NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: GPS and the Chandler Wobble
From: Peter Monta
Date: 2018 Nov 7, 13:31 -0800
From: Peter Monta
Date: 2018 Nov 7, 13:31 -0800
Hi Paul,
> [ GPS CNAV ]
> I don't know how many satellites broadcast the new data, or to what extent consumer
> grade devices apply the corrections.
CNAV is not yet fully operational, but a subset of the CNAV messages has been carried by about half of the constellation for a while now in a pre-operational mode. The full support for CNAV is part of OCX, the control-segment software that has had some serious budget and schedule problems, so much so that there's a contingency plan to retrofit the current software to support GPS III (and presumably CNAV and L1C's CNAV-2 as part of that).
The NAVCEN web page says this about CNAV:
> CNAV Message Types 10, 11, 30 and 33 are currently transmitted on seven GPS IIR-M (L2C) & eight GPS IIF satellites (L2C & L5).
Message type 32 for Earth orientation is missing from this list, so maybe it is not transmitted yet. Even if Earth orientation were transmitted, end-user devices would be wary of using it for anything for fear of confusion; it would not be part of the navigation computation. The only application I can think of might be telescope pointing (initial pointing anyway). Even there, you need a really good telescope mount to be interested in sub-arcsecond refinements to initial pointing, and this hypothetical user on Mauna Kea would already have access to IERS data directly.
A better CNAV is just for autonomous GPS, though, which is increasingly rare in the market. Most devices are "assisted" in one way or another, in that they get data from non-satellite side channels (usually Internet sources) for better orbits and clocks and ionosphere data. The format of the assistance data is of course totally unconstrained by the GPS standards, and it is usable by the device much, much quicker (fraction of a second rather than 30 seconds). And precision users are doing RTK or PPP, so the broadcast data is even more irrelevant for them.
Cheers,
Peter
> [ GPS CNAV ]
> I don't know how many satellites broadcast the new data, or to what extent consumer
> grade devices apply the corrections.
CNAV is not yet fully operational, but a subset of the CNAV messages has been carried by about half of the constellation for a while now in a pre-operational mode. The full support for CNAV is part of OCX, the control-segment software that has had some serious budget and schedule problems, so much so that there's a contingency plan to retrofit the current software to support GPS III (and presumably CNAV and L1C's CNAV-2 as part of that).
The NAVCEN web page says this about CNAV:
> CNAV Message Types 10, 11, 30 and 33 are currently transmitted on seven GPS IIR-M (L2C) & eight GPS IIF satellites (L2C & L5).
Message type 32 for Earth orientation is missing from this list, so maybe it is not transmitted yet. Even if Earth orientation were transmitted, end-user devices would be wary of using it for anything for fear of confusion; it would not be part of the navigation computation. The only application I can think of might be telescope pointing (initial pointing anyway). Even there, you need a really good telescope mount to be interested in sub-arcsecond refinements to initial pointing, and this hypothetical user on Mauna Kea would already have access to IERS data directly.
A better CNAV is just for autonomous GPS, though, which is increasingly rare in the market. Most devices are "assisted" in one way or another, in that they get data from non-satellite side channels (usually Internet sources) for better orbits and clocks and ionosphere data. The format of the assistance data is of course totally unconstrained by the GPS standards, and it is usable by the device much, much quicker (fraction of a second rather than 30 seconds). And precision users are doing RTK or PPP, so the broadcast data is even more irrelevant for them.
Cheers,
Peter
