window (the window may be used as a feedthrough port if required). At present one feedthrough is being used to fill a 50l reservoir, with a 30l nitrogen fill capacity, located in one end section. This in turn fills the cryo-panels and any other test equipment (shrouds and target) using gravity feed. The use of the internal reservoir minimises the direct line cooldown and hence minimises the boil-off. The internal and external liquid nitrogen system is presently undergoing tests.

The pumping system is shown in fig. 4 and consists of two 17.5" CTI 400 on-board cryo pumps (mounted on the fixed base section) with compressors located in the pump room. Two 20& pneumatic gate valves are used for isolation. For rough pumping an integral Stokes 2000cfm roots blower backed by a 500cfm rotary pump are located in the pump room. An 8" pneumatic gate valve is used to isolate this from the chamber. Furthermore, one oil free Drytel 31 turbo pump is used to regenerate the cryo-pumps. There are also four 48x24" cryo-panels located in the end sections. At present only two panels are being used. Instrumentation consists of a pirani guage, a cold cathode and a Bayard Alpert guage. Furthermore, a Vascan residual gas analyser (RGA) is also available for leak checking and to monitor contaminants. The chamber has automatic, interlocked, pump down and vent control electronics with manual overide options. With the chamber clean and empty the roughing system pumps to 10-3 torr in about 30 mins. At this point the cryo-pumps take over and can pump down to 3x10-7 torr in under 3 hours. Filling the cryo-panels will bring the base pressure down below 10-7 torr in a further 15 mins (a typical base pressure is 7x10-8 torr).

ICF Main Calibration Equipment

The ICF must be capable of radiometric, field of view (FOV), spectral and thermal characterisation of an instrument (in this case MOPITT). All equipment will be located inside the vacuum chamber in the desired configuration required to characterise the particular aspect of the instrument being investigated.

Radiometric Targets

There will be three radiometric targets, two blackbody sources and a solar simulator.

The main calibration blackbody target (MCBB), shown in fig. 5, consists of a 25° retro-cone base with a 22cm inner diameter with a 75cm long two section tapered baffle. The insides will be anodised using Martin Marietta (MM) Enhanced Optical Black and the outside will be covered with a minimium 10 layers of mylar multi layer insulation (MLI). An interchangable rectangular aperture will define the FOV. The MCBB temperature will be monitored by six Rosemount four wire 118MK, 100 PRT's, four located on the base section and two along the baffle. They will be monitored using an ASL F250 AC bridge with a 16 channel switching box. The MCBB will operate over a 220-520K temperature range. From 220-350K it will use the coolant ring and jacket arrangement with a Neslab ULT80 re-circulating chiller in conjunction with Galden HT110 fluid. From 350-520K it will use the thermostatted polyimide kapton heaters. Specifications are given in Table 1.

The second blackbody, the space view blackbody (MSBB), shown in fig. 6, consists of a 29 x 34cm rectangular aluminium linear V grooved baseplate (the V grooves have a 15° half angle, 2cm pitch and are 3.73cm deep) with a 35cm long tapered baffle. The V grooves and inside of the baffle are sprayed with Aeroglaze Z306 diffuse black paint and the outside will be covered with 20 layers of MLI. An interchangable rectangular aperture will define the in field response. The MSBB temperature will be measured and monitored as described above. This blackbody is a fixed point "zero radiance" cavity designed to operate at 80K by flooding the rear cavity and baffle jacket with liquid nitrogen. Heaters are mounted on the base section for fast warm up. Table 1 summarises the blackbody specifications.

The solar simulator (MSRS) shown in fig. 7, is a stainless steel cylinder 24cm long with a 42cm outer diameter. It consists of a 21cm diameter Spectralon diffuser plate located at one end with a 19cm window at the other end. A twenty four lamp ring arrangement consisting of twelve 20W quartz iodine (QI) and twelve 10W QI bulbs is used as the source. This arrangement allows the radiance to be stepped down by using different sub-rings (the ring power can be stepped down in 60W steps from 360W). The MSRS is filled and sealed with an inert gas to assist in heat transfer to the coolant loop.

Field Of View (FOV)

The instantaneous FOV (IFOV) will be measured by using a collimator system to simulate the far field response. The baseline at present is a reverse Newtonian telescope. It will consist of a parabolic mirror, an optical flat to relay the beam and a pinhole source at the focus. The pinhole source will be rigidly fixed to the collimator and the whole unit will move as one. The collimator system will be gimballed at the collimator output, in order to be as close to the instrument entrance pupil. The pinhole will be scanned across the detector by translating the rear end of the collimator and hence changing the incident angle of the collimated beam onto the entrance pupil.

The Newtonian telescope will use a f/3 66cm diameter parabolic mirror. The mirror will be gold coated with a final surface finish of at 632nm. The optical flat will be elliptically shaped with a 14cm minor axis and a 20cm major axis. It will have the same coating and surface flatness figures as the parabolic mirror. The source will consist of a 50W QI bulb with a 1mm diameter exit aperture. A slow chopper will be positioned between the bulb and exit aperture in order to subtract the background radiance. Heat will be conducted from the bulb base to a re-circulating loop using the Neslab ULT80 chiller with the Galden HT110 fluid. The source will be fixed onto a translation mechanism to allow it to be positioned at the focus point of the collimator system.

Spectral

A monochromator will be used in conjunction with the collimator system in order to spectrally calibrate an instrument. At present for ambient bench tests a Czerny-Turner with a 240mm focal length is being used. The collimating and focussing mirrors on either side of the grating are gold coated to maximise transmission and hence throughput. The entrance and exit slits are 20mm long and vary in width between 10µm and 2mm. An indium antimonide order sorting filter has also been incorporated. The present monochromator gratings optimise the efficiency in the 2-5µm range.

For spectral characterisation of an instrument the monochromator exit slit is placed at the focal point of the collimator system. The radiance from a 50W QI bulb source, similar to that described above, is focussed onto the monochromator entrance slit.

Thermal

Thermal characterisation can be broken down in thermal vacuum tests and thermal balance tests. Thermal vacuum tests "shake down" an instrument by cycling and soaking it over its survival temperature range. During the cycling process the instrument will be brought into its operating temperature range and run. Thermal balance tests simulate the on-orbit instrument environment and monitor the instrument performance. It also verifies the instrument thermal model. Conditions and hence equipment requirements for both tests are instrument dependent and cannot be generalised. At the ICF the MOPITT instrument thermal configuration is being presently considered.

For thermal vacuum tests this instrument has a survival temperature range from -25° to 60°C whilst the operating range is 20° to 25°C. To cycle over this range a heat exchange plate affixed to the instrument baseplate is being considered. The heat exchange plate will use the Neslab ULT80 re-circulating chiller with a Galden HT110 fluid to operate over the desired temperature range.

For thermal balance tests MOPITT is essentially adiabatic other than the nadir and cold space faces of the instrument. The cold space load will be simulated using a liquid nitrogen cryo-panel whilst the nadir load can be simulated using a temperature variable plate. The temperature of this plate can again be controlled, to simulate orbital fluctuations, using the re-circulating chiller set-up described above.

Summary

The University of Toronto calibration facility and major calibration equipment has been described. The ICF is capable of radiometric, FOV, spectral and thermal characterisation of an instrument requiring a large vacuum chamber for test purposes. The clean room is class 10000 and can be easily upgraded. The vacuum chamber is capable of pumping down to 10-7 torr in under 4 hours when clean and empty, additional cryo- pumps and cryo-panels can be readily added to the system. The main radiometric target covers a wide temperature range and has the option of changing baffle length and input defining apertures. The collimator system uses a large parabolic mirror and can map IFOV's larger than the MOPITT IFOV.

The University of Toronto ICF is a comprehensive and flexible test facility. It is hoped that it will be put to this use to calibrate and characterise other satellite instruments.

Acknowledgements

The MOPITT project is funded by the Canadian Space Agency (CSA) with Com Dev Atlantic being the instrument prime contractor. The Principal Investigator is J. R Drummond (University of Toronto) who heads an international MOPITT science team.

References

Drummond. J. R., 1994, Sounding the Troposphere: The MOPITT Instrument, Eighth CASI Conference on Astronautics, Ottawa, Ontario.