[time-nuts] Do GPS augmentation services improve 1pps?

[jimlux]

On 4/15/19 4:45 AM, Bob kb8tq wrote:
> Hi
> 
> I have never seen any manufacturer recommend SBAS for improved timing. The
> simple answer seems to be that the reference system does not have time sources
> in it and that the software that generates the corrections does not consider time to
> be important.
> 
>


Indeed - the signals for WAAS, SBAS, etc are just bent piped through a 
transponder in GEO. They are formatted in a way that GPS receivers can 
easily receive them and decode the messages but I don't think there's 
any particular claim to the usability of any observables. (although they 
do broadcast a NAV message, so maybe you can.?)

I suppose that, as Bob points out, that it would improve the position 
estimate of your receiver, and there might be ionosphere corrections 
that a receiver might use.  I don't know enough about the *details* of 
the WAAS messages to know.


The ESA navipedia says (in part)

Wide Area Differential (WAD): differential corrections to the existing 
GPS and GEO navigation services computed in a wide area to improve 
navigation services performance.

Satellite orbit and clock corrections to the existing satellite 
navigation services (GPS and GEO), as well as the estimation of errors 
associated to satellites (UDRE).
Ionospheric corrections for a given grid of points, as well as the 
estimation of errors associated to ionosphere (GIVE).
Tropospheric corrections. Satellite orbit/clock corrections and 
ionospheric corrections are dynamically modelled. The SBAS shall 
communicate the user the corrections that are available to be used by 
the receiver. The information of the models is packed on messages to be 
sent to the user. On the other hand, tropospheric corrections are 
statically modelled, which means that corrections are tabulated and the 
information does not depend on any external behaviour but the user 
position (a mean troposphere is assumed). The algorithm for computing 
the tropospheric correction is available to the global community 
(section A.4.2.4 of MOPS).[7]


https://gssc.esa.int/navipedia/index.php/The_EGNOS_SBAS_Message_Format_Explained


It looks like the information in the WAAS message might be usable to 
improve the timing performance (in particular, the ionospheric 
correction grid)

They do provide a nav message so you could use the SBAS satellite in GEO 
in your position solution, which implies it should have good timing - 
that's an interesting feat, since I'm pretty sure the signal is 
generated on the ground, radiated up to the satellite, transformed to a 
different frequency, then radiated down to the user.  I suppose all 
those delays are measurable and predictable, so it can work.


> 
> Bob
> 
>> On Apr 14, 2019, at 9:55 PM, Skip Withrow <skip.withrow at gmail.com> wrote:
>>
>> I'm wondering if GPS augmentation services improve a receiver's 1pps
>> output?  My specific interest is with the Novatel OEM6 series of
>> receivers (but this issue is applicable to other receiver types as
>> well).  With this receiver an external 10MHz oscillator can be steered
>> by the GPS receiver, so there is no sawtooth correction.  But there
>> are also capabilities to enable WASS corrections, and even the
>> TerraStar PPP positioning service that will give centimeter
>> positioning accuracy (with subscription $$$).  The question is, are
>> these corrections just applied to the position solution or are they
>> incorporated into the 1pps solution  somehow as well?
>>
>> If someone has some enlightenment it would be appreciated.
>>
>> Thanks in advance,
>> Skip Withrow
>>
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