Are low powered radio signals ever used to share a timing pulse locally? - Page 1
Forum section: electronics / projects

author: cdev
author_rank: Super Contributor
timestamp: October 29, 2018, 01:59:31 am
content: Suppose I have a GPSDO that gives me a nice, low jitter high precision 1PPS and 10 MHz carrier. Or even just a GPS.Given two identical GPSs their PPS signals are usually (very very) close but they do vary sometimes, probably its reduced to a very low value within the computers (better GPSs also allow you to adjust the cable delay setting) and then if one is connecting them over a network, the network introduces error. If the packets need to go through a router or traverse subnets within a switch or possibly two switches, thats more possible variability between machines. NTP compensates for it and for the most part, does very a good job. But it has inherent limitations.For example, it seems that cable modems are inherently problematic in a way other networking equipment isnt preventing any really precise measurement of that last bit of delay.Also, has anybody ever found that different parts of the world may observe slightly different times?Two GPSs with 1PPS is probably as good as it gets for networking. And as good as it needs to get. But are there other ways (radio) to get a timing pulse that have been standards in the past, or which are maybe used in some settings? (places where GPS signals dont go on Earth, space outside of the Earth's gravity, and GPS signals reach, or where spoofing is a concern)

author: dmills
author_rank: Super Contributor
timestamp: October 29, 2018, 12:34:06 pm
content: I can hit 100ns of timing jitter on a 10Gig network connection, but you don't do it with NTP, you want PTP with the network cards having suitable support (Many of the modern ones do), and ideally a switch that knows how to play transparent clock.As far as RF goes, there are plenty of good frequency references broadcast by the national broadcasters that can be used to derive good local frequency standards, time is a little harder as you really need to know the offset between the time standard transmitter and your location, and that is only sort of predictable (Particularly below VHF the ionosphere can reflect things with somewhat variable delay, which can be very annoying if you also have a groundwave path).The cell phone network has a relatively good timebase but converting that to wall clock turns out to be annoying.A telescope pointed at a pulsar or two works in space for just about any space we can reach, and given sufficiently accurate tracking of at least 4 pulsars can give you time as well as position and orientation.Regards, Dan.

author: Berni
author_rank: Super Contributor
timestamp: October 29, 2018, 01:11:47 pm
content: Cell phones actually have very accurate timing.They time there signals so accurately that they can measure the time of flight and determine the distance from the tower. The reason for this is that cell towers have a limited number of frequency channels available to them so they use one radio channel to talk to many phones simultaneously by assigning each one its time window. To allow so many timing windows to still be efficient they pack them together closely. The problem is that when a phone is far away from the tower it gets its signal delayed so much that it overshoots its timing window and might jam the signal of the phone in the next window (that maybe happens to be right next to the tower). So the way this works is the phones synchronize there own oscillators to the accurate rubidium or GPSDO clocks in the cell towers while accurately measuring round trip times to determine the exact delay to the tower. With this they know exactly when there transmit slot begins at the tower and they start transmitting earlier than that so that once there signal reaches the tower its spot on in the beginning of the time slot.Given the large ranges possible with GSM this gives a pretty large area of time locked devices trough direct radio communication. Tho don't think it you could make much use since you need a bulky GSM radio module for it and you would need to hack it to get it to provide a timing signal.In any case if maximum accuracy is important you can't beat GPS. The GPS system was designed for timing from the ground up and is incredibly good at it.

author: madires
author_rank: Super Contributor
timestamp: October 29, 2018, 03:00:01 pm
content: In central Europe there's DCF77 which is used by tons of clocks for synchronizing. You can get inexpensive receiver modules for a few bucks and also professional receivers, all supported by the classic ntpd. A GPS module, a DCF77 receiver plus a few reliable ntp sources (like the ones from PTB in Germany) should keep you up to date

author: cdev
author_rank: Super Contributor
timestamp: October 29, 2018, 03:27:07 pm
content: Maybe one of you can help. I am having weird lack of DNS connectivity that seems un-amenable to anything, it may have to do with my use of my own DNS servers which are supposed to use DNSSEC. That part is working. But some domains dont resolve. I am trying to trace it down. My ISP claims to not support IPV6.However if I left everything on default I would be connecting via ipv6 much of the time.I am not super comfortable with IPV6 because I feel its creating a loss of privacy. Also i am concerned about security. Mobile devices in particular, tablets are convenient for surfing (although I hate typing into them and basically almost never do that) but especially they seem to be problematic from a security point of view and maybe the companies that sell and design them may be doing that on purpose. In any case it seems that the settings I want to turn off certain features dont exist. (or if they do they are very difficult to find)I am just speculating here, and many of you know I do that a lot -Because some mobile devices and even some browsers involuntarily seem to do things in the ipv6 domain I am not comfortable with, and I am unable to resolve some domains and cant figure out why, among other things, I am trying to learn more about the network, something I should have done decades ago but never really did. In part I hope to be able to have precise timing as one more bit of info that I could use. For what its worth.Except I don't know where to even start.

author: cdev
author_rank: Super Contributor
timestamp: October 29, 2018, 03:29:54 pm
content: Madires, thats not precise enough. I am using NTP and have been for a long time, I have several GPS-connected home NTP(+chrony) servers. (several to serve as a check on one another, and also to serve different subnets)Quote from: madires on October 29, 2018, 03:00:01 pm

author: dmills
author_rank: Super Contributor
timestamp: October 29, 2018, 04:11:38 pm
content: WTF does timing have to do with IPV6?If you don't want to be transmitting IP6 outside your local network just drop it at the border router already, you will then likely be doing CGNat at your ISP instead.Personally I quite like V6, the internet was meant to be end to end damn it, enough with NAT everything already!73 Dan.

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2018, 07:03:40 pm
content: IPv6 is no less secure or private. A firewall which blocks all incoming IPv6 connections substitutes for IPv4 NAT and the IPv6 privacy mode randomizes IPv6 IPs although IPv6 NAT could also be used. If you are that worried, use a separate internal wired network for sensitive traffic; I use a separate higher speed wired Ethernet network for my file servers. Oh, and do not use any consumer or commercial WAN to LAN router; build your own running BSD or Linux.Unless the GPS includes a jitter correction function which is rare, the asychronous clock between different GPSes will include a clock width's worth of jitter which will be 10s of nanoseconds. Separate GPSDOs should not have this problem and I would hope that they maintain timing better than usable for any computer.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2018, 08:37:29 pm
content: Aviation and maritime nav, like LORAN, might be a good example.https://en.wikipedia.org/wiki/Loran-CAlthough at the power levels used, they're not so much "local"!I forget if there are other, finer-timed, short range systems.Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2018, 11:53:05 pm
content: Some enterprising people have build GPSDO to 60kHz WWVB converters to keep their old atomic clocks running after NIST screwed everybody with their new patented phase modulation scheme.

author: LaserSteve
author_rank: Super Contributor
timestamp: November 02, 2018, 06:31:07 am
content: There are low power, privately owned , modest frequency, (72-74 Mhz) local time distribution systems sold in the US. There also GPS correction subcarriers with precise time and really good accuracy on FM stations in Agricultual areas available for subscription.Steve

author: cdev
author_rank: Super Contributor
timestamp: November 02, 2018, 07:09:35 pm
content: What are your suggestions for flexible but inexpensive (Gigabit-Ethernet capable) hardware?Also I'd be interested in hearing your opinion on why shouldn't somebody buy a pre-packaged router box. I do think that is probably a good idea, BTW.Quote from: David Hess on October 31, 2018, 07:03:40 pm

author: cdev
author_rank: Super Contributor
timestamp: November 02, 2018, 07:25:34 pm
content: Thank you for this info!Quote from: LaserSteve on November 02, 2018, 06:31:07 amHow accurate are they? Do any brand names or models come to mind?That sounds kind of like what I was wondering about.Quote from: LaserSteve on November 02, 2018, 06:31:07 amOne could do that on a small scale at home. Robotic lawnmower, now, that seems like a fun project. Good way to learn more about a number of different technologies too.

author: Berni
author_rank: Super Contributor
timestamp: November 02, 2018, 08:37:08 pm
content: You can get GPS modules that support differential operation to a base station. And you don't need the subscription services to do it. Usually you just need to buy a 2nd one of these RTK capable GPS modules, stick it in a fixed position somewhere around your house and somehow get its UART data over to your lawnmower or whatever. It needs a good antenna and a clear view of the sky without trees and such to get a good enough signal but once you do you can get up to 2mm positioning accuracy in relation to your DIY base station.I thought about doing a robotic lawnmower before but i never got around to it as i expect it being a lot of work before it works reliably.

author: LaserSteve
author_rank: Super Contributor
timestamp: November 03, 2018, 06:17:35 am
content: SPECTRACOM makes some amazing gear if your a prefered customer.Steve

author: cdev
author_rank: Super Contributor
timestamp: November 03, 2018, 08:29:36 pm
content: Quote from: Berni on November 02, 2018, 08:37:08 pmYes, I have 3 RTK-capable GPSs now and have used RTKlib successfully mostly to learn about GPS antennas. Its been a few years since I had it running, but recently, in order so that I could use it to learn more about robotics, Ive thought about setting up my base station permanently. Its not a high priority project though. What would make me want to do it a lot more would be finding a project which I could participate in. Other people doing the same thing. Also, cost is an issue, not spending a lot of money. The part that interests me the most is the navigation, more so than the shortening of the lawn.Quote from: Berni on November 02, 2018, 08:37:08 pmAround two or three months ago I had an exchange with somebody on here who had bought a bunch of Skytraq S1315F RTK GPS modules very inexpensively. They are the unit I used and they can deliver very high accuracy. I forget how much he paid but it was under $10/unit. Its a GPS-only module but my own experience with them was quite good. The one thing that you need to do however is either let them run continuously or use the AGPS feature on them to download ephemeris data (which I never did) Otherwise when you turn on RTKlib and it starts collecting data if its from a complete cold start the accuracy at the beginning is typical for a cold start GPS and it takes some time to smooth out, but once its been running for a while its extremely accurate. I could give you the settings I used, i recently found the archive of stuff I saved then. the software has improved significantly since then also. You can also use Ublox and a bunch of other brands with RTKlib. performance is quite good with newer multi-band receivers. Skytraq is very good though. In my experience, more stable than Ublox.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2018, 08:26:22 am
content: Quote from: cdev on November 02, 2018, 07:09:35 pmThere are several reasons:1. At least in my experience, they are unreliable but maybe that is only D-Link.2. They have an appalling security record.3. If you want real security, then you need something with VLAN support. See below.QuoteRepurposing x86 PC hardware allows using a BSD based router distribution like m0n0wall (1) or pfsense and if you want much better security, this allows the use of VLANs in one of two ways. Either multiple ethernet ports can be installed or a VLAN capable external ethernet switch can be used as a port expander to turn one ethernet port into many although this limits total throughput.The advantage of a VLAN versus using a switch for your internal LAN is that all traffic between the LAN ports can be routed. So you get to decide what can access the internet and what can access other LAN ports. If one of your systems is compromised, then it no longer has unrestricted access to everything on your LAN.For instance your PCs and other systems could be granted access to a single file server while *not* having access to each other and the file server itself would not necessarily be able to create connections to the various PCs but can only respond to connections from them. Stateful firewalls are so fun.(1) m0n0wall is deprecated but has spawned several replacements.

author: Berni
author_rank: Super Contributor
timestamp: November 06, 2018, 09:23:39 am
content: Yes almost all of these home grade routers are shit. Stay away from them.The solution is to simply step up one price grade to more professional network gear meant for use in a small company. This sort of gear is a good sweet spot in that it has all the advanced features and proper firmware, but its not one of these big expensive rack mount routers with a ton of ports and screaming fans for cooling.At home i use a MikroTik routerboard with 5 ports. In the many years of using it i only needed to reboot it once and that was because a firmware update requires a reboot. It never starts behaving strangely after running for a long time, changing any setting in the configuration interface never needs a reboot to apply it. The downside tho is that configuring a lot of things is a lot more complicated so you pretty much need to google everythyng.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2018, 10:10:36 am
content: My router for the past 10+ years has been a repurposed Pentium 2 workstation optimized for low power. Its only unscheduled downtime was when the icemaker upstairs leaked water which dripped into it and I had it running again 24 hours later. During that event, it took all of 15 minutes to transfer the ethernet cards and FreeBSD boot drive to a spare Gateway Pentium 4 that I happened to have.

author: madires
author_rank: Super Contributor
timestamp: November 06, 2018, 10:47:10 am
content: Quote from: cdev on November 02, 2018, 07:09:35 pmA TP-Link supported by OpenWrt.

author: madires
author_rank: Super Contributor
timestamp: November 06, 2018, 10:52:47 am
content: Quote from: Berni on November 06, 2018, 09:23:39 amMikroTik's firmware had a few security issues lately:https://www.cvedetails.com/product/23641/Mikrotik-Routeros.html?vendor_id=12508