| The orbit of Landsat 7 is repetitive, circular, Sun-synchronous, and near polar at a nominal altitude of 705 km (438 miles) at the Equator. The spacecraft crosses the Equator from north to south on a descending orbital node from between 10:00 AM and 10:15 AM on each pass. Circling the Earth at 7.5 km/sec, each orbit takes nearly 99 minutes. The spacecraft completes just over 14 orbits per day, covering the entire Earth between 81 degrees north and south latitude every 16 days. Figure 5.1 illustrates Landsat's orbit characteristics.
Landsat 7 and Terra were launched and injected into identical 705 kilometer, sun-synchronous orbits in 1999. This same day orbit configuration will space the satellites ideally 15 minutes apart (i.e. equatorial crossing times of 10:00 to 10:15 AM for Landsat 7 and 10:30 for Terra). A multispectral data set having both high (30 meter) and medium to coarse (250 to 1000 meter) spatial resolution will thus be acquired on a global basis repetitively and under nearly identical atmospheric and plant physiological conditions.
|5.2 Swathing Pattern|
| Landsat 7 orbits the Earth in a preplanned ground track as illustrated in Figure 5.1. The ETM+ sensor onboard the spacecraft obtains data along the ground track at a fixed width or swath as depicted in Figure 5.2. The 16-day Earth coverage cycle for Landsat 7 is known as the swathing pattern of the satellite (Figure 5.3). As seen in this figure for daytime acquisitions, the adjacent swath to the west of a previous swath is traveled by Landsat 7 one week later (and the adjacent swath to the east occurred one week earlier and will recur nine days later). After familiarization with the data acquisition cycle or swathing pattern, it becomes quite straight forward to select Landsat 7 scenes or subintervals required for a specific project.
At the Equator, adjacent swaths overlap at the edges by 7.3 percent. Moving from the Equator toward either pole, this sidelap increases because the fixed 185 km swath width. Table 5.1 shows the amount of sidelap from 0 to 80 degrees latitude in 10 degree increments.
|5.3 The Worldwide Reference System|
The standard worldwide reference system as defined for Landsat 4 and 5 was preserved for Landsat 7. The WRS indexes orbits (paths) and scene centers (rows) into a global grid system (daytime and night time) comprising 233 paths by 248 rows.
The term row refers to the latitudinal center line across a frame of imagery along any given path. As the spacecraft moves along a path, the ETM+ scans the terrain below. During ground processing, the continuous data stream or subinterval is framed into individual scenes each 23.92 seconds of spacecraft to create 248 rows per complete orbit. The rows have been assigned in such a way the row 60 coincides with the Equator (descending node). Row one of each path starts at 80° 47' N and the numbering increases southward to latitude 81° 51' S (row 122). Then, beginning with row 123, the row numbers ascend northward, cross the Equator (row 184) and continue to latitude 81° 51' N (row 246). Row 248 is located at latitude 81° 22'N, whereupon the next path begins. The Landsat satellites are not placed in a true polar orbit but rather a near polar orbit which means the path/row numbers do not coincide with latitudes 90° north and south. Figure 5.6 graphic depicts the Landsat path/row schema.
Successive orbits and spacecraft attitude are controlled to assure minimal variation to either side from the intended ground track and framing of scene centers is controlled through LPS processing so that successive images of a specific scene or scenes can be registered for comparison purposes.
The WRScornerPoints.xls file lists the latitude and longitude for the scene center and four corners of each WRS scene. This includes the ascending rows. Positive latitude is North, negative latitude is South. Positive longitude is East, negative longitude is West. It is sorted in Path/Row order.
WRS path/row maps are available from EROS Data Center and have the LANDSATs 1-3 WRS on one side and the LANDSATs 4-5-7 WRS on the reverse. The map sheets are at 1:10 million scale and 26 are needed for global coverage. Contact EDC Customer Services to request WRS maps:
An Excel spreadsheet allows users to look up the calendar date on which a particular path will be followed by Landsat 7 (listed in blue) or by Landsat 5 (listed in red). It starts on May 27, 1999 and has been extended out to February 22, 2006. A second chart, with larger type, is also available in this file that contains the path flow/calendar dates in 3.5 month chunks. The latest revision number for these charts is 7, dated January 13, 2004. The charts are easily updated by the user to extend further in the future. (Format available: Excel 98).
|5.4 Long Term Acquisition Plan|
The Landsat-7 Long Term Acquisition Plan (LTAP) automates the selection of Landsat scenes to periodically refresh a global archive of sunlit, substantially cloud-free land images. By applying a set of algorithms on a daily basis, the LTAP is designed to ensure optimal collection of Landsat-7 Enhanced Thematic Mapper-Plus (ETM+) imagery for scientific applications, while minimizing the effects of cloud-cover and system constraints. This section provides a brief overview of the LTAP and details the specific algorithms and input files used for calculating scene acquisition priorities. Those wishing a more complete review of the LTAP, including the science justification for the approach and recent performance results, should consult Goward et al. (1999), Gasch and Campana (2000), and Arvidson et al. (2001).
5.4.2 LTAP Overview
The ETM+ does not acquire data continually. Instead, acquisitions are scheduled in advanced using a Long Term Acquisition Plan (LTAP) in conjunction with a software scheduler. The WRS-2 system divides the Earth into a grid of 57,784 scenes, with each scene centered on the intersection of a path (groundtrack parallel) and row (latitude parallel). The Landsat-7 observatory is operated such that it follows the WRS grid within tight tolerances, overflying the entire global WRS scene grid every 16 days. A database of just over 14,000 scenes containing "land" was compiled for the LTAP , including continental areas, shallow coastal waters, Antarctic sea-ice, and all known islands and reefs. Within any given 24-hour period, approximately 850 of these WRS land scenes (descending, sunlit paths only) are in view of the ETM+ and are candidates for acquisition. Mission resource limitations (discussed later) restrict the daily acquisition volume for the U.S. archive to 250 scenes. The mission scheduler must select the "best" 250 scenes for acquisition each day within these constraints. The scheduler automatically schedules acquisitions in accordance with the LTAP, basing these decisions on cloud-cover forecasts, urgency of acquisition, and availability of resources to optimize fulfillment of the Landsat mission goals.
The Landsat-7 LTAP includes several aspects that influence whether a particular scene should be acquired or not. These include:
- Seasonality of vegetation, niche-science communities
- Predicted vs. nominal cloud-cover
- Sun angle
- Missed opportunities for previous acquisitions
- Quality (cloud-cover) of previous acquisitions
- Scene clustering
- System constraints (duty cycle, ground station locations, recorder capacity, etc).
Factors 1-6 feed directly to calculating the priority of each potential acquisition, while the actual acquisition list is obtained by incorporating system constraints (factor 7). Factor 4 requires feedback between the list of archived scenes and the LTAP, while factor 5 requires feedback between cloud cover assessment of archived scenes and the LTAP. Each of these factors is discussed in the following sections.
5.4.3 Seasonality. An objective of the LTAP is to schedule acquisitions more frequently during periods of change, such as growth and senescence of vegetation, and less frequently during relatively stable periods, such as when full growth canopy exists or during winter quiescence. An 8-year AVHRR Normalized Difference Vegetation Index (NDVI) data set was used to determine where and when change was occurring (Goward, et al., 1994, 1999). To flag seasonal change within a WRS scene, a statistical test was applied to each NDVI 45-km sample mapped to that scene. Comparing each sample or pixel to the same pixel two months later revealed periods of significant change in the NDVI mean and standard deviation, when multiple acquisitions are meaningful, and periods of minimal change, when only a single acquisition is necessary.
Thus, the year is broken into a set of temporal "windows" for each path-row location. During each window, a location may be labeled as "acquire once", "acquire always", or "never acquire", and this information is stored in the LTAP seasonality file. It should be noted that "acquire always" does not imply that an actual acquisition will occur for every overpass during that time window. Rather, that scene will always be a candidate for acquisition, and other factors within the LTAP (cloud-cover, system constraints, etc) will govern whether or not an acquisition actually takes place. Conversely, "acquire once" means that once a successful acquisition occurs during the time window, that path-row location is no longer eligible for acquisition.
Niche Science Communities. Defining acquisitions solely through semi-monthly NDVI change does not fully capture the science and user interest in the Landsat mission. To supplement the NDVI-based seasonality metric, specific niche science communities have requested locations for more frequent acquisitions. These locations currently include:
- 282 agriculture (acquire every season if CC predict < 60%)
- 35 calibration (acquire "always")
- 896 reefs (acquire from 2x to 6x each year)
- 30 fire (acquire "always")
- 1392 land ice (acquire once during certain months)
- 3601 Antarctica (acquire once during Jan-Feb)
- 60 oceanic islands (acquire twice each year)
- 1175 rainforest (acquire "always" all year)
- 352 sea ice (acquire from 1x to 3x each year)
- 11 Siberia (acquire "always" over 9 months)
- 72 volcanoes (acquire from 2x to 12x during year, incl. night)
The exact makeup of these niche science acquisitions varies each quarter of operations, depending on user needs and science priorities. In general, approximately 27% of the 250 scenes/day acquired by Landsat-7 are devoted to satisfying "niche" requests, approximately 70% are devoted to satisfying routine acquisitions from the seasonality file, and 3% are devoted to night images or special high-priority acquisitions (natural disasters, national needs, etc). Requirements for night imaging and high-priority acquisitions are dealt with elsewhere in the data specification.
5.4.4 Predicted Cloud-cover. The cloud-avoidance strategy is based on the relative predicted cloud cover with respect to the seasonal average cloud cover for each scene. The LTAP scheduling software compares near-term predictions to the historical average cloud cover for that scene for that month, to determine acquisition priorities each day. Scenes with better-than-average cloud-cover predictions are given a priority boost, while those with poorer cloud cover predictions are given a lower priority. Global historical cloud data were obtained from ISCCP records (Rossow et al., 1996), that give monthly estimates of cloud-cover percentage for 2.5° grid cells from 1989 to 1993. Monthly averages over the 5 years were computed and mapped to the Landsat WRS grid to produce a cloud climatology (Figure 5). The NOAA National Centers for Environmental Prediction provide daily global forecasts of cloud-cover percentage at a 0.7° grid scale (Campana, 1994). The forecasts are compiled at 0600 UT each day and apply to each 3-hour interval up to 84 hours. Once received from NOAA, these cloud predictions are translated to the Landsat WRS coordinate system. The forecast nearest in time to a candidate acquisition is compared to the ISCCP climatology to determine whether the predicted observing conditions are better or worse than typical for that location. Acquisition priority is then adjusted upward to favor the better conditions or downward to avoid the worst conditions. Figure 5.7 illustrates the cloud-free nature of the first year ETM+ archive.
5.4.5 Sun Angle. Requests for high-latitude scenes are rejected during local winter when the solar elevation is below a threshold. This threshold was set at 5° during the initial years of the survey. The cutoff angle was changed to 15° for the northern hemisphere on July 24, 2002 because of duty cycle concerns and the fact that snow dominates scenes acquired at lower angles. The cutoff angle remains at 5° for the southern hemisphere.
5.4.6 Missed Opportunities. A request is granted a priority boost as a function of the number of consecutive past cycles in which the opportunity to acquire the requested scene was not fulfilled. This occurs when the scene was not scheduled for acquisition, or when the acquired image fails to meet minimum quality standards, such as cloud cover. For example, if the last successfully acquired image of a scene was 48 days ago, then a request for this scene is granted a priority boost based on two missed opportunities from 32 and 16 days ago. In addition, all new requests (ie. when an acquisition window opens) are given a priority boost.
5.4.7 Image Quality: Quality (cloud-cover) of the best or most recently acquired image of a path-row from past cycles factors into the relative demand for another acquisition. At high latitudes, the scheduler also considers the quality of the best image that could be spliced together as a mosaic of imagery of adjacent scenes acquired in the recent past. Quality is a function of cloud contamination in the image, as determined during image processing.
5.4.8 Clustering: Within the Landsat-7 LTAP, a higher priority is given to scenes that form "clusters" - contiguous groups of along-path acquisitions. This (i) reduces the on-off cycling of the ETM+ instrument and (ii) promotes the archiving of continuous swath data, which in turn allows "floating scene" subsetting of user-defined areas without regard for scene boundaries.
As of July 31, 2002 the number of scenes successfully archived was 252,208 (Figure 5.8). A more recent archive scene count (as of January 31, 2003) is presented in Figure 5.9. The lowest cloud cover scores for each WRS location are plotted in Figure 5.10.