[ntp:questions] Re: First Galileo satellite launched

David L. Mills mills at udel.edu
Wed Dec 28 22:32:50 UTC 2005


Hal,

The original GPS claim with Selective Availability (SA) was to the same 
order of accuracy as LORAN-C, about one microsecond. Some folks figured 
out how to improve that using sophisticated algorithms. With LORAN-C 
assist, my $16,000 Austron receiver with SA switched on generally shaved 
the time within 50 ns. My Arbiter GPS receiver with SA seitched off is 
generally within 50 ns. But, this is jitter, not absolute accuracy, as I 
have not measured and corrected for the antenna cable delay, about 1.5 
ns per foot.

Light travels about one foot per nanosecond. Crudely put, translate the 
above numbers to feet, then to meters and you get something near the 
figure you quote. I was thinking in GDoP (sic) of feet and forgot the 
meter transformation.

GLONASS and P-code capabilities are available for some high-end geodetic 
receivers. Neither one of us can afford to buy them. By the way, GLONASS 
  uses a different geodetic than GSS WG84. Their web site says a 
parameter conversion is not available. I assume Galileo will use WG84. I 
see GLONASS uses FDMA, while GPS and presumably Galileo uses CDMA. I 
assume that would make multichannel receivers for GLONASS hugely expensive.

Dave

Hal Murray wrote:
>>I'm not a defender of any political agenda here, but I would like to 
>>correct the general assumption, as evident in the interviews last night, 
>>that the Galileo system is more accurate than the GPS system. The GPS L1 
>>signal available to the general public has a nominal accuracy (PDoP) of 
>>30 meters; however, with the L2 signal and P code with capable receiver, 
>>it has a nominal PDoP of one meter, comparable to Galileo. The P code is 
>>available only to the USA military at present. I would assume once 
>>Galileo reaches operational status there is no need to keep the P code 
>>secret.
> 
> 
> Does position accuracy translate directly into timing accuracy?
> 
> Is 30 meters still the right number for GPS?  I thought they turned
> off the scrambling a while ago and L1 only is now good for 10 meters.
> 
> 
> 
>>Folks might forget the Russian GLONASS system, which is very similar to 
>>GPS and Galileo, has been around for over twenty years. It is not clear 
>>how long it will last or whether it can be maintained to the order of 
>>GPS and Galileo.
> 
> 
> Are any GLONASS receivers available at a modest price?
> 




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