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Re: [gpsd-dev] gpsd for time sync on Ubuntu remix too hard for normals -
From: |
Greg Troxel |
Subject: |
Re: [gpsd-dev] gpsd for time sync on Ubuntu remix too hard for normals - how to improve? |
Date: |
Tue, 07 May 2019 08:09:30 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/26.1 (berkeley-unix) |
Hal Murray <address@hidden> writes:
>> I am seeing well within 10 ms with a cheap recent USB dongle, after
>> calibrating the offset.
To be fair, I have not been rigorous. But, I calibrated offset, and
left a machine with just the GPS, no network, for two days. "jitter" as
reported by NTP was low, when checked on 5ish times a time, less than 5
ms.
I then plugged it to the net after the 2 days, and observed the offsets
obtained as it acquired net peers. They were in a clump from maybe -6
to +1, and the VFAN/gpsd offset was in the middle of that.
> What brand/model of device?
VFAN Model BG-803.
Attaches (NetBSD) as:
umodem0 at uhub10 port 3 configuration 1 interface 0
umodem0: u-blox AG - www.u-blox.com (0x1546) u-blox GNSS receiver (0x1a8),
rev 1.10/3.01, addr 8, iclass 2/2
umodem0: data interface 1, has CM over data, has no break
umodem0: status change notification available
ucom0 at umodem0
ubxtool -p VER returns:
UBX-MON-VER:
swVersion: ROM CORE 3.01 (107888)
hwVersion: 00080000
extension: FWVER=SPG 3.01
extension: PROTVER=18.00
extension: GPS;GLO;GAL;BDS
extension: SBAS;IMES;QZSS
marketing description says it has "UBX-M8030".
Here's a line where I didn't have the offset dialed in:
+SHM(0) .GPS. 5 l 1 16 377 0.000 13.707 0.703
But you can see low dispersion for 8 continuous samples.
I get it that it could be low dispersion locally in time and then have
mode changes when something crosses over a boundary and really be
drifting slowly with a 100 ms sawtooth or worse. But so far I have not
observed that.
Given the speed/latency of USB, with care by the chipset manufacturer to
emit messages at the same time relative to when PPS would have been,
several-ms accuracy should be possible.
[gpsd-dev] Best way to avoid systemd woes for NTP?, Greg Troxel, 2019/05/05
- Re: [gpsd-dev] Best way to avoid systemd woes for NTP?, Michael J. Tubby B.Sc. MIET, 2019/05/06
- Re: [gpsd-dev] Best way to avoid systemd woes for NTP?, Gary E. Miller, 2019/05/06
- Re: [gpsd-dev] Best way to avoid systemd woes for NTP?, Greg Troxel, 2019/05/06
- Re: [gpsd-dev] Best way to avoid systemd woes for NTP?, Gary E. Miller, 2019/05/06
- Re: [gpsd-dev] Best way to avoid systemd woes for NTP?, Greg Troxel, 2019/05/06