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LANDSAT 7 - MISSION STATUS



June 7:

From the USGS Landsat page:

The Landsat 7 instrument and spacecraft continue to perform well. The Landsat 7 Team is nearing completion of the on-orbit checkout phase, and we are looking forward to preparing for operational status.

Landsat 5 Underflight

A Landsat 7 underflight of Landsat 5 was successfully completed during the period of June 1-3. The Landsat 7 Program Management Team wishes to extend our gratitude to Space Imaging for their support of the Landsat 5 Underfly Project, and their efforts in developing the draft "statement of cooperation" with the USGS and NASA to support the Landsat 5/7 Underflight Project. This project was designed to collect Landsat 5 and Landsat 7 data simultaneously to cross-calibrate the two Landsat systems. Several ground station operators (Australia, Argentina, Brazil, Canada, ESA, South Africa, and Saudi Arabia) participated by collecting Landsat 5 data. In addition, Spacing Imaging collected Landsat 5 data of Canada, USA, and Mexico. These data will be sent to the EROS Data Center for processing and analysis. The NASA Landsat 7 Project Science Office has responsibility for analysis of these data to characterize cross-calibration between ETM+ and the Landsat 5 Thematic Mapper. Results of these analyses will be distributed as they become available.

International Cooperation

The pace of international ground system development has accelerated with the successful launch of Landsat 7. Six international cooperators (Australia, Argentina, European Space Agency, Germany, Japan, and Thailand) requested and received a Landsat 7 Level 0 subinterval test data set. In addition, several International Cooperators have established an interface with the Mission Operations Center (MOC) and requested and received Landsat 7 test downlinks. The European Space Agency successfully received data at Kiruna and Fucino, Canada successfully received data at Gatineau and Prince Albert, and Brazil and Australia successfully received data. China has requested a test downlink in the near future.

On-Orbit Initialization and Verification

As might be expected following launch and integration of a complex satellite, instrument, and ground system, we have encountered technical difficulties and problems during this On-orbit Initialization and Verification (OIV) period, and these problems are being analyzed and appropriate system modifications have been initiated and/or completed. We again express our thanks to the NASA and USGS engineering team and their associated contract support (AlliedSignal, Raytheon, Computer Sciences Corporation, SSAI, Lockheed Martin Missiles and Space, Scientific Atlanta; Datron) for their technical competence as we work through this busy period of on-orbit checkout.

Near-Term Plans

Nominal Orbit Positioning

By June 28, Landsat 7 will be positioned in its nominal operational orbit (an altitude of 705 kilometers). This final maneuvering process will place Landsat 7 in an orbit consistent with and eight paths to the east of Landsat 5. This will result in a Landsat 7 and Landsat 5 overflight of the same location eight days apart during routine operations. The OIV plan calls for six additional orbital maneuvers during the last two weeks of June to position Landsat 7 in this operational orbit by June 28, and only minimal acquisitions will be received during this period.

Operational Readiness

As stated in previous updates, Landsat 7 will not begin to collect data for the global archive, nor will we be able to proceed with subsequent production operations, until this final orbit adjustment sequence is completed. All data acquired prior to that time will be considered "engineering" data. We will continue to select scenes, sub-scenes, or swaths acquired during this period for evaluation and/or media or other non-scientific interests, but these data will not be offered as "standard" products for general distribution. A prior update indicated that we planned to make an "engineering" data set available for distribution by mid-May, however, we became convinced that release of such a data set would be counter-productive given the near-term availability of a more accurate "final" test data set. We expect to make this test data set available within a few days after reaching operational orbit.

If activities proceed as planned, Landsat 7 data products should be available for sale and distribution on or about July 15, 1999. This operational date has been adjusted slightly to allow the USGS to operate the acquisition/processing systems for one full 16-day data acquisition cycle prior to beginning production operations. On-orbit operations will be scheduled according to parameters defined by the Long Term Acquisition Plan (LTAP) to assure relevant coverage of all major land masses of the world. Detailed information about the LTAP is available on request, and it is also described at the following web site: http://ltpwww.gsfc.nasa.gov/IAS/htmls/igs.html.

Current Data Characteristics

Data Anomalies Affecting Landsat 7 During the OIV Period

The following characteristics have been noted during the on orbit initialization and verification operations. These anomalies will disappear once the spacecraft achieves its final orbit, or as a result of system modifications prior to production operations. These include:

  1. Non-Standard Spacecraft Orbit: The Landsat 7 spacecraft is not in its operational orbit. This has two important implications:
    1. The images are not centered on a standard WRS-2 path. Software that attempts to frame standard WRS scenes will compute large horizontal display shifts between the data and the closest nominal WRS path.
    2. The scan-to-scan underlap (gap) is larger than normal because the target (Earth) is closer to the spacecraft than normal. The scan line corrector (SLC) is designed for optimal operation at the final spacecraft altitude. While the spacecraft is lower and faster than normal the SLC angular field of view under-compensates for spacecraft motion during the active scan time.
  1. Spacecraft Clock Offset: The spacecraft clock is not yet within its operational accuracy specification. Although the absolute time is likely to be within the specified 145 millisecond accuracy, the clock correction parameters which allow users to correct the time to within 15 milliseconds were not available in the payload correction data until approximately 30 days post-launch.
  2. Prelaunch Sensor and Spacecraft Characterization: The current Calibration Parameter File is based on prelaunch test and analysis results and should be used with caution. The characterization of on-orbit performance is ongoing and will provide better values for instrument characteristics such as lamp radiance, detector gain, coherent noise, band position, detector delay, and sensor to spacecraft alignment, among others. A post-launch calibration parameter file containing improved parameter values will be issued prior to the end of the on-orbit initialization and verification period.
  3. Fill Bytes in L0Rp SLO and CAL Files: The Level 0R product (Level 0Rp) includes several Hierarchical Data Format (HDF) external files which contain multiple HDF elements (scientific data sets or VData tabular data). These include the scan line offset (SLO) VData files and the internal calibrator scientific data set (SDS) files. There are two of each of these files in a L0R product (one per format), each containing the corresponding VData or SDS elements for all bands in that format. The at-launch version of the DAAC subsetting software inserts 20 fill bytes between each VData in the two SLO files and between each SDS in the two CAL files. This is transparent to users who access the data through the HDF software interface but must be accounted for by users who access the data directly. The presence of the extra bytes will be apparent in the SLO and CAL file sizes.

Permanent Characteristics of Landsat 7 Data

These anomalies are permanent characteristics of the data and will be documented appropriately:

  1. Inertial Measurement Unit Data Sign Convention: The sign convention for the IMU (gyro) data samples in the payload correction data is the opposite of that described in the Landsat 7 System Data Format Control Book (DFCB) Volume IV – Wideband Data, Revision K. It should also be noted that the IMU axes are not the same as the attitude control system axes. A matrix which relates the IMU coordinate system to the ACS coordinate system is provided in the Calibration Parameter File.
  2. Gyro Drift Values: The payload correction data gyro drift samples are actually provided in two’s complement form rather than sign magnitude form as indicated by the Landsat 7 System Data Format Control Book (DFCB) Volume IV – Wideband Data, Revision K. The gyro drift values are also provided in units of radians per second rather than radians per 512 milliseconds as stated in the DFCB.
  3. Early Calibration Shutter Obscuration: Due to a slight increase in the ETM+ scan mirror turnaround time, the calibration shutter obscuration interval begins before the active scan is complete for reverse scans. This has the effect of partially obscuring image data for approximately 40 minor frames at the end of each reverse scan. The effect is most noticeable for band 8. This anomaly is a permanent characteristic of the data and is expected to grow somewhat with time as the ETM+ bumpers wear.
April 27:

The Landsat 7 mission began after a spectacularly successful launch on April 15, 1999. Since then, in addition to the normal post-launch checkout activity, a great deal of effort has been devoted to extracting and publicizing a few scenes for media and public relations purposes. The Landsat 7 Team is now returning to the critical task of completing the on-orbit independent verification plan required to acclimate the instrument to its space environment

The immediate action is to complete the "out-gassing" process, a key phase that allows the instrument to achieve its nominal operating condition in space. That process was initiated on Friday, April 16 but has been interrupted once or twice daily to obtain imagery. Now, the process will continue uninterrupted for approximately one week. During this time, the ETM+ instrument will not be activated.

Additional on-orbit activities include preparing for the opportunity to underfly Landsat 5. To successfully execute this activity requires meticulous planning to position the Landsat 7 spacecraft in proximity to the Landsat 5 spacecraft using orbit adjust maneuvers to raise the orbit and slow down the drift. Once this proximity has been achieved, the two Landsats will collect selected observations simultaneously to allow cross-calibration, thus establishing a baseline for understanding the respective imaging characteristics of the two instruments. The Landsat 7 Team anticipates that it may take as long as 50 days after launch to complete this objective.

Upon completion of the Landsat 5 underfly maneuver, Landsat 7 will then be positioned in its nominal operational orbit (an altitude of 705 kilometers versus the current 670 kilometers), and at its nominal drift rate, using 6 more orbit adjust maneuvers. This final maneuvering process is required to ensure that Landsat 7 is positioned to collect observations consistent with the orbital path of Landsats 4 and 5.

It is important to note that until final orbital positioning is completed, Landsat 7 will not begin to collect data for the global archive, and subsequent production operations. All data acquired prior to that time will be considered "engineering" data. Selected scenes, subscenes, or swaths acquired during this period will be extracted and used for evaluation and perhaps in some cases for media or other non-scientific interests, but will not be offered as "standard" products for sale. Also, during this period, selected data will be extracted and provided to international cooperators and system developers for testing purposes, but with caveats pertaining to their use.

The Landsat 7 system (spacecraft, instrument, and ground system) is performing well and we are delighted with the quality of the early results, but we are not in production mode, and will not be until we have completed the on-orbit verification plan. If activities proceed as we have planned, Landsat 7 data products should be available for sale and distribution on or about July 1, 1999. The long-term acquisition plan assures repetitive coverage of all major land masses of the world. Therefore, the ground data system has not been configured to accept individual acquisition requests.

April 22:
USGS and NASA releases on Landsat:

April 21:
Landsat 7 is doing, "amazingly well," according to Landsat 7 Project Scientist Dr. Darrel Williams. The first image from the system will be unveiled by NASA Administrator Daniel S. Goldin on Thursday, April 22, as part of NASA's Earth Day activities.

April 20:
(From the Landsat 7 Gateway:) "All systems are nominal and the spacecraft and instrument are being prepared for operational imaging. Test data have been collected and processed. Browse images of these data will be available for viewing on Earth Day, Thursday, April 22."

April 19:
First browse image from Landsat 7:

The data were acquired on Sunday, April 18, 1999 over the southeastern corner of South Dakota. This is a Landsat 7 "Browse" image, i.e, what users will be able to view on-line when searching the catalog and selecting acquisitions to order.

The image is Level 0R, sub-sampled every 8th pixel and every 8th line. Resolution is about 250 meters; bands 4, 3, and 2 (red, green, and blue).

April 16, 1:00pm PDT:
Landsat 7 had a near-perfect launch yesterday, April 15. Launch was .288 seconds after the opening of the launch window at 11:32am PDT. So far, everything looks good with the spacecraft. Today the IMU (Inertial Measurement Unit) will be enabled and precision attitude determination will be made. The Ground Stations and TDRS had a number of shake-out glitches, but at this time no serious problems are evident. The EROS Data Center has tracked Landsat 7 based on new ephemeris and contact schedule; satellite house-keeping data have been downloaded.

The Landsat 7 mission is a joint effort of NASA and the U.S. Geological Suuvey. The mission is now entering into a 75 to 90 day checkout period. If everything goes as planned, the first test image will be received by the EROS Data Center, Sioux Falls, South Dakota, on Sunday, April 18, at noon.


FOR MORE INFORMATION:

USGS Landsat 7 page NASA Landsat Gateway


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