DMSP Instruments: A 50-Year Legacy
DMSP Instruments: A 50-Year Legacy
Aerospace expertise has been instrumental in maximizing the utility of the military’s preeminent weather-forecasting system.
The mission of the Defense Meteorological Satellite Program (DMSP) is to collect and disseminate global visible and infrared (IR) cloud-cover data and other specialized meteorological, oceanographic, and solar-geophysical information to support defense operations and other high-priority programs. Timely data from two primary satellites in low Earth orbit are supplied to the Air Force Weather Agency, the Navy Fleet Numerical Meteorology and Oceanography Center, and the National Oceanic and Atmospheric Administration (NOAA) and finally to deployed tactical users, both military and civilian. Though Aerospace involvement was originally limited, DMSP eventually became one of the more important programs for Aerospace—particularly in the area of sensor development and validation.
The 1960s and Early 1970s: Blocks 1–4
In the early 1960s, the reconnaissance community initiated a program using low-altitude satellites as an interim measure to collect cloud-cover data. The highly classified system, known as Program 417, was to support the operational needs of the Corona satellites, designed to provide photo imagery of the Soviet Union. Corona satellites used conventional film to record their data, and the film canisters were jettisoned and returned to Earth via parachute and recovered by aircraft. The DOD wanted to maximize the usefulness of the film and needed a satellite program to predict cloud cover. Thus began Block 1 of DMSP. The program initially involved a small number of Aerospace scientists and engineers in the Electronics Research and Space Physics Laboratories. Their tasks involved improving elements of the primary sensor and developing secondary sensor concepts as well as general science and technology support. Secondary sensors developed in the laboratories flew on numerous DMSP missions and were, in many ways, precursors of the secondary sensor complement now flying on DMSP.
The first DMSP satellites employed a simple spin-stabilized design. They carried a video camera with a 1.27-centimeter aperture sensing in the 0.2–5-micron regime and two IR systems—the medium-resolution “C” system with 16 channels from 5 to 30 microns, and the high-resolution radiometer working in the 7–14-micron domain. A set of horizon sensors were also used for attitude control and for triggering the camera shutter each time it turned to face Earth. Through the 1960s and into the 1970s, a total of 34 DMSPs were launched, all flying the simple rudimentary payloads. It was not until the design of the Block 5 satellites that more instrument capability began to emerge.
The Mid-1970s: Block 5B/C
The Block 5B/C satellites launched between 1972 and 1976 offered increased instrument capability. The vidicon camera was replaced by a constant-speed rotary-scan radiometer. Spin stabilization was abandoned; instead, instruments were mounted on a platform that kept a constant angle between the direction of motion and Earth. The primary instrument was called the Sensor AVE Package, or SAP, and it consisted of three main data collection and processing channels—one IR channel, one high-resolution visible channel, and one very-high-resolution visible channel. Additional meteorological parameters were measured via the new concept of “special sensors.”
Special Sensor B (SSB), supplied by Sandia National Laboratory, was a gamma tracker used to map the fallout from nuclear tests. Sensing between 10 and 15 kilometers in the atmosphere, it detected the fission gamma particles emitted by the fallout debris. It gave rise to numerous successors on later DMSP models.
Special Sensor E (SSE) was an eight-channel scanning filter radiometer for vertical temperature profiling. Six of the channels were in the carbon-dioxide band (~15 microns), one was in the IR window band at 12 microns, and one was in the rotational water-vapor band near 20 microns. Radiance measurements of the atmosphere were processed to obtain vertical temperature profiles. The instrument weighed 8.2 kilograms and had subsystems such as a chopper filter assembly, a scanner system, and an electronics module. The SSE was capable of measuring scenes in temperatures up to 330 K. The first prototype SSE was flown on F30.
Special Sensor J (SSJ) and SSJ2 measured precipitation electrons with six energy bins from 300 to 18,000 electron volts. This instrument was built by Aerospace and flew on units F30 through F34.
Special Sensor L (SSL) was an experimental lightning detector that flew on F31. The instrument operated at night to map lightning strikes in the visible range from 0.4 to 1.1 micron. The peak response of the SSL was at 0.8 micron, and its field of view was approximately 2500 2500 kilometers.
The Harmonic Oscillation Generator (HOG) was an experimental vehicle roll-rate gyro. It was flown on the F27 satellite to provide advance information concerning the dynamic interactions caused by uncompensated momentum in the design of a future oscillating primary sensor.
The Independent Roll Sensor (IRS) was added to the spacecraft design to provide backup roll attitude information. The SAP sensor provided the primary roll attitude data.
The Late 1970s to the 1990s: Blocks 5D1 and 5D2
While Blocks 1 through 5C represented a gradual evolution in capability, Block 5D was essentially a new system, with far more sophisticated instruments and processing capability onboard. The 5D satellites had a different bus configuration and a different three-axis stabilization system. The first of these, the 5D1 satellites launched between 1976 and 1980, introduced a new primary instrument, the Operational Line Scan (OLS) sensor. It is still the primary sensor on the latest incarnation, Block 5D3. The 5D2 satellites, launched throughout the 1980s and 1990s, flew the first microwave sounder.
As with previous generations, the Block 5D continued the tradition of using special sensors of increasing complexity and utility. Some of these—particularly the microwave sounders—focused on terrestrial or atmospheric weather, while others focused on the space environment. Some flew once, some flew many times. Several were built at The Aerospace Corporation.
First flown in 1976, the OLS provided global cloud-cover imagery to military weather forecasters. The OLS operated at two resolutions in the visible spectrum: smooth (2.77 kilometers) and fine (0.55 kilometers). Smooth data processing onboard the spacecraft decreased the resolution and data rate by a factor of 25. The visible channel could detect smoke and dust storms—information that can be critical to strategic planning—as well as ice cover. The instrument was unique in being sensitive enough to view clouds by moonlight. The low-light sensing capability could capture city lights and distinguish lights from fires. This feature could support battlefield damage assessment by enabling commanders to compare the light in a specific area before and after a strike. Thermal IR viewing enabled nighttime cloud viewing at a lower resolution than daytime visible fine-mode data, but provided mission planners with critical 24-hour information about cloud cover and weather conditions. Aerospace has been instrumental in the design of user applications for the OLS. For example, the Cloud Depiction and Forecast System (CDFS) was prototyped by Aerospace to include high-resolution cloud-detection schemes.
The Special Sensor Microwave Imager (SSM/I) was a breakthrough design based upon the JPL/NASA Scanning Multichannel Microwave Radiometer (SMMR) that flew on Nimbus 7 (launched October 24, 1978) but with a higher-frequency capability and a much larger swath. The SSM/I was a seven-channel dual-polarized passive microwave radiometer operating in the frequencies from 19 to 85 gigahertz. Aerospace wrote all the specifications for its construction. The instrument was a conically scanning imager having a swath width of approximately 1400 kilometers. It provided key surface imagery as well as information about the amount of soil moisture (the Army likes to call this “trafficability”), the amount of water in a column of air from the surface to the top of the atmosphere, and ocean wind speed, to name a few. SSM/I rain-rate products were used to chart tropical cyclones and evaluate their strength. The ability to “see” storm rain bands was important for meteorologists, and both the Air Force and the Navy relied on the SSM/I to provide global products. The SSM/I has been the subject of hundreds of scientific journal articles (many by Aerospace).
The Special Sensor Microwave/Temperature (SSM/T-1) was a passive microwave radiometer that scanned cross-track to the velocity vector of the spacecraft, through nadir. The design had seven channels in the oxygen-absorption band at 50–60 gigahertz. The SSM/T-1 provided vertical temperature profiles globally and was used by numerical weather prediction models to initialize a forecast run and provide boundary conditions. Resolution was approximately 175 kilometers at nadir, growing as the instrument scanned through cross-track. This instrument covered a swath of approximately 1500 kilometers. SSM/T-1 was the first instrument for which DMSP performed a detailed “calibration/validation.” This process first evaluated the quality of the calibration of the instrument and then validated the vertical temperature profiles against “truth” data, which were derived from the global radiosonde network.
The SSM/T-2 was a companion to the SSM/T-1, designed to retrieve vertical water-vapor profiles. It, too, was a passive microwave radiometer that scanned cross-track to the velocity vector of the spacecraft, through nadir. The design had five channels spanning frequencies from 91 to 200 gigahertz. The atmosphere has a strong water-vapor absorption feature centered at 183 gigahertz, which this instrument was able to exploit. The symmetry of the spectroscopic line—characterized by double-sided channels on either side of the line center—was also exploited. For example, the absorption 7 gigahertz to the left and right of the peak are exactly the same; this allowed the averaging of both sides of the peak to help drive down system noise. The calibration/validation for SSM/T-2 was more complicated than for SSM/T-1. Aerospace designed and built a mobile LIDAR (light detection and ranging) facility that was deployed to Barking Sands Navy Facility on the island of Kauai, Hawaii. The LIDAR instrument could scan three-dimensionally as the SSM/T-2 flew overhead, and products from the SSM/T-2 could be validated with high confidence. The mobile LIDAR facility has been reused many times for calibration/validation. In addition, the SSM/T-2 data were used to assist in the forecasting of aircraft contrails, which can put stealth flying assets at risk of detection. The SSM/T-2 calibration/validation was a multiagency activity led by Aerospace and was used as a model for other similar activities.
Special Sensor C (SSC) was the snow/cloud discriminator. It helped weather forecasters determine whether a white background represented snow or clouds.
Special Sensor D (SSD), an atmospheric density sensor, measured major atmospheric constituents (nitrogen, oxygen, and ozone) by making Earth-limb observations of ultraviolet (UV) radiation from the thermosphere. Aerospace developed this instrument, which flew on 5D1 Flight 4 in 1979.
Space Environment Sensors
The SSB/A (the “A” is for Aerospace) was a scanning x-ray spectrometer composed of a high-energy and a low-energy scanning x-ray sensor, a Lyman-alpha sensor, and Geiger counters for monitoring the electron background. It flew on 5D2 Flight 6 in 1982.
This SSB Omindirectional (SSB/O) sensor was a successful experiment to determine whether more accurate atmospheric measurements could be obtained by measuring the co-orbiting particles and the upward flux and subtracting it from the subsatellite scene. The SSB/O was sensitive to x-rays in the energy range of approximately 1500 electron volts. Aerospace developed this instrument as well; it flew on 5D1 Flight 2 in 1977.
This SSB Scanning (SSB/S) x-ray detector determined the location, intensity, and spectrum of x-rays emitted from Earth’s atmosphere. It included an array of four 1-centimeter-diameter mercury-iodide crystals collimated to a 10-degree-wide radial field of view.
The SSB/X, SSB/X-M, and SSB/X-2 was an array-based system for detecting the location, intensity, and spectrum of x-rays emitted from Earth’s atmosphere. The array consisted of four identical and independent directional detectors.
Special Sensor H (SSH) and SSH-2 were IR/carbon dioxide spectrometers that could infer the vertical distributions of temperature, water vapor, and ozone in cloud-free conditions. DMSP did not pursue the infrared technology, but decided to concentrate on microwaves because of their ability to “see” through clouds.
The Special Sensor Ion and Electron Sensor (SSIES) measured ambient electron density and temperature, ambient ion density, and average ion temperature and molecular weight. It consisted of a Langmuir probe and planar collector, a plasma drift meter, and a scintillation meter. These served to characterize the in situ space weather—an important parameter that supports, for example, power-grid management during solar maximum.
The Special Sensor Ionospheric Probe (SSI/P) was a scanning radio receiver that mapped the man-made radio spectrum to determine the critical (breakthrough) frequency of the upper layers of the ionosphere. The instrument automatically scanned from 1 to 10 megahertz in 20-kilohertz steps at a rate of one step per second.
The next generations of the SSJ—the SSJ3 and SSJ4— had augmented capabilities for measuring not only electrons, but ions in 20 energy channels ranging from 30 to 30,000 electron volts. The data were used to provide the position of the equatorial and polar boundaries of the aurora at high latitudes, both north and south.
The SSJ* space radiation dosimeter measured the accumulated radiation produced by electrons in the 1–10 million electron volt energy range, protons of greater than 20 million electron volts, and the effects of the occasional nuclear interactions produced by energetic protons. The analogy of the SSJ* to everyday life is the radiation badge that a medical radiologist wears, or the radiation badges that fly on airliners to measure how much total dose radiation the pilots and flight crews are experiencing. Aerospace also built this instrument, which flew on 5D1 Flight 1 in 1976.
The New Millennium: Block 5D3
The newly instrumented Block 5D3 satellites began operation in 2003, with the launch of Flight 16. On these spacecraft, the three microwave sounders—SSMT-1, SSMT-2, SSMI—were combined to create the Special Sensor Microwave Imager Sounder (SSMI/S), which also added a new temperature sounder for upper-air analysis. Two new space environment sensors were also added to the Block 5D3, as well as new versions of the SSIES and SSJ (SSJ-5).
The SSMI/S suite, with its ability to see through clouds and darkness, provides important surface and atmospheric data that traditional visual and IR satellite sensors cannot. The comprehensive SSMI/S also improves upon past instruments by having a wider swath, greater sounding resolution from 175 to 38 kilometers, and more data channels. Imaging data from the SSMI/S provide information on sea surface winds, rain rate, moisture-laden clouds, and soil moisture. SSMI/S data also provide information on severe storms such as typhoons and hurricanes and help forecasters determine their direction, size, and intensity. The imager can also detect ice and snow cover, and in some cases can provide estimates of ice edge, age, and concentration—a valuable tool for Navy efforts. The SSMI/S also provides important land data such as soil moisture and enables users to distinguish areas of bare soil and identify vegetation types. This is particularly useful for forecasters supporting Army efforts. In addition to its military uses, SSMI/S data supports a variety of users in the public sector. Current and past data records aid climatologists by providing an extensive continuous record of sea ice coverage. This data can be compared on an annual basis to determine changes in ice coverage and depth.
In September 2006, Donald Boucher was presented with the corporation’s highest award, the Trustees’ Distinguished Achievement Award, “for outstanding technical leadership in recovering functionality of a new Defense Meteorological Satellite Program (DMSP) sensor in support of a national program.” During Flight 16’s 18-month calibration/validation period, Boucher led a national team of experts on a fact-finding mission to overcome a variety of anomalies and design flaws, ultimately turning over the instrument on schedule. The lessons learned became a valuable tool not only for future SSMI/S instruments but for additional microwave imaging/sounding sensors. The Aerospace Corporation has led a team of scientists and engineers during the calibration/validation process for all flights. As it did during Flight 16’s calibration/validation, the team has continued to identify performance shortfalls and establish solutions for each one. The latest SSMI/S on DMSP Flight 18 is operating well, with all performance shortfalls eliminated.
The Special Sensor UV Spectrographic Imager (SSUSI) scans Earth’s atmosphere across the satellite subtrack, including Earth’s limb (when viewed from space, Earth looks like a flat circle or disk surrounded by a bright halo of atmosphere known as the Earth limb). It consists of two sensing systems—the spectrographic imager and the photometers. The imager obtains horizon-to-horizon images in the wavelength range of 1100 to 1800 angstroms with a viewing area of 3700 153 kilometers using a scan mirror system. SSUSI can also operate in a fixed-mirror mode to collect spectrographic data. The photometers operate in three wavelengths—6300, 6290, and 4278 angstroms—and provide information on auroral energy deposition and measurements on the nightside of Earth. The SSUSI measures UV emissions and provides information on auroral emissions and airglow and electron and neutral density profiles, among many other products. SSUSI adopted many characteristics of an experiment known as the Global UV Imager (GUVI) developed by The Aerospace Corporation and the Johns Hopkins Applied Physics Laboratory; it flew on NASA’s TIMED mission in 2002.
Like the SSUSI, the Special Sensor UV Limb Imager (SSULI) scans the Earth limb in the orbital plane using silicon carbide scan mirrors. It views tangent altitudes from 750 kilometers down to the Earth disk, with a field of view of 5 kilometers vertically and 100 kilometers cross-track. The SSULI measures UV airglow profiles and produces information on electron density profiles and neutral densities, on both the dayside and nightside of the satellite orbit, as well as other products.
Aerospace is leading the calibration/validation for both the SSUSI and the SSULI. Many challenges have been experienced by the team, and performance shortfalls identified on Flights 16 and 17 were corrected on the current Flight 18.
Aerospace research and expertise in remote sensing technology has helped make DMSP one of the most successful and enduring military satellite programs. The future may bring changes to the way national weather satellites are acquired and deployed, but the need for timely and reliable meteorological data will surely not diminish.
R. C. Hall, A History of the Military Polar Orbiting Meteorological Satellite Program (Office of the Historian, National Reconnaissance Office, Sept. 2001).
J. Bohlson, L. Belsma, and B. Thomas, Cloud Cover Over Kosovo, Crosslink, Vol. 2, No. 2 (Summer 2000).
Back to the Spring 2010 Table of Contents