Beyond RTK: What OPUS, CSRS-PPP, and PPK Actually Require
Everything so far in this companion has focused on everyday RTK work, where a single, simple offset is enough. Static sessions processed through services like OPUS or CSRS-PPP, and PPK reprocessing, work differently, and it is worth knowing exactly how before submitting a file to one of them.
For static and post-processed work, select the correct antenna model directly in the processing software or service, and enter only the height to the ARP. The software adds the full antenna offset itself, including the small direction-dependent variation that RTK does not need to worry about at all.
A Different Convention From Everyday RTK
Everyday RTK field software, as covered earlier in this companion, conventionally works with a single average offset. Static and post-processed services like OPUS and CSRS-PPP work differently in practice: both require selecting the antenna's specific model and rely on its full calibration, including the direction-dependent variation this companion has not needed to open up until now.
Orient the Antenna to Match Its Own Calibration
A static antenna's calibration data is not orientation-independent. It was measured with the antenna facing a known direction, and professional and geodetic antennas carry a physical North Orientation Mark for exactly this reason. Setting up a static occupation with that mark actually facing north, matching how the antenna was oriented during its own calibration, is what keeps the direction-dependent part of that calibration meaningful rather than applied to the wrong actual directions.
Selecting the correct antenna model in OPUS or CSRS-PPP is not the whole picture for a static session. The antenna also needs to be physically oriented to its North Orientation Mark, since the calibration being applied assumes that orientation was actually used.
What OPUS and CSRS-PPP Specifically Expect
NGS's own OPUS instructions are explicit: enter the height to the ARP alone, select the correct antenna type, and OPUS adds the appropriate offset automatically. NGS states plainly that choosing the wrong antenna type can produce a height error as large as eighty centimeters. CSRS-PPP works similarly, reading the antenna type directly from the submitted file's header and reporting the offset it applied in its results. PPK reprocessing software generally asks for an antenna selection as well, though the exact behavior varies enough by product that it is worth confirming directly in that software's own documentation rather than assuming it matches OPUS or CSRS-PPP exactly.
Entering a combined ARP-plus-offset total into OPUS or CSRS-PPP is not simply extra caution. Since these services add the antenna's offset themselves once the correct model is selected, submitting an already-combined total would double-count it, the same mistake covered earlier in this companion, just showing up on the post-processing side instead of the field side.
Calibration, ANTEX, and Reference Frames, Briefly
The antenna-specific values these services apply come from calibration, most often performed today by moving a test antenna through a wide range of orientations against a reference antenna, and are distributed in a standardized file format called ANTEX. This calibration data is itself tied to a specific reference frame realization, and national geodetic agencies have noted that combining calibration data from one realization with processing tied to a different one can introduce several centimeters of error. For virtually all ordinary use, choosing the correct antenna model in an actively maintained, up-to-date service handles all of this automatically, and none of it needs to be managed by hand.
A file is submitted to OPUS with the correct antenna type selected, but the antenna height field contains a combined total that already includes the manufacturer's offset. Is this safe? No. OPUS adds its own offset once the antenna type is selected, and a combined total in the height field would double-count it, producing a real, avoidable error in the result.
Static and post-processed services like OPUS and CSRS-PPP require selecting the antenna's specific model and applying its full calibration, including direction-dependent variation, unlike the single average offset RTK conventionally uses. That calibration is itself referenced to a known antenna orientation, which is why professional antennas carry a North Orientation Mark and why static setups should actually face that mark north. Enter only the ARP height and select the correct antenna model in services like OPUS or CSRS-PPP, and confirm the same expectation for any PPK software in use, since getting this backward silently doubles or drops the correction.
One short chapter remains, to bring this companion's central lesson to a close.