Inmarsat C interference with POS MV GNSS antennas

[kjerram]

Ongoing topic from RVTEC email 2023-05-16 (Paul Duncan, SOI), posted with permission:

Symptoms:
POS MV errors traced down to interference from Inmarsat C transmission close to the GNSS antennas.

Suggestions from industry and RVTEC community:

1. Replace with Trimble GA-830 or Zephyr 3 Rugged antennas with better rejection of Iridium frequencies.
2. Install choke rings and General Dynamics L1/L2 filters
3. Move Inmarsat C antennas higher so POS MV antennas are out of main transmissions

SOI will attempt replacing these with GA-830 antennas at next opportunity, then report back and take further action if necessary.

[JuliannaDiehl-SOI]

Wanted to give a quick update on this issue as we had a large breakthrough recently.

The Inmarsat C transmissions that were causing Posmv errors were actually due to a dropout of our Marinestar G2 corrections. This in turn would cause a position shift of tens to (on a few occasions) hundreds of meters. We reached out to the RVTEC community and they gave the above information, which prompted us to replace our antennas (AeroAntenna AT1675-540S) with GA-830's as they have better rejection of Iridium frequencies. These are currently on order and have not been implemented yet.

We also reached out to Darren Billard at Applanix. In March, he advised us to upgrade our Posmv firmware from v11.25 to 11.26, which he thought might address our position jumps. This firmware upgrade did not fix our issue and we continued having position jumps associated with Inmarsat Transmissions. After more communication with Darren, he said he suspects the jumps we we're seeing is actually an error in the solution status of Posmv during the period in which it has lost its corrections. He said he'd look into this, and we ended up not following up after a few months, focusing instead on replacing the antennas.

In June, 2023 Applanix released v11.40 which has some substantial GNSS Receiver changes per the release notes:
- A new version of GNSS receiver firmware is deployed as part of this release. For PCS-84/86/92 (BD982) the receiver firmware is updated to v5.60. For PCS-100/101/105 (BD992), the receiver firmware is updated to v6.20.
- For compatible systems (PCS-100/101/105), this release sees the adoption of new IN-Fusion+ technology, featuring the latest Trimble® ProPoint™ algorithms. This allows for even more robust high-rate aided-INS position output at the centimetre level even in the most challenging signal environments, and for even faster initialization when using RTX in RTX FAST areas.

We updated the firmware to v11.40 and have been 10 days without any position drops. We are still losing corrections with inmarsat transmissions (hopefully the GA830's will fix this) but the "solution status" bug during corrections drop issue seems to no longer be associated with a major position shift since the firmware update. At most we are seeing ~1 meter differences between our two nav systems, Seapath and Posmv, as opposed to the ~1km differences we'd occasionally see.

Side note regarding the firmware upgrade, we did have to contact Marinestar to have them to resend our G2 corrections because after the firmware upgrade our subscription got reverted back to RTK. Marinestar was very responsive and had the corrections back within a few minutes. Not sure if this is common with firmware updates or just was a fluke during ours.