Nimbus is a NASA meteorological research-and-development satellite program (parallel to the operational TIROS program) which started in 1963 with the prime objective to test new instrument concepts (introduction of sensor technology), a secondary objective was to provide atmospheric data for improved weather forecasts. Eventually, the series grew more into a major Earth sciences program (study of oceans, land surfaces and atmosphere) with the availability of better sensing instrumentation. The ERTS (Earth Resources Technology Satellite, later renamed to Landsat-1) program, developed in the early 1970s, is a direct descendent of the Nimbus program.
Figure 1: Line drawing of the Nimbus-7 spacecraft
Application: Oceanography, pollution of the atmosphere, meteorology; measurement of trace gases and particles in the atmosphere; distribution phenomena of air pollution; observation of the ocean surface color and of the temperature; ice observation, in particular in the coastal regions.
The original Nimbus satellite design life expectancy was 6 months. Due to the increased understanding of the impact of the space environment that led to improved space-hardened electrical components and lubrication techniques, the application of more conservative electrical designs, and the incorporation of redundancy of critical components, the lifetime expectancy for the later satellites increased to five years.
Some background of the Nimbus series missions:
Over the Nimbus Project's thirty-year history, the satellites increased the scientific community's knowledge of the Earth's atmosphere, land surface and ecosystems, weather, and oceanography. When Nimbus-1 launched in 1964, it gave meteorologists their first global images of clouds and large weather systems. By the time the Nimbus-2 mission went into orbit in 1966, the sensors onboard were sophisticated enough to measure the temperature of the ocean. Nimbus-3 in 1969 added atmospheric temperature observations and the ability to measure solar radiation above the atmosphere. In 1970, Nimbus-4 began collecting the first global observations of the ozone layer, and by 1972, scientists and engineers had incorporated into Nimbus-5 the capability to measure rainfall over the world's oceans and to map and monitor sea ice. Taking the atmosphere's temperature became more sophisticated with Nimbus-6 in 1975, which gave scientists the first satellite-based measurements of atmospheric temperature at different altitudes. The final mission, Nimbus-7, which was launched in 1978, collected data on ozone, the stratosphere, ocean conditions, and global weather until 1994. Nimbus-7 ceased operations in 1995.
NASA transferred the technology tested and refined by the Nimbus missions to the National Oceanic and Atmospheric Administration (NOAA) for its operational satellite instruments. The technology and lessons learned from the Nimbus missions are the heritage of most of the Earth-observing satellites NASA and NOAA have launched over the past three decades. Even today, scientists use data from the Nimbus missions to study the Earth system and climate change. 1) 2)
Table 1: Overview of Nimbus series missions
A major success factor in the Nimbus Program was the effective 20-year partnership that existed between the government managers and engineers that managed the program and the over 40 industrial companies that built the satellite equipment and instruments. Designs met performance requirements, individual element schedules were adhered to without impact to the overall integration schedule, and costs came in at or below the rather slim cost estimates for the challenging deliverables.
Two companies were major participants in the Observatory design and development effort. The General Electric Corporation in King of Prussia, PA built the spacecraft structure and the attitude control system, integrated the Observatory and conducted the critical, complex environmental testing that validated the Observatory readiness for launch. The RCA Corporation Astro-Electronics Division in East Windsor, NJ built the spacecraft power systems, spacecraft camera systems, spacecraft communication electronics, high data rate tape recorders, the ground RF signal demultiplexers, and the ground system for converting the visible camera electrical signals into film pictures. A General Electric team of engineers, technicians, and software programmers operated the Nimbus Control Center for thirty years (Ref. 2).
Table 2: Overview of Nimbus spacecraft series technology introduction
Table 3: On October 8, 2014, NASA remembered first Earth observations on the 50th anniversary of the launch of Nimbus-1 on Aug. 28, 1964. Nimbus-1 was the first of seven Earth-observing satellites in a series that spanned 30 years. 3) 4)
Figure 2: This graphic gives an idea of some of the many satellite missions, past and present, that can trace their heritage to instruments that flew as part of the Nimbus program (image credit: NASA, Mike Marosy and Jay O'Leary)
Table 4: Nimbus Program instrument complement, characteristics, and satellites on which each instrument flew (Ref. 4)
Table 5: Number of experiments and spectral channels on each Nimbus satellite. Note the growth in the number of electromagnetic spectral regions from Nimbus-1 to Nimbus-7 (Ref. 4)
Initial design: The GSFC Nimbus Project, initially established as an R&D Project, was tasked to build the Nimbus spacecraft to serve as the ESSA (Environmental Satellite Service Administration) operational spacecraft replacing the TIROS experimental meteorological satellites operated by ESSA. ESSA helped fund the Nimbus development. The Project was reverted to the NASA Earth remote sensing research Project in 1963 after development failures induced ESSA to drop their support of the Nimbus Program (Ref. 2).
Nimbus, named after the Latin word for rain cloud, was originally conceived as a GSFC in-house satellite development Project. Bill Stroud led the Project development that started in 1959. John Licht was the designer of the Nimbus spacecraft architecture: Earth viewing, an attitude control concept for achieving three-axis stabilization, (previous spacecraft were spin stabilized.), solar panels that tracked the sun,and a platform structure (sensory ring) with compartments designed to accommodate new configurations that housed instruments and electronic boxes. Rudolf Stampfl was the system designer who conceived the spacecraft bus concepts and the active cooling system for the electronics and small instruments installed in the sensory ring. The complex attitude control system that applied a combination of 3-axis reactive momentum wheels and the inherent gravity-gradient stabilization to provide 1o 3-axis attitude control with low-jitter represented a major development challenge. This fine attitude control requirement was necessary to be able to accurately locate the high-resolution scientific data that was planned. A large three-axis stabilized air-bearing platform was conceived for dynamic testing of the attitude control system (Ref. 2).
The General Electric (GE) Company in King of Prussia, PA was contracted to develop, build, and test the attitude control system. GE was also contracted to build the structure and serve as the Observatory integration and test contractor.
Eventually, all Nimbus series S/C were built and integrated by GE. The RCA Corporation built the spacecraft power systems, cameras, communication electronics, and the high data rate/volume tape recorders (3). The basic Nimbus series spacecraft structure is butterfly-shaped (some call it `ocean-buoy shaped') when deployed in orbit, designed to serve as a 3-axis stabilized, Earth-oriented platform. It consisted of three major elements: 1) a sensory ring, 2) two solar paddles, and 3) the control system housing. 5) 6) 7) 8) 9)
The solar paddles and the control system housing were connected to the sensory ring by a truss structure. The Nimbus-7 S/C was 3.04 m tall, 1.52 m in diameter at the base, and 3.96 m wide with the solar paddles extended. The sensory ring at the base of the satellite housed the electronics equipment and battery modules. The lower surface of the sensory ring provided mounting space for sensors and telemetry antennas. An H-frame structure mounted within the center of the torus provided support for the larger sensors and tape recorders.
The ACS (Attitude Control Subsystem) provided stabilization about the spacecraft's roll, pitch, and yaw axes and control of the solar paddles' orientation, maintaining them nearly perpendicular to the nominal sun-line. The ACS consisted of four attitude control loops and associated switching logic, telemetry and test modes, electrical manifolding, and thermal environmental control. This system maintained spacecraft alignment with the local orbital reference axes to within 0.7o of the pitch axis and 1o of the roll and yaw axes. The system kept the instantaneous angular rate changes about any axis to less than 0.01o/s. The three-axis active ACS used horizon scanners for roll and pitch attitude error sensing. The rate gyros sensed yaw rate and, in a gyro compassing mode, sensed yaw attitude. A torquing system used a combination of reaction jets to provide spacecraft momentum control and large control torques when required; flywheels were utilized for fine control and residual momentum storage.
The spacecraft power system consisted of solar arrays with nickel-cadmium (NiCd) batteries, providing regulated power of which approximately 186 W (orbit-averaged) was available to the instrument payload. The S/C had a wet mass of 965 kg (832 kg on-orbit dry mass).
Figure 3: Artist's concept of one of the Nimbus satellites (image credit: NASA)
The structure and thermal control system designs proved to be extremely effective in support of the multi-mission Nimbus program that materialized. It accommodated seven different instrument manifests (as many as 9 instruments per spacecraft) with minimum structural changes while providing the required viewing and thermal control requirements.
After the first spacecraft, this provided an extremely favorable payload to spacecraft funding ratio compared to other spacecraft programs; on Nimbus-7 with a new complex instrument mission it was 1.6 compared to 0.2-0.5 for typical new spacecraft programs. This accommodative Nimbus platform enabled the new "Earth Resources" Landsat satellite program to be undertaken with low platform development costs. Landsat 1, 2, and 3, which demonstrated the utility of what became major NASA and commercial satellite programs, used the Nimbus structure and the temperature control, power, attitude control, telemetry, and command subsystems for its platform.
The original Nimbus satellite design life expectancy was 6 months. (Nimbus-1 lasted 28 days, but Nimbus-2 and 3 actually operated for 2 1/2 years.) An increased understanding of the impact of the space environment on the satellite components led to improved space-hardened electrical components and improved lubrication techniques. Starting with Nimbus-4, the incorporation of these measures, the application of more conservative electrical designs, and the incorporation of redundancy of critical components increased the lifetime expectancy for the later satellites to five years. The spacecraft lifetime of the last four Nimbus spacecraft ranged from 6 to 15 years. Except for two instruments with cryogenic coolers that had a 6-month design life, and the Filter Wedge Spectrometer that never functioned properly, the instruments also had lifetimes ranging from 6 to 15 years.
RF communications: The S-band communication system included the S-band command and telemetry system, the data processing system, and the command clock. The S-band command and telemetry system consisted of two S-band transponders, a command and data interface unit, four earth-view antennas, a sky view antenna, and two S-band transmitters (downlink frequency of 2211 MHz). Commands were transmitted to the observatory by pulse code modulation, phase-shift keying/frequency modulation/phase modulation of the assigned 2093.5 MHz S-band uplink carrier. S/C data were received via NASA's STDN (Spacecraft Tracking and Data Network) and transmitted to GSFC for processing, archiving, and distribution. GSFC provided also the function of S/C operations. The data of the SAMS instrument were processed in the UK.
Launch: Nimbus-7 was launched Oct. 24, 1978 on a Delta vehicle from VAFB (Vandenberg Air Force Base), CA.
Orbit: Sun-synchronous polar orbit, apogee = 954 km, perigee = 941 km, inclination = 99.15o, period = 104.16 minutes, local equator crossing time on the ascending node at 12:00 hours [and midnight (descending) equator crossings], with 26.1o of longitude separation.
Figure 4: Artist's rendition of the deployed Nimbus-7 spacecraft (image credit: NASA, Ref. 1)
Mission status of Nimbus-7:
• The Nimbus-7 spacecraft was turned off in 1995 after more than 16 years of service. During its long period of operation, the Nimbus-7 mission was regarded the single most significant source of experimental data from Earth's orbit relating to atmospheric and oceanic processes.
• Nimbus observations began to point to a drop in ozone (blue areas) as early as 1980, with more extreme decreases developing in 1985 (Ref. 3).
Figure 5: These globes show ozone concentrations over Antarctica in selected Octobers from 1979-85 and 2000-2003 (image credit: NASA/GSFC, Paul Newman, Richard Stolarski, Mark Shoeberl, Arlin Krueger)
• Figure 6 is a composite map of 60,000 images (1978-1986) taken by NASA's Nimbus CZCS (Coast Zone Color Scanner) showing average ocean phytoplankton chlorophyll-a concentration. Purple and deep blues represent low concentrations. Yellow, orange and red are high concentrations. The Land vegetation data is from NOAA's AVHRR (Ref. 3).
Figure 6: Average ocean phytoplankton chlorophyll-a concentration observed by CZCS instrument in the period 1978-1085 (image credit: NASA/GSFC, Gene Carl Feldman, Compton J. Tucker)
Sensor complement: (CZCS, ERB, LIMS, SAM-II, SAMS, SBUV/TOMS, SMMR, THIR)
CZCS (Coastal Zone Color Scanner):
The CZCS instrument of NASA was built by Ball Brothers Research Corp., Broomfield, CO (USA) - the predecessor of Ball Aerospace and Technologies Corporation (BATC). Objectives: a `proof-of-concept' experiment for the measurement of ocean color (chlorophyll concentration, sediment distribution, gelbstoff concentrations as a salinity indicator), and the temperature of coastal waters and open ocean (all parameters were optimized for ocean color sensing). CZCS was in fact the first spaceborne instrument devoted to the measurement of ocean color. 10) 11) 12)
Figure 7: Photo of the CZCS instrument (image credit: NASA)
CZCS is a multispectral radiometer (a whiskbroom instrument of 6 spectral bands) utilizing a rotation plane mirror at a 45o angle to the optical axis of a Cassegrain telescope. Band wavelengths: 0.433-0.453 µm (chlorophyll absorption), 0.51-0.53 µm (chlorophyll concentration), 0.54-0.56 µm (Gelbstoff concentration), 0.66-0.68 µm (aerosol absorption), 0.70-0.80 µm (land and cloud detection), and 10.5-12.5 µm (TIR, surface temperature). The mirror scanned 360o but only the ±39.34o of data centered on nadir were collected for ocean color measurements. Observation of backscattered (or reflected) solar radiation in five bands, and sensing of emitted thermal radiance in band 6.
Table 6: Performance characteristics of the CZCS instrument
The instrument viewed deep space and onboard calibration sources (incandescent lamps) during the remainder of the scan. The incoming radiation was collected by the telescope and divided into two streams by a dichroic beam splitter. One stream was transmitted to a field stop that was also the entrance aperture of a small polychromator. The radiance that entered the polychromator was separated and re-imaged in five wavelengths on five silicon diode detectors in the focal plane of the polychromator. The other stream was directed to a cooled mercury cadmium telluride detector in the thermal region (10.5-12.5 µm). A radiative cooler was used to cool the thermal detector. To avoid sun glint, the scanner mirror was tilted about the sensor pitch axis on command so that the line of sight of the sensor was moved in 2o increments up to 20o (forward or aft) with respect to the nadir.
Figure 8: Optical layout of CZCS (image credit: NASA)
CZCS instrument characteristics: multispectral cross-track scanning radiometer, spatial resolution: 825 m x 825 m (each band) at nadir or IFOV = 0.865 mrad, swath width = 1566 km, centered on nadir. Data quantization = 8 bits, data rate = 800 kbit/s. Data products: global maps of chlorophyll concentration, sediment distribution, gelbstoff concentrations as a salinity indicator, and temperature of coastal waters and the open ocean. Prelaunch calibration used an integrating sphere and a blackbody source for the TIR channel. Inflight calibration used an incandescent light source for the first five bands. The TIR channel was calibrated by viewing the blackened housing of the instrument.
The CZCS instrument operated on an intermittent schedule due to overall power demands. The TIR channel failed within the first year after launch. CZCS was operational for eight years before it was turned off in Dec. 1986.
Figure 9: Alternate view of the CZCS instrument (image credit: NASA)
ERB (Earth Radiation Budget):
ERB is an improved follow-on instrument version first flown on Nimbus-6. Objective: to determine the Earth radiation budget (Earth and solar irradiances in UV, VIS, IR regions) on both synoptic and planetary scales by simultaneous measurements of incoming solar radiation and outgoing Earth-reflected (shortwave) and emitted (longwave) radiation.
The ERB instrument consisted of a 22-channel radiometer containing separate subassemblies to perform the required solar, Earth-flux (wide angle), and scanned Earth radiance (narrow angle) measurements. The systems used optical filters for spectral discriminations, and four uncooled thermal detectors, thermopile detectors in the solar and fixed-Earth-flux channels, and pyroelectric detectors in the scanning channels. 13) 14) 15)
The 10 solar channels observed the sun in front of the observatory in the X-Y plane (use of a 10 channel solar telescope). The solar channels obtained usable solar data only during a period of about 3 minutes in each orbit when the spacecraft was over the Antarctic region. Their full response FOV was 0.18 rad. The solar channel subassembly was pivoted ±0.35 rad in the X-Y plane to compensate for sun-angle deviation when required. The channel 10c (solar channel) was a model H-F (Hickey-Frieden) self-calibrating cavity thermopile used for monitoring the total solar irradiance in the spectral range 0.2 - 50 µm. The technology for long-term solar radiation monitoring utilized is referred to as ESCC (Electrically Self Calibrating Cavity).
A special goal of the experiment is to provide data related to the angular distribution of Earth albedo. For solar measurements each of the 10 channels is an independent, individually replaceable modular element with a mated amplifier. The sensors are advanced versions of the Eppley-JPL thermopiles. There are no imaging optics, only filters, windows and apertures.
Table 7: ERB instrument characteristics 16)
The four fixed WFOV (Wide Field of View) Earth-flux channels (numbered 11-14) were mounted so that they could continuously view the total Earth disk, and record data at 0.25 s intervals.
The eight narrow FOV (NFOV) scanning channels were mounted in a cylindrical scanning head (4 telescopes). The NFOV channels consisted of four shortwave (0.2 - 4.8 µm) and four longwave (4.5 - 50+µm) channels. The scanning head was gimbal-mounted in the radiometer unit main frame. The FOVs of the telescopes were asymmetric (4.4 x 89.4 mrad) and those of the shortwave and longwave channels were coincident. The 89.4 mrad FOVs of the four pairs of channels were not contiguous, but covered only alternate 89.4 mrad angular intervals along the horizon. - ERB instrument mass = 32.7 kg, power = 36.2 W, bit rate = 4.5 kbit/s.
The improved ERB radiometer on Nimbus-7 was stable enough to detect for the first time short-term and long-term solar irradiance variability (this could not be done with ERB data from Nimbus-6). ERB was the first long term solar monitor utilizing the ESCC (Electrically Self Calibrating Cavity) technique. - The ERB instrument operation was stopped on Jan. 4, 1994. 17)
LIMS (Limb Infrared Monitor of the Stratosphere):
The LIMS design is of LRIR heritage flown on Nimbus-6 (PI: J. M. Russell III). Objective: Measurement of vertical gas concentrations and temperature profiles in the stratosphere. Observables: O3, H2O, NO2, HNO3 and temperature. Channel center wavelengths: 6.25, 6.75, 9.65, 11.35, 15.25 µm and one broad channel from 13.3 - 17.2 µm.
The LIMS limb-sounding instrument was a six-channel radiometer consisting of two electronic boxes and the radiometer unit (Hg:Cd:Te detectors were used, cooled by a two-stage solid cryogen cooler). Radiance from the Earth's limb entered the OMP (Optical Mechanical Package) aperture, reflected off the scan mirror to the 18 cm diameter off-axis parabolic primary mirror where the radiation was focused and chopped at 945 Hz. The radiation was re-collimated by the secondary mirror and directed through a Lyot stop to a folding mirror and into the detector capsule assembly (DCA). The radiation was then focused through a cadmium telluride lens and through interference filters, which defined the FOVs, and onto an array of discrete mercury cadmium telluride detectors. The detectors were maintained at about 63 K temperature by the cryogen. 18) 19)
The LIMS began a scan near 153 km altitude, taking about 12 s to move near 38 km below the solid limb, then retraced its motion upward. After every second scan pair, the scan mirror was placed in a position to observe radiation from a small cavity blackbody for inflight warm calibration after which the instrument viewed space to obtain a cold calibration point. Instrument mass = 68.4 kg, power = 27 W, bit rate = 4 kbit/s.
The LIMS instrument operated successfully for about 9 months from October 25, 1978 to May 28, 1979. The instrument was turned off due to depletion of cryogen, as planned, in June 1979.
SAM-II (Stratospheric Aerosol Measurement II):
The NASA/LaRC instrument is a sun photometer that views a small portion of the sun through the Earth's atmosphere during S/C sunrise and sunset. The single-channel instrument measured the distribution of particles in the stratosphere, making measurements only at high latitudes (a characteristic of occultation instruments in polar sun-synchronous orbits) from 1978 to 1995.
SAM-II measured aerosol extinction and extinction ratio profiles, and stratospheric optical depth as a function of altitude, latitude and longitude. Channel wavelength: 0.98-1.02 µm. Spatial resolution is 1 km (vertical); swath width is 5 - 40 km vertical. Measurement calibration technique: the Langley technique is used to calculate zero air mass solar intensity. By measuring the intensity of the sun over a several-hour period on a clear, optically stable day and using the calendar day and latitude/longitude of the measurement location, one can plot intensity versus air mass. 20)
The SAM-II instrument package consisted of optics and electronics subassemblies. The optical assembly consisted of gimbals, a flat entrance window (which filters out UV radiation), Cassegrain optics, a flat scanning mirror, sun acquisition sensors, and a sun-photometer detector package. Solar radiation was reflected from the scan mirror into the Cassegrain telescope forming a solar image at the slit plate, which contained two solar edge sensors for monitoring solar limb crossings on either side of the detector aperture. Solar radiation passed through the aperture, was collected by a field lens, passed through an interference filter for wavelength discrimination, and finally measured by a silicon photodiode detector. The optics assembly was gimbaled in azimuth. After acquisition in azimuth, the mirror servo scanned in elevation until the Sun was acquired. The Sun was then scanned back and forth. The photometer viewed a portion of the solar disk with a 0.145 mrad IFOV and a sampling rate of 50 samples/s. Instrument mass = 17 kg, power = 0.8 W, bit rate = 4 kbit/s.
Data products: global maps of the concentration and optical properties of stratospheric aerosols as a function of altitude, latitude, and longitude, which prove valuable for studies on radiative transfer and climatic effects; aerosol transport sources and sinks in the stratosphere; seasonal variations and sudden warming phenomena; and volcanic injection phenomena. When no clouds were present in the instrument's IFOV, then tropospheric aerosols were also mapped.
SAMS (Stratospheric and Mesospheric Sounder):
SAMS was provided by Oxford University, Oxford, UK (PI: F. W. Taylor). Measurement of vertical gas concentrations (H2O, CH4, CO and NO) and temperature profiles in the stratosphere and mesosphere. Observation of `resonant scattering of solar radiation.' Channel wavelengths: nine channels defined by gas cell modulation 4.1 to 15 µm and 25 - 100 µm. Radiation from the limb of the atmosphere was incident on a scan mirror in front of a 15 cm aperture telescope. The scan mirror scanned the limb, viewed space for calibration, and viewed the atmosphere obliquely to obtain vertical profiles.
There were three adjacent FOVs, each 28 mrad x 2.8 mrad (corresponding to 100 km by 10 km at the limb). The FOVs were focused onto a field-splitting mirror by the telescope which directed radiation to six detectors. Separation into channels was accomplished through dichroic beam splitters. There were seven pressure modulator cells (PMC), two containing CO2, the remainder N2O, NO, CH4, CO, H2O. Pressure in the cells could be varied on command by changing the temperature of a small container of molecular sieve material attached to each PMC. The spectral parameters for the H2O channel were 2.7 µm and 25 - 100 µm. All other channels lay within the range 4.1 to 15 µm. A chopper operating at 250 Hz within the telescope, allowed the measurement of two separate signals from all detectors, one at 250 Hz and one at the PMC frequency. Instrument mass = 23.6 kg, power = 20 W, bit rate = 25 kbit/s. 21) 22) 23)
SAMS was operational until April 9, 1985 (an anomalous behavior of the limb scan motor in the SAMS instrument resulted in the instrument being turned off).
SBUV/TOMS (Solar Backscatter Ultraviolet/Total Ozone Mapping Spectrometer):
Instrument mass = 20 kg, power = 20 W, bit rate = 4 kbit/s. Measurement of vertical O3 profiles, total column amounts of atmospheric O3, solar irradiance, and terrestrial radiances.
• SBUV was a double Ebert-Fastie spectrometer and filter photometer, similar to the BUV on Nimbus-4. SBUV used three detectors: a photomultiplier tube (PMT) and a photodiode for the monochromator, and one photodiode for the photometer. Both the monochromator and the photometer have chopper wheels operating at 25 Hz. The SBUV used a depolarizer to eliminate the sensitivity of the grating monochromator to polarization of the backscattered radiation. The instrument's field of view (FOV) at nadir was 0.20 rad. A roughened aluminum diffuser plate viewed the sun for solar-spectral irradiance measurements and for calibration by viewing a mercury-argon lamp. In one mode, SBUV serially monitored 12 selected narrow wavelength bands in the spectral region from 0.250 - 0.340 µm. 24) 25) 26)
The SBUV spectrometer had a second mode of operation that allowed a continuous solar-spectral scan from 0.16 - 0.4 µm for detailed examination of the extraterrestrial solar spectrum and its temporal variations. A parallel photometer channel at 0.343 µm measured the reflectivity of the atmosphere's lower boundary in the same 0.21 rad FOV. Spatial resolution of SBUV = 11.3o in each band. Swath width = 200 km, separated by the 26o longitude interval between successive orbits. - The SBUV instrument was operational until 1990.
• TOMS (Total Ozone Mapping Spectrometer) is a nadir-looking instrument that measures the albedo (backscatter technique) of the Earth's atmosphere at six narrow spectral bands in the near-ultraviolet region. The albedo is measured by comparing the radiance of the Earth with the radiance of a calibrated diffuser plate. The TOMS instrument (the first in a series of TOMS instruments) was developed by NASA/GSFC, PI: R. D. McPeters. 27) 28)
Figure 10: Illustration of the TOMS instrument on Nimbus-7 (image credit: NASA/GSFC)
TOMS was a single stage Ebert-Fastie spectrometer with a fixed grating and an array of exit slits. TOMS step-scanned across the orbital track of ±51o from the nadir in 3o steps with a IFOV of about 0.052 rad. Spatial resolution is 3o x 3o (scanned through the satellite subpoint and perpendicular to the orbital plane) resulting in a footprint of 50 km x 50 km at nadir. Swath width of about 3000 km providing contiguous mapping of ozone data. At each scan position, the Earth radiance was monitored at six wavelengths between 312 and 380 nm (centered at 312.5, 317.5, 331.3, 339.9, 360.0 and 380 nm) to infer the total ozone amount. TOMS completed a cross scan in eight seconds, with one second for retrace, to record 35 scenes per scan. At each scene, a chopper sequentially sampled all six wavelengths four times. TOMS used the same type of PMT as SBUV, and had a separate mercury-argon lamp for wavelength calibration and a separate depolarizer. TOMS shared the diffuser plate with SBUV. Both SBUV and TOMS had five scanner modes and a shared electronics module.
The TOMS instrument provided reliable, high-resolution maps of global ozone amounts on a daily basis from October 1978 to May 1993.
Data products: vertical distribution of ozone, global maps of total ozone and 200-mb height fields; incident solar ultraviolet irradiance and ultraviolet radiation backscattered from the Earth. SBUV helps to determine the total amount of atmospheric ozone in a vertical column above the subsatellite point; vertical profile of ozone above the ozone maximum; measurements of ultraviolet solar spectral irradiance and its temporal variability over the 160 - 400 nm range (with a spectral resolution of 1 nm). TOMS exploits the polar orbit to yield global, contiguous maps of total ozone concentrations.
Note: The TOMS instrument aboard Nimbus-7 mapped the extent of the phenomenon known as the "ozone hole." Data from TOMS were part of the scientific basis for treaties banning the manufacture and use of ozone-depleting chemicals, known as CFC (chlorofluorocarbon) compounds.
Figure 11: Nimbus-7/TOMS total ozone distribution, May 6, 1993 (image credit: NASA)
SMMR (Scanning Multichannel Microwave Radiometer):
SMMR is a NASA instrument (conceptual design by GSFC based on ideas by C. R. Laughlin and K. Richter). Objective: observation of sea-ice parameters, ocean surface conditions, atmospheric conditions, land parameters, glacial features. Microwave brightness temperatures were observed with a 10-channel (five-frequency dual polarized) scanning radiometer operating at frequencies of 37 (0.81 cm), 21 (1.42 cm), 18 (1.66 cm), 10.69 (2.8 cm), and 6.6 (4.54 cm) GHz. The SMMR instrument consists of five hardware elements: 29) 30)
• The antenna assembly consisting of the reflector, fabricated of graphite epoxy, and the feedhorn.
• The scan mechanism, including momentum compensation devices
• An RF module containing the input and reference switching networks, the mixer-IF preamplifiers, and the Gunn local oscillators
• An electronics module containing the main IF amplifiers, all the post-detection electronics, and the power supplies for the scan and data subsystems
• A power supply module which contains the dc-to-dc converters and regulators for the rest of the instrument.
Microwave radiation from the Earth's surface and its atmosphere is collected by a 42o offset parabolic reflector that focuses the received power into a single feedhorn covering the entire range of operating wavelengths and provides coaxial antenna beams for all channels. Scanning is accomplished by oscillating the parabolic reflector about the vertical axis between local azimuth angles of ±25o with a period of 4.096 s. The various integrate-and-dump times of the radiometers combined with the oval IFOV result in roughly circular beam spots ranging in diameter from about 30 km at 0.81 cm to 150 km at 4.6 cm wavelength. Cross-track/along-track sampling intervals range from 14 km x 14 km to 58 km x 28 km over the wavelength interval. The incoming microwave beam is at a constant angle of 42o from nadir, corresponding to an Earth incidence angle of 50.2o at the Nimbus-7 orbital altitude.
The SMMR instrument is forward viewing and scans 390 km to either side of the orbital track. It is operated on alternate days so that it maps the entire globe twice every 6 days. The antenna beam scan lies along a conical surface with a 42o half angle so that the distance to the surface of the Earth is constant over the scan. The angle of incidence at the Earth's surface is about 50.2o.
Calibration: By observing both hot and cold reference sources, the instrument is calibrated in flight. A radio frequency source at the ambient temperature serves as a hot reference, and deep space viewed by a special antenna horn provides a cold reference. This two-point reference system allows the measured antenna counts to be converted to the observed radiances. Radiances are computed with a calibration equation from the radiometric signal from the Earth's surface, the hot and cold calibration counts, and several instrument temperatures in the SMMR microwave circuity. This calibration equation was developed using prelaunch calibration data.
Figure 12: Schematic illustration of SMMR (image credit: NASA/JPL)
Six separate Dicke-type radiometers were utilized. Those operating at the four longest wavelengths measured alternate polarizations (H, V) during successive scans of the antenna; the others operated continuously for each polarization. The motion of the antenna reflector provided observations from within a conical volume along the ground track of the spacecraft. An identical instrument was flown on SEASAT (launch June 28, 1978). SMMR instrument mass = 52.3 kg, power = 60 W, bit rate = 25 kbit/s.
Status: The SMMR operations on Nimbus-7 ended on July 6, 1988.
THIR (Temperature Humidity Infrared Radiometer):
The instrument was built by Hughes SBRC. A similar instrument was flown on Nimbus-4, -5, and 6. The Nimbus-7 THIR operation ended on Nov. 30, 1987.
Objective: detection of emitted thermal radiation in two spectral ranges, measurement of daytime and nighttime surface and cloud-top temperatures as well as the water vapor content of the upper atmosphere. The instrument consisted of a 12.7 cm Cassegrain system, a scanning mirror common to both channels, a beam splitter, filters, and two germanium-immersed thermistor bolometers. Incoming radiant energy was collected by a flat scanning mirror inclined at 45o to the optical axis. The mirror rotated through 360o at 48 rpm and scanned in a plane normal to the spacecraft velocity vector. The energy was then focused into a dichromatic beam splitter, which divided the energy spectrally and spatially into two channels (10.5 - 12.5 µm TIR window, and 6.5 - 7.0 µm for water vapor). Direct readout infrared radiometer (DRIR) data could be transmitted to APT ground stations for both day and night portions of the orbit. The TIR window channel had a ground resolution of about 7 km and the water vapor channel about 22 km at nadir. - Instrument mass = 9.0 kg, average power consumption = 8.5 W. 31) 32)
1) Rebecca Lindsey, "Nimbus: 40th Anniversary," NASA Earth Observatory, July 19, 2005, URL: http://earthobservatory.nasa.gov/Features/Nimbus/nimbus.php
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).