HJ-1B

Mean elements of every TLE set over the past 365 days, plotted against the epoch of the set. Each source is its own line; click it in the legend to hide it. A jump in altitude is a manoeuvre or a bad set; lines drifting apart are a disagreement between sources. “TLE age” is how many hours have passed since the epoch of the source's freshest set, plotted against when the set reached us, on a logarithmic scale. A set the source repeats unchanged does not reset the age.

Each TLE set is propagated with SGP4 to the epoch of the next set from the same source and compared with it, plotted against the epoch of the next set; the tooltip shows how many hours ahead the prediction ran. The logarithmic scale shows the size of the divergence and the tooltip its sign: plus along-track means the prediction is ahead of the satellite. This is how well the sets agree, not the true error: the reference is a TLE too, off by about a kilometre. An outlier is a manoeuvre between sets. Sets more than a week apart are not compared.