Landsat Chapter 5

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 10:00 AM on each pass. Each orbit takes nearly 99 minutes, and 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.
Figure 5.1 The Landsat Orbit

Landsat 7 and EOS-AM1 are both scheduled for launch and injection 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 EOS-AM1). 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.

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, 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.

Table 5.1 Image Sidelap of Adjacent Swaths

Latitude (degrees)	Image sidelap (%)
0	7.3
10	8.7
20	12.9
30	19.7
40	29.0
50	40.4
60	53.6
70	68.3
80	83.9

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 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 5.4 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.

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:

In order to create and periodically update the global archive, a Long Term Acquisition Plan (LTAP) has been developed for the Landsat 7 mission data collection. To maximize the quality of the data collected, the LTP specifies the frequency of acquisition for each WRS scene by filtering out scenes with inadequate lighting for the time of year and assigning climatology-based maximum allowable cloud cover for each scene by time of year. ETM+ scheduling algorithms are built on a database of scene parameters from the Long Term Plan, each adjustable. They include:

Under operational conditions, the LTP process will be fine-tuned based on data collection results and data quality metrics. The scheduling software will get feedback on the performance of the predicted cloud cover versus the calculated cloud cover for each successfully acquired scene and the LTP will be adjusted accordingly. The emphasis in the LTP is on optimizing acquisition performance, and continuously improving the quality and usefulness of Landsat 7 data archive contents.