> In short, the sawtooth correction describes, to the best > guess of the receiver, how early or late the hardware > 1 PPS was.
Let me go on a bit more. If the GPS OEM board knows what nanosecond the 1 PPS should be generated, why then doesn't it just issue the 1 PPS pulse precisely at that instant? Well, if there were a 1 GHz oscillator and ASIC on the GPS board then this would be easy, yes? But it doesn't surprise me that market economics argues against this level of overkill. Few people need ns granularity of their 1 PPS; those that do often don't care if it comes in fine hardware or in coarse hardware with fine software correction. And besides, 1 ns granularity doesn't always make sense when the actual UTC accuracy of the pulse is on the order of 10 to 100 ns. Also none of this makes sense if you're applying post corrections to your GPS data; e.g., to back out UTC errors in UTC(USNO), or GPS orbit errors, etc. So most GPS boards just issue the 1 PPS synchronous to the main clock; which is on the order of tens of MHz; giving 1 PPS jitter on the order of tens of ns. Now, another way to handle this is to run the hardware 1 PPS output, with all its jitter, though a programmable nanosecond-resolution digital delay generator. Each second you use the firmware reported future negative sawtooth as the programmed delay value for the next 1 PPS pulse. Tom Clark and I talked about this years ago and Rick Hambly implemented this as an option in his CNS Clock II ( http://www.cnssys.com/ ). Note this hardware trick only works for GPS OEM boards that predict sawtooth error for the next 1PPS (vs. reporting the sawtooth error of the previous 1PPS). Motorola is OK here. If you're tempted, see some programmable delay lines at: http://www.maxim-ic.com/products/timers/delay_lines.cfm /tvb http://www.LeapSecond.com _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
