Contents

EO-1 User's Guide

Orbit and Data Coverage

EO-1 Orbit

The EO-1 satellite follows a repetitive, circular, sun-synchronous, near-polar orbit with a nominal altitude of 705 km (438 miles) at the Equator. The spacecraft travels from north to south on the descending (daytime) orbital node, maintaining a mean equatorial crossing time between 10:00 AM and 10:15 AM for each daytime pass. The satellite circles the Earth at 7.5 km/sec, with an orbit inclination of 98.2 degrees and an orbital period of 98.9 minutes. Each orbit takes nearly 99 minutes, and the velocity of the EO-1 nadir point is 6.74 km/sec. EO-1 completes just over 14 orbits per day, with a repeat cycle of 16 days.

EO-1 follows the same orbit as Landsat 7 , trailing the latter by one minute (+/- five seconds). This orbit has been very useful for obtaining cross comparisons of instrument performance from the two satellites. Because EO-1 is much smaller and lighter than Landsat 7, periodic burns are required in order to maintain this distance, thus preventing EO-1 from overtaking Landsat 7.

Figure 1. Schematic diagram showing the EO-1 orbit. Note that this is nearly identical to that of Landsat 7.

Figure 2. EO-1 is flying in formation with Landsat 7, trailing the latter by approximately one minute. Pointable sensors onboard EO-1 allow off-nadir viewing capability outside the current (nadir) WRS path.

EO-1 Scene Size

The cross-track width of an ALI scene is 37 kilometers, and the cross-track width of a Hyperion scene is 7.7 kilometers. The along-track scene length for both ALI and Hyperion will generally be either 42 kilometers or 185 kilometers, depending upon the dimensions that are specified when the scene was scheduled. A standard ALI or Hyperion scene will cover 42 kilometers in the along-track direction, which represents approximately 6 seconds of image collection. If an extended scene is requested, the scene will cover 185 km in the along-track direction, representing approximately 25 seconds of image collection. Note: For some archived images, the scene length may differ from the two along-track dimensions described above.

Figure 3 shows the relative footprint area for a standard Landsat 7 scene, compared to a pair of standard and extended scene collects for ALI and Hyperion. Note that the total geographic coverage for any EO-1 scene will be significantly smaller than that of an equivalent Landsat 7 scene. The cross-track width of an ALI scene is approximately one-fifth the cross-track width of a standard Landsat 7 ETM+ scene, and the cross-track width of a Hyperion scene is less than one-tenth the width of a standard Landsat 7 scene. For any standard ALI or Hyperion collect, the along-track scene length is also significantly smaller than that of a standard Landsat 7 scene. An extended collect (185 km) will be slightly longer than one full Landsat 7 ETM+ scene (170 km).

Figure 3. Schematic drawing showing the approximate scene footprint size for EO-1 ALI and Hyperion standard and extended scene collects.

Note that all EO-1 scenes acquired simultaneously from both sensors (ALI and Hyperion) will be geometrically fixed relative to one another. Within any simultaneous collect, the Hyperion scene will always lie along the westernmost edge of the co-acquired ALI scene, with approximately 6 km of scene overlap. In addition to the area of overlap, the Hyperion scene will also extend approximately 1.7 km beyond the west extent of the co-collected ALI scene.

Cross-track Pointing Capability

The EO-1 orbit follows the World Reference System-2 (WRS-2) path/row grid, with a 16-day potential repeat cycle for scenes within the current flight path (nadir). However, the EO-1 instruments are also capable of cross-track pointing, thus allowing potential imaging of specified targets that fall outside the current WRS-2 (nadir) flight path. ALI and Hyperion scenes may be tasked within one full adjacent WRS path in each direction (west or east) from the current nadir flight path (Figure 4).

Figure 4. Example of potential ALI or Hyperion imaging opportunities for a target location within path 31.

The pointable sensors on board EO-1 will therefore allow three potential scheduling opportunities for any single target within every 16-day orbit cycle:

nadir (overhead) pass (once every 16 days)
west path pointing east (7 days after nadir pass)
east path pointing west (9 days after nadir pass)

The EO-1 Overpass Schedule provides a list of all potential WRS-2 path-row collects for any particular date. This schedule includes the current nadir path, as well as east- and west-pointing off-nadir potential collects for any given date and location.

EO-1 Look Angle

Almost any EO-1 scene will be acquired with a small non-zero look angle. This is because in most cases, the target point will not fall precisely along the nadir path for the sensors. True nadir acquisitions are very rare, occurring less than 1% of the time.

The look angle associated with a scene will depend upon the specific location of the image within the WRS-2 path, as well as the latitude of the scene. In general, the range of potential look angles will be lowest near the polar regions and increase with descending latitude. Table 1 shows the approximate range of potential look angles at specific latitudes.

Table 1. Range of potential look angles at specific latitudes.

Specific information or estimates on how geometric distortion may increase as a function of look angle are not available at this time. However, users should be aware that a larger look angle may have an adverse impact on the geometric characteristics of a particular scene. For this reason, we do not recommend off-nadir acquisitions from more than one adjacent neighboring path, particularly within low-latitude regions (i.e. near the equator).