Dear Community Members,
After scanning through all posts I still have some question marks regarding the extraction of the correct GPS time stamp from the Data Flash logs. My observations are as followed.
I am currently analyzing hexacopter flight data (data flashlog) for which was also recorded the NMEA streams in parallel using the GPS output functionality (Ctrl-G). The Mission Planner indicated time stamp and the GMS and GWk calculation are equivalent and the following:
Mission Planner and GPS GMS (308113800) and GWk (1921) (UTC Correction -1 hour (-3600 secs))
November 02 2016 2:34:56.800 PM
The realtime NMEA Output is about 3 mins offset and as following:
GPS NMEA Time Output
November 02 2016 2:37:58.251 PM
I am wondering where this offset comes from as I expected the NMEA data to be transperantly streamed by using the Ctrl-G functionality. It could also be that the NMEA stream is recreated making use of the system time of the computer running Mission Planner what could explain the large offset between the dataflashlog and the NMEA serial port output. If so this could be a problem for realtime applications requiring a very precise GPS timestamp.
Additionally I also observed for older recordings that the Log Browser indicated time (second column) differs from the time calculated using GMS and GWk. For example:
GPS GMS (397269400) and GWk (1859)
August 27 2015 4:20:52.400 PM
Mission Planner Time Stamp
2015-08-27 16:20:51.900
If I interpret the forum entries correct the Mission Planner Log Browser indicated time originates from the TimeMS since epoch (switch on). But the question is does the GMS and GWk represent the exact time stamp of the indicated GPS position.
As my application is time sensitive and requires a precise as possible GPS timestamp these questions are critical to me.
I hope that you can help me answering them as I didn’t find a conclusive answer yet.