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Platform Description

The Nimbus 7 Spacecraft System

The following information has been adapted from National Aeronautics and Space Administration. 1978. The Nimbus 7 Users' Guide. C. R. Madrid, editor. Goddard Space Flight Center.


Summary

Launched on 25 October 1978 from Vandenberg Air Force Base, California, the Nimbus-7 spacecraft was the last in a series of operational weather satellites operated by the US National Oceanic and Atmospheric Administration (NOAA) and the US National Aeronautics and Space Administration (NASA). Nimbus-7 was placed in a sun-synchronous orbit at an altitude of 955 km. Equatorial crossings are local noon for ascending node and local midnight for descending node. Spacecraft inclination is 99.1 degrees, with a leeward latitude of 80.77 degrees. Orbital period is 104.15 minutes, and consecutive equator crossings are separated by 26.1 degrees longitude.

The spacecraft has three major structures that house power, attitude control and information flow components. The spacecraft's base is a hollow, torus-shaped sensor mount containing electronics equipment and battery modules. The lower surface of the torus provides mounting space for sensors and antennas. Larger experiments are held by a box beam structure mounted in the center of the torus. A control housing unit is connected to the top of the sensor mount by a tripod truss structure. Above the control housing are sun sensors, horizon scanners and a command antenna. Duplicate solar paddles complete the configuration, which is similar to an ocean buoy's.

The Nimbus-7 weighs 965 kilograms, is 3.04 meters tall, 1.52 meters in diameter at the base and 3.96 meters wide with solar paddles fully extended.

The spacecraft supported the following seven experiments and subsystem (THIR):

Nimbus 7 Observatory Schematic

Table of Contents

1. Document Information
2. Platform or Data Collection Environment Information
3. Ground Segment Information

1. Document Information

Document Type: Platform Document
Revision Date: December 1995

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 Stratospheric 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:

In addition, the NETS defined the following goals:

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

3. Ground Segment Information

Tracking and Control

Please 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:

Latitude Crossing Times

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