Here's an example of using PPP-tools to compare PPP solutions from NRCan gpsppp, ESA gLAB, and RTKLIB.
It seems that all programs fix the lat/lon output. However both gLAB and RTKLIB leave the height as a variable parameter. Interestingly it seems there is some peak at the end of the PTBG data and gLAB compensates by raising the height while RTKLIB compensates by raising the clock value.
Update3 on 2014-02-20: by popular demand, screenshots of options (at least roughly correct...) used in RTKPOST to get the results shown:
Update2: gLAB data now at 30s intervals, and an error-plot that compares CSRS to gLAB clock offset.
Update: using the latest RTKLib from github on Ubuntu, I now got this, with the lat/lon/height results between CSRS-PPP and RTKLib in better agreement. There is still a 4ns offset between the clock results - which curiously is about 0.5*ZTD, but this may be just a coincidence.
GPS based Precise Point Positioning is used worldwide to compare clocks against each other and maintain UTC.
The idea is to connect your clock to a dual-frequency GPS receiver that collects data (in RINEX format). The RINEX data is then post-processed by using satellite orbit and clock information available (with delay) from the IGS. While normal GPS has a precision of maybe 1-10 meters (~10 ns clock offset/noise), PPP can achieve <1 cm precision (<1 ns clock offset).
I tested a few of the PPP algorithms I found by running them on the same datafile (KAJA2900.13O) and got the results shown below.
If anyone wants to try this at home you will need from the IGS (I used RTKGet, a tool that comes with RTKLib to download these): clock data CLK file igs17624.clk, orbit data SP3 file igs17624.sp3, NAV file brdc2900.13n, antenna correction ANTEX file igs08.atx, earth rotation parameters ERP file igs17627.erp, and possibly also an ocean tide loading BLQ file available from Onsala Space Observatory.
The CSRS-PPP and ESA-gLAB clock offset solutions agree reasonably well, but the clock offset of the RTKLib/JPL-APPS solutions differ by 3-4 nanoseconds - which is a lot if we are using this for monitoring/steering an atomic clock. I'm not sure why this happens - if anyone knows how to reproduce the CSRS-PPP results with RTKLib please let me know!
See also: A Comparison of Free GPS Online Post-Processing Services