NavList:

Alexandre,

I was not specifically aware of any INS's/IRS's presence on-board GPS satellites.

100 %True ! if not updated their positions will quickly go anywhere, and especially their altitudes will start diverging extremely fast and go ballistic.

Nonetheless, it does not look totally unrealistic because you can use IRS's in a smart way through regularly refreshing their positions.

Such updates are performed through Kalman filtering and such procedures are often dubbed "hybrid navigation" in which you always need to maintain altitudes in a very narrow corridor.

These are very common systems now found on-board most if not all Long-Haul Aircraft (with 3 IRS's).

So, I would not be overly surprised to hear that there are IRS's / INS's actually on board GPS satellites. They are probably required to smooth positions out. Another reason for this is that their actual positions are not - and cannot be - continually monitored / radar refreshed.

So, mulling over this subject, the presence of some "inertial" systems on board GPS satellites does make sense. You do need some kind of instantaneously and immediateley available canned/DR position. And,

nonetheless I will listen to more competent members here.

We never stop learning, don't we ?

Regards,

Kermit