# Large shifts in elevation with RS3 and PPP

Hi,

I’m using an RS3 and logging for ~4 hours, then submitting the log to the NRCan PPP site. I am seeing some very large shifts in elevation to ~13m.

The A priori was 250.193m
The estimated ellipsoidal is 237.141m
The orthometric height makes more sense at 254.369m

ARP to marker or antenna height was 1.6m which is correct, so it isn’t just antenna height error. I was wondering if anyone has any thoughts on what I might be missing?

Thanks
Jake

“A priori” (before) means approximate position before any post processing reduction. This is the approximate location of the receiver in the rinex header when you download the data.

“A posteriori” (after) means the final result after post processing the data (final computed position).

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Thanks, they use estimated for the processed results though, at least as far as I understand?

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@EBE111057 is right. The A priori position is from the RINEX header. There is a note in the screenshot you shared, “Coordinates from RINEX header used as a priori position”.

The RINEX header is the last averaged epoch coordinate with an accuracy of a few meters. This would explain the huge difference in ellipsoid height.

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I don’t know if this comment was supposed to be based on it matching the a priori value or just from inexperience, but orthometric and ellipsoïdal will seldom match. The separation between the two varies greatly from place to place. If it isn’t already in your skillset, I suggest reading up on geoid separation and the different ways we define the shape of the Earth.

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I realize that the orthometric and ellipsoidal shouldn’t match. The 13 meter shift in the ellipsoidal which puts is suspiciously close to matching is what bothers me.

@EBE111057 cleared that up.

BEFORE (250.193m) and AFTER (237.141m) the solution is calculated.

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