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Technology Identification, Evaluation, Selection and DemonstrationProcesses for Space-Qualification of Subsystems for FTS Sensors

Ronald J. Glumb, Norman H. Macoy, David C. Jordan, and Joseph P. Predina

ITT Industries, Aerospace 1CommunicationsDivision, 1919 West Cook Road, Fort Wayne, IN 46801

ABSTRACT

Development of space-qualified Fourier Transform Spectrometer (FTS) systems for long-life operational space missionsrequires development of new technologies. ITT Industries has been developing these new FTS technologies for the past 5years, in anticipation of their use in FTS systems for operational meteorological satellites and other long-life spaceapplications. Our objectives are to identify FTS technologies that have important mission advantages, design and build newcomponents using these technologies, and prove the new technologies in a complete FTS interferometer technology testbed.This paper describes the process used at ITT to identify and develop these new technologies, the Dynamically Aligned PorchSwing (DAPS) interferometer technology testbed used to prove the new technologies, characterization tests of the DAPSused to verify the performance of the new technologies, and space qualification efforts now underway to verify that the newtechnologies can survive space environments.

Keywords: interferometers, fourier transform spectrometers, dynamic alignment, porch swing, space qualification

1. TECHNOLOGY SELECTION AND EVALUATION PROCESS

ITT Industries is continuously seeking improvements to the remote sensing systems that it produces. The goal is to find newtechnologies or design approaches that improve performance, lower cost, simplify the design, improve reliability, or enhancerobustness. Other special program considerations also sometimes play a role, such as special spacecraft accommodationneeds, modular-design flexibility, schedule or risk considerations, or future growth requirements. Figure 1 illustrates theprocess used to identify and develop new technologies for our FTS systems.

TechnologyDevelopments(Government,

ITT, Other)

FTSOptimization

Trade Studies

MissionUtility

Studies

Figure 1. Technology Identification, Evaluation, and Demonstration Process Used for ITT FTS Systems

The evaluation process begins with an identified need for better technology or a recognition that an available technology mayimprove system performance. New FTS technologies can emerge from a number of different sources. First, developmentsfrom ITT, government, university, or other technology programs can yield new ideas or design approaches that arepotentially usable in our FTS systems. Second, as we perform trade studies to optimize the performance of our FTS systems,we will often identify alternate design approaches that could enhance the performance of our FTS systems, if only a certainpiece of technology were available and proven. Third, mission utility studies often identify key performance drivers, whichlead to identification of high-value technology insertions that can provide important mission benefits.

Part of the SPIE Conference on Optical Spectroscopic Techniques and Instrumentation36 for Atmospheric and Space Research III • Denver, Colorado . July 1999

SPIE Vol. 3756 • 0277-786X/99/$1 0.00

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Once the initial evaluation process determines that a candidate technology offers increased value, it is developed, validated.and made available for insertion into our flight-model manufacturing process. ITT Industries uses three steps to validate thenew technologies. First, the new hardware or software approach undergoes detailed design to tailor the technology for use inour FTS systems. Second. the new technology is built and placed into our Dynamically Aligned Porch Swing (l)APS)technology testhed for validation testing. Third, the new technology undergoes environmental testing to verify that it canwithstand space environments. These test generally include vibration, thermal-vacuum, radiation, and life testing. Uponsuccessful completion of these tests, the new technology is considered proven, and ready for insertion into flight instruments.

2. THE DAPS TECHNOLOGY TESTHED

Recognizing the need for a technology testbed to prove new FTS technologies. ITT Industries began developing the DAPSinterferometer technology testbed in 1994. The goal of the DAPS development effort was to produce a modular, flexibleFTS system that could be used to test and evaluate new FTS technologies as part of an end-to-end interferometric instrument.Initial mission analysis studies identified the basic architecture used in the DAPS. It employs a flat-mirror Michelsonconfiguration with dynamic alignment to ensure excellent radiometric accuracy, a moderate aperture size (3.8 cm) typical ofLEO sounding systems, two deep-cavity calibration blackbody sources (one warm and one hot), a single-element infrareddetector (either SWIR or LWIR), a porch swing assembly with voice coil drive motors to generate the optical path difference(OPD) within the interferometer. and control electronics to coordinate instrument operations. Figure 2 is a photograph of theoverall E)APS technology testbed. The DAPS was constructed as a compact, modular "brassboard" (i.e.. similar in size andconfiguration to an actual flight instrument). DAPS is also equipped with a complete set of signal processing electronics andsoftware needed to control operations. record interferograms. and process the interferograms into calibrated spectra

Electronics

Detector(LN2 cooled)

Calibration Blackbodies(MOPITI Design)

MotorizedPointingMirror

Dynamic AlignmentAssembly

Figure 2. DAPS Interferometer Technology Testbed

Porch swing Beam spi itter

Assembly Enclosure


The initial DAPS configuration utilized several components that were later replaced as part of subsequent technologyinsertions. For example, the original DAPS used a flex-pivot porch swing assembly (later replaced with a flexure-basedunit), a HeNe laser for metrology (replaced by DFB laser diodes), and a KC1 beamsplitter Icompensator (replaced with ZnSeelements). The key characteristics ofthe DAPS instrument are summarized in Table 1.

Table 1. Key Characteristics of DAPS Technology Testbed

Parameter ValueEntrance aperture diameter 3.8 cmField ofview 12 mradEtendue 0.0012 cm2-srMaximum optical path difference +1- 1 .4 cmMaximum resolution (unapodized) 0.36 cm'OPD velocity 7.8 cm/secSpeed fluctuation (1 sigma) 0.73 percentCarriage position tracking Quadrature fringe countingSampling frequency 9 1 kHzSamplingjitter < 5 nsec

Dynamic range 16 bitsMax interferogram size (Double-Sided) 32,768 pointsModulation mdex (at ZPD) 0.97

A more complete description ofperformance characterization data for the DAPS instrument is provided in Section 4.

3. TECHNOLOGY UPGRADES VALIDATED IN DAPS

After construction of the DAPS technology testbed, several advanced FTS technologies were identified as having high valuefor remote sensing applications. Each of these new technologies was integrated into the DAPS testbed, and fullycharacterized. The following paragraphs describe each ofthe technology upgrades proven in DAPS.

852 nm DFB Laser Diodes. The original DAPS employed a HeNe gas laser, which is not a viable option for spaceapplications. Trade studies performed on the GHIS program' identified Distributed Feed-Back (DFB) laser diodes as theoptimum choice for space-based interferometers. This is due to their high power output, excellent wavelength stability, andresistance to mode-hopping. It is also easy to control the wavelength output of the device via a combination of temperatureand current control. A trade study was also performed to select the best operating wavelength, which impacts the samplingrate of the instrument. The trade resulted in the selection of an 852 urn operating wavelength. This wavelength minimizesthe sampling rate (and thus the data rate), and allows for sampling on every positive-slope laser fringe zero-crossing, whichminimizes sampling jitter. The selected diodes are manufactured by Spectra Diode Labs (SDL). Two laser diodes, in aredundant optical injection package, have been installed in DAPS and are operating as expected. The laser diode assemblydesign is shown in Figure 3.

Neon Lamp Wavelength Calibration System. Meteorological mission utility studies identified the importance of precisespectral calibration of FTS spectra, with requirements of <5 parts per million (ppm) knowledge. Since laser diodes maychange wavelength with operating conditions and with age, another method was needed to provide accurate information ondiode wavelength over extended mission lifetimes. ITT Industries has developed a patented technique2 for measuring theoutput wavelength of the laser diode on-orbit, throughout mission life, to within an accuracy of 2 ppm. Energy from a neonlamp is injected into the metrology beam path, and detectors compare the number of neon fringes to laser fringes over the fullOPD sweep. Since the neon wavelength is extremely stable, the ratio of the number of fringes can be used to accuratelycalibrate the laser wavelength. A redundant neon lamp system has been installed in DAPS, and has been used to demonstratethe wavelength calibration technique.

38


Output T&escope

L

c -—- -\

I

Diode ControlElectronics

Figure 3. Redundant 852 nm DFB L.aser l)iode Assembly Validated in DAPS

Flexure Porch Swing Assembly. Precise alignment of the moving interferometer niirror with respect to the stationary mirroris critical in an FTS system. Optical tilts of less than 50 prad are desired to maintain high modulation efficiency. Theoriginal DAPS employed a flex-pivot type of porch swing. which used 8 flex pivots in a ftur-lmk parallelogram structure.However, we Ibund that this configuration was bulky, difficult to assemble and align, and achieved alignment accuracies nobetter than 100 prad (before DA correction). GI-IIS trade studies' ideiititied a superior porch swing conliguration. which usedtlexures in a simple parallelogam structure. Stiffeners were added to the centers of each Ilexure blade to increase the naturalfrequency of each blade and to prevent unwanted bending in other axes. Blade thicknesses are selected to ensure infinitemechanical life. Proprietary adjustments built into the base of the porch swing allow the unit to he aligned to extremely highaccuracies. Recent units have demonstrated alignment accuracies of between 5and 10 trad. The flexure porch swing alsouses a voice coil linear actuator with a moving magnet configuration. which provides precise linear motion, zerohysteresis.

and extremely high reliability. The porch swing features a "passive caging" approach to withstand launch loads that is, theporch swing is free to move during launch, but soft bumpers are placed at the limits of the porch swing travel (in 3 axes) toensure that the flexures are not overstressed. A flexure porch swing has been installed in DAPS. and has been operating forover a year without degradation. Figure 4 shows a photograph of the flexure porch swing assembly installed in I)APS.

Two DFB LaserDiodes With BeamCombining Optics

MovingMirror '.

Stiffener

FlexureBlade

Voice Coil—' Linear

Actuator

—. Base

Figure 4. Flexure Porch Swing Assembly Validated in DAPS Testbed

j (3


Dynamic Alignment Assembly. Misalignment of the two plane mirrors in the two branches of an interferometer relative toeach other and the beamsplitter reduces the modulation index ofthe signal. This is particularly true for FTS systems havinglarge apertures (>3 cm) or operating at short wavelengths (>2500 cm5. A dynamic alignment (DA) subsystem is used toactively maintain precise alignment between the two plane mirrors. The DA system senses tilts in the interferometer, anduses a closed-loop control system with a two-axis steering mirror to remove the relative tilt errors, typically to within I grad.DA is the key to producing a robust sensor that can operate over a wide range of environments. The DA system originallyused in DAPS employed galvanometric actuators which are not well suited for space applications. Trade studies performedon the GHIS program' identified a superior type ofmechanism utilizing a central torsion-bar pivot (using a monolithictitanium flexure), coupled with redundant voice-coil linear actuators. This design is much more reliable, eliminates stictionissues, and has very high bandwidth capabilities. The resultant design, shown in Figure 5,was installed in DAPS, and hasproven to be a reliable and high-performance design. Life tests, currently underway, are described in Section 5.

Actuotor Fied

Tilt Actucto, Redundont

- --- j__ Thermof Strop

Fexure CIomp_—._..

Mirror CeII

Retordotion

Retaining Hooks.____.4

nFigure 5. Improved Dynamic Alignment Mechanism Demonstrated in DAPS

ZnSe Beamsplitter. The beamsplitter assembly is the heart of an FTS system. Therefore selecting a beamsplitter which canmeet the system and life requirements, and can be manufactured in a cost effective manner is a critical part of interferometerdesign. The original DAPS was equipped with a KC1 beamsplitter. This material (along with a similar material, KBr) has theadvantages of low refractive index, and excellent transmission at wavelengths out to beyond 16 tm (the typical limit ofinterest for meteorological applications). Unfortunately, these materials are also highly hygroscopic, and require specialpolishing techniques and special handling procedures to prevent absorption of moisture. ZnSe is a superior material in termsof strength and durability, and has a low coefficient of thermal expansion. It also is not hygroscopic. However, ZnSe doeshave reduced transmission at wavelengths beyond about 1 5 .tm, which can have negative impacts on mission utility. GHIStrade studies' determined that the impact on meteorological mission performance is negligible. As a result, the DAPS wasrefitted with a ZnSe beamsplitter and compensator to prove the durability and robustness ofthe ZnSe material. Coatings usedon the ZnSe components effectively compensated for the high refractive index of ZnSe, providing good overall transmission.To date, the ZnSe materials have been in place in DAPS for over 4 years, with no noticeable degradations, and with nospecial handling or storage provisions. The DAPS experience has clearly indicated that ZnSe is the preferred material ifwavelengths beyond 15 tm are not mission-critical.

40

InSulating

Monolithic Flenure—.-

"X" Voice Coil Actu at or


4. DAPS ChARACTERIZATION TEST RESULTS

The performance of the DAPS interferometer has been well characterized to prove the feasibility of the new technologiesemployed. Complete characterization test programs have been conducted and reported."4 In the following paragraphs,results from several key characterization tests are summarized.

Porchswing Speed Stability. A high level of speed stability (typically less than I %) is required in FTS systems, since non-uniformities in sampling of the interferogram act as a major source of interferogram noise. Characterization tests of theDAPS testbed, using the improved flexure porch swing with voice coil linear actuators, demonstrated a 1-sigma speedstability during interferogram sampling of less than 0.3%. Porch swing turnarounds are accomplished in less than 29 msec,with peak accelerations during turnaround ofapproximately 023 cm/s2.

ZPD Tracking / Phase Stability. Measurements were also made to confirm that DAPS can maintain an accurate Zero PathDifference (ZPD) knowledge, and that phase stability is good over extended time periods. Continuous DAPS data collectedover a 12 hour period confirmed that ZPD knowledge was maintained perfectly without loss of fringe count. In addition, thedata shows that the phase is highly stable over a 12-hour period, with a standard deviation over the period of less than 0.0 1%.

NEdN Measurements. Noise Equivalent Spectral Radiance (denoted NEdN) is a measure of the sensitivity of an FTS system.The DAPS NEdN has been fully characterized, and compared to model predictions, as shown in Figure 6. Good agreement isfound between predicted and measured noise, with values of about 1.5x105 mW/(m2-sr-cm'). Figure 6 shows NEdN datafor the SWIR band; data is also available for the LWIR band4, where NEdNs are in the 1.5-3.5 mW/(m2-sr-cm') range.

Ill I'

'ii1I

III 1' I

(Measured / CalcuLated) 0.73

2300 2400 25001800 1900 , 2000 2100 2200Wavenumber (cm)

Figure 6. Comparison ofPredicted and Measured DAPS NEdN Performance (SWIR Band)

Modulation Index. One of the key contributors to NEdN is the interferometer modulation index, which is closely related tointerferometer cavity misalignment and optical surface quality. A modulation index > 0.95 is a good indicator of excellentinterferometer alignment and performance. DAPS modulation index was measured utilizing an external 3.39 im He-Nelaser and an incoherent narrowband blackbody source3. The measured modulation index was 0.97 1 0.036, confirming theexcellent alignment achieved by the DA system. Typical raw data near ZPD is shown in Figure 7. The near perfectsymmetry about ZPD illustrates well-corrected optical and electrical phase characteristics, and the lack of "chirping" effects.

41

Predicted NEdN atTwo InstrumentTemperatures

(320 and 321K)

MeasuredDAPS NEdN

EQU)

E(.)

a)UCU)

U)ccU)

0a)(J)

Ill


6000 Pt. Avg. = 1.000 024; ZPD Maximum = 1.97102; M = 0.971First Minimums = 0.05127 and 0.02145

Figure 7. DAPS Interferogram Near ZPD, Indicating Near-Perfect Symmetry

Instrument Line Shape (ILS). ILS is an indicator of the spectral resolution capabilities of an FTS system. Moreover, a stableILS is required if accurate spectra are to be constructed from the measured interferograms. Data have been taken to measurethe ILS of the DAPS, and to ascertain its stability over extended time periods. Figure 8 illustrates the measured LWIR ILS ofthe DAPS testbed. This data was collected by viewing a gas cell containing NH3, placed in front of a blackbody source;thus, the data shows loss of transmission near the NH3 line at 908. 1 851 cm1. The measured data points (circles) are in closeagreement with the theoretically predicted ILS for DAPS, indicating that the spectral resolution of the DAPS system isachieving near-ideal performance. ILS changes over a 7-hour period were also measured. Figure 9 shows the ILS curves ofeight interferograms taken over that time period, superimposed on one graph. Analysis of the data indicates that variations inthe ILS were less than 1%, which is less than the noise inherent in the data, suggesting that the change in ILS over this timescale is very small.

1.02

1.01

1.00

Cl) Q99

0.98

) 0.97

0.96

0.95

0.94

42

905 906 907 908 909 910 911

0.40

0.20

Wavenumber (cm1)920

2.0

En 1.5cy)0a)a)

ci)00C)

2 0.5

0.0-0.30 -0.20 -0.10 0.00 0.10 0.20

flme (rrilliseconds)

0.30

1.208 tLS curves

1.00

Center pixel- ,1

912

Wavenumber a [cm1

0.60U)-J

0.00

910

Figure 8. Measured DAPS ILS (Circles), Compared Figure 9. Eight Superimposed ILS Curves TakenTo Theoretical Predictions (Line) Over a 7-Hour Period

915


Channel Spectra. Window-like structures within the optical train of an FTS system can act as etalons, producing secondaryinterferograms (or channel spectra) that can corrupt the radiometric accuracy of the instrument. DAPS employs wedging ofthe interferometer beamsplitter and compensator to move any reflections from these transmissive components to outside theFOV of the detector, thus virtually eliminating the effects of channel spectra. Interferograms collected during DAPScharacterization testing show no signs of measurable channel spectra.

Outdoor Field Tests. The measurement of downwelling atmospheric radiance can be instructive in validation of end-to-endsensor performance and calibration. DAPS was used to collect outdoor downwelling radiance data. Figure 10 displays thedownwelling radiance over the spectral region from 600 to 2000 cm1. Also included for reference is the Planckian functionfor the instrument temperature. Telluric atmospheric absorption lines appear as well defined pinnacles of width dictated bythe instrument line shape (ILS) function, at 0.946 cm' apodized resolution. The valleys correspond to high altitudeemittance but also include local aerosol and molecular scattering. The right half of Figure 10 displays the water vapor,ozone, N20 and CH4 emittance over the spectral region from 1100 to 1200 cm'. Each apparent line is the combined effect ofseveral unresolved lines. Note that the aerosol continuum emittance is about 0.22 tW/cm-sr at 1142 cm.

E

a)

a)

8

Figure 10. Downwelling Radiance Measured By DAPS Testbed. Fort Wayne, Indiana, 3 October 1996, 0940 hrs EST,Clear Sky, RH =48%., Visibility > 10km. Pasquill stability category C.

5. SPACE QUALIFICATION OF DAPS COMPONENTS

For new FTS technologies to be practical for long-life space applications, it is necessary to demonstrate that each technologyhas an operating lifetime that is compatible with typical space missions, and can survive space and launch environments.Thus, our approach for each promising new technology is to perform life tests and environmental testing to verify that thetechnology is suitable for operational space use. Lifetime requirements can be as high as 7 years on-orbit (plus up to 8 yearsof ground storage time). Environments include space radiation, extreme thermal conditions, and stressing launch loads. Thefollowing paragraphs describe ongoing space qualification efforts for four of the new technologies described in earliersections.

Neon Lamps. The wavelength calibration system described in Section 3 requires neon lamps as the illumination source.While these components are rugged, and have been used in previous space missions (and thus are capable of surviving launchconditions), they typically are not used for long-life applications. ITT has therefore been pursuing a life test program todemonstrate that the lamps can survive the extended operating times required. Fortunately, the wavelength calibrationsystem is not operated continuously, but rather only intermittently (typically for a few seconds every few hours). This greatlyreduces operating times, to approximately 500 hours. However, there currently is very little data on the lifetimes of these

43

1 .2E-02

1 .OE-02

E8OE-03

6.0E-03

4.OE-03

C

82.0E-03

0.0E+00600

4

3

2

01100 1120 1140 1160

Optical Frequency (1/cm)800 1000 1200 1400 1600 1800 2000

Optical Frequency (WN)

.- 1-120,03

.v. 1-120, 03,

N20

.,. 1-120, N20,

C1-

1180 1200


devices (the data that does exist is typically for continuous operation at much lower current levels), and no data at all isavailable for the cycled type of operation needed for space FTS missions. Moreover, the failure mechanisms of these devicesare not well understood. Also, the lamp does not need to totally fail in order to he a prohlem if its power output decreasesby more than a factor of 3-5 over the mission life, the wavelength calibration system SNR drops to levels that makemeasurements impractical. rendering the calibration system ineffective.

the objectives of our life test program are to quantify the lifetime of neon lamps under cycled operation at current levels of2-5 mA, determine the cause of any lamp failures or degradations, and develop effective screening processes to select lampswith the longest possible lifetimes.

The test program began with a series of neon lamp characterization tests prior to the start of extended operational tests.Measurements of the voltage-versus-current curve were made, with the finding that lamp breakdown and maintenancevoltages tend to increase with age. Measurements of optical power output versus current, voltage, and operatine teniperaturewere also made. Next, an initial set of 10 lamps operated under three different types of operating conditions. Five lampswere operated at current level of4.0 mA and a very fast 50% duty cycle (I second on. one second oW. Four other lamps alsoused a 4.0 mA current level. but were operated using a slow 50% duty cycle (I nhinute on. I minute off) l'he final lampused a 5mA current level and the fast 5000 duty cycle. All of the lamps were tested in a vacuum chamber to better representoperation in space. and to avoid issues related to convective cooling of the lamps in an ambient environment. A photographof the test setup is shown in Figure 11. along with typical data tiom a good lamp that meets lifetime requirements (tipperright), and one that does not (lower right).

Neon Lamp #2 Power at 711324 nni 02-Oct-i 998 13:59:38 to 1 0-Jun-i 999 09:36:58

0 50 100 150 200 250

100 150 200 250

Figure 11. Neon Lamp Lifetime Test Setup and Typical Results

lhe initial round of testing led to three major conclusions. First, lifetime is a strong inverse function of operating current(as found in previous testing) and current levels above 4 mA result in unacceptably short lifetimes Second, theic was nomeasurable difference in lifetimes associated with the different duty cycle periods. 'Ihird. by observing the initial 2-1i) hoursof lamp operation. lamps that are likely to have short lifetimes can be identified froni the shape and continuity of theirvoltage-current curves, and screened out.

A second round of life tests has just been completed with an additional 10 lamps. at au operating current of 2.5 mA, andusing the screening procedure described above. After accumulating more than 6000 hours of Operation at 50% duty cycle, all

44

40

120

100a, 800a-600,0

20

a

100

Ca'

0a-

0a-0

T,me [Days)

Noon Lamp #0 Power at 703.24 nm 02-Oct-i 998 13:59:38 to 10-Jun-1999 09:36:58

0 50Time IDays]


of the lamps continued to produce the required power output. Further tests are planned in the f'uture to conlirm these results

using larger samples of lamps. and to verify the effectiveness ol the screening procedure.

Laser Diodes. As discussed in Section 3, the DFI3 laser diode source used within the interferonieter metrology system isessential for operation of an intertérometric sensor. Ihe 852 urn DFE3 laser diode is manufactured by Spectra Diode labs(SDI.), hut is not currently space qualified. Four types of qualification tests are currently planned for these diodes: I ) lifetests, 2) radiation total dose tests. 3) vibration tests. and 4) thermal-vacuum tests. The life tests are currently underwav theother tests are planned for next year.

The life test program is intended to verify that the 852 DFB laser diodes can operate continuously for a 7-year mission life.Although life data exists for other types of SD!.. laser diodes that are very similar in construction and materials to the 852 nm[)FB, lifetime data for this exact configuration is desired. The laser diode life tests are now underway. We are using twodiodes, both operated at an operating current of 75 mA. and a teniperature of 35C' (vell above that planned for space rise).the elevated temperature is expected to produce an acceleration factor of approximately 3 during the life tests, allowing us tosimulate a full 7-year lifetime in just over two years of life testing. Full characterization tests on both diodes were completedprior to the start of life tests, and will be repeated at the end of the life tests.

During the life tests, we are collecting data on diode junction voltage, injection current, optical power. and operatingtemperature. Figure 12 is a photograph of the test setup (at left) and a sampling of some of the diode data that has been

collected. To date, the tests have shown no significant aging effects. Very minor changes have occurred in unction voltage(lo change) and operating current (<0.05% change). but power outputs have been nearly steady. In addition, the diodetemperature control circuitry has operated as planned, providing better than 20 rnK temperature stability over a 2 hour period.

Laser Diode 1 Current vs. Time

Figure 12. Laser Diode Life lest Setup and 1 vpical Results

Porch Swing Assembly. Another critical element within an EN interferonieter is the porch swing assembly. which keepsthe moving interferometer mirror in motion on a continuous basis for up to 7 ears. (.)hviouslv. an' failure in either themotor or mechanical assembly could cause the motion to stop. or could produce unacceptably large tilts in the mirror motion

4

I !'' I

7445 :...

F

C7447435 I

0 200 400 600 800 1000 1200 1400 1600 1800

Laser Diode #1 Junction Voltage vs. Time

212O9

2.08

2.o7 :

"206 - I.. .. I . . I.' I.

0 200 400 600 800 1000 1200 1400 1600 1800Laser Diode #1 Module Output Power with Attenuator Glass and Optical Isolator

0.28-.9,4 26

p.0.24

0.22ci

02

0 200 400 600 800 1000 1200 1400 1600 1000

Time [hours]


that could not be compensated for by the dynamic alignment system. Thus, both life tests and vibration tests of thisassembly are considered essential in order to fully validate the feasibility of FTS system for long-life space missions.

Life tests of a porch swing assembly similar to that shown in Figure 4 have recently begun. The life tests have so farachieved a total of 2 million cycles, which is sufficient to confirm that stress levels in the flexures are below the fatigue limit,and thus should exhibit infinite life. Tests will continue for the next several years to fully confirm the lifetime requirementsof all components within the assembly. Two other porch swings of the same design are now being fabricated, and will jointhe life test program later this year. Prior to the start of life testing for these units, one of the porch swings will be subjectedto vibration levels similar to expected launch loads, to confirm its ability to survive without changes to its alignmentcharacteristics, and to verify that the passive caging approach works as designed.

Dynamic Alignment Assembly. The dynamic alignment (DA) assembly of the interferometer is another critical element thatcould potentially cause failure of the instrument. The DA system is used to correct for any residual misalignments and tilts ofthe porch swing and moving mirror assembly. For space applications requiring a high degree of radiometric accuracy, theDA system is operated continuously, so as to correct for tilt errors throughout the sweep ofthe moving mirror. This resultsin a very large number of DA tracking cycles, albeit with very small motions of the DA mirror. As a result, life tests of theDA mechanism over extended periods are considered necessary in order to verify the feasibility ofthis approach for long-lifeFTS space applications.

The demonstration process for the DA assembly closely parallels that of the porch swing assembly. One DA life test unit hasalready been constructed, similar in design to that shown in Figure 5,and has undergone 2 million cycles to verify that theflexures in the DA mechanism are not subject to fatigue. Life tests will continue for the next several years. Two other lifetest units now under construction, and will begin tests shortly. One of these units will also undergo vibration testing tosimulate launch loads. The DA mechanism uses a passive caging approach similar to that of the porch swing assembly.

6. SUMMARY AND CONCLUSIONS

New FTS technologies that offer improved performance, superior reliability, or lower cost are under development at ITT foruse in future space remote sensing applications. This paper has described the process used to select new technologies, andthe use ofthe DAPS technology testbed to verify the feasibility of each new technology. The qualification process has begunfor the flexure porch swing, dynamic alignment assembly, neon lamps, and 852 nm DFB laser diodes. Results to dateindicate that each ofthese technologies is a viable option for future long-life space missions.

7. ACKNOWLEDGEMENTS

ITT Industries gratefully acknowledges the participation of Bomem in the development of DAPS, and in the demonstration ofthe new technologies described in this paper. Bomem manufactured the original DAPS unit, and has been responsible forinstalling the upgrades described above. In particular, we recognize the assistance of Martin Chamberland, Francois Aube,Jacques Giroux, Jacques McKinnon, Serge Forttin, and Jean Giroux.

8. REFERENCES

1. GOES High-Resolution Interferometric Sounder (GHIS) Final Report, Volume 3, September, 1997.2. Optical Frequency Stability Controller, United States Patent Number 5,757,488.3. Macoy, Norman H., et. al. "Dynamic Alignment Design and Assessment for Scanning Interferometers," SPIE

Proceedings 2832 126, (1996).4. DAPS Performance Characterization Report, DAPS Upgrade for Technical Demonstration Program, Bomem Document

Number ITT-BOM-017/97, March 1998.

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