[ntp:questions] Re: gps; why pps?

Richard B. Gilbert rgilbert88 at comcast.net
Wed Mar 29 12:21:44 UTC 2006

Tim Keck wrote:
> Richard B. Gilbert wrote:
>> Brian Garrett wrote:
>>> "Folkert van Heusden" <folkert at vanheusden.com> wrote in message
>>> news:20060328155232.GP26463 at vanheusden.com...
>>>> Hi,
>>>> Could someone please explain me why you want to only use the PPS signal
>>>> of a GPS receiver and not the time indication?
>>>> Folkert van Heusden
>>>> -- 
>>> Presumably because of the GPS receiver's tendency to delay the time 
>>> display
>>> anywhere from a half-second to two seconds behind the actual time.  The
>>> processor's man task is calculating location and its firmware is 
>>> dedicated
>>> to that; by the time it gets around to spitting out time-of-day info, 
>>> the
>>> moment has passed.  Even using the raw NMEA data can get you into 
>>> trouble if
>>> you aren't clear about whether each sentence pertains to the second just
>>> passed, or the one coming up.
>>> Brian Garrett
>> Brian,
>> I don't think that's quite correct.   The processor solves a system of 
>> four simultaneous equations in four unknowns: latitude, longitude, 
>> It is essential to understand the relationship of the reported time to 
>> the PPS output if you are writing your own driver.  The reported time 
>> can mark either the last pulse or the next pulse depending on the 
>> design of the receiver.
> As I recall, if SA is on, the accurate position is not known for some 30 
> days; position hold involves a very slow filter to try to remove the 
> affect of sthe clock variation imposed by Selective Availability.  The 
> early units I played with solved the 4 equations with the postion and 
> time to be valid at the next 1 second tick, but those were rack mount 
> Collins units about 20 years ago.  We also had some PCI based units (I 
> think Ashtech) that ran at 10 Hz solution rate, something like 15 years 
> ago.  Have the manufacturers abandoned the "at the mark, the time and 
> postion will be" approach for "at some time in the past, we were at" 
> kind of solutions?  Sounds dumb to me.
> Tim Keck

SA was turned off years ago and is not likely to be turned on again!

The only receiver I'm familiar with in detail is the Motorola Oncore 
M12+T.   The Oncore adds one second to the calculated time and reports 
the time of the next pulse.  It can be done either way and, as long as 
you know which way, it works.

The Oncore driver places the receiver in "0d" or position hold mode 
after determining the position.  If you are setting the receiver up at a 
fixed site, you do a site survey that takes about two hours.  The 
receiver reports its position and the driver logs it.  You then put the 
position in your Oncore driver configuration file and tell the driver to 
load the position from the file.

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