3.2.3.2. Accuracy, maneuvers, and operator data
Most orbital data, including TLEs, have no accuracy or uncertainty information. Some orbital solution data formats allow provision of uncertainties in the form of a time series of position-velocity 6 x 6 covariance matrices.
Satellite operators regularly perform maneuvers to maintain or change their orbits (e.g, ESA performs weekly manoeuvres — “burns” — for its Earth observation satellites; and SpaceX adjusts the orbits of its Starlink satellites frequently for orbit raising and slot relocation). These maneuvers cannot be predicted by external observers and, of course, make existing orbit predictions obsolete. There is now a mechanism by which operators can inform 18SPCS of planned trajectories and maneuvers and allow 18SPCS to make these predictions available to other operators (but not to the public). In addition to incorporating planned burns, operator orbital predictions are often based on on-board GNSS receivers, are more accurate than passive tracking by surveillance systems, and may include covariance data.
GP data are given in ASCII TLE 80-characters-per-line format inherited from punch card days; they are also available in JSON and other representations. Ephemeris data formats include: CCSDS Orbit Ephemeris Message (OEM), NASA ITC, and ILRS CPF. CPF is used by the satellite laser ranging community who often prefer the ITRF frame. More details of the formats are given in the appendix.
3.2.4. PassPredict simple mode: detailed requirements for inputs
- Observatory location. For a ground based observatory, the station location shall be provided either in a geodetic frame using latitude, longitude and height (e.g., in degrees and meters) or in a geocentric Earth-fixed cartesian frame (ITRF) using x,y,z coordinates. For space-based telescopes, an ephemeris is required. For compatibility with the groundbased case, this should also be in geodetic or ITRF frame versus time. Alternatively we may want to support doing all the calculations in inertial TEME or ICRS frames. The exposure times of space observatories are typically very long compared to the time it takes for the observatory position to change significantly.
- Observation schedule.
- The centre pointing of the telescope during exposure shall be defined with azimuth and elevation or topocentric ICRS (or TEME) right ascension and declination.
- The field of view shall be defined using a radius in degrees in the case of a circular shape, horizontal and vertical dimensions for a rectangular shape or using a polygon to allow arbitrary shapes
- The pixel size shall be given in arcseconds.
3.2.5. PassPredict: algorithm
We can search for passes using a brute force grid search with fixed time steps, or by using a root search algorithm (Oltrogge, Kelso & Seago, 2011). Batch or individual requests could be generated by existing tools which support observation requests, but they would need updating to provide an interface.
