Design and Maintenance of Low-Earth Repeat-Ground-Track Successive-Coverage Orbits

R EPEAT-GROUNDTRACKorbits allow a satellite to reobserve any particular spot within the predefined period of time and have been employed in a number of Earth-observation and Earth-science missions, such as LANDSAT, SPOT, ENVISAT, RADARSAT and JASON [1]. Such orbits have, of course, clear military and intelligence applications as well. In the design and maintenance of a repeat-groundtrack orbit, effects from a nonspherical Earth and atmospheric drag need to be taken into account, as these factors are the primary causes of the groundtrack drift of low-Earth orbits. Inmissions that require precise groundtracks, the second term of the zonal geopotential harmonics has proven to be insufficient [1]. With such stringent requirements, higher orders of the geopotential harmonics have to be included in the orbit analysis and control. This was done by Vincent [2], and further investigation regarding orbit evolution caused by tesseral harmonics was presented by Ely and Howell [3]. Their work is useful when considering orbit propagation and has been employed in subsequent studies involving orbit design and maintenance. An example using repeat-groundtrack orbits is found in the work of Aorpimai and Palmer [1], in which a geopotential with an arbitrary number of terms in the spherical harmonics was included. The design and maintenance of a low eccentricity orbit for target access, based on the Gaussian equations, was considered by Sengupta et al. [4]. In all these studies, the groundtrack drift at equator crossing (GDEC) is used to evaluate the performance of the orbit maintenance strategy. However, GDEC does not describe the extent of drift at middle and high latitudes. Consequently, orbit maintenance strategies based on GDEC cannot ensure a high level of accuracy over the entire groundtrack, whichmay be crucial in somemissions, such as JASON [5] and ENVISAT [6]. Responsive orbits are those intended to meet the needs of responsive missions. They have the potential to provide means for communications and high-resolution surveillance anywhere in the world within hours of an identified demand [7]. In visual or radar observation missions, responsive orbits can be used to rapidly acquire information of given ground targets and are always lowaltitude and circular to achieve responsiveness and high ground resolution. A low-Earth successive-coverage orbit, a subtype of responsive orbits, can provide coverage of a given ground target for several successive orbits every day [7,8] and is suitable for Earthobservation and Earth-science missions. If a low-Earth successivecoverage orbit could repeat its groundtrack, it would allow for specific observations to be scheduled under exactly the same sensing conditions on a routine basis [9]. In this paper, a strategy for design andmaintenance of a low-Earth repeat-groundtrack successive-coverage orbits will be presented to achieve high-precision groundtracks. This orbit maintenance strategy is explained based on an analysis of drift over the entire groundtrack (not just GDEC); It not only can keep theGDECbelow a given threshold, but it also ensures the accuracy of the entire groundtrack.