What to Do About the LAT Spacecraft File Data Quality Flag Issue

If your Fermi LAT analysis depends on high time resolution — pulsar timing studies in particular — re-download the LAT spacecraft position and history files and rerun your analysis. Data quality flags were not being propagated into those files after the March 2026 migration to the new SLAC computing cluster, so intervals of poor timing resolution were not correctly marked with DATA_QUAL=2. The problem has been resolved and correctly flagged data are now available from the FSSC, but files you downloaded during the affected window may still carry the old, unflagged values.

What went wrong

When LAT data quality monitoring runs, it sets flags in the spacecraft files that downstream analysis uses to filter out bad intervals. Following the migration to the new SLAC computing cluster in March 2026, those flags stopped being written into the spacecraft files. The practical consequence: intervals with degraded timing resolution were left unflagged instead of being marked DATA_QUAL=2.

This matters most when you are working at fine time scales. A pulsar study that assumes every retained interval has good timing resolution can be biased by intervals that should have been excluded. Lower-cadence analyses are generally less exposed, but the flag is there for a reason and the corrected files are the authoritative version.

Affected dates

Eight periods were affected, most spanning multiple orbits:

If your time window overlaps any of these, treat your current spacecraft files as suspect.

What to do

1. Check whether you are exposed. If your analysis uses high time resolution — pulsar studies are the example given — and your data window covers any of the dates above, assume you are affected.
2. Re-download the spacecraft position and history files from the FSSC. The corrected, properly flagged data are now available there.
3. Rerun the analysis with the new files so the DATA_QUAL=2 filtering is applied correctly.
4. If your analysis does not need high time resolution, the impact is limited, but re-downloading is still the safe choice if you want your files to match the current FSSC release.

Related context worth knowing

The flag issue is separate from the processing pause that followed the Level 1 pipeline migration to the S3DF facility at SLAC. That backlog was cleared and normal LAT data processing and deliveries were fully restored as of the March 27, 2026 announcement, with all LAT data and products up to date on the public server. In other words, the data themselves are current — the problem was specifically the quality flags inside the spacecraft files, not missing or delayed observations.

If you are also running Fermitools, note that version 2.5.3 (released July 31, 2026) fixed a critical bug affecting the accurate generation of radial gaussian and radial disk source maps and patched a Fermipy performance issue from 2.5.2. That is a separate matter from the data quality flag, but if you are reanalyzing data anyway, it is a reasonable moment to confirm your tool version.