Using NTP as the Time Source on a Rock Digitizer

Size: px

Start display at page:

Download "Using NTP as the Time Source on a Rock Digitizer"

Victor Charles
6 years ago
Views:

1 Using NTP as the Time Source on a Rock Digitizer Application Note #60 Dennis Pumphrey June 15, 2011 Rev. NC

2 This application note copyright Kinemetrics, Inc., All rights reserved. Kinemetrics products are covered by U.S. and foreign patents, issued and pending. Printed in U.S.A. Kinemetrics, Inc., 222 Vista Avenue, Pasadena, CA USA Phone: Fax: services@kmi.com Website: Kinemetrics SA, Le Tresi 6B, 1028 Preverenges, Switzerland Phone: Fax: kmi_sa@bluewin.ch

3 Introduction The Rock Digitizers (Basalt and Granite) now support NTP as an alternate time source. Definitions NTP Network Time Protocol. A network based source of time DNS Domain Name Server. How the internet resolves names like to numeric addresses like Stratum NTP servers are categorized by strata. Generally speaking, higher numbers are less accurate. Stratum 0 servers are the most accurate and generally directly based on atomic clocks. Stratum 1 servers are typically GPS based Stratum 2 servers are typically based on stratum 1 servers, and so on What The Rock s NTP Support Means NTP is an alternative time source that can be used with Rock digitizers in situations where using GPS is not possible or not practical, or not cost effective. Although the Rock will make a good effort to track NTP time, the accuracy that is achieved varies depending on the NTP servers available to the digitizer, and the network infrastructure What you can expect Simply put, NTP time on a Rock will never be as good as GPS time on a Rock. Timing via the Rock s internal GPS engine will be the least expensive, most reliable, and most accurate time source available. If you need precise timing, you should do everything possible to use GPS time. Our testing at Kinemetrics suggests that the time provided by NTP from internet based NTP servers improves somewhat over time. After a boot of the digitizer s Operating System, NTP time may differ from UTC time by as much as 200ms. Over the course of a day or two, this improves and NTP time will generally stabilize to less than 10ms from UTC time, with occasional excursions as large as 80ms. When using a stratum 1 time server on a local network, results are significantly better because internet latencies are removed. After initial settling, difference to UTC is typically less than 5ms, with occasional excursions as large as 10ms. The simplest network would be a single digitizer (such as a Granite) and a stratum 1 NTP server on a local network. Using NTP as the Time Source on a Rock Digitizer Page 1

4 Of course, the actual accuracy you experience will depend on having a stable network connection and on the quality of the NTP servers available to your site. By comparison, using GPS the Rock is typically able to maintain lock to within about 200 nanoseconds. Typical time source errors are summarized here: Time Source Typical error vs UTC Expected worst case (after settling) GPS < 200ns < 1us Stratum 1 NTP on LAN 5ms 10ms Internet based NTP 10ms 100ms Stratum 1 NTP servers (which are based on GPS) are available commercially for between $1,500US and $5,000US. How it works It is important to understand that there are several sources of time involved: 1) UTC time. True absolute time GPS time is usually extremely close to UTC time. 2) NTP time. This is the time of the Operating System as adjusted by NTP. 3) Digitizer time. This is the time used to timestamp samples. Digitizer time is adjusted to follow NTP time as it changes. The Rock Operating System gets periodic updates from selected NTP servers via a network connection and uses these updates to adjust Operating System time, which is then heavily filtered so that the digitizer can use it as a time basis. As stated, the ability of the Operating System to track NTP time will usually improve over time. Regardless of which NTP servers are used, Digitizer time can typically track NTP time within 250 microseconds, with occasional following errors (on larger NTP time adjustments) as large as 5ms. Once NTP is enabled, the digitizer operation will be similar to a digitizer with GPS (with some differences): Time source will be shown as NTP rather than GPS Displays and data streams are available for Time Quality, Clock Drift, and VCO Displays indicate drift values in milliseconds instead of microseconds The definition of good time is widened Additional GPS information such as location and satellite data are not available Using NTP as the Time Source on a Rock Digitizer Page 2

What you need to do to use it Using NTP on the Rock is not complicated to set up, but you do need several things: 1) At the present time, NTP timing works only on the Rock digitizers Basalt and

If using internet based NTP servers, NTP time will not be accurately tracked with a periodic internett connection (e. g. dial up).

3) You need to enable DNS to use the internet based NTP servers 4) You need to be using current Rock software: a. Linux update 16 or later b. Rockhound v3.

These are often good enough to provide approximate timing which may orr may not be adequate for your application. A stratum 1 NTP server on a local network will give better results.

6) You need to edit the Time Source parameter of thee Rock Data Interface module to select option 5 NTP: NTP Time Source Selection After restarting with NTP enabled, it should take approximately 5 10

5 What you need to do to use it Using NTP on the Rock is not complicated to set up, but you do need several things: 1) At the present time, NTP timing works only on the Rock digitizers Basalt and Granite 2) You need a full time network connectionn between the Rock and the NTP server. If using internet based NTP servers, NTP time will not be accurately tracked with a periodic internett connection (e. g. dial up). Wireless and satellite internet connections have not been tested but will likely also be significantly worse. 3) You need to enable DNS to use the internet based NTP servers 4) You need to be using current Rock software: a. Linux update 16 or later b. Rockhound v3.8 or later 5) You need to decide what NTP servers you will use. Byy default the Rock is configured to use commonly available internet based NTP servers from pool.ntp.org. These are often good enough to provide approximate timing which may orr may not be adequate for your application. A stratum 1 NTP server on a local network will give better results. You can reconfigure the NTP servers used with the Rock s timeconfig script if you like. 6) You need to edit the Time Source parameter of thee Rock Data Interface module to select option 5 NTP: NTP Time Source Selection After restarting with NTP enabled, it should take approximately 5 10 minutes to get an initial lock with NTP. As stated previously, NTP may not settle in to its best accuracy for a period of one or two days. In normal operation with NTP as your only time source, theree is no easy way to determine how close NTP time tracks UTC time. See the section on What You Cann Expect above for the expected accuracy range. To verify that NTP is configured and working, and to get a rough idea of NTP s current accuracy, Operating System statistics for NTP can be obtained using thee ntpchk command from Linux: # ntpchk This gives a snapshot of the current NTP offset (in seconds) from Operating System time. This value is a single sample point and the values can vary significantly from one second to the next. So use this command more as an indication of overall functionality of thee NTP server and the approximate NTP time offset nothing more. Using NTP as the Time Source on a Rock Digitizer Page 3

6 Conclusion While not as accurate or stable as GPS, NTP timing shows promise for systems with somewhat looser timing requirements. NTP timing may be suitable for: Relative timing within a structure using a single multichannel digitizer (such as a Granite) 1 Standalone free field digitizers (where timing uncertainties of approximately 20ms can still allow for earthquake location) When sample rates are relatively low (200sps or less) At this time we would not recommend NTP timing for: High sample rates (500sps and above) because the inaccuracies of the timing becomes a significant fraction of the sample period Independent timing for a group of instruments where relative timing differences must be very small (such as for structural analysis) 2 Studies of phase differentials between multiple units 2 Note 1: All channels in a single instrument are time stamped from the same clock regardless of time source Note 2: Each digitizer resolves NTP time independently, so channels between independent instruments do not share the same time stamp Using NTP as the Time Source on a Rock Digitizer Page 4

Altus Emulation on Rock Granite/Basalt Digitizers

Altus Emulation on Rock Granite/Basalt Digitizers Procedures for integrating Rock digitizers into an existing network Application Note #56 Dennis Pumphrey November 6, 2008 This application note copyright