[time-nuts] Some results of PRS10 and Trimble Resolution

Magnus Danielson cfmd at bredband.net
Fri Jun 30 16:52:09 EDT 2006

From: bg at lysator.liu.se
Subject: Re: [time-nuts] Some results of PRS10 and Trimble Resolution
Date: Fri, 30 Jun 2006 22:10:42 +0200 (CEST)
Message-ID: <65357. at webmail.lysator.liu.se>

> On Fri, June 30, 2006 20:49, Brooke Clarke said:
> > Hi Björn:
> >
> > I don't think that's that case.  I live in a small town and hired a
> > local surveyor chosen because he's the guy that uses GPS.
> Maybe I was wrong with extrapolating the local situation to a global level.
> With the present dense network of "CORS"-like stations [1]. Surveyors are
> moving away from running their own RTK-base stations. With close to full
> coverage RTK-corrections is distributed via the cellphone network, which
> means the surveyor only need the rover side of the traditional equipment.
> > antenna.  To do this he needed to go back to his office and post process
> > the data.
> That makes his GPS recievers cheaper, than the ones where the software
> options for RTK is enabled. He also does not need a real time radio
> connection. Precision is about the same or better, since he can download
> better ephemeris, and can tinker more with parameters in the software.
> > It's my understanding the the military has the crypto key needed to use
> > the L2 channel to full effect and that's not available to surveyors.
> That is true in theory. In reality Ashtech (and others) early found that
> the crypto code rate is much lower then the chipping rate. Details was
> posted here in the last 6 months?

I might have been the guilty party for that one. I think I posted something on
that here.

> IIRC the known P-code will flip sign every 511 bits depending on the crypto
> code. This makes the highend receivers do much more than intended on L2.

The P-code is being encrypted into Y-code by XOR-ing it with a W-code (also
known as A-code in some papers) which has a chipping rate of 511 kchips/s.

There is several different strategies around this encryption. One of the first
was by squaring the received signal and lock onto that. The W-code will cancel
and P-code tracking is possible. It has however several limits. The Z-tracking
algorithm used by Aztech geodesic receivers is a bit more advanced. By doing
the C/A to P code handover on L1 (where C/A and P correlate closely) a rough
W-code estimade is made by mixing up the received L1 channel with the locally
procduced P-code. The L2 P-channel is then decoded by the W-code estimate and
P-code tracking is acheived by using a delayed variant of the P-code. The time
difference between L1 and L2 will contain the delay-difference due to mainly
ionospheric delay and with that you are enabled to come up with a more correct
model of the delays than normal C/A code ever gives you, and quite similar to
that of what the coded receivers have. What you don't get is the fancy direct
breakin into Y-code which the modern munition receiver do for instance.

The codeless L1&L2 receivers does not provide a much higher threat to the US
military than they anticipate from C/A receivers, so to the best of my
knowledge no real issue is being made of it. They still depend on the C/A code
to operate over time and there are means to deal with that situation anyway,
which the US military isn't particular afraid of applying.

L1&L2 receivers not only have added baseband complexity, it goes from the top
down with antenna etc. If you are a bit fancy, you do GLONASS too. Now we
should start considering L4 and soon GALILEO. Only a few bothers with L3, which
occassionally transmitt C/A, but for quite dedicated uses (erhm!).

> > What surveyors do is record the carrier phase of both the L1 and L2
> > signals and resolve the ambiguity in post processing.  There are a
> > couple of flavors of post processing.
> And there is real time flavors as well, with the best going down below 5cm
> errors.


> > I think what you may be referring to are the differential correction
> > methods, either Low Frequency or satellite broadcasts that improve the
> > position accuracy of GPS receivers.  But I don't think these can be used
> > to improve timing accuracy.
> These are the SBAS (WAAS, EGNOS, MSAT), commercial satellite based, "coast
> guard" DGPS, etc.  Where the good do sub 1m and SBAS a little worse.
> Why would these not make absolute accuracy better? They do improve
> positioning.

Doesn't care, it's summertime so we don't know what time it really is! :P

> [1]   http://swepos.lmv.lm.se/natverksrtk/nat_postj_031012.gif

Discussion has been going on to spread that as a freer service, but I don't
know where it ended.


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