[ntp:questions] Re: First Galileo satellite launched

Bjorn Gabrielsson bg at lysator.liu.se
Thu Dec 29 00:03:32 UTC 2005


Dave, 

A few comments.

The Precise Positioning Service (PPS) is protected by two means. SA -
Select Availability, that we all know is not used any more. AS - Anti
Spoofing, which enables the Y-code, that is an encrypted P-code. Only
keyed military receivers can generate the Y-code for its correlators,
and everyone else is thus denied the 10.23MHz ranging codes on L1 and
L2. Most high end surveying receivers are using clever tricks to use
as much of the encrypted signals as possible, so the denial is not
complete. 

The generation of P-code is described on page 20 in the GPS interface
specification. [1].

The performance for the Standard Positioning Service (SPS) - the L1
C/A signal, is still defined in [2] as,

        100 meters (2 drms) horizontal 156 meters (2 Sigma) vertical
        300 meters (99.99% prob.) horizontal 340 nanoseconds time (95%
        prob.)  

Which is _very_ conservative and the current constellation is
exceeding the spec by a very large margin.

I think the PPS is specified to something like 15 meter rms (3d)
position error. 

Actual performance of the system after the SA removal is presented in
[3]. Interesting data around page A-36. Showing "actual" horizontal performance
in the area 8-20m for SPS and 3-5m for PPS. Note, that often performance
figures are given for Signal in Space, neglecting receiver noise,
ionosphere, troposphere and multipath errors.

I do not think a standalone dual freq Galileo receiver give much
better positions than a similar miltary GPS receiver (or a GPS L1/L5
(or L2C) receiver). To get 1m reliably you probably need some kind of
differential service.

Dilution of precision (DOP) is a measure of the geometry of the
visible satellites. DOP is calculated from the inverse of the normal
equation of the (position&time) solution.

              T  -1
        Q = (A A)

Where Q is a 4x4 matrix. And the diagonal elements are q_xx,q_yy, q_zz
and q_tt. Elements are expressed in the cartesian ECEF system and must
be transformed to get horisontal and vertial DOP values.

 GDOP - geometric dilution of precision is sqrt(q_xx+q_yy+q_zz+q_tt)
 PDOP - position DOP is sqrt(q_xx+q_yy+q_zz)
 TDOP - time DOP is sqrt(q_tt)

To get a measure of your position (or time) error you will have to get
an estimate of your range measurement errors to the satellites. This
estimate is then multiplied with your DOP-factor for a user position error
estimate. 

GLONASS reached a low point a few years ago, with only some 5+
satellites working, now that is up to 11,12,13? and rising. But a
nominal constellation with 24 satellites is not expected until around
2012. 

The next test satellite (Giove-B?), I think, is supposed to also carry
a H-maser besides the rubidiums. GPS satellites are using Cesiums and
Rubidiums but not H-masers.

[1]  http://www.navcen.uscg.gov/gps/geninfo/IS-GPS-200D.pdf
[2]  http://www.navcen.uscg.gov/faq/gpsfaq.htm#Standard
[3]  http://www.navcen.uscg.gov/gps/geninfo/2001SPSPerformanceStandardFINAL.pdf 

--

   Björn

"David L. Mills" <mills at udel.edu> writes:

> Danny & Co.,
> 
> 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.
> 
> 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.
> 
> Dave
> 
> Danny Mayer wrote:
> > Giove-A has been successfully launched and is a precursor to the full
> > Galileo satellite navigation system. Its payload include two rubidium
> > atomic clocks.
> > http://news.bbc.co.uk/2/hi/science/nature/4555298.stm
> > Danny
> > _______________________________________________
> > questions mailing list
> > questions at lists.ntp.isc.org
> > https://lists.ntp.isc.org/mailman/listinfo/questions
> >




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