From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mout.perfora.net (mout.perfora.net [74.208.4.194]) by huchra.bufferbloat.net (Postfix) with ESMTP id 8B00221F0B7 for ; Tue, 22 May 2012 07:07:23 -0700 (PDT) Received: from [192.168.83.2] (c-76-97-152-51.hsd1.ga.comcast.net [76.97.152.51]) by mrelay.perfora.net (node=mrus3) with ESMTP (Nemesis) id 0Lbdxb-1RlhUU2gP5-00kvbK; Tue, 22 May 2012 10:07:13 -0400 References: User-Agent: K-9 Mail for Android In-Reply-To: MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----OWMWM8T71L4K95RI5LHAA34DS2KPP1" From: "Ron Frazier (NTP)" Date: Tue, 22 May 2012 10:07:06 -0400 To: Dave Taht , thumbgps-devel@lists.bufferbloat.net, questions@lists.ntp.org Message-ID: <8daed9ae-8ef6-4b5f-a25e-a5a74b967a4c@email.android.com> X-Provags-ID: V02:K0:diKhldqWwNPY6Ks3yaZ8ZX4bMwVewzc8cr1oxSXJfSy ndPpgHBRJhvVnF35Dgn31Zt6FHMUlcjT9Z9wYnLdahXJdmT+lR 45EhrvZ78l0sK1YzqKLOrF3WIq2CBMpT853TctTUSaLPlxwZNr DYiu1EjcN3V3GOe24jIOiNccy5fz7FGQwnh0YTaM3yhyr3AfGc E4+9/68PMVwbLYT0SFxlRgtQPWoHwmgi/W5tdkxuy7mkCJn7As /Ild9VydVJYT3dun+sjzfIMZ0b74K5T2+gHQLPycPpPmDB+SQE GNtP8+U5VTOsnmkZltLsNs017UwvF4YDbP7D0cYw2mtCpKP/ZL bunDFUZ3Vmnt7tslRJMyFR1aWQDPQLS5JTwY4xnZtekddtd7sk iBv4gPYuCer+GFWNqVKe/1s1Y6NH9y17Y4= 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 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 22 May 2012 14:07:24 -0000 ------OWMWM8T71L4K95RI5LHAA34DS2KPP1 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Hi Dave T, and others, (I'm cross posting my reply to the NTP questions li= st since I think they'd be interested too. The original message was from th= e Thumbgps-devel mailing list.) I enjoyed that article. I'll admit to not = spending 4 hours studying it, and sometimes my eyes glazed over, but I enjo= yed it. It brought up some, perhaps simplistic, questions, but I'll pose th= em anyway. The essential problem of time sync is to observe one or more re= mote time servers, with variable and asymmetric propagation delays between = you and them, and choose what the best time to set your clock is. Obviously= , not simple. However, it occurs to me that the GPS receivers are doing es= sentially the same thing via radio. They are observing numerous satellites = at various locations in orbit. Those satellites have precision clocks all c= alibrated to within 100ns (or .1 us) of true time on Earth. The GPS receive= r, cannot "poll" the satellites, but it can observe their broadcasts. Each = satellite has a variable and at least somewhat asymmetric propagation delay= . That propagation delay can be in the range of .25 sec or 250,000 us. Yet,= the GPS receiver can routinely output a PPS pulse with an accuracy of 1us = or better, taking the Garmin 18 as an example. So, the possibly simplistic= question is, if our network time sync programs used the same algorithm tha= t the GPS receivers use to read their "servers", ie satellites, which all h= ave 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? Thanks in advance for any replies= . Sincerely, Ron -- Sent from my Android Acer A500 tablet with bluetoo= th keyboard and K-9 Mail. Please excuse my potential brevity. (To whom it = may concern. My email address has changed. Replying to former messages prio= r to 03/31/12 with my personal address will go to the wrong address. Please= send all personal correspondence to the new address.) (PS - If you email = me and don't get a quick response, don't be concerned. I get about 300 emai= ls per day from alternate energy mailing lists and such. I don't always see= new email messages very quickly. If you need a reply and haven't heard fro= m me in 1 - 2 weeks, send your message again.) Ron Frazier timekeepingdude= AT techstarship.com Dave Taht wrote: http://queue.= acm.org/detail.cfm?id=3D1773943 -- Dave T=C3=A4ht SKYPE: davetaht US Tel:= 1-239-829-5608 http://www.bufferbloat.net ________________________________= _____________ Thumbgps-devel mailing list Thumbgps-devel@lists.bufferbloat= .net https://lists.bufferbloat.net/listinfo/thumbgps-devel ------OWMWM8T71L4K95RI5LHAA34DS2KPP1 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: quoted-printable Hi Dave T, and others,

(I'm cross pos= ting my reply to the NTP questions list since I think they'd be interes= ted too. The original message was from the Thumbgps-devel mailing list.)
I enjoyed that article. I'll admit to not spending 4 hours stud= ying it, and sometimes my eyes glazed over, but I enjoyed it. It brought up= some, perhaps simplistic, questions, but I'll pose them anyway.
The essential problem of time sync is to observe one or more remote time= servers, with variable and asymmetric propagation delays between you and t= hem, and choose what the best time to set your clock is. Obviously, not sim= ple.

However, it occurs to me that the GPS receivers are doing ess= entially the same thing via radio. They are observing numerous satellites a= t various locations in orbit. Those satellites have precision clocks all ca= librated to within 100ns (or .1 us) of true time on Earth. The GPS receiver= , cannot "poll" the satellites, but it can observe their broadcas= ts. Each satellite has a variable and at least somewhat asymmetric propagat= ion delay. That propagation delay can be in the range of .25 sec or 250,000= us. Yet, the GPS receiver can routinely output a PPS pulse with an accurac= y of 1us or better, taking the Garmin 18 as an example.

So, the po= ssibly simplistic question is, if our network time sync programs used the s= ame algorithm that the GPS receivers use to read their "servers",= ie satellites, which all have variable and perhaps somewhat asymmetric pro= pagation delays, which can be substantial, would we be able to achieve much= greater levels of accuracy doing synchronization via the internet?
Thanks in advance for any replies.

Sincerely,

Ron
<= br>
--

Sent from my Android Acer A500 tablet with bluetooth k= eyboard and K-9 Mail.
Please excuse my potential brevity.

(To = whom it may concern. My email address has changed. Replying to former
= messages prior to 03/31/12 with my personal address will go to the wrong address. Please send all personal correspondence to the new address.)
(PS - If you email me and don't get a quick response, don't= be concerned.
I get about 300 emails per day from alternate energy mai= ling lists and such.
I don't always see new email messages very qui= ckly. If you need a reply and
haven't heard from me in 1 - 2 weeks= , send your message again.)

Ron Frazier
timekeepingdude AT techstarship.com

Dave Taht <dave.taht@gmail.com> wrote:
http://queue.acm.org/detail.cfm?id=3D17739=
43

-- 
Dave T=C3=A4ht
SKYPE: davetaht
US Tel:=
 1-239-829-5608
http://www.buff=
erbloat.net

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Thumbgps-=
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https://lists.bufferbloat.net/listinfo/thumbgps-dev=
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