On 15-Dec-17 07:44, Mukund Sivaraman wrote: On Fri, Dec 15, 2017 at 12:45:11PM +0100, Petr Menšík wrote: >> Hi folks. >> >> I am looking for a way to validate name also on systems, where current >> time is not available or can be inaccurate. > I use a Garmin 18x LVC 1pps GPS receiver device connected to RS-232 > serial port. The device plus cables cost me $70 altogether, and ntpd > works natively with it using the NMEA refclock driver (there's no need > of gpsd). It has a 1s PPS signal accurate to 1us. It is accurate to > within +/- 100us on Fedora where due to no hardpps kernel support > because of tickless kernel, the PPS signal is timestamped and available > on /dev/pps0 but the kernel doesn't use it to directly maintain the > clock and it has to be done from userland which is affected by the > system load. If you were to recompile a kernel that's configured > appropriately, I feel the clock can be synchronized to about 1us > accuracy. > > It is more or less reliable and value for $70 if one wants UTC on their > computer without accessing the internet. This is more than sufficient > for DNSSEC validation and many other network services, and certainly > more accurate than using the ntp.org pools. > > Mukund > I use an 19xLVC too (On Raspbian == Debian). But I also have an RTC. GPS does have outages, can take a while to get a fix, and NTP wants consensus. So I use my GPS receiver as a local clock source (preferred), but also configure several servers from the pools as a sanity check - and to deal with any GPS outages/slow starts. It's worked well for me.
Along those lines, I haven't splurged yet, but Adafruit has an interesting module for ~$40 (US) with a breakout module, ($45 on a Pi Hat - which is cheaper/easier than building your own PCB), which includes a GPS patch antenna. If you need an external antenna, it comes up to about the cost of the Garmin, but draws only 20ma vs. 90, and is a more modern receiver.) On paper it looks good. See https://www.adafruit.com/?q=ultimate%20gps - I'm not affiliated with Adafruit, and while I've looked at the specs, don't have direct experience. YMMV. Enjoy. Timothe Litt ACM Distinguished Engineer -------------------------- This communication may not represent the ACM or my employer's views, if any, on the matters discussed.
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