The National Environmental Satellite Service (NESS) image product processing system is described. Other topics discussed include: (1) image processing of polar-orbiter satellite data; (2) image processing of geostationary satellite data; and (3) quality assurance and product monitoring

Crowe, M.

English

NASA Technical Reports Server

PROCESSING OF SATELLITE IMAGERY
AT THE NATIONAL ENVIRONMENTAL SATELLITE SERVICE
M. Crowe
U.S. Department of Commerce, NOAA
National Environmental Satellite Service
Washington, D.C. 20233
ABSTRACT
The National Environmental Satellite Service (NESS)
image product processing system is a subset of the
polar-orbiter satellite and the geostationary satellite
systems. These systems enable the day and night monitor-
ing of the environment from markedly different perspectives,
thereby fulfilling the needs of an international spectrum
of public and private users in the environmental and earth
sciences.
This paper is divided into four sections:
1. Overview of the Satellite Data Processing Systems.
2. Image Processing of Polar-Orbiter Satellite Data.
3. Image Processing of Geostationary Satellite Data.
4. Quality Assurance and Product Monitoring.
1. OVERVIEW OF THE SATELLITE DATA PROCESSING SYSTEMS
There are many physical components that are functionally common to both
the polar-orbiting and geostationary satellite systems. The earth environ-
mental sensors include the Scanning Radiometer (SR) and the Very High Resolution
Radiometer (VHRR) aboard the polar orbiters and the Visible Infrared Spin Scan
Radiometer (VISSR) on the geostationary series. Each of these instruments is
sensitive to energy in the visible and infrared portions of the electromagnetic
spectrum.
Some nonsensor subsystems deal with spacecraft and ground-based command,
communications, and power to allow the following functions to be performed:
(1) spacecraft command and command verification;
(2) receiving and processing of telemetry (system performance measurements);
(3) tracking of spacecraft for position/orbit determination and subsequent
prediction;
(4) data flow throughout system;
(5) spacecraft attitude determination and prediction; and
(6) data calibration, processing, and display.
All of the above functions and others must be executed properly if image
data processing and analysis are to be worthwhile. For example, although a
sensor is calibrated and tested extensively prior to launch, the inflight
monitoring of its performance is necessary to assure measurement accuracy.
Also, knowledge of the precise position and attitude of a spacecraft is vital
to the accurate earth location of the data and utility of most image products.
The performance of the above functions is handled by the four major
operational elements of the NESS satellite system:
(1) The Satellite Operations Control Center (SOCC).
(2) Two Command and Data Acquisition (CDA) Stations in Wallops Island,
Virginia, and Gilmore Creek, Alaska.
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(3) Ground-based and satellite communications networks.
(4) A quasi-central processing, analysis, and distribution center (the
Office of Operations in Suitland, Maryland, and the Central Data Distribution
Facility (CDDF) in Camp Springs, Maryland).
The SOCC is responsible for the continuous monitoring, evaluation, and
direction of the performance of the spacecraft and data acquisition systems.
The CDA stations handle data acquisition, processing, and dissemination using
a variety of communications subsystems. The central processing and distribution
facilities use a number of computers with manual intervention to ingest,
reformat, digitize, earth-locate, display, and quality control the sensor data
and subsequent derived products.
2. IMAGE PROCESSING OF POLAR-ORBITER SATELLITE DATA
2.1. SR DATA
Reflected and radiated energy from the Earth is continuously detected in
detected in the appropriate wavelengths by the scanning radiometer aboard the
polar-orbiting satellites. The SR mirror scans across the spacecraft track from
horizon to horizon. The spatial resolution of the ground field decreases away
from the subsatellite point, where the visible and infrared channels can resolve
3.2-km and 6.4-km "spots," respectively. The sensor information is recorded on
magnetic tape aboard the spacecraft and is played back to the CDA stations when
the satellite is within transmission range. The data enter the NESS Suitland
facility through hardware/software components, dubbed the Digital Data Handling
System, and are demodulated, separated, digitized, and transferred to the NESS
large-scale computer (IBM S360/195) for further processing.
Image product processing of SR data includes the creation of:
• polar-stereographic mapped mosaics
• Mercator-mapped mosaics
• time-composites of mapped data
• pass-by-pass gridded images
2.2. MAPPED MOSAICS
The mapping of image data can be broken down as follows.
2.2.1. CALIBRATION
The infrared data are corrected using prelaunch instrument calibration
results and continual inflight blackbody versus outer-space temperature
comparisons.
Also, a brightness correction is applied to the visible data using a
prelaunch-determined relationship between the SR instrument response and solar
equivalent brightness.
2.2.2. ENHANCEMENT
The infrared data are also corrected for the attenuation of the signal
passing through the Earth's atmosphere. Water vapor is considered the major
attenuating constituent.
A correction is made to the visible data for differences in solar illumin-
ation of the Earth scene due to varying sun angle.
2.2.3. EARTH LOCATION
The mapping process continues with the earth location of a subset of data
points or "benchmarks," using such information as predicted satellite orbital
position, attitude, time, and a knowledge of sensor geometry. The benchmark
latitude and longitude inputs are transformed into coordinates in the desired
map projection array. Since the mapping is done on a pass-by-pass basis, the
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limits of this array must also be computed. The mapping resolution is sub-
stantially poorer than that of the raw data, creating a surplus of samples for
each mapped location.
2.2.4. MAPPING PROCEDURE
Using the calibrated, enhanced data and the benchmark data array and array
limits as input, the mapped position coordinates for each sample are computed
by a linear interpolation. Redundant plotting of samples into the mapped sector
of desired resolution is done until the input data are depleted. The mapped
sector is then transferred to disk storage, awaiting further product processing.
Three mapped data bases exist for both the polar-stereographic and Mercator
projections:
• visible
• nighttime infrared
• daytime infrared
Each of the polar-stereographic mapped arrays is aligned with the National
Meteorological Center's (NMC) Numerical Weather Prediction grid for ease in
meteorological application of the satellite imagery. The mapped resolution
varies from approximately 15 kilometers at the equator to 30 km at the poles.
The Mercator-mapped arrays are aligned parallel to the equator, covering
a latitude belt from 40°N to 40°S along the full Earth longitudinal span. The
resolution is approximately 10 km at the equator with an improvement poleward.
2.3. PASS-BY-PASS GRIDDING
Due to the computer resource cost and unsuitability for some user appli-
cations of mapped data, NESS also produces pass-by-pass gridded imagery. While
mapping entails the fitting of the data to conform to a mapped projection,
gridding involves the placement of grids into the data stream in the proper
geographical locations (or more precisely, at the proper time). The grids
consist of latitude/longitude lines and geographical/political boundaries. The
earth location (or data stream positioning) of the grids is accomplished once
a day for the following 14 orbits of the spacecraft using predicted satellite
orbit and attitude, time, and geometry. A grid feature file consisting of
geographical/political latitudes and longitudes is used as input to this grid
table production.
A table of grid positions, within the data stream, for each orbit of the
day is produced.
The melding of grids and data is performed during the creation of each
gridded, orbital display product.
2.4. MAPPED TIME COMPOSITES
NESS produces limited area, multiday brightness, and temperature composites
of polar-mapped data for specific user application. Areal coverage of ice and
snow can be deduced from mapped data by saving only the minimum brightness or
maximum temperature value for each mapped point over a multiday—in this case,
10-day—period. This process effectively removes the transient cloud cover,
exposing the surface background features for snow/ice extent analyses. The
composites are produced daily over the north and south polar regions for the
previous 10-day period.
2.5. MAPPED DISPLAYS
The NESS produces a variety of displays of the mapped data bases for user
application. These range from photo reproductions of full-hemisphere mosaics
to facsimile displays of specific earth sectors. The displays are produced in
various map scales, sizes, and areal coverage in response to user needs.
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This imaging process begins in the IBM S360/195 with software which pro-
cesses the data for the Digital Muirhead Display (DMD) devices that produce
25-cm by 25-cm negatives. Photo processing and reproduction are done in the
Visual Products Support Branch.
The facsimile display media include the standard weather facsimile networks
(FOFAX, NAFAX) using such devices as Alden Electrolytic paper recorders and
weather facsimile images rebroadcasted via geostationary satellites (WEFAX) and
recorded by users on either photofacsimile or other facsimile recorders.
2.6. VHRR DATA
Both the visible and infrared channels of the VHRR resolve an 0.8-km spot
at the subsatellite point. Because of the higher data resolution, only about
eight minutes of VHRR data per orbit can be recorded on the satellite. The data
can then be transmitted to either of the CDA stations or a readout station
located in San Francisco, California. The VHRR system also includes a direct
readout capability, the High Resolution Picture Transmission (HRPT) subsystem.
Users with proper ground equipment can receive the data as the spacecraft passes
within transmission range.
The processing of VHRR data at the NESS (by the Environmental Products
Group) consists of a large amount of manual intervention. Hardcopy photographs
are received from the ingest facility and are analyzed to generate a number of
products with generally hydrological application. These include a Gulf Stream
Analysis, Great Lakes Ice Chart, Great Lakes Surface Temperature Chart (in the
summertime), Alaskan and Labrador Ice Analyses, and the U.S. Snow Basin Mapping.
These products are disseminated by mail, facsimile, teletype, and telecopier.
3. IMAGE PROCESSING OF GEOSTATIONARY SATELLITE DATA
The Geostationary Operational Environmental Satellites (GOES) possess
equatorial, earth-synchronous orbits, which allow them effectively to hover over
the same earth location continuously. The respective scenes viewed by the VISSR,
then, aboard the Eastern Satellite with a subsatellite longitude of 75°W and the
Western Satellite, located above 135°W, change very little from picture to
picture. The VISSR mirror sweeps from earth horizon to horizon viewing an 8-km
swath during each sweep. The mirror is stepped down so that adjacent swaths
are viewed during successive sweeps.
The visible and infrared data are transmitted to the Wallops CDA station
and are demodulated, synchronized, preprocessed, and retransmitted to Suitland.
Much of the preprocessing takes place in the Synchronizer/Data Buffer (S/DB)
computer. The visible data, with a raw 1-km resolution, are adjusted for sensor
bias and can be reformatted into resolutions of 1, 2, or 8 km. The infrared
data may be output in either 8-km or 8x4-km resolutions. These data are sent
to Suitland either by retransmission via the Eastern Satellite or landline
communications.
The S/DB computer hosts another important event in the GOES imaging system.
Earth-located gridding information is produced at Suitland in essentially the
same fashion as polar-orbiting satellite grid positions are predicted. In this
case, the picture coordinate locations of the political/geographical boundaries
and latitude/longitude lines are produced for each picture, instead of on an
orbital basis. Inputs to the gridding algorithm include predicted satellite
position (orbit), attitude, -time, and sensor geometry. The gridding data are
transmitted to Wallops via a computer-to-computer link and telephone line named
the Grid Transfer System. The S/DB receives and properly inserts the grids into
the real-time VISSR data stream for each GOES frame.
The VISSR Ingest Computers (VIC) at Suitland create a data tape that inter-
faces with the photofacsimile display devices. The data, then, may be saved \
for later processing and/or displayed in near real time. The NESS Suitland
facilities handle only a portion of the display and distribution of GOES imagery.
The retransmitted signal from the spacecraft is also relayed via line-of-sight
microwave communications to the CDDF in Camp Springs. From there, pertinent
data may be routed to any of the five Satellite Field Service Stations (SPSS)
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located in Washington, D.C., Miami, Kansas City, San Francisco, and Honolulu.
Each of the SPSS's serves as a focal point for analysis and distribution of
satellite imagery to regional environmental centers.
The VISSR data are available to users in full-disk displays in resolutions
of 4-km visible and 4x8 kilometers or 8-km infrared. Sectors of visible data,
covering a subset of the full-disk area, in 1- and 2-km resolution can also be
formatted at ingest time.
3.1. MOVIE LOOPS
Animated sequences of successive VISSR picture frames called "movie loops"
are created daily at NESS using visible and infrared data of different resolu-
tions over varying geographical locations. The process, using an Oxberry 16-mm
recorder (a camera system which is designed to produce motion pictures from
still-scene sequences), is controlled by a minicomputer.
3.2. ADDITIONAL GRIDDING
The "automatic" gridding system described above is used to provide grids
to all GOES images except for 8-km infrared and movie loops. These products
use computer-produced overlay grids, hand-fitted over images, which are then
photographed. The overlay grids are created using earth-location software,
predicted satellite orbit and attitude, time and a master feature file, in a
manner similar to the polar-orbiter satellite grid generation process.
In this case, the IBM S360/195 has been used to create data tapes that
produce the clear film grid overlays, using the DMD's and photographic pro-
cessing.
3.3. MAPPING
Some operational mapping of GOES images is performed, but it is done on a
limited basis. At this time, two Eastern Satellite pictures are mapped daily—
one visible and one infrared, both in Mercator projections. The mapping process
is essentially the same as that for the polar-orbiting satellites, involving
calibration, earth location of benchmark arrays, interpolation through these
arrays, and storage of the mapped information in an area on the S360/195 disk.
Mapped imagery is output in facsimile form for transmission via standard
weather facsimile circuits.
3.4. MANUALLY DERIVED ANALYSES
The Synoptic Analysis Section uses multiformatted VISSR, SR, and VHRR
imagery to prepare a number of daily, useful meteorological analyses for aid to
the analysis and forecast missions of the NWS. Among these are (1) nephanalyses
delineating cloud cover and type, (2) windflow and moisture analyses as input
to NMC numerical analysis and prediction models, and (3) tropical storm warnings
and classifications.
4. QUALITY ASSURANCE AND PRODUCT MONITORING
The quality control function is a necessary part of any system that expects
to deliver products in an accurate, timely, and consistent fashion. The NESS
Operational Products Monitoring Section (OPMS) performs the monitoring and
evaluation of the ingest, processing, and dissemination of data and derived
products from the NESS operational satellite systems.
The monitoring by OPMS personnel includes around-the-clock identification
and evaluation of real and potential problem areas in the entire NESS product
processing system. Such occurrences as the failure of an operational computer
program to successfully complete or a breakdown in the facsimile transmission
of imagery must be diagnosed. A course of action is then taken which may
involve a number of NESS processing components. Many problems are handled
immediately while others may require extensive post facto analysis.
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Review and discussion of problem areas with pertinent personnel is aided
by a weekly briefing conducted by OPMS.
The first year of this quality assurance effort marked an improvement from
75 to 90 percent in the successful output of products from the NESS operational
imagery system, despite a large-scale increase in both the number of products
and the complexity of the overall facility.
5. CONCLUDING REMARKS
For the sake of brevity, this paper has avoided detailed description of
specific image display products. The NESS operational facsimile transmission
schedule alone consists of approximately 30 mapped products covering a range of
geographical locations, map scales, and data types. To help inform users or
potential users of satellite imagery, the National Oceanic and Atmospheric
Administration (NOAA) has published a "Catalog of Operational Satellite Products,"
edited by Hoppe and Ruiz (1974) and will soon publish an updated version, "NOAA
Catalog of Products, Chapter III" (Dismachek, 1977). Each of these publications
deals with satellite image products and other satellite-derived products, such
as alphanumeric messages and digital magnetic tapes, in greater detail than does
this paper.
REFERENCES
Johnson, Jimmie D., Frances C. Parmenter, and Ralph Anderson, "Environmental
Satellites: Systems, Data Interpretation, and Applications," U.S. Depart-
ment of Commerce, National Environmental Satellite Service, Wash., D.C.,
Oct. 1976, 66 pp.
Bristor, C.L., Ed., "Central Processing and Analysis of Geostationary Satellite
Data," NOAA Technical Memorandum NESS 64, U.S. Department of Commerce,
NESS, Wash., D.C., March 1975.
Fortuna, Joseph J. and Larry N. Hambrick, "The Operation of the NOAA Polar
Satellite System," NOAA Technical Memorandum NESS 60, U.S. Department of
Commerce, NESS, Wash., D.C., Nov. 1974.
Hoppe, Eugene and Abraham L. Ruiz, Eds., "Catalog of Operational Satellite
Products," NOAA Technical Memorandum NESS 53, U.S. Department of Commerce,
NESS, Wash., D.C., March 1974.
Dismachek, Dennis C., Ed., "NOAA Catalog of Products, Chapter III" (In
preparation as NESS Technical Memorandum).
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