Aircraft measurements for calibration of anorbiting spacecraft sensor

Warren A. Hovis, John S. Knoll, and Gilbert R. Smith

The best method of establishing a calibration for a satellite sensor is to make simultaneous measurementsalong the satellite view vector from a calibrated instrument on board a high-altitude aircraft. The satellitesensor prelaunch radiance calibration can then be compared to the in-orbit radiance values produced bycomparison with the simultaneous aircraft data. Aircraft measurements were made over the Atlantic Oceanoff the east coast of the United States, and comparison of the prelaunch and in-orbit data indicated a 25%degradation in channel 1 of the Nimbus 7 Coastal Zone Color Scanner.

1. Introduction

The Nimbus 7 Coastal Zone Color Scanner (CZCS)after 9 months in orbit began to show signs of degra-dation in response, especially in the blue channel cen-tered at 443 nm. The degradation became obviouswhen the Rayleigh correction based on prelaunch cali-bration began to produce values of upwelled radianceat the surface that were too small after atmosphericcorrection. The onboard calibration lamps, used tocalibrate the sensor only beyond the postoptics (afterthe large scan and telescope mirrors), indicated no suchdegradation in response and hence could not be used torecalibrate the entire sensor. In an effort to determinethe degradation and recalibrate the sensor, an aircraftprogram was initiated to fly on board a high-altitudeLear jet a double monochromator specifically config-ured to cover the wavelength region of the CZCSchannels in question. The monochromator viewed thesame surface area of the ocean through the atmosphereas being viewed by the spacecraft during near simulta-neous measurements. The results showed a very strongspectral characteristic to the degradation. A largedegradation was indicated in the 443-nm channel of theCZCS which dropped off quite rapidly in amount in thelonger wavelength channels. This result has implica-tions both for the CZCS and other optical instrumentsin space that may operate into the blue region of thespectrum. The strong spectral characteristic of thedegradation will change the shape of the spectral re-sponse of a band, degrading the blue side more than thered side, as well as reducing the overall responsivity ofthe channel.

The authors are with NOAA National Environmental Satellite,Data, and Information Service, Washington, D. C. 20233.

Received 3 August 1984.

11. Onboard Calibration Status

Instruments such as the CZCS on Nimbus 7 and theMulti Spectral Scanner (MSS) used on Landsats 1, 2,3, 4, and 5 as well as the Thematic Mapper flown onLandsat 4 and 5, all have calibration lamps internal tothe system. In all the above the calibration lamps donot illuminate the large collecting optics and scan mir-rors that are most exposed to the atmosphere of thespacecraft (schemes illustrated schematically in Fig. 1).The CZCS actually has two calibration lamps with abeam splitter so that either one of the two can be usedalternatively. The Thematic Mapper has three cali-bration lamps that can be used in various combinationsto monitor the performance of the detectors and am-plifiers. The MSS also has two calibration lamps thatcan be used alternatively. The first of the MSS in-struments on Landsat 1 also had a solar calibrationmirror that was intended to monitor sensor degradationby viewing the sun during a pass over the Pole. Theusable calibration mirror diameter was limited to 0.5mm to reduce the direct solar image intensity on thedetectors to a level that would not damage the detectors.This calibration method was never successful, and, infact, when the Landsat 1 MSS was activated on orbit 21,-35 h after launch, it was found that the solar calibra-tion mirrors were profoundly degraded by the solar ir-radiance. In the shortest wavelength channel, 500-600nm, the signal reaching the detectors was -9% of thatexpected. The degradation was somewhat less severein the longer wavelength channels but nevertheless sosevere that this method of calibration was found to beunusable.

After 5/2 yr in orbit, a test utilizing the alternatecalibration lamp of the CZCS showed degradation ofless than one digital count on average from that mea-sured before launch. This degradation included theoptics subsequent to the insertion of the lamp, the de-tectors, and amplifiers. Nevertheless, the images

1 February 1985 / Vol. 24, No. 3 / APPLIED OPTICS 407

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Fig. 1. Calibration schemes MSS and CZCS.

produced, especially with the 443-nm channel of theCZCS, show an obvious degradation. This indicatesthat loss of response is due to loss of reflectance in thelarge mirrors prior to insertion of the calibration sourcein the optical path. The mirrors are fabricated with aberyllium base over which there is a Kanigen coatingcovered by an evaporated silver film that is protectedby a proprietary coating of the Samson Co. The mirrorsmost exposed to the environment of the spacecraft arethe scan mirror and the primary and secondary area ofthe Cassegrain telescope system. The exact mechanismof this degradation is unknown at this time with spec-ulation centered around possible particulate contami-nation due to outgassing from the spacecraft itself ormicrometeorite impact roughening the surfaces of themirrors. Until one of these instruments, or a test mirrorrecovered by the shuttle, is returned to earth there ap-pears to be no way of determining the mechanism of thedegradation of the mirror surfaces.

III: Aircraft Measurements

A number of methods are possible for estimatingdegradation of sensors in space including calculationsbased on ground-based measurements such as done byGordon et al. 1 One fundamental problem exists withsuch measurements in that the backscattering charac-teristics of the aerosols cannot be measured directlyfrom the surface except possibly with the use of a mul-tiband laser measurement. Measurements from ahigh-altitude aircraft which can fly above most of theatmosphere, and, hence, most of the aerosols, empiri-cally include the optical charactereistics of the aerosols.It is important to carry out the spectral measurementsimultaneously with the spacecraft overflight and toview the same ground surface area since the solar ele-vation is changing rapidly with time when the Nimbus-7spacecraft passes overhead.

A double monochromator2 was chosen for this par-ticular measurement since measurements down to 433nm with a single monochromator are highly susceptibleto scattered light within the instrument itself. The

460 500 540 580 620 660 700

Wavelength In Nanometers

Fig. 2. CZCS spectral bands.

740 780 820

1/8-m double monochromator (spectrometer) used a camdriven sine drive to scan the grating every 5 sec. Theaverage spectral resolution of the instrument is 7 nm.The spectrometer is calibrated in radiance in the labo-ratory using a 76.20-cm diam white sphere source3 il-luminated by twelve internal quartz-halogen lamps.The sphere output is traceable to the NBS. The spec-trometer foreoptics includes a quartz-wedge depolarizerand a lens system which defines the surface field of viewat -2 X 2 km. Some distortion in the shape of the fieldof view is caused when the look angle is set at other thannadir. The field of view of the CZCS is -800 X 800 m,and the aircraft spectrometer field of view provides anaverage over several CZCS fields of view. The CZCSdata when viewed with an image analyzer were averagedover an area of approximately the same size as the air-craft spectrometer footprint.

It should be noted that the spectrometer was chosendeliberately rather than trying to attempt to build amultichannel radiometer simulating the CZCS. Therationale for this decision was that it would be difficultto duplicate the CZCS spectral bands exactly, whereasthey were well measured before launch as shown in Fig.2. It is then a relatively simple matter to integrate theupwelled radiance measured at the aircraft over themeasured spectral response of the instrument. Band5, from 700 to 800 nm, is relatively insensitive in thecalculation of the characteristics of water, and no at-tempt was made to recalibrate it.

The double monochromator chosen for this mea-surement was designed for NOAA by W. G. Fastie of theJohns Hopkins University and is shown schematicallyin Fig. 3. It was produced for NOAA by ResearchSupport Instruments Inc. (RSI) of Cockeysville, Md.

The double monochromator was flown on the NASALear jet aircraft of the NASA Lewis Research Center.It was mounted in a specially constructed downwardobservation pod so that the monochromator was ad-justable, and the optical axis could be pointed off to theside of the aircraft to be parallel to the line between thesatellite scan and the target area. This was necessarybecause in some cases the subsatellite track of thespacecraft was so far off the Atlantic coast that it wouldhave been impractical to fly the airplane out directlyunder the subsatellite track which was beyond theround trip flight range of the aircraft. The scan of the

408 APPLIED OPTICS / Vol. 24, No. 3 / 1 February 1985

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Fig. 3. One-eighth-meter zero dispersion Ebert doublemonochromator.

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Fig. 4. Aircraft data.

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Fig. 5. CZCS channel 1 Lear jet calibration.

CZCS of 400 to either side of nadir allowed the aircraftinsrument to be adjusted in elevation to the off-nadirCZCS view vector at a point close enough to shore to bewithin the flight range of the aircraft. The Lear jetoperated at a pressure altitude of -200 mbars with theexact pressure altitude determined by utilizing both theaircraft's pressure altimeter and measurements ofsurface pressure at the National Weather Service Sta-tion nearest to the aircraft track. The contribution bythe atmospheric backscatter above the aircraft wascalculated by determining the Rayleigh phase functionfrom the known solar elevation and phase angle at thepoint of measurement.

Data were recorded on board the aircraft during flightutilizing a digital system and a cassette recording devicewhich could be immediately entered into a Hewlett-Packard 9836 computer on return to the laboratory.The computer was programmed to produce a calibratedspectrum, in mW/cm 2 sr Aim vs wavelength and to in-tegrate under the CZCS spectral response functions todetermine the radiance within the four bands of interestof the CZCS. A sample of the computer printoutshowing all the various information that is produced isshown in Fig. 4. Numerous spectra were taken on eachflight of the Lear jet before and after coincident mea-surement with the spacecraft. These spectra will notbe shown in this paper for the sake of brevity; however,investigators who may have use for such data may ac-quire copies of these spectra by contacting the au-thors.

Digital magnetic tapes of the CZCS imagery for thedays in question were produced by NASA GoddardSpace Flight Center, Nimbus Operations ControlCenter, immediately after the flights. They were ex-amined to determine the digital output count of theCZCS while viewing the same area as viewed by theaircraft instrument. Calculations were made of theRayleigh backscattered radiation along the path fromthe aircraft to the spacecraft, not included in the mea-surement. This backscatter was added to the measuredupwelled radiance as seen by the aircraft. Figure 5compares the digital output count vs radiance for the433-453-nm channel of the CZCS as measured prior tolaunch and as measured during the period of Lear jetflights between 4 May and 13 May. This figure showsthat the 433-453-nm band has degraded -25% in re-sponsivity, over a period of 4 years and 7 months, be-tween prelaunch calibration and this calibration.Figure 6 shows the results for the channel from 510 to530 nm, indicating a degradation of -9.5% as comparedto prelaunch measurements. The spectral band from540 to 560 nm was found to be degraded by -3%, andthe band from 660 to 680 nm of the CZCS showed nomeasurable degradation within the accuracy of thisexperiment.

Figure 7 summarizes the measured degradation after4 years and 7 months in orbit and shows a rather star-tling spectral dependence of the degradation. Thisspectral characteristic of degradation has implicationfar beyond the CZCS program. It implies that any in-strument with a spectral band extending into the blue

1 February 1985 / Vol. 24, No. 3 / APPLIED OPTICS 409

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Fig. . Thematic Mapper channel 1.

region, such as the Landsat Thematic Mapper channel1 illustrated in Fig. 8, will undergo not only a degrada-tion in response, but a change in the shape of the spec-tral response function, should it continue to functionfor a period of time equivalent to the CZCS. Presentschemes and those being planned for onboard calibra-tion of optical sensors such as the CZCS, the ThematicMapper, and the MSS of Landsat, will provide onlytotal responsivity information and will not have thecapability of determining if there has been a change inspectral response. If spectral response, especially in theblue region of the spectrum, is to be an important part

of a measurement, it would appear that it is necessaryto add another calibration source, namely, a source thatcan measure the response as a function of wavelengthto determine the degree of alteration with time.

IV. Future Plans

The instrument will be flown again in the summer of1984 on the Lear jet and then be transferred to theNASA ER-2 aircraft which can reach a pressure altitudeof 50 mbars. This will reduce the amount of calculationthat is part of these calibration measurements and willbe used to monitor further the degradation of the CZCSoptics in flight.

V. Conclusion

Degradation in the response of satellite instruments,sensing in the visible wavelength region, is not beingadequately monitored by onboard calibration devicessince these devices do not include all the optical ele-ments of the system. The results from the CZCS pro-gram and from the aircraft calibration measurementsconfirm that the major source of degradation is loss ofreflectance from the large optics that are exposed to thespacecraft environment, namely, the scan mirror andthe primary and secondary of the telescope. If quan-titative measurements are to be made with such in-struments in the future, the onboard calibration sourceshould pass a beam through all the optical elements anditself must be protected so that any observed degrada-tion is indeed in the instrument and not in the optics ofthe calibration soruce. Devices such as white diffuserplates would have to be enclosed and exposed to thesunlight only for very brief periods of time to avoid solardegradation. The French HRV instrument scheduledto fly on the SPOT satellite will utilize a collimator withan active light source. The lens of this collimatorshould be protected so that it does not undergo degra-dation due to whatever mechanism is degrading theCZCS, and, therefore, if any degradation is observed,it can truly be attributed to the instrument performanceand not to the performance of the calibration sourceitself. Such additions will certainly be expensive, butif quantitative measurements are to be made from spacethey appear to be essential.

The authors wish to thank our colleagues in the Sat-ellite Experiment Laboratory: Lee Johnson, FrancescoMignardi, Kenneth Hayes, and Robert Levin, for theircontributions to the instrument electronic design, as-sembly, calibration, and data gathering in the field; andJohn Bray and Robert Koyanagi for their contributionto instrument mechanical design, fabrication, and as-sembly.

References1. H. R. Gordon, J. W. Brown, 0. B. Brown, R. H. Evans, and D. K.

Clark, "Nimbus 7 CZCS: Reduction of its Radiometric Sensitivitywith Time," Appl. Opt. 22, 3929 (1983).

2. G. R. Smith et al., "The NESDIS-SEL Lear Aircraft Instrumentsand Data Recording System," NOAA Tech. Memo., NESDIS 9(June 1984).

3. W. A. Hovis and J. S. Knoll, "Characteristics of an Internally Il-luminated Calibration Sphere," Appl. Opt. 22, 4004 (1983).

410 APPLIED OPTICS / Vol. 24, No. 3 / 1 February 1985