Achim Gratz via devel <devel@ntpsec.org>: > True, however the "cheap GPS connected over USB" one can actually buy > are mostly using USB serials that don't even allow DCD to be handed > through and hence have no PPS. PPS over USB serial is still rare to find.
Not any more. You can buy them from Mark on Etsy for $50. https://www.etsy.com/listing/501829632/navisys-gr-701w-u-blox-7-usb-pps He's sold about 150 of them. That happens to be my design inside that Navisys case. The 1PPS gets mapped to a USB priority packet that will arrive at the host on the next poll. Maximum latency is the 1ms poll interval, average is half that. This is measured performance, not theoretical. What's amusing about this is that it's literally a one-wire patch to a bog-standard PL2303 reference design; I figured it out by looking at specsheets. > For the benefit of other readers: The Macx-1 GR-601W seems no longer > obtainable, but the successor products GR-701W and GR-801W may be. I've > instead switched to NavSpark mini modules plus an FTDI breakout board > that has the full set of serial signals. Correct, the 601W (the original Macx-1) has been EOLed. Mark was selling the GR-701W last I checked. The 1PPS-to-DCD patch is simple enough not to care what rev of the u-blox is on one end of it. They might have upgraded what they're shipping him to the u-blox 8 and we would not necessarily have noticed. All you have to do look for the W suffix. Yes, most of the resto of the USB GPS world has not caught up with this simple patch. But there was one other back in 2012 when I designed this, a USB GPS stick assembled in Brittany of all places. > So, what makes 2ms a number that lets you throw a driver out and 1ms a > number that lets you keep it? That's not the predicate I'm using. The Neoclock4x driver is now gone from my personal repository not because of its accuracy limit alone but because of a combination of factors. The hardware has vanished off the face of the net - you can't even find it on eBay, which will cheerfully sell not just 57 flavors of obsolete GPSes but pre-modulation-change WWVB receivers that don't even *work*. The vendor has vanished too (this is what's news since 2015). The driver is unsupported since 2009, ten years ago. According to our project policy, a driver is eligible for removal when it's been dead for seven years. The Neoclock4x gives every appearance of having been dead for at least that long. Reading between the lines, it was a small-batch side project by a small software company; I wouldn't be surprised if fewer than 100 of the things ever existed. It *is* considered a strike against retention if a device has performance significantly worse than the state of the art in cheap GPSes (and in 2019 that means a Macx-1) but there's a specific exception to that for radio clocks with holdover. Thus, the Neoclock4x's 2ms jitter isn't a deal-killer by itself, but it is... marginal. Not good enough to make an argument for keeping it in support. I haven't pushed the deletion yet. I'm not in a hurry about this; someone could pop up with news that the hardware is not dead. I am going to be eying the other drivers that only ship 2-digit years with a view to removing them as soon as other circumstances justify it. Spectracom Type 2, Arbiter, TrueTime, and OnCore are tops on that hit list. > >> Except that GPS still needs clear view of a relatively large portion of > >> the sky and VLW doesn't, aside from all the interference and signal > >> propagation issues that it has too, because it is operating just on a > >> different band of RF. > > > > What you say in true in theory. > > Well, it's true in practise as well. This is a result of the physics of > electromagnetic wave propagation and the constraints on where you can > put the computers and an antenna for the receiver. If you care to look > what stratum-1 servers you get back from the NTP pool you'll see that > certain colocation centers have only VLF and no GPS among their clients. I'm sure that's true. I'm also sure the share of GPS-only colos is much larger. Especially in the US, where most of the most densely populated part of the country - the northeastern seaboard - is out of reliable propagation range for WWVB. > > In practice, experience in the U.S. > > tells us pretty clearly that the tradeoff is in favor of GPS. > > > > How do we know this? After the WWVB modulation change in 2012, all > > the Amwerican clock-radio vendors moved to GPS-conditioned units *and > > never looked back.* Longwave receivers are no longer worth the NRE > > to build them here. > > That example is irrelevant to the discussion since the application > (clock radio) has completely different operational and economical > constraints from the one we're discussing (NTP stratum-1 refclock). No, it's the same conversation. By "clock radio" I didn't mean the cheap wall clocks, I meant the high-precision Stratum 1 radios that used to exist in the U.S. but don't any longer. That whole market segment was poleaxed by the WWVB modulation change and never came back. I just recently found the first WWVB receiver board that can decode the new phase modulation after seven years of keeping an eye out for same, but it's not a product you can put in a datacenter - it needs system integration with some sort of host SBC and looks like hobbyist gear. -- <a href="http://www.catb.org/~esr/";>Eric S. Raymond</a> _______________________________________________ devel mailing list devel@ntpsec.org http://lists.ntpsec.org/mailman/listinfo/devel
