From: [EMAIL PROTECTED] Subject: Re: [time-nuts] Some results of PRS10 and Trimble Resolution Date: Fri, 30 Jun 2006 22:10:42 +0200 (CEST) Message-ID: <[EMAIL PROTECTED]>
> 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. Indeed. > > 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. Cheers, Magnus _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
