The Nimbus 7 Spacecraft System

Revision Date:

March 1994

Review Date:

2. Platform or Data Collection Environment Information:

Platform or Data Collection Environment Long Name, Acronym:

Nimbus 7

Platform Introduction:

Collection Environment:

The satellite was placed in a 955 km sun-synchronous polar orbit on 25 October 1978. Its repeat cycle allowed for global coverage every six days, or every 83 orbits. Because of power limitations aboard the spacecraft, sensors were not run simultaneously, but were scheduled on a priority basis.

Platform Program Management:

Seven Nimbus Experiment Teams (one for each NASA-provided sensor program) plus the United Kingdom team for the Startospheric and Mesospheric Sounder experiment met at frequent intervals from the inception of each committee through at least one year post-launch. Each team consisted of five to ten members and was supported by applications scientists and data processing support personnel. Each NET was also supported by the Nimbus-7 data applications system manager or an appointed representative.

NET members advised on all aspects of their respective sensor programs and performed related studies and tasks during pre- and post-launch phases. They determined the principal research and development requirements of each experiment.

Platform Mission Objectives:

The Nimbus-7 platform allowed a number of experiments related to pollution control, oceanography, and meteorology to be conducted. Mission objectives were:

• To observe gases and particulates in the atmosphere for the purpose of determining the feasibility to map sources, sinks, and dispersion mechanisms of atmospheric pollutants (SBUV/TOMS - SAM II - SAMS-LIMS)
• To observe ocean color, temperature, and ice conditions, particularly in coastal zones, with sufficient spatial and spectral resolution to determine the feasibility of applications such as:

  detecting pollutants in the upper level of the oceans

  determining the nature of materials suspended in the water

• To continue to make baseline measurements of variations of long wave radiation fluxes outside the atmosphere and of atmospheric constituents for the purpose of determining the effect of these variations on the earth's climate (ERB-SBuv/TOMS -- LIMS).

In addition, the NETS defined the following goals:

• to develop, use and test processing and science algorithms
• to define the general content of the film and tape product
• to verify sensor calibration and performance
• to participate in the planning for data acquisition, time schedule sharing of sensor operations
• to certify the quality of data output products by comparison measurements with other data (ground truth measurements), and
• to perform initial post-launch experiement investigations and issue appropriate reports and publications.

Platform Parameters:

The Nimbus-7 was maintained in a near polar, sun-synchronous orbit at an altitude of 955 km. Equatorial crossings are local noon for ascending and local midnight for descending nodes. Spacecraft inclination is 99.1 degrees, with a maximum poleward latitude of 80.77 degrees. The orbital period is 104.16 minutes. Equator crossings on consecutive orbits are separated by 26.1 degrees longitude.

Coverage Information:

The Nimbus-7 observatory provides global coverage every six days, or every 83 orbits.

Attitude Characteristics:

Spacecraft inclination is 99.1 degrees, with a maximum poleward latitude of 80.77 degrees. The Nimbus-7's attitude control subsystem provides stabilization about the spacecraft's roll, pitch and yaw axis and control of solar paddle orientation, maintaining them nearly perpendicular to the nominal sunline.

Consisting of four attitude control loops and associated switching logic, telemetry and test modes, electrical manifolding, and thermal environmental control, this system maintains spacecraft alignment with the local orbital reference axes to within 0.7 degrees of the pitch axis and one degree of the roll and yaw axis. The system keeps the instantaneous angular rate changes about any axis to less than 0.01 degree per second.

The three-axis ACS uses horizon scanners for roll and pitch attitude error sensing. The rate gyros sense yaw rate and, in a gyro compassing mode, sense yaw attitude. A torquing system uses a combination of reaction jets to provide spacecraft momentum control and large control torques when required; flywheels are utilized for fine control and residual momentum storage.

Data Collection System:

The communications and data handling subsystem, which manages all information flow for the Nimbus-7 platform, is composed of the S-band communications system and tape recorder subsystem. The S-band communication system includes the S-band command and telemetry system , the data processing system and the command clock. The S-band command and telemetry system consists of two S-band transponders, a command and data interface unit, four earth view antennas, a sky view antenna, and two S-band transmitters (2211 MHz). Commands are transmitted to the observatory by pulse code modulation, phase-shift keying/frequency modulation/phase modulation of the assigned 2093.5 MHz S-band uplink carrier. Stored command capability provides for command execution at predetermined times.

List of Instruments:

• Coastal Zone Color Scanner
• Earth Radiation Budget
• Limb Infrared Monitor of the Stratosphere
• Stratosheric Aerosol Measurement II
• Stratospheric and Mesospheric Sounder
• Solar Backscatter Ultraviolet/Total Ozone Mapping Spectrometer
• Scanning Multichannel Microwave Radiometer
• Temperature Humidity Infrared Radiometer

   

3. Ground Segment Information:

Tracking and Control:

See Telemetry and Ranging

Data Acquisition and Processing:

A data handling and processing complex established at the Goddard Space Flight Center, designated the Nimbus Observation Processing System, distributed payload data (except for that pertaining to Stratospheric and Mesospheric Sounder) among several facilities for processing, then converted the results into data products. Responsibilities of the Nimbus Observation Processing System were:

• to calibrate, quality check and geographically locate the raw sensor observations
• to convert observations into meaningful parameters through the application of scientific algorithms
• to establish a broad data base by correlating observations from related sensors
• to display the derived parameters in the most useful forms (products) for scientific investigations and correlations
• to distribute the generated products to Nimbus Experiment Teams and selected investigators on a limited basis, and
• to distribute archival quality tapes and film products to archive centers for their dissemination to all interested users.

Latitude Crossing Times:

The Nimbus-7 Observatory's equatorial crossings are local noon for ascending node and local midnight for descending node.

4. Glossary of Terms:

No further information at this time.

5. List of Acronyms:

See acronym list.