FreeFlyer is a flexible script-based Mission Design tool. It was first used in flight operations to successfully plan the eight ascent burns required to boost the Landsat-7 spacecraft to its desired mission orbit. FreeFlyer’s scripting capability helped in the automation of this procedure thus allowing the work to be done quickly.

FreeFlyer has been and still is being used by the Landsat-7 project to plan routine “ground track” maneuvers. Landsat-7 has a requirement that its ground-track (projection onto the Earth Surface) follow a desired path dictated by the so-called World Reference System (WRS). The WRS divides the Earth surface into 233 equally spaced longitudinal bands. This WRS requirement further states that when Landsat-7 passes the equator on its descending node, that the ground track must be within +/- 5 km of the center of the appropriate band. Thus, Landsat-7 is required to stay within a +/- 5 km “Error Box.” Because of the affects of atmospheric drag on Landsat-7, the orbit will naturally decay thus decreasing the orbital period, and causing Landsat-7 to drift towards the Eastern edge of its Error Box. An orbit-raising maneuver is needed to prevent the Eastward drift from exceeding the 5 km maximum. The FreeFlyer software is used to estimate when this maneuver is needed. In addition, the FreeFlyer software is used to determine the proper size of this maneuver. If the maneuver is too large, then the ground-track may drift too far Westward and thus leave its Error Box. This is depicted in the Figure below.

Finally, FreeFlyer has been used by Landsat-7 project to plan all of the inclination maneuvers. In addition to Landsat-7, the FreeFlyer software has also been used successfully for many other missions including Terra, EO-1, and AQUA.

3D View of Path differences between Aqua and Terra in FreeFlyer

2D View of Path differences between Aqua and Terra in FreeFlyer