Neither the PPS signal nor the NMEA data streams are in any way part of the GPS signal itself. A GPS receiver may choose to emit NMEA messages, or Rockwell binary, or a military IDS, or a PPS but all of those are generated by the receiver itself.
The time stamp on a NMEA message should reflect the time at which its data fields were valid and will only be loosely correlated with UTC by the time it gets queued and then sent out the serial port. Time of receipt of, say, a $GPZDA sentence was never intended to be precisely synchronized to UTC.
Yes, of course. I should have said "the signal from the GPS module".
I also worked with a similar time code used in Military radios and it also was a single stream combining the data (time value) with the 1 pps marker. We built a GPS unit to give the radio location info. I don't know if they ever used the GPS module to provide time data. I guess if they did they would have found this issue.
That sounds a lot like weasel wording. A GPS receiver does not have the same sort of time and priority problems that a PC will have. If they can't provide a few ms of messages in a timely manner, then the product is defective. So I don't buy the idea that I should not expect the messages to be sent in a timely manner. If there are other issues involved... no, I don't think anyone has pointed out any issues that are not *design* issues in the receiver. There is a 1 PPS signal that is recovered from the GPS signal. This is coordinated to "real" time to within microseconds according to all the docs I have read on GPS receivers. The calculations are typically done on a 1 sec interval. The receiver should be keeping a PLL for updating the time data. Then the time output by the receiver can be closely correlated to the 1 PPS signal. If they don't do this, then it is a design issue, not an inherent flaw.
The timing of the sentences reporting time should be closely correlated to the 1 PPS signal (which *IS* correlated to UTM) that it can be matched to the correct 1 PPS pulse. Otherwise, what is the point of the sentence?
I just don't know how the GPS module makers implement their time sentences. I know that my work with retail units show they have any number of issues, but I guess that is to be expected. I bet the $10,000 mil units don't have *any* of these problems. I don't know how much the surveying units cost, but they are accurate enough to locate a point to within a few cm. That's pretty durn amazing!
In a general purpose GPS one should be able to rely on the time stamp as pretty darn accurate with respect to "data good", and most of the time I'd expect the sentences to appear at regular, periodic intervals and at their proper place between PPS events. However, unless otherwise spec'd by the manufacturer, there's no guaranteed relationship between the leading start bit (or trailing stop bit) of any particular sentence to the PPS. The NMEA spec does not require one (or even mention a pulse per second).
Indeed, unless the device *specifies* bounding values for delta-T from UTC along with jitter, rise time, etc. I'd not count on the PPS pulse itself to have any particular relationship to UTC or even to have a periodicity that's well defined.
There absolutely are receivers that are well-behaved in this regard, but if it ain't in the datasheet ...
They put it in the specs. ;-) For example, "When 1PPS UTC is selected, the leading edge of the PPS output will be within a period of time from the UTC 1 second rollover that is RCVR specific, but shall not exceed [redacted] ns ([redacted]%), when the Time Figure of Merit (TFOM) is [redacted] or less." which goes with "Message xxxx is associated with the 1PPS UTC output pulse. It is automatically output whenever the 1PPS output mode is set to 1PPS UTC. ... Message xxxx shall be output after its associated 1PPS UTC pulse is output, and before the next 1PPS UTC pulse." If it ain't in the datasheet ...
It's one of those where if you have to ask, you can't afford it. ;-) But yeah, you get what you pay for -- usually. At the least, it's rare to get $10K performance from a $100 receiver. On the other, other hand, what $100 buys nowadays is pretty amazing.
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