Looking for a GPS Module with 1pps to use on the Ultimate U2 / U3 kits. Then read the Specifications and avoid the StarIII Chipset, I looked into these some time ago and concluded that they were never intended for timing applications.
Last week I became concerned about StarIII module being supplied with reference to them having 1pps output. Not many suppliers tell the lie but at least one Chinese source does.
OK there is LED that flashes at something like 1pps but that is an indication that the thing is on and searching for or located the odd satellites. It is certainly not an indication of 3D lock or that an accurate PPS is available. Little information is available from suppliers or manufactures and some suppliers have got it wrong, you have been warned avoid SRFIII like the plague.
I searched the Manufactures Spec. Sheets and produced this comparison of some known good and bad GPS Modules.
GPS Module Timing Accuracy From Spec. Sheet
In order of timing accuracy
PPS output +/- 50 ns (RMS) accuracy
Half as good as Trimble.
50Hz in 1,000,000,000 (RMS?)
Time: 1us synchronized to GPS time
40 times worse than Trimble
1Hz in 1,000,000
timing accuracy: 30 us
More than 1000 times worse than Trimble. (Could be +/- 2mS Jitter according to GPS Passion)
30Hz in 1,000,000
187.5Hz in 6.25MHz
3.75KHz in 125MHz
Timing Accuracy (1PPS Output) 10 ns RMS Jitter
No actual statement of accuracy. Well at least they admit there is jitter
The last two Modules use the SiRF StarIII chipset it seems that there is no true PPS.
To quote from GPS Passion A jitter this large definitely confirms that it’s not 1PPS but a simple digital output crudely toggled by software at opportune moments. You can already have a similar accuracy from the first start bit on the TX pin
8th December 2013
Things move on, and now the U3 Huff Puff type stabilising has been added to the U3, average the errors for long enough and you end up with a reference. It works a treat with an accurate GPS, not that it needs taming, the thermal drift of the DDS and Reference Crystal benefits.