[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: gpsprof: higher resolution

From: John Ackermann N8UR
Subject: Re: gpsprof: higher resolution
Date: Sun, 16 Aug 2020 11:54:20 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0

On 8/16/20 11:29 AM, Greg Troxel wrote:

John Ackermann N8UR <> writes:

I just did a test of a u-blox M8P with very high quality RTK (data
from same antenna) and high precision (seven decimal place) NMEA
output. Running 12 hours of results through gpsprof yields CEP of a
few millimeters; very cool!

Do you mean you used a splitter, and some other antenna to make an RTK
feed, via ntrip, and then received that feed with gpsd and squirted it
into the receiver, and the M8P did the RTK processing?   If so, posting
the recipe would probably be interesting to many.

Yes, I've been doing a vast series of tests on seven u-blox modules (8 and 9 series). All are fed from a Trimble Zephyr Geodetic L1/L2 antenna via several splitters.

The main focus is on timing performance, but I thought I'd do positioning while I was at it. So as a first step I collected 12 hours of autonomous data and did gpsprof plots of each receiver. (I would have preferred 24 or 48 hours of data, but the deadline for my paper is tomorrow. :-) )

On the same antenna is an old Trimble NetRS geodetic L1/L2 receiver which spits out RTCM v3 messages on its serial port. Ultimately, I want to set up a "real" ntrip caster on a Linux box, but for quick-and-dirty I am feeding the RTCM into an instance of the "Snip" server on Windows. Then the M8P or F9P is connected to u-center running on the same Windows machine and the u-center ntrip client is turned on pointing to localhost:2101. I'm logging the data via u-center, then grep'ing the GGA sentences out and feeding them into gpsfake/gpsprof.

This is *very* ugly but it was a quick and dirty way to do the test. When I'm finished I'll wash my mouth out and start over again to build a proper reference station setup.

It sounds like you are logging NMEA, vs ublox binary via gpsd.  True?  I
am curious why (not challenging; expecting to learn something).

As mentioned, this is a rush job and using NMEA seemed simpler given that I didn't know when I started the collection runs just how I would process the data. Several of the receivers have the "high precision" NMEA enable option that provides seven digits of resolution on the minute value, rather than four, so I don't think it's giving up anything to use NMEA in this instance.

I am also curious if you have compared M8P internal RTK to rtklib.

No, but I did do a PPP measurement using NRCan and that gave a much more reasonable seeming result for a single-freq receiver of 36mm lat, 41mm lon, 65mm alt.

That's why I do wonder if the RTK results are real. But since base and rover are on the same antenna, you probably couldn't get better correction data and maybe mm precision is possible in that case. I wouldn't expect it in the real world, though.


PS -- all this data will be in a paper I'm publishing in the proceedings of the upcoming ARRL/TAPR Digital Communications Conference. The conference is being held virtually on Sep. 11 and 12, and I'll be doing a presentation there. Info at:

reply via email to

[Prev in Thread] Current Thread [Next in Thread]