The objective of the Landsat-6 spacecraft, owned by EOSAT, was to continue the Landsat program. It carried an improved suite of instruments. However, it failed to achieve orbit during launch, and forced the continued operation of the failing Landsat-4 and -5 vehicles.
Figure 1: The Landsat-6 spacecraft configuration
The LS-6 spacecraft builder and integrator was Martin Marietta Astro Space (formerly General Electric Astro Space), S/C procurement and management by EOSAT. The S/C structure consists of aluminum with graphite struts. Hydrazine propulsion system. A single solar array with 1-axis articulation produces 1430 W, two NiCd batteries provide a capacity of 100 Ah (total). Data stored onboard using tape recorders for direct downlink to ground stations at 85 Mbit/s. LS-6 is three-axis stabilized, zero momentum with control to 0.01º using reaction wheels. 1) 2)
Launch: LS-6 was launch Oct. 5, 1993 on a Titan 2 booster from VAFB (Vandenberg Air Force Base), CA. The satellite failed to achieve its orbit; communication with the satellite was never established.
A formal review was conducted by NOAA to investigate the failure. A ruptured hydrazine manifold was the reason. The separation from the booster rocket occurred properly, however, the ruptured rocket fuel chamber prevented fuel from reaching the apogee kick motor. This failure resulted in the spacecraft tumbling instead of accumulating enough energy to reach its planned orbit. 3) 4)
Orbit: polar sun-synchronous orbit, altitude = 705 km, inclination = 98.2º, period = 99 min, repetition cycle (repeat coverage) = 16 days, equatorial crossing time: 9:45 hours, nominal design life = 5 years.
Sensor complement: (ETM)
LS-6 was designed to carry a single sensor, the Enhanced Thematic Mapper (ETM), which includes several new features to significantly improve data quality. Instrument mass (scanner assembly) = 288 kg, auxiliary electronic module = 81 kg, power = 490 W (max).
ETM (Enhanced Thematic Mapper)
LS-6 functional capabilities: Note: an attempted pushbroom version upgrade for the instrument failed due to severe budget constraints.
• ETM (seven TM bands, plus an additional panchromatic band). An instrument upgrade had the following features: Use of a single, monolithic silicon detector array for all the VNIR spectral bands. All detectors were on a common silicon substrate, this approach provided a better band-to-band geometric registration and stability (TM used four separate detector arrays).
• Two onboard recorders (playback to ground in X-band); each recorder is capable of recording/reproducing at a rate of 85 Mbit/s, and each can store ≈15 minutes worth of image data, or 29 scenes.
• Three pointable antennas
• Simultaneous acquisition of TM and PAN data
• ETM generates three different data streams:
- Mode 1 Seven spectral bands
- Mode 2 Panchromatic band (PAN) + channels 4, 5, and 6
- Mode 3 Panchromatic band (PAN) + channels 4, 6, and 7
- Channel PAN (8) : 500 - 900 nm, 15 m resolution; applications: cartography
- Channel 1: 450 - 520 nm (VIS, blue); applications: water penetration, bathymetry (water depth), chlorophyll absorption, distinguishes deciduous/coniferous forests
- Channel 2: 520 - 600 nm (VIS, green); applications: matches green reflectance, peak of healthy vegetation, assessment of plant vigor
- Channel 3: 600 - 690 nm (VIS, red); applications: chlorophyll absorption, plant type discrimination
- Channel 4: 760 - 900 nm (VNIR); applications: plant cell structure, plant vigor, complete absorption by water, shoreline mapping
- Channel 5: 1550 - 1750 nm (SWIR); applications: moisture content, soil mapping, thin cloud penetration
- Channel 7: 2080 - 2350 nm (SWIR); applications: hydroxyl ion absorption, geology
- Channel 6: 10.4 - 12.5 µm (TIR); applications: brightness temperature, soil moisture, plant heat stress.
Ground pixel resolution: 15 m (panchromatic), 30 m (channels 1-5, channel7) 120 m (channel 6). Ground image size: 185 x 185 km.
Table 1: Summary of Landsat-6 ETM bandwidth specifications
Data: Landsat-6 has three X-band frequencies for downlink (8082.5, 8212.5, and 8342.5 MHz) to allow combinations of up to two real time data links (TM and/or a panchromatic mode) and two tape recorder dumps. The ETM outputs two 85 Mbit/s serial composite streams, each consisting of the digital image area, internal ETM calibration data, image-related timing data, S/C time code, ephemeris, and attitude data. The ancillary timing, calibration, and attitude data are used by the Landsat-6 Image Data Processing System S/W to perform geometric image corrections. The X-band downlink communications subsystem permits simultaneous downlinks to three stations using pointable antennas.5)
As of 1995 Landsat-5 is the only remaining Landsat system acquiring TM data (S/C operated by EOSAT). Landsat-5 has no operational TDRSS support and no onboard storage. Data transmission to the ground is by X-band direct downlink only.
2) E. W. Mowle, C. J. Dennehy, "The Landsat-6 satellite: an overview," Aerospace and Electronic Systems Magazine, IEEE Volume 6, Issue 6, Jun 1991, pp. 18 - 23
3) "Landsat-6 failure attributed to raptured manifold," March 10, 1995, URL: http://landsat.gsfc.nasa.gov/pdf_archive/NOAA_landsat6.pdf
4) "Satellite Loss Raises Questions for Eosat's Future," Space News, October 11-17, 1993, p. 3
5) EOSAT Landsat Technical Notes, September 1992
This description was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments and corrections to this article are welcomed by the author.