GPST vs UTC while geotagging in PPK

[sunny.droneman (Vincent)]

Hi Forum,
We are flying a drone with an M2 strapped to it and we want to geotag a bunch of images that have milisecond-level UTC-0 timestamps (tagged by a local gps-ntp-server). We also have an RS2+ on site for the usual PPK setup.
Now I’m learning, that all RS2+ and M2 are logging in GPST:

UTC to Gprst

[image]

GPST-Error-RINEX file Reach M+

Hi @maryamasgariii, I suppose this error is a difference in UTC and GPS time that constitutes 18 leap seconds. The time in Reach M+ LLH log is in GPS format. As for NMEA, it provides the time in UTC. To convert GPS time to UTC time, you need to subtract 18 leap seconds from GPS time.

Linking DJI drone images to Reach RTK timing

There is a difference between UTC (Coordinated Universal Time) and GPS Time. http://tycho.usno.navy.mil/leapsec.html After December 2016, TAI is ahead of UTC by 37 seconds. TAI is ahead of GPS by 19 seconds. GPS is ahead of UTC by 18 seconds. Reach raw position logs are in GPST the PostProcessed position files from RTKLIB are in GPST be default unless you change the option on tab 2 of options to UTC! The DJI drone photos are tagged with UTC, flight logs from programs such as MapPilo…

A)
I’m understanding from the forum here that in PPK, EMLID will convert GPST to UTC-0 in EMLID Studio and geotag UTC-0 images correctly. Is that true?

So that would mean, if I’m PPKing with EMLID studio, I can just trow in my UTC-0 Images, but…

B)
…in case we resort to geotagging the images without EMLID Studio, e.g. via the LLH, we will have to do the GPST-UTC-Conversion by ourselves?

C)
Could someone from EMLID confirm that the M2’s and RS2+'s LLH is “still / also” in GPST, like @kseniia.suzdaltseva says here Weird time difference between dji drond and M+ ?

Sorry for insisting on confirmation, but we have quite a complex project and we really need millisecond level alignment of our images to the drone position.

THANKS!

PS: it’s NOT traditional Photogrammetry, so nevermind tipps like "“use GCPs” for this one

[ruth.bongon]

Hi @sunny.droneman,

I can confirm that the Emlid Studio output files are in GPST by default. The raw data logs from the Reach devices are in GPST as well. So, you are correct that the M2 and RS2+ LLH files are also in GPST.

in case we resort to geotagging the images without EMLID Studio, e.g. via the LLH, we will have to do the GPST-UTC-Conversion by ourselves?

Yes, you need to consider the leap seconds, which are currently 18 seconds.

However, it’s worth mentioning that the LLH is the track of M2 during the entire flight duration. Thus, the LLH coordinates would be more than the number of images. You would need to automate the process to determine which LLH coordinates are closest to when the photo was taken.

But Emlid Studio drone data processing will do this for you.

[sunny.droneman (Vincent)]

Hi Ruth, great, thanks for the clarification!

Yes, if we do use the LLH later on, we would pick two appropriate points in time to interpolate the location of our M2. It might be useful for a higher level of automation going directly through the LLH, but we haven’t fully thought it through yet.

For now, EMLID Studio is a great tool to get up & running fast!

One more thing, while we’re at it: is the antenna height subtracted before or after logging the position in the LLH? according to the logic from above, the LLH is probably logging the antenna position without offset, right?

Vincent

[ruth.bongon]

The height here is the ellipsoidal height to the antenna phase center.

[sunny.droneman (Vincent)]

alright, THANKS, that helps !