From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mout.perfora.net (mout.perfora.net [74.208.4.195]) by huchra.bufferbloat.net (Postfix) with ESMTP id 9B9C92004D7 for ; Thu, 15 Mar 2012 06:20:45 -0700 (PDT) Received: from [192.168.83.5] (c-76-97-152-51.hsd1.ga.comcast.net [76.97.152.51]) by mrelay.perfora.net (node=mrus4) with ESMTP (Nemesis) id 0M1FVw-1SRkn50unA-00tcQP; Thu, 15 Mar 2012 09:20:44 -0400 Message-ID: <4F61ECAA.6010105@c3energy.com> Date: Thu, 15 Mar 2012 09:20:42 -0400 From: "Ron Frazier (NTP)" User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.9) Gecko/20100722 Eudora/3.0.4 MIME-Version: 1.0 CC: thumbgps-devel@lists.bufferbloat.net References: <20120315110540.CD6C040617@snark.thyrsus.com> In-Reply-To: <20120315110540.CD6C040617@snark.thyrsus.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Provags-ID: V02:K0:NousxuCnBHxx1o2sgN6M1t7rHYCdP3QI1bgbxlyhJZh PtQ/Zvz5yGqYQK8RN84OfgAn9ZeO15SfQfVOHCUzZu1SPQeTWz hsBPdt/bvxxIzzbK2TxkT3kye7ZW0lREziRJZyFwxMdshbp6id MFWoRkqxbEOGyxDSyVtTGzC/4oZ04KpW4JE1AXUIAgJ5FMQfaY GhDboFNzZzfB4DaJET3RyS6QAa1Y0ryYIwdgtZ3zhYWiUQA1x8 hRFYEmU+hu2/jTL1Qc5S7XZUv4oDab4v45SVmB+HeiVowFdAXm YO/2zMXnG6yt11J1mPH2Vj6V7ixF1MEI72QXrlhBwjzI14JU/t Sy1zHS2aQNZlxF0UW9iJ+MX/xNKqhzYTGKeGT/YuL Subject: [Thumbgps-devel] Effect of baud rate USB GPS / NMEA wandering effect / GPGGA X-BeenThere: thumbgps-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 15 Mar 2012 13:20:45 -0000 Hi guys, I thought I'd pass along some random info from my testing of my GlobalSat BU-353 USB only GPS. Some of this has been discussed on the NTP questions list, and I thought it might be useful to you. A) I have a nice example of (apparently) NMEA wandering here: http://dl.dropbox.com/u/9879631/drifting01-peerstats.20120312.jpg This graph shows my pc locked to the GPS, which is the jaggy dark line centered around zero vertically. The band of colored lines is a group of internet servers I'm watching. You can clearly see them wandering away from zero. Then, in the middle, my GPS has a heart attack, and the offset of the colored lines dramatically shifts. Then it starts wandering again. So, one of three things is happening: 1) all the internet servers are wandering 2) my gps is wandering 3) neither is happening, and this is a reporting error in the peerstats file #2 seems the most likely. B) The graph above was taken with the GPS set to 57600 baud. I don't really even know what that means in the context of a USB connection. I did that to reduce latency and perhaps jitter. However, it may also reduce the stability of the GPS. I've had two instances where my system clock suddenly jumped to about 52 seconds off. Cause unknown. I reduced the baud rate to 9600 as a test. Results are preliminary, but it looks like the unit may be more stable and may wander less. It didn't seem to affect the jitter much. Regardless of how the data gets through the USB, once the baud rate was reduced, I had to adjust my offset fudge factor by about 90 ms to get the reported GPS time to agree with the internet servers I'm monitoring. C) I believe I have recommended using the GPZDA sentence here before, and indeed, the manual for the Trimble Resolution T also recommends it for timekeeping. However, David Taylor and I got into a discussion on this on the NTP Questions list. He pointed out that the GPZDA doesn't have any validity check field where as GPGGA does. We looked in the C code for NTPD's refclock implementation, and found out that it does check said validity field. I don't know what it does with the data. In any case, I decided to revert my system back to using GPGGA. Theoretically, this should allow a more graceful recovery in case the GPS fails. In my experience, GPGGA does produce slightly more jitter than GPZDA. Sincerely, Ron -- (PS - If you email me and don't get a quick response, don't be concerned. I get about 300 emails per day from alternate energy mailing lists and such. I don't always see new messages very quickly. If you need a reply and have not heard from me in 1 - 2 weeks, send your message again.) Ron Frazier timekeepingdude AT c3energy.com