[time-nuts] GPS position survey
lists at lazygranch.com
lists at lazygranch.com
Mon May 6 00:15:56 EDT 2013
Isn't the other half of the question how useful the information is to the OS? That is how is the time integrated with the application using it. Even if the RTCs had perfect sync, would two apps attempting to read the clock get the same time value in a multitasking system?
I've been looking for the stat on the London stock exchange, which runs on Suse Enterprise. I recall they claimed 100us time stamp accuracy, but can't find a source.
I thought PTP would be more accurate than NTP, but the consensus of the hive is they are equally good.
If you follow ADS-B/mode-s aircraft tracking, a number of vendors are using MLAT techniques to detect the aircraft location in the case of mode-s, and to detect spoofing in the case of ADS-B. Some use a transmitter that all the sites can receive, that is they make their own time sync scheme. But others are using GPSDO. But I assume they time stamp the aircraft signal reception in their own hardware.
From: Chris Albertson <albertson.chris at gmail.com>
Sender: time-nuts-bounces at febo.com
Date: Sun, 5 May 2013 20:40:41
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
<time-nuts at febo.com>
Subject: Re: [time-nuts] GPS position survey
So this is different from the Thunderbolt, even if they both use the
same serial protocol or can the t-bolt also have it's flash rom
programmed from a PC?
The bottle neck in the system in the uncertainty in the interrupt
latency on the PC where NTP runs. After all this I doubt you can
captures the PPS to better than 1 uS.
For a long time I've been wnting to build an external counter for NTP.
But it would have to use some very fast logic family.
On Sun, May 5, 2013 at 6:26 PM, Miguel Barbosa Gonçalves <m at mbg.pt> wrote:
> Hi Chris!
> On 06/05/2013, at 01:21, Chris Albertson <albertson.chris at gmail.com> wrote:
>> If you are talking about using this with NTP. I don't know if you
>> have a choice. The Trimble receiver is going to do whatever it is
>> going to do when you power it up. Software running on a PC can
>> perform a 24 hour survey and report the location but the "Type 29"
>> driver in NTP has no way to tell the Trimble receiver what to do on
>> power up. And there is no way to specify a location you have surveyed
>> by some other means.
> What I am doing at the moment is connecting Trimble's port A (timing
> port) to NTP and configuring the receiver using a Windows laptop
> through port B. I can connect to port B occasionally to check the
> receivers health. The receiver has its configuration stored in an
> EPROM so it will just work anytime unless the configuration gets
> After the survey ends the position can be optionally stored to the
> EPROM. Also, if the antenna changes its position more than 1000 meters
> a new self-survey will run automatically.
>> Perhaps the Trimble GPS has some way to program it's FLASH Rom with
>> changed parameters but NTP only reads the packets. It does not send
>> any. Can LH download a surveyed location r change the length of the
>> survey? NTP can't do any of that
> It does. It is available on the Trmble's FTP site.
>> Other NTP drivers such as the Type 30 Motorola driver are more
>> flexible. Those alow you to specify a lat, long that was surveyed or
>> have the receiver do a survey. The type-30 Motorola driver is a lot
>> more configurable.
> The Trimble software is very nice and allows one thing the Motorola
> NTP driver doesn't allow if I remember correctly. On the Trimble I can
> choose the number of fixes for the survey and can watch it run while
> NTP is receiving time. I can't do this on an Oncore because it only
> has one port.
>> But as was said, light travels about 30cm/nanosecond and NTP works in
>> microseconds. So your location can be "off" by 1000 times 30cm before
>> NTP will care. So 3 or 4 meters of location error will not matter and
>> the 2,000 point self survey will be good enough.
> When surveying accuracy of the PPS will be around 1 us. The accuracy
> will obviously increase when the survey ends.
>> But if yo really so want nanosecond level timing, then you need to
>> care about the survey
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