From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from snark.thyrsus.com (static-71-162-243-5.phlapa.fios.verizon.net [71.162.243.5]) by huchra.bufferbloat.net (Postfix) with ESMTP id A09DA21F0B7 for ; Tue, 22 May 2012 08:16:22 -0700 (PDT) Received: by snark.thyrsus.com (Postfix, from userid 1000) id 7C9C140447; Tue, 22 May 2012 11:15:17 -0400 (EDT) Date: Tue, 22 May 2012 11:15:17 -0400 From: "Eric S. Raymond" To: Dave Hart Message-ID: <20120522151517.GA448@thyrsus.com> References: <8daed9ae-8ef6-4b5f-a25e-a5a74b967a4c@email.android.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Organization: Eric Conspiracy Secret Labs X-Eric-Conspiracy: There is no conspiracy User-Agent: Mutt/1.5.21 (2010-09-15) Cc: thumbgps-devel@lists.bufferbloat.net, questions@lists.ntp.org Subject: Re: [Thumbgps-devel] good paper on timing and delay X-BeenThere: thumbgps-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list Reply-To: esr@thyrsus.com List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 22 May 2012 15:16:23 -0000 Dave Hart : > > So, the possibly simplistic question is, if our network time sync programs > > used the same algorithm that the GPS receivers use to read their "servers", > > ie satellites, which all have variable and perhaps somewhat asymmetric > > propagation delays, which can be substantial, would we be able to achieve > > much greater levels of accuracy doing synchronization via the internet? > > GPS birds tell the receiver the flight plans of all birds, so the > receiver knows the dominant factor in the propagation delay, the > distance between transmitter and receiver. With WAAS reception, even > more information about delay is provided in the form of atmospheric > conditions. NTP has a much more difficult row to hoe. This is true, but Ron might nevertheless be onto something here. The solver algorithm for position given a bunch of pseudoranges is irrelevant to anything NTP does. It's straight-up spherical trigonometry. But... GPSes also use Kalman filtering to try to back out the effects of variable ionospheric delay, which manifests as noise in the pseudorange measurements. The pseudorange noise behaves a lot like jitter in network packet latencies. It's possible that Kalman filtering could be useful for cleaning noise from an NTP server's measurements of propagation delay. It's a general technique used for all kinds of noisy time series. >From Wikipedia: The Kalman filter, also known as linear quadratic estimation (LQE), is an algorithm which uses a series of measurements observed over time, containing noise (random variations) and other inaccuracies, and produces estimates of unknown variables that tend to be more precise than those that would be based on a single measurement alone. More formally, the Kalman filter operates recursively on streams of noisy input data to produce a statistically optimal estimate of the underlying system state. -- Eric S. Raymond