cheap GPS

Jeremy Porter jerry at fc.net
Fri Aug 20 17:37:37 UTC 1999


Most telcos do use use GPS for timing, however they also use
Cesium standards for backup also.  Don't forget CDMA systems use
GPS/Cesium for clocking also, and CDMA won't work at all without
accurate clocking.  There are only about 2 companies in the world
making precision time/frequencey references for high speed telecommunications
networks.  The NTP term "stratum" is derived from the old
primary refence clocks used by the Bell network to provide
timing.

The little birdies tell me that some of this equipment failed its
week rollover test the first time, but patches were quickly made.
Hopefully all the telco's using the equipment read the engineering
field notices....

In message <19990820125326.F24455 at puck.nether.net>, Jared Mauch writes:
>
>	I'm interested in knowing if there are any telcos that
>are using a GPS for their ckt timing, and this will cause that timing to
>break, and those of us that take "clock source line" from M13's, etc..
>will have problems with our channelized ckts (dial, ct3, etc..?)
>
>	Anyone here privy to that type of information, and can
>you comment?
>
>	- jared
>
>On Fri, Aug 20, 1999 at 09:42:30AM -0700, Jerry Scharf wrote:
>> 
>> 
>> 
>> On Fri, 20 Aug 1999, Alex P. Rudnev wrote:
>> 
>> > Through I did not see anything to worry about - your GPC may be show you 
>> > the wrong date, but why it can affect the accuracy at all (except some 
>> > short time around the very moment itself).
>> > 
>> 
>> This is not true. There is things called the catelog and ephemerus, which
>> are time based and give detailed corrections of the orbital position of
>> the satellites that GPS derives locaton from. If the receiver does not
>> handle rollover correctly, it will not correctly return time or position.
>> Most receivers built since the mid 90s have handles this, and even more of
>> the precision time sources have handled this, but nothing is perfect. It's
>> really easy for the manufacturer to test this, but almost impossible for a
>> user to (you need a GPS simulator.)
>> 
>> The good thing about modern NTP systems is that they don't accept times
>> that are way off (there was a bad incident of a wacko clock many years
>> ago) so if the GPS reports a 1980 date, the software would not believe it.
>> That would mean losing synch with the GPS, but that should not be the end
>> of time for a reasonalbly configured system.
>> 
>> I'll be watching all my clocks, but a lot of people won't be.
>
>-- 
>Jared Mauch  | pgp key available via finger from jared at puck.nether.net
>clue++;      | http://puck.nether.net/~jared/  My statements are only mine.
>             |           "Waste Management Consultant"           VOYN
>

--- jerry at fc.net
Freeside/ Insync Internet, Inc.| 512-458-9810 | http://www.fc.net
#include <sys/machine/wit/fortune.h>




More information about the NANOG mailing list