Beidou Satellites Misclassification

Hi everyone,

As noted previously I’ve gone all in on GNSS monitoring. GNSS is the generic term for GPS, Galileo, GLONASS and BeiDou satellites.

Eventually a big blog post will come out, but meanwhile you can see the results on

A few days ago, an unknown GLONASS satellite started transmitting signals on slot R26 and I tried to figure out which one it was. By definition, GNSS satellites broadcast their own very precise location & speed, which makes it possible to match up their orbit to published “TLEs”, which document where objects in space are.

After a suggestion from Octavian Andrei I manually tried if the mysterious R26 satellite matched the orbit of an experimental GLONASS-K satellite, COSMOS 2471.

But this was a lot of work so I then decided to incorporate the most excellent sgp4 library by Daniel Warner so ‘galmon’ could do automated lookups of GNSS orbits against the TLE database.

This worked well and all satellites were automatically and correctly matched to their TLE entries, except for four of them. The Chinese BeiDou satellites C21 & C22 are swapped, as are C25 & C26: Example image This is perfectly understandable, according to this Wikipedia list each of these pairs were launched as a couple. After launch, the various objects that then appear in space are assigned tracking codes, and frequently when two very similar things are launched, these get mixed up.

If we go to the official BeiDou page that describes the constellation we find that:

C21 is MEO03 (launched 2018-02-12)
C22 is MEO04 (launched 2018-02-12)

C25 is MEO11 (launched 2018-08-25)
C26 is MEO12 (launched 2018-08-25)

C27 is MEO07 (launched 2018-01-12)
C28 is MEO08 (launched 2018-01-12)

According to the US 18th Space Control Squadron, which manages these things, the situation is:

C21's orbit matches with "BEIDOU-3 M6" (launched 2018-07-29) 
C22's orbit matches with "BEIDOU-3 M5" (launched 2018-07-29) 

C25's orbit matches with "BEIDOU-3 M12" (launched 2018-08-24 23:52)
C26's orbit matches with "BEIDOU-3 M11" (launched 2018-08-24 23:52)

C27's orbit matches with "BEIDOU-3 M3" (launched 2018-01-11)
C28's orbit matches with "BEIDOU-3 M4" (launched 2018-01-11)

From this, it appears that M11 and M12 have simply been reversed, so that should be easy to fix once verified.