[time-nuts] Leap Second quirk, Another hanging bridge

[Chris Kuethe]

On Tue, Aug 12, 2008 at 4:29 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>> Yep, that's to be expected, given the iterative solver they're using.
>> Try turning on the ZDA message, apparently the $ sign is aligned to
>> the start of the second ... if its implemented in your version of
>> firmware. Since you're running a BU-353 you could have all kinds of
>> broken things in your firmware (not impressed by Globalsat).
>
> Do you expect the ZDA message to be better synchronized than any of the
> others?

Yes. In SiRF receivers ZDA is specifically aligned to the second. I
just checked my copy of the manual I got with my BU-353, and Table 1-1
NMEA Output Messages says "ZDA     PPS timing message (synchronized to
PPS)." RMC is produced after the navigation solution is computed and
that does not happen in constant time.

I found another little gem in my old email: the XTrac (High
Sensitivity) version of the software does not support PPS. XTrac is
often designated by "ES" in the version string. I also found that the
PPS line is just a GPIO. Perhaps in the XTrac build they cut out the
GPIO control code, but left in the scheduling of ZDA... *shrug*

> Is the BU-353 any better or worse than any of the other units using the SiRF
> chips?  I've tried several different brands.  I can't tell them apart unless
> I look at the physical package.

The hardware is about the same, but I don't trust Globalsat to not
load some customized broken firmware...

>> I've used the LVC w/ PPS as the reference clock... not sure that I
>> trust the delay/jitter characteristics of USB enough to give me better
>> time than a wrist-watch.
>
> USB isn't fundamentally evil.  It's polled, so you won't get great response
> to something like a PPS interrupt.  But the polling is handled automagically
> with modern hardware so It's not much worse (maybe better) than the typically
> interrupt batching that RS-232 chips do.

I know it's quick enough... usb ethernet, webcams, sound devices all
work. But I don't think the usb implementation in most GPS receivers
was designed to support timing applications.

> I think USB will be good enough if your target is a few milliseconds rather
> than a few microseconds.

Yes, I've found that with a BU-303 (sirf2), using just NMEA
timestamps, I can sync to about 5ms. Good enough for kerberos and nfs
:)

CK

-- 
GDB has a 'break' feature; why doesn't it have 'fix' too?