Page:SATCON2 Algorithms Report.pdf/21

 * for efficiency our database should avoid requesting fresh orbital data for a satellite which has now re-entered, so it needs to keep track of such events.
 * The orbital solutions for each satellite.
 * A curated list of active satellites and their current status (in orbit, orbit raising, operational, failed, etc.) is desirable. Such lists are currently maintained by unfunded enthusiasts but there is no sustainable project to support this in the long term. One can find the list of all satellites in orbit by filtering the space-track catalog; at a minimum we should provide software to do this and cache it on a weekly basis, rather than running a space-track query every time PassPredict is run. Additional information on the status of each satellite will be needed for assessments of the overall impact of the constellations and such a list should be supported and maintained by the astronomical community. in addition to the usual catalog-numbered satellites, some numbers are reserved for so-called ‘analyst satellites’. These are objects not identified with a specific launch and can be thought of as analogous to unnumbered minor planets. The analyst numbers are arbitrarily re-used. The megaconstellation satellites we are mostly concerned with will probably not be in the analyst list, so it is not urgent to consider them.

3.2.3.1. Orbital Solutions

The position of a satellite at a future time is predicted by an algorithm called a “propagator”, using an orbital solution at some given past epoch. For accurate predictions the epoch used should be only a few days old, at most a week, especially in low orbits (since drag effects and atmospheric density are not predictable on long timescales). The propagator that you need depends on the model used to generate the orbital solution. There are two main sources of orbital solutions:


 * Operator orbit solutions for their own active satellites determined by active tracking of the satellite radio signal or derived from onboard GPS receivers
 * Passive (radar or optical) tracking of satellites, including inactive satellites and debris. This is systematically done by US Space Force 18th Space Control Sqn (18SPCS) and the Russian Space Forces’ SKKP, and is now also being done by commercial companies LeoLabs and ExoAnalytics, and to some extent by ESA. For brighter satellites optical and radio-transmission tracking is also done by hobbyists.

The orbital solutions used are typically one of several types: Rh
 * GP (General Perturbations) mean elements. These are time-averaged Keplerian elements using a model called SGP4 which takes a simple drag model and some other perturbations into account. GP elements used to be provided in TLE format but are now also available in JSON and other formats.
 * SP (Special Perturbation) state vectors — the state vector (position plus velocity) at an epoch in a particular frame. These state vectors are not directly observed but are derived from orbit fits using high-fidelity force models.
 * SP ephemerides — sets of predicted state vectors vs time which can be interpolated using Lagrange or Hermite polynomials.