Ed_Wischmeyer
Well Known Member
I'd noticed that frequently the SAV tower would be talking to traffic that didn't show up on the TIS-B uplink. Once, when I was about 15 nm south of the airport, I watched a bizjet disappear from the traffic display as it got closer to the airport. I called the tower, they double-checked, and all of their systems passed the appropriate tests. I sent in an ASRS report (good for such things), and no results. Finally the right report and the right information got to the right people, and the problem was resolved.
Moral of the story is that if something doesn't seem right, it's not always the pilot!
Below is the final correspondence from the FAA / Harris, the ADS-B contractor.
Ed
Mr. Wischmeyer,
Thank you again for reporting the TIS-B issue you experienced in the Savannah area while flying in December. The analysis was more complex than normal. We found the cause of the issue you reported and it was resolved in late April.
The issue you reported was TIS-B uplinks not available in the SAV area. Our analysis found there were a reduced number of targets being uplinked due to a high occurrence of radar data whose value in the “altitude confidence” field was low in the SAV radar data stream. The SAV radar is one of the radar feeds ingested to generate tracks of TIS-B targets in the SAV area. By design, radar data with low “altitude confidence” is discarded and therefore the targets not uplinked in TIS-B. The already planned introduction of an upgraded digitizer for the radar providing this feed has resulted in an increased number of radar messages with high values in the “altitude confidence” field and therefore a reduced amount of discarded data. The transition to the upgraded digitizer resolved this issue and resulted in improved TIS-B service in the SAV area.
We have investigated other sites with radar configurations similar to SAV and have not found this issue. Your email mentioned other pilots have had similar issues, please pass on that they may report the issues, including details, via this mailbox ([email protected]) and we can research them as well.
We appreciate you bringing this issue to our attention.
Moral of the story is that if something doesn't seem right, it's not always the pilot!
Below is the final correspondence from the FAA / Harris, the ADS-B contractor.
Ed
Mr. Wischmeyer,
Thank you again for reporting the TIS-B issue you experienced in the Savannah area while flying in December. The analysis was more complex than normal. We found the cause of the issue you reported and it was resolved in late April.
The issue you reported was TIS-B uplinks not available in the SAV area. Our analysis found there were a reduced number of targets being uplinked due to a high occurrence of radar data whose value in the “altitude confidence” field was low in the SAV radar data stream. The SAV radar is one of the radar feeds ingested to generate tracks of TIS-B targets in the SAV area. By design, radar data with low “altitude confidence” is discarded and therefore the targets not uplinked in TIS-B. The already planned introduction of an upgraded digitizer for the radar providing this feed has resulted in an increased number of radar messages with high values in the “altitude confidence” field and therefore a reduced amount of discarded data. The transition to the upgraded digitizer resolved this issue and resulted in improved TIS-B service in the SAV area.
We have investigated other sites with radar configurations similar to SAV and have not found this issue. Your email mentioned other pilots have had similar issues, please pass on that they may report the issues, including details, via this mailbox ([email protected]) and we can research them as well.
We appreciate you bringing this issue to our attention.
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