UP CLOSE: THE ATMOS INSTRUMENT
ATMOS INSTRUMENT DEVELOPMENT AND
TESTING
Background
The current ATMOS instrument represents the outgrowth of an atmospheric measurement
program begun at the Jet Propulsion Laboratory (JPL) in 1972 using a flight
model High Speed Interferometer (HSI) designed and built by the Laboratory for
use on aircraft and balloon borne platforms. The original instrument, designated
the Mark I, covered the wavelength region from 1.8 to 5.2 micrometers
with an unapodized resolution of 0.125
wavenumbers. This instrument, which was
flown on a wide variety of aircraft and balloon borne platforms, gathered information
on the mixing ratios of atmospheric minor and trace species from ground level
to a height of 40 km. Among these measurements were several stratospheric "firsts,"
including the first spectroscopic detection of NO, the first NO/NO2 ratio, the
first spectroscopic detection of HCl, the first HF/HCl ratio, and the first
profile for HF in the 20 to 40 km region of the atmosphere.
During the 1976-1977 time period, a Shuttle Definition Study was conducted
both to determine the feasibility of acquiring infrared interferometric data
from the Space Shuttle and to provide a conceptual design for an instrument
capable of making the measurements. This study culminated with the award of
a contract to Honeywell ElectroOptics Center (HEOC) in January 1978 to design
and build the ATMOS sensor. The following paragraphs describe the design and
development of the ATMOS instrument by HEOC.
Instrument
Overview
Radiation enters the instrument via a suntracker and a foreoptics subsystem,
with the latter defining the FOV and the
size of the beam. The energy rejected by the field
stop is reflected to a video camera and is recorded as an image of the
Sun with the position of the FOV of the instrument superimposed. The wave front
passing through the field stop is divided by the beamsplitter
and modulated by the moving elements in the interferometer. The modulated radiation
leaving the beamsplitter passes through one of the eight selectable bandpass
filters and is focused on a Mercury Cadmium Telluride (HgCdTe) detector cooled
to 77 K.
The signal produced at the detector is conditioned and digitized by the Signal
Handling Subsystem (SHSS). The digitized signal is then formatted by the Data
Handling Subsystem (DHSS) into a Pulse Code Modulated (PCM) data stream. This
signal is then output to the instrument Ground Support Equipment (GSE) or, during
a flight, directly to the Shuttle High Rate Multiplexer (HRM). Additional engineering
and housekeeping data is processed by the Engineering Data Handling Subsystem
(EDHSS) and is output as a part of the high rate data stream as well as in a
separate low rate engineering stream. All command and control functions for
the instrument are performed by the Command and Control Interface Subsystem
(CCISS).
Instrument
Description
For purposes of discussion the instrument can be divided into two sections:
(1) the optical sensor, composed of all the optical subsystems and the scan
servo control unit, and (2) the electronic assemblies, including the compressor
for the detector cooler. All the elements of the optical sensor are mounted
to an aluminum baseplate that in turn is mounted via vibration isolators to
a substructure assembly. This assembly attaches to, but is thermally isolated
from, a platform located in the payload cargo bay of the Shuttle. An aluminum
cover, which also mounts to the baseplate, encloses all but the suntracker and
camera. The internal pressure of the sensor is maintained at ambient through
a vent that is fitted with a desiccant filter assembly designed to provide a
clean, dry environment under all conditions.
The electronics section is mounted on a secondary support platform that is
mechanically isolated from the main support structure through vibration isolators.
The electronics are physically separated into two units, one chassis containing
the SHSS, DHSS, EDHSS, and CCISS and the other containing the power supply subsystem.
A thermal control plate mounts to the underside of the secondary platform and
provides a nominal thermal operating range of +5 degrees C to +45 degrees C
under pallet conditions ranging from -150 degrees C to +120 degrees C.
Suntracker and
Foreoptics
Click here to view
the optical layout for the suntracker and foreoptics, including the frame camera
optics. The two axis servo controlled suntracker tracks the Sun to an accuracy
of 0.4 mr with a stability of 0.06 mr. Sun detection and positive feedback are
provided by a silicon quadrant array with an acceptance cone angle of 20 deg.
In the tracking mode, the sensor's FOV is centered on the solar disk; however,
commands can be sent to offset the tracker null position in 1 mr steps to avoid
or seek out sunspot activity. The tracker can be prepositioned to intercept
the Sun at the appropriate time for a subsequent sunset or sunrise encounter
by a commandable pointing capability that provides near hemispheric coverage.
Energy reflected by the suntracker passes through a Zinc Selenide (ZnSe) window
and enters the instrument foreoptics. The cover window is anti reflective (AR)
coated for improved efficiency in the 2 to 16 micron region without significant
loss at the camera wavelength of 0.575 microns. The foreoptics telescope has
a 7.5 cm diameter collecting aperture and consists of two confocal f/3 off axis
paraboloids with a selectable field stop located at their common focus. Field
stops corresponding to instrument FOVs of 1, 1.4, 2, and 2.8 mr can be selected.
Energy passing through the stop is recollimated
by the telescope secondary and transferred to the interferometer. Angular magnification
is restricted to 2.6 because of obliquity
limitations within the interferometer. The rejected energy is reflected by an
ellipsoidal mirror to a camera which records one solar image at the end of each
scan.
Interferometer
A block diagram of the optical layout of the interferometer is shown
here (in a PDF file). The optical system is a modified version of the classical
Michelson interferometer that uses cat's eye retroreflectors in place of plane
mirrors and double passing techniques for maximum alignment stability. This relaxes
component alignment tolerances from a few arc seconds to several arc minutes,
rendering the instrument insensitive to mechanically or thermally induced misalignments.
Energy enters the interferometer through an aperture in the common retro mirror
and impinges on the beamsplitter at an angle of 45 deg. The beamsplitter substrate
is Potassium Bromide (KBr) with a Ge/KRS-5 coating designed for optimum efficiency
over the 2 to 16 micron region. Radiation reflected from the beamsplitter passes
through a KBr compensator plate and goes to the cat's eye retroreflector, which
consists of an f/2.7 paraboloid primary and a slightly convex secondary. The
radiation exits the cat's eye displaced, travels through the compensator, and
is reflected off a gold coated area on the beamsplitter substrate that directs
the energy to the retro mirror. The reflected beam then retraces its path back
to the beamsplitter surface. The radiation transmitted by the beamsplitter follows
a similar path but uses the beamsplitter substrate in transmission rather than
in reflection. The Optical Path Difference (OPD) is varied continuously from
+50 cm to -50 cm at a rate of 50 cm/second by moving both cat's eyes equal distances
but in opposite directions. This has two advantages over a single moving element
system: first, it decreases the mechanical scan rate to 6.25 cm/second, half
its former value; and second, by scanning the cat's eyes in opposite directions,
it rejects common mode forces, thus providing for a first order velocity correction.
Requirements for high spectral data quality dictate precision control of the
OPD scan to a velocity uniformity of 0.1% peak to peak. This level of performance
is achieved through the use of a frequency stabilized HeNe laser for OPD velocity
feedback. The reference laser used in the ATMOS interferometer is a modified
Hewlett Packard HP5501A Zeeman split actively stabilized laser providing single
mode, single frequency output at 0.6330 microns. The laser energy passes through
the interferometer parallel to the infrared beam.
The key elements of the scan servo subsystem are
shown here. The velocity servo is implemented with two control levels: (1)
an outer control that uses a phase locked loop to convert laser feedback to
an error signal and provides the precise velocity control; and (2) two inner
control loops, one for each slide, that use moving coil tachometers for velocity
feedback. The function of the inner loops is to force both slides to operate
at approximately the same velocity. As each slide reaches the end of its travel,
control is switched to position feedback with the loop closed around Linear
Variable Differential Transformer (LVDT) position sensors; the return scan is
initiated after both sliding assemblies are brought to a halt in their hold
positions. To maximize the duty cycle, scan control is provided in both directions
with turnaround times of 150 ms at each end.
The output from the interferometer passes through one of the eight selectable
bandpass filters and is focused by an AR coated ZnSe lens onto a photoconductive
HgCdTe detector, where a pupil image is formed. The detector is a single element,
27 mils on a side; the size of the chip was chosen on the basis of both sensitivity
and linearity requirements under high signal flux conditions. It is AR coated
to improve quantum efficiency and uses a constant bias voltage to improve linearity.
The element is packaged in a sealed glass vacuum dewar and cooled to 77 K using
a split Stirling cycle mechanical cooler with a 1.6 watt cooling capacity.
Electronics
Description
The output from the detector is transferred to the SHSS, where it is amplified,
filtered, and digitized for insertion into the DHSS. The SHSS, seen schematically
here , has been designed to preserve
source noise limited performance while accommodating an expected dynamic range
of 18 bits with minimum phase and amplitude distortion. The 18 bit dynamic range,
high data rate operation is obtained through the use of two 12 bit parallel
data channels offset in gain by 5, 6, or 7 bits, selectable by command. Both
outputs are included in the PCM data stream. During ground processing, valid
data from each channel is interleaved to reconstruct an effective 18 bit signal
that preserves peak signal precision and maintains source noise limited performance
at high spectral resolution. Sampling of the analog interferogram in both channels
is based on delayed reference laser fringe sampling at every second (395 kHz)
or third (263 kHz) laser fringe, selectable by command. Electronic distortion
has been minimized through proper delay of the sampling waveform as well as
through use of multipole compensated Butterworth filters for noise bandlimiting.
The remaining elements of the ATMOS electronics can be viewed
here. The DHSS accepts the digitized high and low gain interferometric data
to be buffered and formatted into a Shuttle compatible Non Return to Zero Logic
(NRZ L) serial 15.76 Mbit/s PCM data stream. The formatting section is a hard
wired design with formatting accomplished by a digital multiplexer. The high
rate data format includes the engineering and housekeeping data and both time
and optical path position references. Also shown in the block diagram are the
main elements of the EDHSS and CCISS. A common feature of these subsystems is
a Motorola MC6802 microprocessor that formats the system engineering and housekeeping
data as well as decoding and implementing the instrument commands received through
the Shuttle interface. In addition to the high rate PCM data channel, the ATMOS
instrument provides a 1.28 kbit/s low rate PCM output containing engineering
and housekeeping data only.
Go back to the ATMOS Home Page.
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