DMSP Block 5D-3 Satellite Series
DMSP (Defense Meteorological Satellite Program) is a long-term operational meteorological program of the US DoD (Department of Defense), managed by the USAF and operated by the 6th Satellite Operations Group at Offutt AFB (Air Force Base), Nebraska. The program originated in the early 1960s with the objective to collect and disseminate worldwide cloud cover data on a daily basis (along with oceanographic and solar-geophysical environment parameters). The DMSP system is being used for strategic and tactical weather prediction to aid the US military in planning operations at sea, on land and in the air. The DMSP series experienced several generations of satellites and instruments (see Table 1). The USAF maintains an operational constellation of two near-polar, sun-synchronous satellites. 1)
In the timeframe 1965 to 2006 (F-17 inclusive), a total of 44 DMSP spacecraft have been launched successfully by the USAF, all were built by Lockheed Martin Corporation.
Table 1: Chronological overview of DMSP Block 5 series satellites
Nomenclature: The DMSP satellites are known as F-xx after launch. Prior to launch, they are designated by S-xx. The 5D-2 series of satellites encompasses F-6 through F-14, while the 5D-3 series of spacecraft begins with F-15. As polar-orbiting spacecraft, all DMSP satellites are being launched from VAFB (Vandenberg Air Force Base), Vandenberg, CA, USA.
DMSP and NOAA-POES programs merge
For a period of over 30 years, the DMSP program of the US military, with its LEO polar orbiting satellites (830 km altitude), was in fact a full parallel system to the NOAA-POES series, the US civil LEO weather satellite program in polar orbit. 2) 3) 4) 5) 6)
In the fall of 1993, the US National Performance Review (NPR) and the subsequent Presidential Decision Directive/NSTC-2 (May 5, 1994) called on DOC, DoD and NASA, to “converge” the US civil and military operational meteorological satellite programs (POES of NOAA and DMSP of DoD), in order to reduce duplication of effort and to generate cost savings. In October 1994, an Integrated Program Office (IPO), consisting of a team made up of NOAA, NASA and DoD representatives, was established organizationally under NOAA (with the IPO Headquarters located in Silver Spring, MD) with the explicit objective to integrate their separate meteorological programs into a single program that includes: planning, development, management, acquisition, and operations. A tri-agency MOA (Memorandum of Agreement) was signed in May 1995. The merged program received the name of NPOESS (National Polar-orbiting Operational Environmental Satellite System). The IPO is a tri-agency office reporting through NOAA to an Executive Committee of representatives from DOC, DoD and NASA.
In May 1998, the US Air Force Space Command transferred its day-to-day operations of the DMSP spacecraft to NOAA. With this action, NOAA assumed full responsibility for the operation of both the POES and DMSP satellite constellations. NOAA maintains and conducts joint satellite control operations at a collocated operations center where it also conducts operations of NOAA's GOES (Geostationary Operational Environmental Satellite) series (from Suitland, MD).
The DoD DMSP program and the POES program convergence takes place in two phases.
• First phase: Starting in May 1998, all DMSP satellite operational command and control functions of AFSPC (Air Force Space Command) were transferred to a tri-agency Integrated Program Office (IPO) established within NOAA. NOAA was given the sole responsibility of operating both satellites programs, POES and DMSP (from NESDIS, Suitland, MD).
• During the second phase, the IPO will launch and operate the new NPOESS satellites (starting in 2009) that will satisfy both the DoD and Department of Commerce (DOC/NOAA) requirements.
DMSP and NOAA polar programs separate again :
In Feb. 2010, the NPOESS (National Polar-orbiting Environmental Satellite System) tri-agency program was terminated by the US government due to severe cost overruns and program delays. The NPOESS program was a joint DoD/NOAA/NASA endeavor that tried to integrate the capabilities and infrastructure of the NOAA POES (Polar-Orbiting Environmental Satellite) program, the DoD DMSP (Defense Meteorological Satellite Program), and NASA’s long-term continuous climate record collection. - The President's FY 2011 budget request for NOAA directed the NPOESS program to split into separate NOAA/NASA and DoD programs.
The major challenge of NPOESS was jointly executing the program between three agencies of different size with divergent objectives and different acquisition procedures.
In 2009, an IRT (Independent Review Team) concluded that the current NPOESS program, in the absence of managerial and funding adjustments, has a low probability of success and data continuity is at extreme risk. The Office of Science and Technology, with the Office of Management and Budget and the National Security Council, as well as representatives from each agency, examined various options to increase the probability of success and reduce the risk to data continuity. 7) 8) 9)
NOAA's restructured satellite program, the civilian JPSS (Joint Polar Satellite System), was created in the aftermath of the White House's Feb. 2010 decision to cancel NPOESS. The development of the new JPSS will be managed by NASA/GSFC while the spacecraft will be owned and operated by NOAA. The launch of JPSS-1 is planned for 2016.
NOAA, through NASA as its acquisition agent, will procure the afternoon orbit assets that support its civil weather and climate requirements and DoD will independently procure assets for the morning orbit military mission - referred to as DWSS (Defense Weather Satellite System). Both agencies will continue to share environmental measurements made by the system and support the operations of a shared common ground system.
The Administration decision for the restructured Joint Polar Satellite System will continue the development of critical Earth observing instruments required for improving weather forecasts, climate monitoring, and warning lead times of severe storms. NASA’s role in the restructured program will be modeled after the procurement structure of the successful POES (Polar Operational Environmental Satellite) and GOES (Geostationary Operational Environmental Satellite) programs, where NASA and NOAA have a long and effective partnership. The partner agencies are committed to maintaining collaborations towards the goal of continuity of earth observations from space.
The restructured JPSS (Joint Polar Satellite System) is planned to provide launch readiness capability in FY 2015 and FY 2018 (with launches of JPSS-1 in 2016 and JPSS-2 in 2019, respectively) in order to minimize any potential loss of continuity of data for the afternoon orbit in the event of an on orbit or launch failure of other components in the system. Final readiness dates will not be baselined until all transition activities are completed. 10)
The following restructured concept scenario emerged in 2010:
• NOAA/NASA Joint Polar Satellite System (JPSS) covers the afternoon (13:30 hours) orbit
• JPSS flies an “NPP-like” satellite bus
• DoD covers the early morning (05:30 hours) orbit. A DMSP Follow-on program (beyond DMSP-F-20) is in planning.
• NOAA manages the JPSS ground system.
The European polar weather satellite program of EUMETSAT, namely the MetOp series, is also involved in this new overall scenario through the EPS (EUMETSAT Polar System) agreements (Figure 2).
Figure 1: Overview of the polar meteorological program evolution in 2010 (image credit: NOAA)
Figure 2: Scenario of Polar Operational Satellite Programs in 2010 (image credit: NOAA)
Space segment of Block 5D-3 series:
Figure 3: Artist's view of the deployed DMSP 5D-3 spacecraft (image credit: USAF)
The DMSP satellites were built under USAF contract by LMSSC (Lockheed Martin Space Systems Company), Sunnyvale, CA. They are three-axis stabilized. Pointing of the satellites is maintained by three orthogonal reaction wheel assemblies. A precision mounting platform provides a pointing accuracy of 0.01º. The S/C structure has a diameter of about 1.2 m and a length of about 6.4 m (deployed). The spacecraft is separated into four sections or modules: 11)
• A precision mounting platform for sensors and equipment requiring precise alignment
• An equipment support module containing the electronics, reaction wheels, and some meteorological sensors
• A reaction control equipment support structure containing the third-stage rocket motor and supporting the ascent phase reaction control equipment
• A deployable sun-tracking solar array of size 9.29 m2 (10 panels), spacecraft power of 2.2 kW.
The spacecraft stabilization is controlled by a combination flywheel and magnetic control coil system so that sensors are maintained in the desired earth-looking mode. One feature is the precision-pointing accuracy of the primary imager to 0.01º provided by a star sensor and an updated ephemeris navigation system. This allows automatic geographical mapping of the digital imagery to the nearest picture element. The Block 5D-3 series spacecraft have a design life of 5 years.
Figure 4: Line drawing of the DMSP Block 5D-3 spacecraft
The F-15 and F-16 DMSP satellites are Block 5D-3 series that accommodate larger and more advanced sensor payloads than earlier generations. They also feature a more advanced attitude control system for precision pointing; a more powerful onboard computer with increased memory - allowing greater spacecraft autonomy; a higher rate command link for shorter ground contact times; an increased battery capacity that prolongs the mission duration, and an improved onboard autonomy (60 days). The F-16 S/C is in fact the first full Block 5D-3 satellite. Although the F-15 spacecraft features the new Block 5D-3 bus, it is flying the legacy 5D-2 sensor complement.
Figure 5: Photo of the DMSP Block 5D-3 spacecraft (image credit: LMSSC)
Orbital Parameters of DMSP: 12)
RF communications: DMSP downlinks mission data stored onboard Stored Mission Data (SMD) once per orbit and has two real-time data transmissions, the Real-time Data Smooth (RDS) and Real Time Data fine (RTD).
Onboard preprocessing of data by the OLS sensor system provides for the various modes of data output. OLS data consists of both visual or Light data (L data) and infrared or Thermal (T data) modes. Infrared Fine resolution data (TF data) can be collected continuously, day and night. Visible fine (LF data) is collected during daytime only. Storage capacity and transmission constraints limit the quantity of fine resolution data (LF or TF) which can be provided in the SDF (Stored Data, Fine) mode. Data smoothing permits global coverage in both the infrared (TS) and visible (LS) spectrum to be stored on the primary tape recorders in the SDS (Stored Data Smoothed) mode.
The data rate is 1.33 MBit/s for SDF (TF or LF) or 2.66 Mbit/s if SDS is interleaved bit-by-bit (TF/LF or TS/LS). The DMSP RTS Mux accepts either data rate and formats Equipment Status Telemetry data with the incoming stored data stream. This 3.072 Mbit/s data stream is then transmitted via a DOMSAT communications satellite link to AFWA (Air Force Weather Agency) and FNMOC (Fleet Numerical Meteorology and Oceanography Center) for processing.
DMSP transmits two S-band downlinks in real-time: Real Time Data (RTD) link containing OLS fine (TF/LS or LF/TS) data; and Real Time Smooth (RDS) data stream containing OLS smooth data. - The F-16 spacecraft features a dedicated S-band for RTD at a frequency of 2222.5 MHz. The date rate of the RDS increases to 88.75 kbit/s unencoded (177.5 kbit/s encoded). Additionally, there is the capability to transmit a UHF RDS transmission at 400.328 MHz and at 400.822 MHz. All DMSP downlinks are encrypted.
Figure 6: Overview of the F-16/-17 spacecraft sensor complement (image credit: SMC)
Operational status of DMSP series:
• In 2009, the DMSP satellite series continues to provide timely worldwide meteorological and ionospheric data to military users and the civilian community. The current constellation consists of two primary operational (F-16, and F-17) and three partially operational satellites (F-13, F-15). 15)
Table 2: Overview of operational DMSP spacecraft
• The SSMI instrument on the F-14 mission lost onboard storage in August 2008; F-13 shows worsening seasonal receiver gain fluctuations and tape recorder problems. 16)
- The SSMIS sensor series (F-16 since 2003, F-17 since 2006, F-18 since Oct. 2009, (and F-19 to -20 yet to be launched) will be a key constellation member in the GPM (Global Precipitation Mission) era. 17)
- The SSMIS instruments of the F-16 and F-17 missions are affected by two significant instrument problems: solar intrusions into the warm load, and thermal emission from the reflector.
• As of Jan. 29, 2007, the F-17 spacecraft had completed its on-orbit checkout phase and was declared “operational” (launch of DMSP F-17 on Nov. 4, 2006). The checkout team included representatives of the Air Force, NOAA, Northrop Grumman, and the Aerospace Corporation. This implies that NOAA can begin a regular operational service with DMSP F-17. 18) 19) 20)
• As of Nov. 20, 2003, the F-16 spacecraft had completed its on-orbit checkout phase and was declared “operational” (launch of DMSP F-16 on Oct. 16, 2003). 21)
Description of Block 5D-2 and 5D-3 sensor complement
The sensor suite of 5D-3 differs from that on 5D-2. Though F-15 is the first of the 5D-3 spacecraft, F-16 is the first spacecraft in the 5D-3 series to carry the complete new 5D-3 sensor complement. 22) 23) 24)
Please consult Table 1 for the sensor complement of the various DMSP missions.
OLS (Operational Linescan System)
OLS is the primary sensor on each satellite, built by Northrop Grumman, Westinghouse Corporation. Objective: day and night cloud cover imagery. The OLS instrument consists of two telescopes and a photomultiplier tube (PMT). The detectors sweep back and forth in a “whiskbroom” fashion (a “flying spot design” is employed - a subset of whiskbroom scanners). The continuous analog signal is sampled at a constant rate so the Earth-located centers of each pixel are roughly equidistant, i.e., 0.5 km apart, 7,325 pixels are digitized in the cross-track direction.
Swath width = 3000 km from a nominal 833 km orbit altitude. OLS provides global coverage in both visible (L data) and IR (T data) modes. Fine resolution data with a nominal linear resolution of 0.56 km are collected as needed day and night by the IR detector, and as needed during daytime, by a segmented silicon diode detector (LF data). A high resolution photometer tube is used for nighttime visible imagery (used for fire detection).
- Band 1: VIS wavelength = 0.4 - 1.1 µm (0.58 - 0.91 µm FWHM). The visible channel allows for a very large dynamic range of illumination (approximately 107). The gain is continuously adjusted across an image to compensate for the wide differences in solar illumination that occur near the terminator. This is achieved automatically using a switchable gain amplifier controlled by a digital processor in addition to information regarding the scan angle and solar geometry.
- Band 2: TIR wavelength = 10.0 - 13.4 µm (8 - 13 µm, old prior to 1979), resolution = 0.56 km for fine resolution data (stored data is smoothed to 2.7 km resolution), continuous data collection, polar stereographic image products have ground resolution of 5.4 km. Swath = 3000 km. The IR system counts are automatically calibrated to vary between 190 and 310 K of effective blackbody or brightness temperatures.
- The PMT is sensitive to radiation from 0.47 - 0.95 µm (0.51 - 0.86 µm FWHM). The PMT provides visible imagery (0.5 - 0.85 µm) at night, which makes possible the optical detection of man-made and natural fires and lightning events, among other phenomena.
Figure 7: Illustration of the OLS instrument (image credit: SMC)
The scanning telescope of OLS is a f/5.8 Cassegrain design with a 20.3 cm clear aperture and an effective collecting area of about 185 cm2. The telescope has an effective focal length of 122 cm. Two telescope calibration mirrors intercept the normal FOV at the edge of scan with hot and cold loads of known temperatures. The light from the telescope is split into two channels by a beam splitter, and sent via relay optics to the visible and infrared focal planes, as well as to the photo multiplier tube that provides useful nighttime visible imagery (approx. 0.5 - 0.85 µm half-power response point bandpass) down to a lunar illumination level of about a quartermoon. The telescope images over a scan angle of ±56.25º which corresponds to a swath width on the ground of 2960 km (some overlap at the equator from orbit to orbit).
Glare suppression is built into the system by a variety of sun shades, field stops, low scatter surfaces, and aperture stops. This minimizes the amount of data lost in the orbit due to sunlight saturating the visible detectors. OLS provides the ability to command gain adjustments to overcome slow degradation of the thermal transmission of the system. The system is designed to produce near-constant high resolution imagery for most DOD applications because the location of clouds, fog, ice floes, etc., is much more important than accurate radiometry. The near constant resolution across the swath is accomplished through a combination of the natural rotation of the detector footprint and detector segment switching. On-board data smoothing (averaging) can be done to reduce the data rate by a factor of 25, smoothing electronically the pixels in the cross-track direction and digitally averaging in the along-track direction. However, smoothing is only done in cases to cope with current recorder limitations. When this mode is used, the original high-resolution imagery cannot be recovered.
OLS utilizes three types of detectors:
• A silicon photoconductive diode is used for daytime VIS imagery. Three segments in the detector provide for a nearly constant resolution across the swath. Two smaller segments are used for scan angles > 41º, all three segments are summed in the middle portion of the scan about nadir.
• A two-segment HgCdTe photoconductive detector is used for the TIR channel. The detector is cooled to 108 K by a cone cooler. The two detector segments are used on the far right and left parts of the scan and are summed over the middle portion of the scan (within ±41º).
• A single PMT detector is used for nighttime visible data [a GaAs opaque photocathode and multiple dynode PMT).
The OLS instrument has a unique capability to collect low-light imaging data of the Earth during the nighttime period in its orbit (Figure 8). The low light sensing capabilities of the OLS at night permit the measurement of radiances down to 10-9 W/cm2/sr with a nominal spatial resolution of 2.7 km. The light intensification enables the observation of faint sources of visible- near infrared emissions present at night on the Earth's surface including cities, towns, villages, gas flares, heavily lit fishing boats and fires (Figure 9). 25)
Figure 8: OLS low-light imaging of the Earth at night taken on Dec. 30, 1999 in the VNIR range (left) and in the TIR range (right), image credit: NOAA
Figure 9: Fire detection capability of three US instruments with regard to the fire front size (image credit: NASA)
OLS instruments are being flown on all DMSP spacecraft since 1976; they are expected to continue flying until ~2015.
SSM/I (Special Sensor Microwave Imager)
The SSM/I is a seven-channel, four-frequency, linearly-polarized, passive microwave radiometer (a total-power instrument configuration) which measures atmospheric, ocean, and terrain microwave brightness temperatures (similar to NIMBUS-7 SMMR) which are converted into environmental parameters such as: sea surface winds, rain rates, cloud water, precipitation, soil moisture, ice edge, and ice age. SMM/I data is used to obtain synoptic maps of critical atmospheric, oceanographic and selected land parameters on a global scale. The archive data consists of antenna temperatures recorded across a 1400 km swath (conical scan), satellite ephemeris, Earth surface positions for each pixel and instrument calibration. The electromagnetic radiation is polarized by the ambient electric field, scattered by the atmosphere and the Earth's surface, and scattered and absorbed by atmospheric water vapor, oxygen, liquid water and ice. 26) 27)
The SSM/I instrument was developed and built by Hughes Space and Communications Co. [now BSS (Boeing Satellite Systems)]. The SSM/I project represents a joint Air Force/Navy operational program to obtain synoptic maps of critical atmospheric, oceanographic and selected land parameters. SSM/I consists of an offset parabolic reflector (61 cm x 66 cm) that is fed by a corrugated broadband seven-port horn antenna. The reflector and feed are mounted on a drum which contains the radiometers, digital data subsystem, mechanical scanning subsystem, and power subsystem. The entire reflector, feed horn, and drum assembly rotates about the axis of the drum by a coaxially mounted BAPTA (Bearing and Power Transfer Assembly). SSM/I consumes 45 W of power, it has a mass of 48.5 kg.
The SSM/I sensor executes a 45º conical scan of the Earth's surface from nadir. This gives a nominal incidence (zenith) angle of 53.1º to the Earth's surface from the nominal orbit. Only part of the possible 360º scan in azimuth is used to collect data. The active azimuth scan angle is 102.4º is ahead of the S/C for an afternoon ascending orbit and behind the S/C for a morning ascending orbit. The sensor electronics perform an integration and hold sequence on each channel, timed so that the odd 85 GHz reading is centered with the 37 GHz reading. The sensor conically scans the Earth and atmosphere at a scan rate of 31.9 scans/min (or 1.88 s/scan). The sampling scheme results in 128 samples (called scene stations) per scan for the 85 GHz channels, and 64 scene stations per two scans for the other channels. The footprint sizes are listed in Table 3. The footprints are sampled every 25 km for the 19, 22, and 37 GHz channels, and every 12.5 km for the 85 GHz channel.
Figure 10: Illustration of the SSM/I instrument (image credit: BSS)
Table 3: Specification of some SSM/I parameters
Figure 11: The scan geometry of the DMSP SSM/I sensor
In Figure 11, the rotating antenna sweeps the surface in two alternating modes - one in which all four frequencies are recorded, and another in which only 85 GHz data are recorded. The use of a single antenna results in different ground resolutions for each frequency.
SSM/I calibration: A small mirror and a hot reference absorber are mounted on the BAPTA and do not rotate with the drum assembly. They are positioned off axis such that they pass between the feed horn and the parabolic reflector, occulting the feed horn once each scan. The mirror reflects the cold cosmic background radiation (3 K) into the feed horn, thus serving, along with the hot reference absorber, as calibration references for the instrument. The scheme provides an overall end-to-end absolute calibration which includes the feed horn. A total-power radiometer configuration is used which provides a sensitivity with a factor two better over a conventional Dicke-switched system. Each scan consists of a cold reading, a warm load reading, and the scene stations. The cold reading utilizes a view to deep space (3 K black body), and the warm load temperature (variable over an orbit) is read by three precision thermistors. Each sensor has a different set of coefficients that enters into the retrieval algorithms. Both the calibration scheme and the use of total-power radiometers are innovations (so-called external calibration scheme) which significantly improve the instrument performance as compared to previous spaceborne radiometric systems. 28) 29)
The first SSM/I instrument was flown on the F-8 spacecraft of the DMSP series, it became operational in July 1987. Other DMSP missions carrying the SSM/I are: F-10, F-11, F-12, F-13, F-14, and F-15.
Table 4: SSM/I environmental data products
Figure 12: Photo of the SSM/I radiometer (image credit: DMSP Program Office USAF, NOAA)
SSM/T-1 (Special Sensor Microwave Temperature Sounder)
SSM/T-1 is in operation since 1979, first flown on F-4, built by the Aerojet Corporation. The SSM/T-1 is a seven channel passive microwave sounder consisting of a rotating 7 step antenna reflector (rotating once every 32 seconds), a Dicke-switched seven channel radiometer, a digitizer, and signal processor.. It measures the Earth's surface and atmospheric emission in the 50 to 60 GHz oxygen band. The SMM/T-1 is a cross-track nadir scanning radiometer having a FOV of 14.4º. At nominal altitude (833 km) the subtrack spatial resolution is an approximate circle of 174 km diameter at nadir elongating to an ellipse of 305 x 313 km at the extreme viewing angles toward the limb. There are seven total cross-track scan positions separated by 12º with a maximum cross-track scan angle of 36º. Swath width = 1500 km (data coverage gap between successive orbits). The SSM/T-1 is a step and stare type sensor which dwells on each of the seven scene stations, then observes a cold (3 K) reference, followed by a warm (300 K) reference. The daily data volume is 1.5 MByte per satellite.
Table 5: Parameters of the SSM/T-1 instrument
SSM/T-2 (Special Sensor Microwave Water Vapor Profiler-2)
The SSM/T-2 is a modification of SSM/T-1 for water vapor sounding. It was built by the Aerojet Corporation. The first flight of the instrument took place on F-11 in 1991. The instrument is a cross-track scanning, five channel, passive total power microwave radiometer system which consists of a single, self-contained module with a step-scan motion in the cross-track direction of ± 40.5º. The SSM/T-2 scan mechanism is synchronized with the SSM/T-1 so that the beam cell patterns of the two sensors coincide. The observation rate is 7.5 scans/minute. There are 28 observations (beam positions) per scan for each of the five channels, with each observation having a spatial resolution of about 48 km. All five channels have coincident centers. The swath width is about 1500 km.
The instrument employs a single offset parabolic reflector with a 6.6 cm diameter projected aperture. The reflector is shrouded to eliminate the possibility of rays from the sun striking either of the calibration paths and causing unwanted thermal gradients. The feedhorn is a corrugated pyramidal horn with a flare designed to minimize phase center separation over the bandwidth (91 to 183.3 GHz), while providing a spherical wave illumination of the reflector. To achieve the cross-track scanning, the reflector alone rotates. The rotation of the reflector produces a rotation of the plane of polarization of the upwelling scene which is permitted provided that the polarization remains identical for the two window channels and 183.3 ± 7 GHz. These channels must have the same polarization characteristics because they measure contributions from both the atmosphere and the surface. During each scan period, and for all five channels at 28 discrete earth viewing positions, four discrete calibration measurements of a hot-load target (about 300 K), and cosmic background radiation (about 3 K) are monitored.
The sensor has footprints at nadir (resolutions) ranging from 48 km (for highest frequency) to 120 km. The noise levels (NEDT) for the five channels are 0.6 K to 0.8 K. The peaking measurement heights from 0 to 10 km vary because they are dependent on the total atmospheric water content. This variation, together with the changing viewing geometry, complicates the retrieval of water vapor profiles. Like the SSM/T-1, the SSM/T-2 is a step stare system, but with 28 steps per scan versus 7. The scan period has been decreased to 8 seconds per scan from 32 seconds. Each scan consists of a warm load reading, a cold load reading, and 28 scene stations. - The data is used to determine water vapor mass in seven layers, and specific and relative humidity at six levels. When combined with the SSM/T-1 data, meteorological conditions can be analyzed, such as types of cloud formations, icing potential, and the location of weather systems, based on differences in air mass temperature and moisture.
Table 6: Parameters of the SSM/T2 instrument
SSMIS (Special Sensor Microwave Imager Sounder)
SSMIS is a joint USAF/Navy multichannel passive microwave instrument. SSMIS was built by the Aerojet Corporation of Azusa, CA, a company of GenCorp Inc. of Sacramento, CA. The SSMIS instrument combines the observation capabilities of the heritage sensors: SSM/I + SSM/T-1 + SSM/T-2 (Block 5D-3 sensor).
In the Block 5D-3 satellite era, the Block 5D-2 passive microwave sensor suite is combined into a single new sensor package - the SSMIS. SSMIS measures microwave energy from the Earth's surface and its atmosphere in 24 channels with center frequencies ranging from 19.35 GHz to 183.31 GHz. The objective is to retrieve synoptic atmospheric temperature and moisture profiles using a conical-scan geometry resulting in constant horizontal spatial resolution and polarization over the entire scan swath. In addition to its atmospheric sounding capability, the SSMIS also includes the heritage SSM/I channel set allowing concurrent measurement of atmospheric, land and ocean parameters (integration of imaging and sounding capabilities in SSMIS). 30) 31)
SSMIS was first flown on DMSP flight F-16 (launch Oct. 18. 2003) with an orbital altitude of ~833 km in a 98.8º inclined sun-synchronous polar orbit with an ascending nodal crossing time of 20:03 hours local time.
Figure 13: Illustration of the SSMIS instrument (image credit: Aerojet)
The sensor generates a conical scan pattern at 45º from nadir. This scan pattern, combined with the 144º clear field of regard, results in a constant angle of incidence on the Earth of 53.1º and a swath width of 1,700 km (Figure 14). The sensor collects data to the aft of nadir for a morning ascending node spacecraft orbit and collects data forward of nadir for a morning descending node spacecraft orbit.
Improvements include 24 channels of data which are all coincident, increased resolution range, increased FOV, and enhanced ground processing software. The sensor adds one additional channel over the SSM/T to improve the measurement of the tropopause temperature and height. The frequencies chosen (channels 1-7 and 24) provide near uniform coverage in height to about 32 km. With the addition of five more channels (channels 19-23), the temperature retrievals are extended up to about 80 km.
The instrument has a mass of 96 kg, power = 135 W, the data rate is 14.2 kbit/s. The antenna has a diameter of 61 cm, the beam width is in the range: 0.4º-1.9º. The reflector is inclined at approximately 23º to the horizontal. Upwelling radiance is focussed onto an up-looking feedhorn array which is fixed relative to the main reflector. This arrangement results in a beam direction of 45º to nadir at the spacecraft ground track.
Figure 14: SSMIS conical scan geometry showing 1707 km swath width and footprint geometry (image credit: The Aerospace Corp.)
Table 7: Some parameter accuracy requirements of SSMIS
Table 8: SSMIS frequency and resolution characteristics
The scene spacing for the sounder channels has been improved from 120 km to 480 km of the earlier instruments to 50 km for the lower atmosphere and to 75 km altitude for the upper atmosphere measurements. SSMIS uses almost the same channels as SSM/I for the environmental parameter extraction. The frequency of 85 GHz was changed to 91 GHz to save an extra channel in the system. SSMIS augments the rain retrieval and cloud amounts with channel 8 at 150 GHz.
The sounding data is only partially smoothed onboard. The 183 GHz channels are downlinked at 12.5 resolution, they have to be averaged to the 50 km spacing for the sounding retrievals. The 12.5 km data is used for the display of water and vapor data.
SSMIS features redundant Gunn diode oscillators and phase lock loop oscillators. The sensor conically scans the Earth and atmosphere at a scan rate of 31.9 scans/min (or 1.88 s/scan).
Instrument calibration: SSMIS employs a two point (hot and cold load) calibration taken once during each scan of 1.88 seconds duration in all 24 channels. The scan width has been increased from 104º to 144º (FOV) by positioning the sensor farther outboard on the S/C. The six separate feed horns for each frequency require the flight software in the sensor to align the data prior to transmission to the ground (via OLS). SSMIS operates asynchronously with OLS, the data to OLS is transmitted in two blocks/s.
SSMIS underflights: During March-April 2004, six SSMIS under-flights (5 hours each) were conducted with the NASA CoSMIR (Conical Scanning Millimeter-wave Imaging Radiometer) instrument on board the NASA ER-2 aircraft over the coastal region of California. CoSMIR has nine channels at the frequencies of 50.3, 52.8, 53.6, 91.665 (V and H polarization), 150, 183.3 ± 1, 183.3 ± 3, and 183.3 ± 6.6 GHz. All except the two 91.665 GHz channels are horizontally polarized. Three of the aircraft flights passed over Lakes Pyramid and Tahoe that could be used to validate the in-flight sensor calibration. Immediately after these flights, an inter-comparison of the calibrated SSMIS and CoSMIR brightness temperatures (T(sub b)) followed.
Status: The SSMIS Cal/VAL (Calibration/Validation) period began upon the F-16 launch and was completed on schedule 18 months after launch, on 27 April 2005 with the release of validated Environmental Data Records (EDRs) and a stable configuration of the sensor and ground processing software. 32) 33) 34) 35)
The Cal/Val effort was divided into 6 major milestones or gates, including 1) basic functionality, 2) Early orbit validation, 3) initial assessment of Temperature Data Records, Sensor Data Records and geolocation, 4) detailed system calibration with aircraft under-flights, 5) EDR validation including Lidar and NWP comparisons, and finally Gate 6 which includes basic improvements to the algorithms based on data collected within the Cal/Val before data release.
SSULI (Special Sensor Ultraviolet Limb Imager)
SSULI was designed and built at NRL (Naval Research Laboratory), Washington, DC. The optical instrument is a spectrograph with the objective to measure extreme and far ultraviolet radiation (vertical profiles) from the Earth's limb. The primary observations, ranging from 80 - 170 nm (FUV/EUV), with 1.5 nm resolution, are of radiation from atomic oxygen and nitrogen, and molecular nitrogen, resulting in direct measurements of the electron density vertical profile as well as ion and neutral densities. The vertical profiles in the upper atmosphere and ionosphere are obtained by viewing the Earth's limb at a tangent altitude of approximately 50 km to 750 km.
The LORAAS (Low Resolution Airglow/Aurora Spectrograph) instrument on ARGOS (Launch Feb. 23, 1999) is a SSULI prototype instrument. LORAAS data of ARGOS is being used to validate SSULI algorithms that convert raw measurements into useful environmental parameters that characterize the upper atmosphere. The first SSULI instrument is being flown on the F-16 spacecraft (launch Oct. 18, 2003).
SSULI is mounted on the +x panel of the spacecraft. The sensor consists of a folded telescope, a scan mirror assembly with a mounting interface to the UV Wadsworth spectrograph, the collimator, a grating element (focuses the radiation onto the detector and disperses the light into a spectrum from 80 - 170 nm), a detector (imaging microplate detector with a wedge and strip anode that characterizes photon events), control electronics (to operate the scan mirror assembly, etc.), a high voltage power supply, and the harness assembly.
A scanning mirror sweeps the FOV across the Earth limb to obtain vertical profiles of the natural airglow radiation from atoms, molecules and ions in the upper atmosphere and ionosphere. The scan is performed in the orbital plane, looking either forward or aft. The primary scan mode extends from 10º to 27º below the satellite horizon (with the capability to go from 10º to 40º in an alternate scan mode), forming vertical images of the Earth's atmosphere from 750 km to 50 km every 90 seconds. The scan rate is faster at high tangent altitudes and slows down at lower altitudes where there is more structure in the airglow. The intensity at these wavelengths is so low, both sensors count individual photons on the focal plane using wedge and strip microchannel plate detector technology. 36) 37) 38) 39)
Figure 15: Illustration of the SSULI instrument (image credit: NRL)
Radiation into SSULI is reflected off the scan mirror to a grid collimator. The radiation is dispersed off a grating into its spectral elements, focused onto a detector, and converted into an electrical signal. The SSULI electronics use this charge to determine the wavelength and amplitude of the photons striking the detector.
Figure 16: The SSULI optical path (image credit: NRL)
Table 9: SSULI parameter specification
Table 10: SSULI observables
SSUSI (Special Sensor Ultraviolet Spectrographic Imager)
SSUSI was was designed and built by JHU/APL. SSUSI is a nadir-pointing instrument that measures UV radiation from the Earth's atmosphere and ionosphere, it also measures visible radiation (airglow and terrestrial albedo). The instrument provides the 5D-3 satellite series with the ability to obtain photometric observations of the nightglow and nightside aurora. Note: SSUSI and GUVI (Global Ultraviolet Imager) of the TIMED mission (launch Dec. 7. 2001)) are nearly identical instruments. The SSUSI instrument consists of three subassemblies (Figure 17): 40) 41)
• SIS (Scanning Imaging Spectrometer)
• NPS (Nadir-looking Photometer System)
• Support module
SIS in turn consists of a cross-track scanning mirror at the input to the telescope (folded design) and spectrograph optics. There are redundant 2-D photon-counting detectors at the focal plane (detector size: 16 pixels in along-track and 160 pixels in the cross-track direction). The detectors employ a position sensitive anode to determine the photon event location. The scan mirror sweeps the 16 pixel footprint from horizon to horizon, producing one frame in 22 seconds. The imaging mode performs simultaneous measurements in five wavelength bands from 115 - 180 nm. The imaging mode scan cycle consists of a limb-viewing section followed by an Earth viewing (nadir) section. Limb-viewing imagery is collected from -72.8º from nadir to -63.2º from nadir. The limb-viewing section has a cross-track resolution of 0.4º per pixel, it consists of 24 cross-track pixels and 8 along-track pixels (at five bands). The Earth-viewing section has a cross-track resolution of 0.8º.
Figure 17: Block diagram of the SSUSI instrument (image credit: JHU/APL)
Figure 18: A more detailed block diagram of the SSUSI instrument (image credit: JHU/APL)
Figure 19: Schematic of the SIS (SSUSI Imaging Spectrograph), image credit: JHU/APL
Figure 20: Observation scheme of the SSUSI instrument (image credit: JHU/APL)
Figure 21: Alternate presentation of the SSUSI observation scheme (image credit: JHU/APL)
Table 11: SSUSI performance parameters of imaging spectrograph (SIS)
Figure 22: Photo of the SIS housing (image credit: JHU/APL)
NPS consists of three nadir-looking photometers. It operates in the visible portion of the electromagnetic spectrum, monitoring airglow at 427.8nm and 630nm and the terrestrial albedo near 630nm. NPS operates only on the nightside of the orbit. Its data determine the auroral oval location and provide information to help determine electron densities in the F-layer, energy deposition in the auroral region (day and night), photoelectrons, neutral composition, and equatorial electrojet. Each photometer unit includes an integrated detector package consisting of a photomultiplier tube, high voltage power supply, and pulse amplitude discriminator electronics. 42)
Table 12: NPS performance parameters
Table 13: NPS detector parameters
SESS (Space Environment Sensor Suite)
The SESS sensors provide data on the geophysical environment of the upper atmosphere and ionosphere. 43)
SSI/ES-2 (Special Sensor Ionospheric Plasma Drift/Scintillation Monitor), a PL sensor built by the University of Texas. Measurement of the ambient electron density and temperatures, the ambient ion density, and the average ion temperature and molecular weight at the DMSP orbital altitude. - The instrument consists of an electron sensor (Langmuir probe) and an ion sensor mounted on a 2.5 meter boom. The ion sensor is a planar aperture, planar collector sensor oriented to face the spacecraft velocity vector at all times. In addition to the Langmuir probe and planar collector which make up the SSI/E, the SSI/ES has a plasma drift meter and a scintillation meter. The data volume of SSI/ES-2 is 12 MByte/satellite-day.
SSJ/4 (Precipitation Electron/Proton Spectrometer), a PL sensor built by Ampek Inc. The SSJ/4 is a next generation sensor of the SSJ/3. SSJ/4 has been flown starting with F6 (launch in Dec. 1982) until F15 (launch Dec. 12, 1999). Objective: Measurement of transfer energy, mass, and momentum of charged particles through the magnetosphere-ionosphere in the Earth's magnetic field. The instrument looks toward the satellite zenith. - The SSJ/4 sensor consists of four electrostatic analyzers that record the flux of precipitating ions or electrons at 20 fixed energy channels between 30 eV and 30 keV. The curved plate detectors allow precipitating electrons and ions to enter through an aperture of about 20 x 10 (FWHM). Electrons and ions of the selected energy are deflected toward the target by an imposed electric field applied across the two plates. The two low energy detectors consist of 10 channels centered at 34, 49, 71, 101, 150, 218, 320, 460, 670, and 960 eV. The high energy detector measures particles in 10 channels centered at 1.0, 1.4, 2.1, 3.0, 4.4, 6.5, 9.5, 14.0, 20.5 and 29.5 KeV. Each detector integrates each channel for 0.09 s from high energy channel to low. A complete cycle is sampled each second. The primary sources of the particles precipitating into the upper atmosphere are the northern and southern auroral zones. The daily data volume is approximately 1 Mbyte per satellite. The sensor data also supports missions which require knowledge of the polar and high-latitude ionosphere, such as communications, surveillance, and detection systems that propagate energy off or through the ionosphere.
SSJ/5 (Precipitation Electron/Proton Spectrometer). The instrument is an electrostatic analyzer detector developed and built by Amptek Inc. of Bedford, MA (first flight on DMSP F16, launch Oct. 18, 2003). SSJ/5 is an upgrade of SSJ/4. Detects and analyzes electrons and ions that precipitate in the ionosphere to produce an aurora display (0.3-30 keV low energy range). The instrument uses a nested spherical deflection plate system to simultaneously analyze electrons and ions over a 90º field of view. It utilizes a space qualified microprocessor that permits customizing data rates, measurement ranges, on board storage, and specific analysis algorithms, such as auroral boundary detection or real time charging measurements.
Figure 23: Illustration of SSJ/5 instrument (image credit: Amptek)
SSM (Special Sensor Magnetometer), a triaxial fluxgate magnetometer, PL sensor built by NASA/GSFC. The SSM measures geomagnetic fluctuations associated with solar geophysical phenomena (i.e., ionospheric currents flowing at high latitudes). In combination with the SSI/ES (or SSI/ES-2) and the SSJ/4, the SSM provides heating and electron density profiles in the high-latitude ionosphere. SSM takes and reports 12 readings/s for the Y and Z axes. Only 10 readings of the 12 readings per second are reported for the X axis due to telemetry limitations. The SSM's axes are aligned with the spacecraft's axes where X is downward and aligned to local vertical within 0.01 degree, Y is parallel to the velocity vector for spacecraft with ascending node in the afternoon/evening sector, and Z is away from the solar panel and anti-parallel to the orbit normal vector.) The measurement range is ±65535 nT for each axis, with a one-bit resolution of 2 nT. The first SSM flight started with F12 (launch 29.8, 1994).
Note: The magnetic field has three sources: 1) the magnetic field from the solid Earth, 2) the magnetic field from electrical currents flowing in the ionosphere and magnetosphere, and 3) the magnetic field from the spacecraft. Measurement of source 2 is the principal objective of the SSM, the measurement of source 1 is a secondary objective, and measurement of source 3 is a nuisance which is eliminated from the data as much as possible during data processing.
SSM-Boom (Special Sensor Magnetometer-Boom), an upgrade to SSM. Measures geomagnetic fluctuations associated with solar geophysical phenomena (i.e. ionospheric currents flowing at high latitudes). Combinations of data from SSI/ES (or SSI/ES-2), SSJ/4, and SSM will provide more complete specification of heating and dynamics of the high-latitude ionosphere and neutral atmosphere.
SSF (Laser Threat Warning Sensor). The instrument is an operational static Earth viewing laser threat warning sensor.
The NOAA/SOCC (Satellite Operational Control Center) in Suitland, MD is the primary satellite operations center for DMSP. It is charged with supporting launch, early orbit, and day to day operations. The IPACS (Integrated Polar Acquisition and Control Subsystem) real-time system installed at SOCC provides the telemetry processing, spacecraft commanding, and ground system configuration and management functions for the DMSP spacecraft. The SOCC can communicate directly with the DMSP satellites through enhanced remote tracking stations.
DMSP uses one NOAA and three USAF (United States Air Force) Satellite Control Network (AFSCN) Remote Tracking Stations (RTSs):
• NOAA Fairbanks, AK Tracking Station
• HTS: Hawaii Tracking Station, Kaena Point, HI
• NHS: New Hampshire Station, New Boston, NH
• TTS: Thule Tracking Station, Thule, Greenland
C3S (Command Control, Communications Segment). The C3S is made up of many geographically separated elements, linked by communications networks. The C3S has two Satellite Operations Centers (SOCs); the primary Satellite Operations Control Center (SOCC), Suitland, MD; and the backup Environmental Satellite Operations Center (ESOC), SOC 81, Shriever (Falcon) AFB, Colorado Springs, CO. NOAA/NESDIS/OSO Office Satellite Operations and 6 Space Operations Squadron use the resources of these SOCs to perform the planning, processing, and command generation functions of the C3S.
Control of the new DMSP spacecraft was transferred on December 23, 1999, to the NPOESS Integrated Program Office (IPO).
The User Segment consists of Air Force Global Weather Central (AFGWC), the Navy Fleet Numerical Meteorology Oceanography Center (FNMOC), and the Tactical Terminals (Mark IV Series Transportable Terminals, AN/SMQ-11 Shipboard Receiving Terminals, and RDITs). The Tactical Terminals receive DMSP mission data in realtime. DMSP, operated by NOAA, is used for strategic and tactical weather prediction to aid the U.S. military in planning operations at sea, on land and in the air.
Figure 24: Schematic overview of the DMSP network
Figure 25: Overview of DMSP ground segment (image credit: SMC)
DMSP Data Availability - Visible and Infrared Imagery
NOAA/NESDIS-NSIDC (National Snow and Ice Data Center) and NOAA-NGDC (National Geophysical Data Center) in Boulder, CO have established a collection of digital satellite imagery acquired from the DMSP series of the US Air Force under NOAA-NESDIS contract. Hence, DMSP imagery data are available for the general user community (among them the SSM/I, SSM/T and SSM/T-2 sensor data). These data are prepared from a global, digital intensity file used operationally by the Air Force in forecasting and are subsequently archived at NGDC [DMSP imagery are archived after operational use (usually 45 to 60 days)]. A digital archive of DMSP data has been operational at NGDC since April 1992 (NGDC receives two 5 GByte tapes in compressed format every day). Archival services are continually upgraded. 44) 45)
The imagery collection consists of three positive transparency products produced by USAF.
1) Limited coverage (at 0.6 km resolution) is available for selected areas
2) Global coverage is available on single-orbit strips (at 2.7 km resolution)
3) Mosaics compiled from several orbits, with latitude and longitude grids added, are available globally (at a resolution of 5.4 km).
Mosaics are the only pre-gridded product available. Single-orbit strips can be custom gridded for an additional fee. Each of these products has been produced since 1973, except the mosaics, which were available from December 1975 on.
The AMSU-A (Advanced Microwave Sounding Unit) instrument of the NOAA POES series was first flown on NOAA-15 (launch May 13, 1998). It's primary objective is to provide significantly improved temperature soundings compared to its predecessor (MSU). However, since AMSU-A features similar window channels as the SSM/I sensor, it permits also the determination of surface and hydrological products similar to those derived from SSM/I. 46)
Table 14: Environmental products of the SSM/I sensor47)
Table 15: Comparison of AMSU-A and SSM/I channels
Table 16: Comparison of two sensors - NOAA AVHRR/2 and DMSP/OLS 48)
Early history of DMSP program:
DMSP, originally known as the Defense System Applications Program (DSAP) and the Defense Acquisition and Processing Program (DAPP), is a long-term USAF effort in space to monitor the meteorological, oceanographic and solar-geophysical environment of the Earth in support of DoD operations. All spacecraft launched have had a tactical (direct readout) and a strategic (stored data) capacity. In March 1973, DMSP data was declassified and made available to the civil/scientific community. The USAF maintained an operational constellation of two near-polar, sun-synchronous satellites. The DMSP mission was to provide global visible and infrared cloud data and other specialized meteorological, oceanographic and solar-geophysical data in support of world wide Department of Defense (DoD) operations.
The early DoD weather program began with first launches in 1962 with a spin-stabilized satellite of 90 kg mass equipped with a shutter-style TV camera (a top secret classified program at the time). The satellites in this series were built by RCA (Radio Corporation of America) for the National Reconnaissance Office (NRO). Various code names were used for the entire program such as Program 2, Program 694BH, and Program 417.
The first successful launch of the “P35 series” occurred on Aug. 23, 1962 (Scout launch vehicle from Point Arguello (VAFB), CA; orbit: perigee of 578 km, apogee of 752 km, inclination of 98.5º, period of 98.1 min, spacecraft mass of 91 kg). The COSPAR designation of this flight was: 1962-A-Omicron-1. Another launch of Program 35 took place on Feb. 19, 1963 (Scout vehicle; orbit: perigee of 488 km, apogee of 810 km, inclination of 100.5º, period of 98.7 min; spacecraft mass of 40 kg). COSPAR designation: 1963-005A. The various spacecraft identified in the P35 series were grouped in generations known as 'Blocks'. Blocks 1 and 2 must be considered as experimental satellites. RCA manufactured the spacecraft of Blocks 1, 2, 3, 4A and 4B. 49) 50) 51)
All the successful satellites in the P35 series were placed into a sun-synchronous orbit. This was the first time ever that a sun-synchronous orbit was demonstrated.
Table 17: Overview of DMSP Block 1 satellites
Table 18: Overview of DMSP Block 2 satellites
Figure 26: Illustration of an early spin-stabilized Block 1 series S/C (image credit: USAF)
Table 19: Overview of DMSP Block 3 satellite series
The imagery of these first satellites was received at two command/readout stations established at retired Nike Missile sites located near Fairchild Air Force Base (FAFB), near Spokane, Washington, and Loring Air Force Base, Limestone, Maine. From these acquisition sites, the imagery was sent to AFGWC (Air Force Global Weather Central), now AFWA (Air Force Weather Agency), located at Offutt Air Force Base, near Omaha, Nebraska.
Declassification of the program, previously known as DSAP (Defense System Applications Program) and as DAPP (Defense Acquisition and Processing Program), took place in March 1973 along with a new name, namely DMSP (Defense Meteorological Satellite Program). In December 1972, DMSP data was made available to the civil/scientific community. 52) 53)
Block 4 series spacecraft: From mid-1965 to early 1970, the series of DMSP satellites was known as the Block 4 series (launches on a Thor DM-18 vehicle from VAFB, CA). Spacecraft of this series, built by RCA (Radio Corporation of America), were of the shape of a spin-stabilized octagon with a mass of about 150 kg. The payload consisted of two vidicon cameras for collecting high-resolution TV pictures (resolution: 2.4 km at image center to about 16 km at the edge of a picture). Some later S/C also had a low-resolution VIS (0.4 - 4 µm) and infrared (8.0 - 12 µm) sensor. From these, better sensors emerged with first attempts to combine reflected and emitted energy for the cloud analysis problem. 54)
Table 20: Overview of DMSP Block 4A satellite series
Table 21: Overview of DMSP Block 4B satellite series
Figure 27: Illustration of a DMSP Block 4 satellite (image credit: The Aerospace Corporation)
Block 5 series spacecraft: Beginning in the early 1970s, new generations of satellites were inaugurated, the Block 5 series. These were three-axis stabilized spacecraft (starting with Block 5D-1), based on the TIROS bus of GE (General Electric Company) Astro Space with a design life of 3 years.
Table 22: Overview of Block 5A satellite series
Figure 28: Illustration of the DMSP Block 5D-1 spacecraft series (image credit: Ref. 50)
The primary sensor on this series was OLS (Operational Linescan System) employing a scanning VIS/IR radiometer as imaging sensor (two channels). The VIS channel had a spectral band from 0.4 - 1.1 µm, and the infrared channel lower resolution channel (approx. 3.6 km at nadir). Also use of a low-light level amplification system. These two design factors considerably enhanced the utility of the system.
Figure 29: Illustration of the deployed DMSP Block 5D-2 spacecraft (image credit: USAF)
Spacecraft technology in the seventies brought about improved optics, signal processing along with larger payloads. With new optics capabilities, gaps in the photos were eliminated and the meteorological data could be gathered along the poorly illuminated horizon where the sun was rising or setting. 55)
Figure 30: Line drawing of the DMSP Block 5D-2 spacecraft (image credit: Ref. 4)
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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.