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Modeling the potential swath coverage of nadir and off-nadir pointable remote sensing satellite-sensor systems.

Article Excerpt
Introduction

Determination of the potential ground area imaged by a satellite-sensor system on future revisit dates was once a somewhat simple endeavor. Early earth resources satellites (e.g., Landsats 1-5 and 7, or the Defense Meteorological Satellites) carried sensor systems that only imaged in a nadir pointing fashion. Also, their orbital characteristics were such that their ground tracks were repetitive (e.g., the track sequence was repeated every 16 days). The familiar path-row maps (either hardcopy or the online web form) for the Landsat series, for example, were appropriate for such a stable, repetitive collection sequence. Such systematic orbits and nadir-only imaging allowed for elegant conic or cylindrical map projection solutions showing the ground-projected satellite tracks (Snyder 1987). Evaluation of if and when an area could be imaged by a specific satellite sensor were almost trivial.

The pointable sensor systems onboard many earth resources satellites, particularly the higher spatial resolution sensors (e.g., Quickbird 2, Orbview-3), provide a near infinite set of collection opportunities. The satellite orbits of these systems are not systematic repetitive tracks. They do not need to be repetitive as the sensors can point off-nadir and thus, "revisit" geographic locations. Predicting future collection opportunities requires predicting where the satellite will be and then computing the potential swath coverage from a pointable sensor along its orbit. While each agency or company models its own satellite-sensor systems, few publicly available sources (e.g., web sites) exist for mapping the future satellite ground tracks. Evaluating collection opportunities from multiple satellite-sensor systems is problematic. Those web sites offer, for the most part, very small-scale maps of future ground tracks, making precise, future collection opportunity predictions difficult. The goal of this research was to develop a generic approach for modeling future satellite-sensor collection opportunities.

The problem studied here is the computation of a polygonal shape representing an area that may be imaged by a satellite with a pointable sensor on a single overpass. We refer to this as a potential swath coverage area in a 2-dimensional form and potential swath coverage in 1-dimensional form (figure 1). The relevant sensor angles are instantaneous field of view (IFOV), field of view (FOV), and the maximum off-nadir angle of the pointable sensor. The ground-projected distances for these angles are pixel size (for instantaneous field of view), swath width (for field of view), and potential swath coverage (for the maximum off-nadir angle of the pointable sensor). The astrodynamics and lidar communities use the term boresight angle to refer to the off-nadir angle for which the FOV is centered (Finite 1). The field of view is the angle that encloses the imaged area (e.g., analogous to the CCD array size or radar pulse width) (Jensen 2007). The instantaneous field of view is often reported, while the field of view is seldom reported but described in the ground-projected form as the swath width. The potential swath coverage, to our knowledge, is rarely derived or reported (Jensen 2007).

[FIGURE 1 OMITTED]

The parts of a complete solution to the problem presented here are:

1. Collect satellite-sensor viewing geometry possibilities (i.e., off-nadir and fore/aft angles);

2. Model the position (altitude, latitude, longitude, and heading) of satellite at times [t.sub.1] through [t.sub.2];

3. Determine potential visible swath below satellite position at times [t.sub.1] through [t.sub.2];

4. Integrate all potential visible swaths below multiple satellite positions along a path to form a potential swath coverage area; and

5. Map the potential swath coverage area with appropriate cartographic symbology (or analyze swath coverage polygon in a GIS context).

In this article, we develop 1) formulae for computing the potential swath coverage based on the field of view and satellite characteristics, and 2) an algorithm for constructing the potential swath coverage area based on the satellite position, heading, and potential swath coverage. Thus, we provide solutions for parts 3) and 4) above. This problem differs somewhat from the familiar image navigation problem where automatically registering the image is the goal (Salamonowicz 1986; Emery...

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