The JPL CNEOS Scout service provides predictions for current NEOCP objects independently of the Minor Planet Center (MPC).
There area few important differences between the two services:
- The Scout service takes better account of planetary perturbations than the MPC NEOCP service, so for close-approach NEOs, or suspected artificial satellites the Scout prediction may often be closer to reality than the NEOCP.
- The MPC's NEOCP has the concept of a nominal orbit for an object, Scout does not.
- The NEOCP calculates a nominal orbit, generally the orbit producing the smallest residuals when compared to the underlying astrometric positions it was derived from. A number of variant orbits (often several thousand) are also calculated using slightly different assumptions, giving larger residuals than the nominal solution overall, but still within the probable errors of the underlying astrometric positions. The variant orbits define a cloud of possible positions (an uncertainty area) the object may be found in, scattered around the nominal position.
- Scout does not have the concept of a single nominal (best) orbit for a NEOCP object. Instead it calculates a number of orbits (maximum 1,000) for the object using slightly differing assumptions and again producing a set of predictions defining a cloud of possible positions the object is likely to be found within. However, the most likely position predicted by Scout is the median position of all the calculated set of orbits. If only 10 orbits are requested, the median of those 10 is unlikely to be a good prediction of where the object will be in reality. Requesting 1,000 orbits will generally result in the median position being a much better prediction, at the expense of taking more time to compute.
When the Retrieve Scout ephemeris button is pressed, the Ephemeris Interpolator requests 1,000 orbits be calculated by Scout and returns an ephemeris derived from the median solution. This process generally takes 5 - 10 seconds to generate and during that time the button is replaced by a red "stop" button . If the process needs to be cancelled before it completes, press the red button.
The Scout concept of calculating the median from a number of predictions for the most likely position to find an object works well in most instances but can very occasionally result in discontinuities in an ephemeris. This can occur when a significant fraction of the positions used in the median calculation cross the 0h / 24h border in RA from one ephemeris line to the next, causing the median value to jump.
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