Landsat-4 and Landsat-5

The Landsat spacecraft series evolved from an experimental program (LS-1 to LS-3) to an operational program (LS-4 and LS-5) as NASA developed and launched considerably improved spacecraft that were placed into lower orbits than the previous Landsat spacecraft and carried improved instrument suites. ¹⁾

Spacecraft:

A new spacecraft design (2nd generation) was introduced with LS-4 based on the MMS (Multimission Modular Spacecraft) bus design \[first introduced by the SMM (Solar Maximum Mission) of NASA with a launch Feb. 14, 1980\]. Both Landsat satellites were built by GE Astro Space, Fairchild (now Lockheed Martin Missiles & Space) for NASA. The spacecraft mass of LS-4 is 1407 kg (1,941 kg launch mass); LS-5 = 1407 kg (launch mass of 1.938 kg).

The flight segment consisted of two major systems:

• The instrument module, containing the instruments together with the mission unique subsystems, such as the solar array and drive, the TDRS antenna, the wide-band module, and the global positioning system (GPS)

• The multimission modular spacecraft that contained the modularized and standardized power, propulsion, attitude control, and communications and data handling subsystems. The flight segment was designed with 3 years nominal lifetime.

The spacecraft structure consists of aluminum panels with graphite struts. A hydrazine propulsion system is being used for orbit maintenance. Electric power of 1430 W (BOL) is provided by a single solar array with a 1-axis articulation mechanism. Two NiCd batteries provide 100 Ah total during the ecliptic orbit phase.

A retractable boom (4 m long) with 2 powered joints supports the articulated HGA (High Gain Antenna) which downlinks data via TDRS (Tracking and Data Relay System). The RF communications system uses S-, X-, L-, and Ku-bands. The S/C is 3-axis stabilized, zero momentum with control to 0.01º using reaction wheels.

Table 1: Overview of LS-4 and LS-5 spacecraft parameters

Figure 1: Illustration of the Landsat-4 and 5 spacecraft

Launch: Landsat-4 was launched on a Delta 3925 vehicle (Thor Delta) from VAFB (Vandenberg Air Force Base), CA on July 16, 1982.

Launch: Landsat-5 was launched on a Delta 3925 vehicle from VAFB (Vandenberg Air Force Base), CA on March 1, 1984.

Orbit: Landsat-4 and -5 were placed into sun-synchronous near-circular orbits with an altitude of 705 km, inclination of 98.2º, repeat cycle of 16 days. The local equatorial crossing time is between 9:30 and 10:00 hours on a descending node.
Note: This differed from the orbital altitude of LS-1 to LS-3 (about 910 km). However, the instrument swath width of MSS (heritage instrument) remained the same as before by increasing the FOV (Field of View) of the sensors from 11.56º to 14.92º.

Figure 2: Alternative view of the LS-4 and LS-5 spacecraft (image credit: NASA)

While NASA was in charge of spacecraft and instrument development as well as the launch of the LS-4 and LS-5 satellites, management of the spacecraft operations was transferred from NASA to NOAA with Landsat 4. On October 1, 1982, NOAA assumed responsibility for Landsat data production and archiving activities at the Department of Interior's EROS Data Center. On January 31, 1983, NOAA also took over the MSS operation and maintenance of the Landsat spacecraft and ground system resources from NASA. The RBV operation was under NOAA as of Oct. 1, 1984.

Operational status of missions:

Landsat-4: Overall the mission lasted from mid-1982 until mid-2001 when the spacecraft was decommissioned.

• Landsat 4 began experiencing numerous spacecraft malfunctions which limited spacecraft functionality in its early mission life. The first of these occurred on July 27, 1982 when the high gain Ku-band antenna was commanded to deploy but failed to do so. Many attempts were subsequently made to drive the antenna free, and success was finally achieved on August 15.

• On October 29, 1982, the redundant central unit in the spacecraft CCU (Command and Control Unit) failed - the primary unit on Sept. 22, 1982, and the redundant on February 15, 1983. Intermittent power loss began in one of the four cables of the solar array on March 18, 1983. Since June 1983, the S/C had been operated at reduced power levels. This situation prompted the early launch of Landsat-5 to guarantee continued coverage.
Note: Within a year after launch, Landsat-4 lost the use of two of its solar panels and both of its direct downlink transmitters. So, the downlink of data was not possible until the TDRS (Tracking and Data Relay Satellite) system became operational: Landsat-4 could then transmit data to TDRS using its Ku-band transmitter and TDRS could then relay that information to its ground stations. ²⁾

• In 1987, after the Landsat-5 Ku-band transmitter failed and Landsat-5 was no longer able to downlink international data to the U.S. via the TDRSS relay, Landsat,4 used its functional Ku-band transmitter to downlink the acquired international data via the TDRSS. This continued until 1993, when this last remaining science data downlink capability failed on Landsat-4.

• Landsat-4 was kept in orbit for standby and for housekeeping telemetry command and tracking data (which it downlinked via a separate data path, the S-band) until it was decommissioned on June 1, 2001. Subsequently, Landsat-4 was placed into a disposal orbit to reduce collision risks with other resident space objects.

Landsat-5 status: Landsat-5 is still "operational" as of 2008 (LS-5 will complete its 24th year of operational service life in March 2008!). LS-5 is being used to collect data over the United States and for selected international ground stations. The expected end-of-life of the LS-5 mission, based on fuel reserves, is projected for 2010. ³⁾

• On Oct. 6, 2007, the Landsat 5 Flight Operations Team (FOT) noted that battery #2 was automatically taken off-line the previous evening. The satellite continues to operate on battery #3, but is not collecting imagery. In this configuration, battery #3 appears to be operating normally and maintaining an adequate charge to operate the mission to required health and safety standards. Also, the FOT has uplinked a new command load that ensures health and safety of the spacecraft operating with only one battery. It is expected that the spacecraft can operate indefinitely in this configuration (barring any further complications). ⁴⁾

• In 1987, the LS-5 TDRS transmitter (Ku-band) failed. This failure made downlinking data acquired outside of the US data acquisition circle (i.e., range of US ground receiving antennas) impossible; Landsat-5 has no on-board data recorder to record acquired data for later downlink. ⁵⁾

• On Nov. 26, 2005, the back-up solar array drive on LS-5 began exhibiting "unusual behavior" according to USGS (as a precaution, imaging operations were suspended until the problem was identified and potential solutions were evaluated and tested). By the end of January 2006, Landsat-5 was back in operation. USGS and NASA engineers were able to make adjustments to operating procedures for the solar array drive mechanism that now allows the solar array to provide enough power for the mission to resume normal operations.
The primary solar array drive of LS-5 had failed under similar circumstances in January 2005 - and the secondary drive was engaged. It is feasible that after 22 years of operations, the various lubricants are depleted or breaking down. Other mechanical issues such as the clutch slipping and wear on the teeth of the harmonic drive could also cause the array to stop functioning properly. ⁶⁾

• The LS-5 TM is operating "nominally", although it has experienced degradation over the years, particularly in the scan mirror bumpers and the internal calibration lamps. The bumpers are worn to the point that synchronization cannot be maintained between the internal shutter and the scan mirror in the primary scan mirror operating mode. The backup "bumper mode" was initiated in 2002. Each of the three internal calibration lamps has degraded differently over the years, such that the original calibration scheme for the lamps no longer works effectively. ^{7) 8)}

• USGS is reporting in January 2006, that Landsat-5 is back in operation. USGS and NASA engineers were able to make adjustments to operating procedures for the solar array drive mechanism that now allows the solar array to provide enough power for the mission to resume normal operations. ⁹⁾

• With Landsat-4's 11 year and Landsat-5's 20+ year data record, there is a need to understand the historical behavior of the instruments in order to verify the scientific integrity of the archive and processed products. Performance indicators of the Landsat-4 and -5 thermal bands have recently been extracted from a processing system database allowing for a more complete study of thermal band characteristics and calibration than was previously possible. ¹⁰⁾

Sensor complement: (TM, MSS, GPSPAC)

TM (Thematic Mapper):

TM is a whiskbroom instrument designed and built by SBRC (Santa Barbara Research Center) of Hughes Aircraft Company in Goleta, CA; PI: John L. Barker. TM is being regarded as a second generation imager for monitoring Earth's resources with considerably improved spectral and spatial resolutions over those of the MSS instrument. TM is being flown on LS-4 and on LS-5.

TM is a multispectral mechanically scanning optical imager operating in the visible and infrared regions of the EMS (Electromagnetic Spectrum). The instrument consists of the following elements or subsystems:

• SMA (Scan Mirror Assembly)

• Telescope

• SLC (Scan Line Corrector)

• Primary focal plane detector array

• Relay optics

• Cooled focal plane detector array

• Internal calibrator.

The telescope is of the type Ritchey-Chretien with 40.6 cm in diameter \[primary mirror clear aperture diameter = 41.15 cm, secondary mirror baffle diameter = 15.7 cm, telescope clear aperture = 1056 cm², effective focal length = 243.8 cm (f/6)\].

The scan line corrector compensates for the forward motion of the S/C, allowing the scan mirror to produce usable data (parallel scans) in both scan directions. The bidirectional scan and the use of detector arrays for each spectral band provides scan efficiency. Scan period = 7 Hz cross-track; FOV = ±7.2º (swath width = 185 km); IFOV = 42.5 mrad; overall instrument size of 2.0 m x 1.1 m x 0.7 m with a mass = 258 kg; power = 385 W (peak); quantization = 8 bit. Design life = 2 years (with a goal 3 years). ¹¹⁾

Whiskbroom bi-directional scanning method: The TM detector array features a total of 96 parallel line arrays (16 each for the seven spectral bands), oriented in the along-track direction. This new arrangement technology provides a parallel coverage of 480 m along-track in one scan sweep (cross-track direction). The wide along-track coverage permits sufficient integration time for all cells in each scan sweep.

Figure 3: Illustration of the Thematic Mapper (image credit: NASA)

Instrument inflight calibration is done at the start and end of each scan by using solar and lamp-based approaches (three lamps) for the solar reflective bands. Onboard blackbodies are used to calibrate the thermal bands. TM instruments are operational since 1982 (first flight on LS-4). ^{12) 13) 14)}

Figure 4: Functional block diagram of the TM instrument (image credit: NASA)

Background: The Thematic Mapper got its name from the intended "thematic" applications of its data. Its images were used to produce maps tailored to different Earth-observation themes, such as agriculture, hydrology, geology, etc. In addition, the TM bands were chosen on the basis of a comprehensive analysis of spectral reflection features for a variety of vegetation types and surface minerals. Spectral classification accuracy was a key determinant for selecting the specific band edges and bandwidths. In this context, band number 7 (2.08 - 2.35 µm) was added to the TM specifications much later in the design process then the other bands. It was an extra for the geology community, therefore it required contract modifications.

Applications: Landsat TM data can be used in fields such as: global change research, agriculture, forestry, geology, resources management, geography, water quality, and oceanography.

▪ Band 1: Coastal water mapping, soil/vegetation differentiation, deciduous/coniferous differentiation, chlorophyll absorption

▪ Band 2: Green reflectance, peak of healthy vegetation, plant vigor

▪ Band 3: Chlorophyll absorption, plant type discrimination

▪ Band 4: Biomass surveys, water body delineation

▪ Band 5: Vegetation moisture measurement, snow/cloud differentiation

▪ Band 6: Plant heat stress, thermal mapping, soil mapping

▪ Band 7: Hydrothermal mapping, geology

Table 2: TM parameter definition (LS-4/5)

TM data image size: 185 x 172 km; 5760 lines x 6928 pixels. Transmission: frequency = 8215.5 MHz (X-band); data rate = 84.9 Mbit/s (246 MByte per scene). There is no capability for onboard recording of TM data. Data transmission from the S/C to the ground is in realtime via a network of licensed X-band stations. An alternate data transmission link is via TDRS (Ku-band) to GSFC (Note: LS missions with TDRS support do not provide an onboard recorder).

Figure 5: Configuration of the Thematic Mapper (image credit: NASA)

TM calibration: The TM thermal calibration system consists of a single onboard cavity blackbody and a black, highly emissive shutter. As the shutter sweeps onto the optical axis, a toroidal mirror on the shutter reflects the radiation from the blackbody onto the optics and through to the cooled focal place (Figure 6). The non-mirror part of the shutter is coated with a high emissivity paint and is not temperature controlled. Thermistors are located on the shutter and blackbody and their outputs are included in the downlinked housekeeping data. The shutter oscillates in synchronization with the scan mirror and crosses the optical axis at the end of each scan. ¹⁵⁾

Figure 6: Optical layout of the TM instrument (image credit: NASA)

Legend of Figure 6: the calibration wand, or shutter, swings across the optical path, allowing the focal plane to see both the cold target (ambient black shutter) and the hot target (reflected energy from the off-axis blackbody).

MSS (Multispectral Scanner):

MSS PI: V. V. Salomonson. See instrument description under LS-1 to LS-3. However, with a new and considerably advanced TM sensor on the same platform, the MSS instrument was able to assume only a secondary observation role on the LS-4 and LS-5 satellite missions. For technical reasons, the MSS instrument was turned off on LS-5 in August 1995.

Table 3: Some MSS parameters

Comparing TM data with MSS data resulted in the following conclusions: ¹⁶⁾

• TM images cover a wider range of applications than Landsat MSS images, due to more spectral bands and improved spatial resolution

• MSS images are better for large area analyses (geologic mapping)

• More specific mapping (detailed land cover) is difficult on MSS because many pixels are "mixed" pixels

• TM's decreased IFOV produces less mixed pixels

• Incorporation of mid-IR bands (5 & 7) has increased the vegetation discrimination of TM data.

• Due to the line scanning system (one-dimensional relief displacement), Landsat images can be viewed in stereo only in areas of sidelap on adjacent orbit passes.

Figure 7: Cutaway view of the multispectral scanner (image credit: NASA)

GPSPAC (GPS receiver and processor Package):

The DCS systems of LS-1 to LS-3 missions were replaced by experimental GPS receiver systems (DoD program) on LS-4 and on LS-5. GPSPAC on LS-4 represents in fact the first spaceborne GPS receiver onboard operation in history (a satellite-to-satellite navigation system). The GPSPAC on LS-4 was operated in two phases. The first phase (approximately 90 days) was an experimental one to validate and calibrate the position and timing information provided by GPSPAC. The second phase called for operational use of the GPS data by LS-4. Note: GPSPAC malfunctioned soon after launch of LS-4. - Three more GPSPAC units were successfully launched: one on LS-5 in March 1984 and two on DoD host vehicle satellites in 1983 and 1984. ^{17) 18) 19) 20)}

GPSPAC (designed at JHU/APL, NASA as co-sponsor, built by Magnavox) is an integrated GPS receiver/processor assembly (R/PA), whose tracking data is used for spacecraft time system synchronization and for post-event (on-ground) orbit determination.

GPSPAC on LS-4 has the distinction to be the first spaceborne GPS receiver in history. Despite the sparse GPS satellite constellation in orbit (in the early mission phase of LS-4 and -5), GPSPAC demonstrated navigational accuracy of better than 50 m over 10- to 30-minute arcs on 88% of the revolutions. Since those early days, many GPS receivers have been installed on all types of satellites. ²¹⁾

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