I have tried several GPS modules for Navigation, APRS, Computer Timing and more recently Time, Location and Frequency setting in the Ultimate series TX's.
Not all GPS Modules have 1PPS, few that do have accurate PPS Timing.
The SkyLab SKM52 is not generally available for sale in one off or small quantities, the specification looks good,10ns PPS accuracy. It uses the MT3339 chipset which claims high sensitivity, I had the opportunity to try one out.
The module has a small back up cell on board.
I used all the decoupling and pull up resistor(s) as recommended in the Data Sheet and a 3v3 low dropout linear regulator. My standard set up includes a Battery Back Up supply.
The module is a SMD but soldering wires to the provided tabs was no problem. I used 5 wires all the same colour (green) as this was the only wire to hand which was soft enough, to connect the module to my standard, PSU / interface board design. A good source of fine flexible insulated wire is from thin multicore screened cable although in some the insulation can be quite stiff.
I set GPS Mode = 1, Cal HP = 00, Cal Time = 010. Initial acquisition took a few minutes and 3D lock followed.
No variation was seen in the accuracy of each Cal. session.
My now standard set up, Cal HP = 02, Cal Time = 240 gives an advantage to combat DDS oscillator thermal drift, from observation of the 125MHz signal.
To check sensitivity I removed the module from the shack window for 20 minutes and 3D Lock continued.
As far as I can tell the SKM52 Module would appear to perform to the published Spec. and within the test parameters tried, as good as the Trimble Sylvania and Reyax UP501.
The Module is available in one off quantities from Hans Summers QRPLabs.