Jack L. Engel, Howard W. Eyerly, Hugh H. Kieffer, Frank Don Palluconi,
The infrared thermal mapper (IRTM) was designed to measure the emitted and reflected radiance of Mars.Carried by the Viking Orbiter, the IRTM contains four small Cassegrainian telescopes which each image thesame, seven circular areas. There is a total of twenty-eight channels in four surface and one atmosphericthermal bands from 6 /im to 30 um and a broad solar reflectance band. All channels are sampled simulta-neously, using the spacecraft scanning capability to map the radiance over small and large areas of the plan-et. All channels use thermopile detectors; spectral passbands are determined by a combination of interfer-ence filters, detector lense materials, antireflection coatings, and restrahlen optics.
Introduction
The objective of the Viking Orbiter infrared thermalmapper (IRTM) is to measure the thermal emission ofthe Martian surface and atmosphere and total reflectedsunlight with high spatial and flux resolution.' Fourbands are used for surface thermal emission: 6.1-8.3 ,.m;8.3-9.8 Mm; 9.8-12.5 Am; and 17.7-24 Am. There arethree detectors in each of the first two bands and sevenin each of the last two. One detector sensitive at14.56-15.41 gim, centered on the CO2 vibration band, isused to measure atmospheric temperature, and sevendetectors are used in the 0.3-3.0-gm solar reflectanceband. The IRTM contains four telescopes each withseven detectors. Three of these, each having an aper-ture of 5.8-cm diam, are used for the thermal bands, andone having an aperture of 3.7 cm is used for the solarreflectance channels. The field of view for all channelsis 5.2 mrad in diameter, determined by stops at the focalplane. Evaporated thermopile detectors were used forall channels. The array of seven detectors in eachtelescope is arranged in a chevron pattern, and the fourtelescopes are boresighted making the chevrons coin-cident in object space.
The IRTM is mounted and boresighted with theimaging system and the Mars atmospheric water de-tector (MAWD) on the Orbiter scan platform to allow
S. C. Chase and J. L. Engel are with Santa Barbara Research Cen-ter, Goleta, California 93017; H. H. Kieffer is with University ofCalifornia, Los Angeles, California 90024; the other authors are withJet Propulsion Laboratory, California Institute of Technology, Pas-adena, California 91103.
simultaneous scanning of the Mars surface with thethree instruments. Platform slews and orbital motionallow essentially contiguous scans.
Except for the increased number of telescopes anddetectors the IRTM design is similar to that used in theradiometers flown on Mariners 6,7,9, 10 in 1969,1972,2and 1973,3 respectively. The IRTM, however, wasdesigned to have much sharper field of view responsewhich allowed observations of high contrast scenes, suchas near the planetary limb or polar caps, to be usedwithout correction for out-of-field response. The majorchanges from the instrument configuration shown in theinitial description of the thermal mapping experimentsresulted from use of a two-axis science scan platform onthe Viking orbiter rather than a three-axis platform, asinitially planned. These changes are the use of fourtelescopes and a chevron array pattern in order to pro-vide coincident fields of view and reasonable spatialcoverage regardless of the direction of apparent mo-tion.
System Description
An exploded view of the IRTM is shown in Fig. 1, anda summary of salient design characteristics is presentedin Table I. The object-space mirror, oriented at 450with respect to the optical axes of the telescopes, hasthree discrete viewing positions 90° apart which allowthe telescopes to view the planet, space, or an internalreference surface. During normal operation the mirroris stepped through a sequence which is controlled by atwo-bit register in the spacecraft flight data subsystem(FDS). The normal sequence consists of four ap-proximately 1-min planet viewing periods separated bybrief views of space during which thermal system offsetsare restored out. At the start of each sequence the in-ternal reference surface is viewed by telescopes A, B,
Fig. 1. Exploded view of the Viking IRTM. In flight, the instrument is covered by a solar blanket.
Table I. IRTM Design Summary
Instrument type:
DC1C2BC3A
Telescope type:
Field of view:
Detector type:
Signal processing:
Integrate time:Dynamic range:Noise equivalent
temperature (K)6.1-8.3,um8.3-9.8,um9.8-12.5 pm
14.56-15.41,um17.7-24.0 um
Dimensions:Weight:Power:
Multichannel radiometerSpectral bands/channels:
0.3-3.0 pm 7 Channels6.1-8.3 um 38.3-9.8jlm 39.8-12.5,um 7
14.56-15.41,um 117.7-24.0)um 7
285.8 cm, f/3.5 Cassegrainian, except D above
which is 3.7 cm, f/5.75.2-mrad round, seven fields of view per
telescope arranged in chevron patternEvaporated thermopile, 0.030-cm diam
D* = 2 X 108 cm-Hz 1/2 -W- 1
Integrate, sample, and reset; ten-bitsdigitization
981 msec; sample cycle, 112 sec120-330 K
1.07 at 200 K 0.07 at 320 K0.52 at 200 K 0.07 at 320 K0.32 at 200 K 0.08 at 310 K0.96 at 200 K 0.36 at 310 K0.19at200K 0.12at300K
33 X 26.7 X 17.8 cm8.4 kg6 W average, 13 W maximum
and C for approximately 2 sec to obtain a thermal cali-bration; the solar channels view a filament lamp sourceimbedded in the reference surface opposite telescopeD. The other FDS controlled operational modesavailable for diagnostic reasons, for situations where oneviewing direction is obscured, or for protecting the in-strument, are: fixed planet; fixed space; and fixedreference. A modified normal mode is also available.In this mode, the only space view is the one which im-mediately precedes the reference view. The FDS au-tomatic modes have a basic period of 256 * 1.12 sec.
The functional block diagram (Fig. 2) shows one ofthirty-two analog channels (twenty-eight active andfour spares). All are sampled in 135 msec every 1.12 sec.Thirty-two housekeeping channels-are multiplexed intothe data stream during the time that the mirror is inmotion. Before the analog data samples are sent to theFDS for insertion into the spacecraft data stream, theyare converted to pulse-width modulated format by theanalog-to-pulse width converter (A/PW) controlled bythe FDS.
The gains of all signal channels in A, B, and C tele-scopes can be automatically reduced by a factor of 0.75when the mirror is in the reference position so that thesechannels will not be overranged when the instrument
m----- - - --- - , -- 1I 31 CHANNELS IDENTICAL TO CHANNEL I ABOVE) L
I (THE INPUTS OF 4 CHANNELS ARE SHORTED)…--~~~~_ _ - Ii-
1,~~~~~~~EPRTR SUPPLY~~~~~~_5 (F - I ~~~~~~~~~~~~~~MONITORS I
Fig. 2. Functional block diagram.
CONE
Fig. 3. Telescope schematicdrawing. Except for filters thefour detector packages are identi-cal. The D telescope is of smaller
diameter. TELESCOPE TUBE
is at the upper end of its temperature range. -Thisfunction is inhibited by ground command to increasethe resolution of in-range signals and in fact was notneeded after the spacecraft reached Martian heliocen-tric ranges.
Except for the hybrid preamplifiers, the IRTMelectronics packaging is conventional, and almost all of
it is located in the module which makes up the base ofthe instrument. The hybrids, consisting of the inputfield-effect transistor choppers and differentialpreamplifier and gain/temperature compensation cir-cuit, are packaged in four modules, eight hybrids to amodule. These are located close to the detector pack-ages on the telescope mounting plate.
Fig. 4. Fields of view of the IRTM, MAWD, and imaging systems:(a) Ideal registration of the IRTM, MAWD, and the VIS cameras.The actual array is about 5% smaller due to a telescope focal lengthdesign change. (b) Alignment of VO-1 IRTM and VIS determinedby in-flight observations of Mars. The dashed extension of spot 1shows the motion during the IRTM integration time for a /4 o-sec-D
scan platform slew. The triangle is the C axis (boresight) of theIRTM; the small circle is the scan platform reference direction. (c)Same as (b), but for VO-2. The dimensions of the IRTM array are
identical to VO-1, they are given here in degrees for convenience.
Optical Design
- The A telescope (17.7-24 gim) is shown schematicallyin Fig. 3. It is an f/3.5, 20.3-cm focal length Casse-grainian design with an aperture diameter of 5.8-cm,spherical surfaces, and, except for mirror materials, isidentical to the B and C telescopes. The D telescopeshas a reduced aperture (3.7 cm) but the same focallength making it f/5.5. By using relatively slow fore
optics, degradation of filter sharpness normally causedby operating an interference filter in a low f-numberbeam is negligible. The focal plane contains a field-defining aperture plate with seven 0.107-cm diam holesare arranged in a chevron pattern. The fields of viewthus defined are nested with those of the MAWD andimaging systems (Fig. 4). Behind each hole in the fieldstop plate is a lens which produces a 0.0254-cm diamimage of the telescope aperture on the detector, whichitself is about the same size. The final optical speed atthe detector is f/1.
Optical materials used in the four telescopes areshown in Table II, and the resulting spectral responseis shown in Fig. 5. Mirrors are made of aluminized andSiO overcoated fused silica except for the A telescope,which uses hot-pressed uncoated zinc oxide for bothprimary and secondary mirrors. The reststrahlen re-flection properties of ZnO are the major factor in the Atelescope.spectral response. The spectral bandpass ofthe other ir channels is determined by interferencebandpass filters and AR coated detector lenses. Thespectral limits of the D telescope are determined by thedetector lenses and a uv cutoff filter.
The out-of-band response for the B and C telescopesis less than 0.1% of full scale for an object of 10-6 theradiance of a 5800-K blackbody, the level expected forreflectance from the subsolar region of Mars. The Aband has less than 0.1% response for wavelengths lessthan 16 ,gm, and wavelengths longer than 30 Am arelimited by the Irtran 6 field lens. The transmissionelements of the D telescope insure that it is thermallyblind. Special coatings on the D telescope mirrors wereused to obtain a reasonably gray response to solar ra-diation.
Optical Image Quality and Alignment
Minimizing extrafield sensitivity (EFS) was an im-portant aspect of the optical design since on previousMariner radiometers EFS contribution seriously com-promised observations of scenes near large temperaturecontrasts (points near the planetary limb and polarcaps). During instrumentation development, IRTMimage quality was determined in two angular regions.In the near-field region, a laboratory collimator and irsource were used to measure the 2-D spatial responseout to 16-mrad diam (three fields of view). Point sourcefield of view measurements in this region are shown inFig. 6. For far field measurements, sensitivity con-straints dictated an approach in which the fraction ofenergy within a given angular annulus is measured. A30.5-cm diam, concentric grooved, blackbody plate witha series of restricting apertures was used at severaldistances (30.5-cm, 140-cm, and 610-cm) to define an-gular response regions from about one field of view outto 1-rad diam. That is, with the telescope focused at610-cm, a disk 3.17-cm in diameter at that distancedefines one half-response field of view (5.2 mrad). Thesource was held at 950 C by a heater/regulator and in-tegral water jacket. To prevent difficulties with at-mospheric transmission, the entire apparatus was
Primary mirror ZnO Fused silica Fused silica Fused silicaCoating Al and SiO Al and SiO Al and enhanced
Secondary mirror ZnO Fused silica Fused silica Fused silicaCoating Al and SiO Al and SiO Al and enhanced
Detector lens IRTRAN-6 Ge Ge SapphireCoating Special AR AR AR none
Filter substrate N/A Ge C1,C2 : Irtran 3 + Ge UV-22C3 : Irtran 4 + Ge
1. 0
0�110r'.
t�
1.
2"I
:511::
0 1 ' ' ' ' j ' -I I d A X \ ' I' " . I' L I L . a I L 1 ' I I IJ o0 2.0 4.0 6.0 8. 0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0 26.0 28.0 30.0
WAVELENGTH [m]
Fig. 5. IRTM relative spectral response; data are for VO-1; VO-2 differs neglibibly.
contained in a polyethylene bag flushed with dry N2 .The EFS problem was more severe for for the longerwavelength A telescope than the others, possibly owingto the higher reflectance at longer wavelengths of theblack paint used inside the telescope.
Tests using this apparatus led to several telescopemodifications designed to reduce EFS (see Fig. 3):
(1) A postfocal baffle was placed between the fieldlens and the detectors to confine energy to the sensitivearea of the detectors.
(2) A spider baffle, added to the outer edges of thesecondary mirror support spider, was designed to reducereflection off the sides of the spider legs.
(3) A cone baffle coated with CTL 15 black paintwas placed on the central dead spot of the secondarymirror. This was designed to prevent focal plane re-flections from falling on the detectors.
Of these three modifications, only the cone baffle gavesignificant improvement, although all three were in-corporated in the design.
These results of the final EFS measurements areshown in Fig. 6. The calculated response due to dif-fraction and the measured values are shownm The in-
tegrated EFS response betweenl2 mrad and 1-rad diamwas about 4%. Of this, about 1/2 is due to diffractioneffects.
The effect of response outside of the nominal field ofview can be estimated directly from data obtained onscans across the hot (subsolar) planetary limb. As-suming that the response is circularly symmetric, andall evidence indicates this to be closely followed, thesignature of a half space would also be symmetric.
A plot of fractional energy derived from a Viking 1IRTM scan across the sunlit limb of Mars is shown inFig. 6.
The alignment was determined using an 20.3-cm(8-in.) collimator to illuminate all four telescopes witha small source of high temperature blackbody radiation.Measurements were taken simultaneously in twenty-eight channels over a 1.5-mnrad square grid pattern. Foreach channel, a parabolic ellipsoid was fit to data wherethe measured intensity was more than 10% of the peakintensity in that channel. The alignment of each tele-scope was ascertained by combining the center of re-sponse so determined for the seven channels in thetelescope. This procedure allowed by the alignment of
Fig. 6. IRTM normalized spatial response; (a) and (b) are computedfor Fraunhofer diffraction for the A telescope at 20 ,im: (a) Responseto an on-axis uniform source as a function of source radius (left-handscale). (b) Response to a uniform half space as a function of off-axisdistance (right-hand scale). (c) Observed A telescope response tocircular sources in laboratory EFS test. Small circles are collimatordata, square are for distant blackbody. The change beyond 95 mradwas less than system uncertainty (left-hand scale). (d) Observed Atelescope response crossing a hot limb of Mars normalized to the on-planet radiance. This corresponds to the theoretical curve (b). Thedashed curve shows the result of correcting for atmospheric opacityby assuming that the edge of the radiance source is 19 km (2.9 mrad)
above the planetary surface (right-hand scale).
the four telescopes to be determined with an estimatedprecision of 0.1 mrad. The back of the secondary mirrorof the B telescope was aluminized and used as thealignment reference for this procedure and for instru-ment alignment on the spacecraft. The instrumentpointing direction was verified in the same manner justprior to planetary encounter using Mars as a 5-mraddiam source and using the science platform motion togenerate a 5-mrad spaced grid. The in-flight alignmentis shown in Fig. 4.
thermopile array used in the IRTM is shown in Fig. 7.The chevron arrangement was based on the need foruniform coverage irrespective of scan platform orien-tation; it also allowed the detectors to all be approxi-mately the same distance from the telescope optic axis.In this application thermopiles were found to be betterthan other thermal detectors because they operate todc and exhibit no 1/f noise. Thus, no optical chopperis needed. Also, no bias supply, another potentialsource of 1/f noise, is needed. Cooled quantum detec-tors were not practical, considering the duration andweight constraints of the Viking Mission. The arraywas made by evaporating the various components ontoa sapphire film using photoetched masks for dimen-sional control. The film, about 200 nm thick, is sup-ported by a sapphire disk. The film was made byanodizing aluminum foil and etching away the alumi-num. The black circular dots in the figure are the
sensitive areas overlaid with bismuth oxide smoke whichhas good ir absorptivity but low thermal mass. Char-acteristics of the detectors are
Active areaNumber of junctionsResistanceTime constantResponsivityDetectivity (D*)
7 X 10-4 cm 2
613 KQ80-100 msec130 V/Watt2 X 108 cm Hz1 /2 W-.
To obtain full sensitivity the detectors must beevacuated. Therefore, during ground testing the de-tector packages were pumped down through a perma-nently attached manifold. At other times the detectorpackages were backfilled with xenon to protect the de-tectors while still allowing gross sensitivity checks. Toavoid exposure to moisture during the long period priorto launch when the IRTM was mounted on the space-craft and could not be sealed, the manifold was kept ata slight positive pressure by a continuous flow of highpurity nitrogen. The manifold was opened to space bylaunch vehicle separation.
Signal Processing
The signal channels use a synchronous demodulationscheme to provide good stability and to avoid 1/f noisein the preamp. The input FET chopper is a full-wavetype operating at 471 Hz. This and the center-tappedthermopile allow voltage doubling of the detector signaland noise and thus reduce the preamp noise contribu-tion which otherwise would be significant. The dif-ferential input connection, while suffering a -V2 noisedisadvantage compared to single-ended input, providesexcellent common mode rejection of chopper spikes and
Fig. 7. IRTM thermopile array. The sapphire substrate disk is 1.25cm in diameter. There are six Sb-Bi junctions under each of the
other input noise. Temperature dependence of thethermopile, about -0.5%/0 C, is compensated by athermister network external to the hybrid package.Preamp gain is adjustable with an external resistor.
Following the half-wave synchronous demodulatoris an integrate, hold, and reset circuit with an integratetime of 981 msec. The integrator serves as a low passfilter while the hold feature ensures spatial simultaneityof corresponding detectors in each telescope. Aftercompletion of sampling by the multiplexer, all channelhold circuits are reset to ensure independence of datasamples.
The IRTM analog signals, which have a range of 16V, are digitized by the analog-to-pulse width converterand flight data subsystem (FDS) counter into ±29 levels,yielding 1023 data numbers (DN) which are nearlylinear with radiance in each channel.
The IRTM multiplexer consists of sixty-eight FETswitches and a buffer signal amplifier. In addition tothirty-two data channels (twenty-eight active and fourspare), thirty-two channels of engineering data are alsosample. These include eight temperature measure-ments from thermisters located at four locations on thereference plate, the electronics module, and each of thethree ir detector packages (telescopes A, B, C). Threepower supply voltages and the pre-dc restore voltage oftwenty-one channels (telescopes A, B, and C) aremonitored. The pre-dc restore monitors are diagnosticto determine the presence of large thermal or detectoroffsets.
Motor and Control Logic
The scan mirror is driven by a four-position steppermotor through a 50/1 gear reduction. A motor drivepulse duration of 40 msec allows a 90° mirror rotationin 2 sec. The mirror position is sensed by a two-bitencoder on the motor shaft; the contacts at the threedesired positions are about half of the width of 1.80mirror step. The motor stepping is controlled by theFDS using a comparison of the encoder readout with thedesired position originating either from the FDS normalmode clock or direct ground command; the motor can-not be directly commanded.
In addition to the restore which occurs automaticallyin the normal model when the mirror reaches the spaceposition, restores can be ground commanded when theIRTM is in the fixed planet or fixed space mode; in ei-ther case housekeeping data are multiplexed into thedata stream during the 1-sec restore period.
Whenever the mirror reaches the reference position,the calibration lamp is turned on for the next two inte-gration periods. The lamp is at full radiance through-out the second integration period, which is used for gaindetermination of the D telescope channels. In the fixedreference mode, science and housekeeping data aresampled alternatively.
Spectral Calibration
Relative spectral response of all channels was mea-sured end to end using a Perkin-Elmer 16 U mono-chrometer with appropriate gratings and order filters.
A globar at 1400 K was used in the 2-25-,m range;shortward of 2.0 ,m a tungsten source at 2700 K wasused. The reference detector was a thermocouple forall but the 0.4-1.1-gm range, where a calibrated siliconphotodiode was used. Out-of-band measurements weremade by replacing the spectrometer grating with a planemirror and ir materials having known cutoff and cutonwavelengths.
Flux Calibration
Flux calibration of the IRTM was performed undera simulated space environment using a vacuum chamberoperated typically at a pressure of 10-6 Torr. TheIRTM was operated by means of a console which sim-ulated the interfaces and functions of the spacecraftFDS. A minicomputer was used to provide all opera-tional sequences and modes. Data were recorded onmagnetic tape for subsequent computer processing.
The calibration fixture consisted of two identicalblackbodies, one located in front of the space port andmaintained at LN2 temperature and the other in frontof the plant port and adjustable in temperature from 77K to 350 K; eleven setting from 140 K to 330 K wereused. Blackbody temperatures were measured withplatinum resistance thermometers having an absoluteaccuracy of ±0.10 C traceable to the National Bureauof Standards. The digitizer used in the test consoleprovided ten times the resolution of the FDS digitizer,thus making the digitizing uncertainty during calibra-tion insignificant compared to the noise. The calibra-tion data thus produced are IRTM output in digitiza-tion level (DN) as a function of blackbody temperature.Radiometrically measured internal reference surfacetemperatures showed close agreement (0.5 0 C) withthose measured independently with a thermister.
The IRTM temperature was controlled by regulatingthe temperature of a mounting base plate and thethermal shield inside the vacuum chamber. Calibrationwas performed at 100C spacing across the range of op-erating temperatures expected during flight.
Typical IRTM channel response to scene brightnesstemperatures is shown in Fig. 8. The one-sample noiseon the thermal channels is less than 1 DN except for the15-gm channel where it is about 2.5 DN.
The dynamic ranges of the surface thermal bands arebased on temperatures expected for the Martian sur-face. The 300 K maximum chosen for the A telescopemight be exceeded by midday summer temperatures,but temperatures above the 310-K limit limit of the Btelescope should not be exceeded unless active volcanicareas were found; temperatures to 320 K and 330 Kcould be measured by the 9-gm and 7-gm bands. The15-gm band dynamic range was set quite large as itsresolution is noise limited rather than digitizationlimited.
Telescope D channels were calibrated using a dif-ferent method. The radiance source was a mercury-xenon lamp and narrowband filter centered at 0.896 gmwith a bandwidth of 425 nm. The in-band radiance ofthe lamp was known by direct comparison with a stan-dard lamp acquired from the National Bureau of
Standards, using a silicon photodiode as a transferstandard. The relative spectral response measure-ments then allowed extension of the one-point absolutecalibration to the entire passband. Gains for the Dchannels were set to give full scale for 75% of the diffusereflection of solar irradiance at Mars average distancefrom the sun.
Using integrals of the Planck function and the mea-sured spectral response, the flux response of the IRTMis found to be close to linear in the thermal channels.The best fit quadratic functions, normalized to fullscale, typically had constant and quadratic coefficientsof 0.002 and 0.02, respectively.
The solar band channels, which had much higherabsolute flux levels at full scale, showed a decrease inresponse at high signal levels corresponding to a qua-dratic coefficient of 0.07. With the IRTM in the vac-uum chamber, the instrument response was measuredat four lamp currents. An additional series of wideband measurements utilizing a NBS standard lamp anda BaSO 4 diffusing screen, in which only the lamp-screendistance was changed, was used to determine in detailthe solar band nonlinearity.
During spacecraft thermal-vacuum testing and inflight, a small drift of about 1-min duration was foundto be induced when the scan mirror moved to the ref-erence position in normal mode. This appears to becaused by the decrease in radiative heat loss from theinstrument when the telescopes do not view space. The
Fig. 8. IRTM radiometric re-sponse. Calculations are for VO-1spectral response and ignore thesmall nonlinearity measured dur-
ing calibration.
shape of this postreference drift was accurately deter-mined during normal mode sequences when thespacecraft was well away from Mars, and this effect isremoved in the data reduction.
The change of the thermal state of the IRTM causedby large scan platform slew or planetary radiation nearperiapsis can introduce significant drifts of the zero-fluxlevel. These shifts have a time constant of 1-2 min orlonger, and their magnitude increases with inbandwavelength and preamplifier gain. It is probably dueprimarily to very small temperature gradients inducedin the detector packages as the general instrumenttemperature changes. A significant design feature ofthe IRTM is that the space DN level of each channel ismeasured immediately prior to and after the restorewhich occurs each minute in normal mode. A linearinterpolation between these zero-flux DN levels is usedin data decalibration. The remaining quadratic andhigher order drift is generally negligible.
In-Flight Anomalies
During developmental testing, it was found that theIRTM signal was affected by 2295-MHz (S-band) ra-diation which was to be used for orbiter communicationsto the earth. Susceptibility was presumed to be in thedetector area, where the 1-DN equivalent signal levelis on the order of 10 nV. By grounding each detectorcenter tap at the detector and sealing with conductiveepoxy all accessible openings between the IRTM ap-
1250 APPLIED OPTICS / Vol. 17, No. 9 / 1 May 1978
erture and the area which houses the preamplifiers anddetectors, the IRTM immunity to S-band radiation wasraised above 0.1 W/m2 , the level predicted for in-flightoperation.
Special S-band on/off sequences performed duringearly cruise showed that the VO-1 IRTM was not af-fected, but that VO-2 was measurably affected. Themaximum effect, 15 DN in one channel, occurred withthe scan mirror in the space position pointing directlyat the side of the high gain antenna. Additional testingshowed that the VO-2 IRTM was affected measurablyby high gain antenna radiation only in a small set of scanplatform positions and then only for a few channels.For the antenna/scan platform positions used forscience observations, the effect was expected to be lessthan 2 DN. While in orbit about Mars, offsets betweenthe planet and space port zero-flux levels were checkedby operations when the planet was not visible in eitherdirection; no offset above the noise level was found.
Commencing on 2 October 1976, the VO-2 IRTM didnot stop correctly about 13% of the time when com-manded to the space position. This was probablycaused by a small change in the rest position of thestepper motor, as transmitted to the encoder throughthe gear box, resulting in occasional lack of electricalcontact at the space position. When this occurs, atimed restore then incorrectly occurs on the referencesurface (at approximately 275 K rather than at the 4 Kof space) and results in a loss of useful data until at leastfour correct space restores have been accomplished, aperiod of at least 41/2 min. The use of normal mode hasbeen greatly reduced, and a special mode which delaysthe restore until the space position is attained has beenimplemented in the spacecraft computer. In accordwith the general nature of things, the anomalous be-havior has not recurred since implementing a routinewhich could handle it.
Conclusions
The IRTM was designed to measure radiance overnearly a factor of 100 in wavelength with the stringentconstraints on weight, power, and reliability associatedwith planetary missions. The use of identical ther-mopile detectors, a relatively old technology, allowedthis over a long mission without requiring high speedoptical chopping or the large weight and/or power re-quirements of quantum detectors. The instrumentshave obtained the performance of quantum detectors.The instruments have obtained the performance re-quired for the science objectives and have operated es-sentially. as expected throughout the nominal Vikingmission. Initial scientific results from these instru-ments have been reported by Kieffer et al. 4 and in pa-pers cited therein.
G. Neugebauer, E. Miner, and G. Munch have mademajor contributions throughout the development of theIRTM. The IRTM was developed by the Santa Bar-bara Research Center. Technical personnel at the JetPropulsion Laboratory aided in detailed design andperformed major testing. This work was supported bythe National Aeronautics and Space AdministrationViking Project.
References1. H. H. Kieffer, G. Neugebauer, G. Munch, S. C. Chase, Jr., and E.
Miner, Icarus 16, 47 (1972).2. S. C. Chase, Jr., Appl. Opt. 8, 639 (1969).3. T. C. Clarke, Jet Propul. Lab. Tech. Mem 33-719 (1975).4. H. H. Kieffer, T. Z. Martin, A. R. Peterfreund, B. M. Jakosky, E.
D. Miner, and F. D. Palluconi, J. Geophys. Res., 82, 4249 (1977).
The Adolph Lomb Medal for 1978 will be presented to Eli Yablonovitch of
the Division of Applied Sciences of Harvard University. The Lomb Medal was
established in 1940 to honor a person who has made a noteworthy contribution
to optics before reaching the age of thirty. Yablonovitch is honored for his
pioneering contributions to laser physics and technology, including the under-
standing of the optical breakdown strength in laser window materials, the
development of ultrashort CO laser pulses, and the identification of laser2
plasma heating processes. Yablonovitch was educated at McGill University and
Harvard University. He was a staff scientist at the Bell Laboratories before
joining Harvard in 1974. The medal will be presented at the 1978 fall meeting
