From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yw0-f43.google.com (mail-yw0-f43.google.com [209.85.213.43]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id 5534A200252 for ; Tue, 13 Mar 2012 16:30:28 -0700 (PDT) Received: by yhkk6 with SMTP id k6so1906669yhk.16 for ; Tue, 13 Mar 2012 16:30:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:from:date :x-google-sender-auth:message-id:subject:to:cc:content-type; bh=gHb0eT809BshWReFSJ8k1I215EGYWQ0H3j+7p9bGfBI=; b=FiN0gkRQjv6FZ0nO7BnZxCYKId63+sDKEe31zqPdJfehBaSEslWLktWgllxInQ29VH E5+4NKP7zdHN2PWRrMpNLLWaKOLhhNyDt9gRgF5XYdZKNaAkuu4dSQ52a1vGk1FQFBDG w10UI5GyOT24FrZyLijtFgskXlN9Tw8Es5fmLm84sNG17kkZxspiiHDU9a8JK2H3zsW0 TzgfSjttMuXvixwsBaamCwzlCy9NAtmAgzNCz5GhJOm09ZX6AweMV4bhn9cPFDqJ0FNz +5Z5yIEfSWl7jrQ/m6+qcobM7V6vRXidf7jIuo/WeCOE8D7UVhqxx+1nPya4914ENAn5 0rHg== Received: by 10.182.119.71 with SMTP id ks7mr129895obb.9.1331681427105; Tue, 13 Mar 2012 16:30:27 -0700 (PDT) MIME-Version: 1.0 Sender: tz2026@gmail.com Received: by 10.182.68.164 with HTTP; Tue, 13 Mar 2012 16:30:07 -0700 (PDT) In-Reply-To: <20120313230612.GA24800@thyrsus.com> References: <20120313230612.GA24800@thyrsus.com> From: tz Date: Tue, 13 Mar 2012 19:30:07 -0400 X-Google-Sender-Auth: BFJ_WCzrdeTPxdelC3Dy_ofWma4 Message-ID: To: esr@thyrsus.com Content-Type: text/plain; charset=ISO-8859-1 Cc: thumbgps-devel@lists.bufferbloat.net Subject: Re: [Thumbgps-devel] Project clarification X-BeenThere: thumbgps-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 13 Mar 2012 23:30:28 -0000 A side note - Sparkfun has the Venus638 on a breakout board and it has a PPS and even a "sync to UTC" so in 1Hz mode the first sentence start bit is at a small fixed offset from UTC, so it has a few ways which might make things easier downstream,. I've been playing with them and it is a reason I've suggested their use. There also is some consideration that these things should be in some kind of enclosure. They may end up inside a router on a console port though. That is part of the spin-up. Some milli-micro clarifications: On Tue, Mar 13, 2012 at 7:06 PM, Eric S. Raymond wrote: > One the one hand, ordinary GPS can yield time-service accuracy down > to a second with jitter on the close order of a hundred milliseconds. > This is not as good as NTP, which is generally believed accurate to 10us > (but may not be - that's part of what we want to check). Maybe LAN NTP can go 10uS - microseconds, but pool.ntp.org is more like 10mS - milliseconds > To bogon-check NTP, we need time sources with about 1us accuracy that > are cheap enough to be deployable in flocks. 1uS - One Microsecond? > ...The product concept Patrick Maupin > (our principal hardware designer) is focusing on is a device the size > and shape of a thumb drive that delivers GPS info *and* PPS over USB. > (USB would add some jitter due to polling latency but, we think, not > enough to bust our 1ms goal.) 1ms - One Millisecond goal? Measuring an edge inside a kernel interrupt (linuxPPS) is probably under 10uS, maybe well under as long as interrupts aren't disabled. It would depend on the polling interval, but USB2.0 should have lower latency.