Passes vs. Composites

A satellite pass is one satellite's single overpass: real holes, usually a few hours old. A composite is a gap-filled daily analysis product blended from several sensors: no holes, usually a day or two old. The map shows both; here's which to trust when.

What's a satellite pass?

A pass is exactly what one satellite saw during one overflight: nothing filled in, nothing smoothed over. If a cloud sat over a stretch of ocean when the satellite went by, that stretch shows up transparent on the map, because the satellite genuinely couldn't get a reading there. See why there are holes for more on that. Passes are much fresher than composites (typically a few hours old) and higher resolution: as fine as ~750m for VIIRS chlorophyll passes, ~1km for MODIS, ~2km for VIIRS SST, and ~25km for the microwave AMSR2 sensor.

What's a composite?

A composite is a single blended picture of the whole ocean, built once a day from multiple sensors, with cloud gaps filled by combining them. For SST that's the GHRSST MUR L4 analysis, 1km resolution; for chlorophyll it's a VIIRS Level-3 daily product, 4km resolution. Sentinel-3 OLCI feeds its own separate chlorophyll composite, not a blend into the VIIRS one, roughly 8km as processed. A composite is a modeled estimate where a pass has real data, not a direct observation. Full detail on data sources. The tradeoff for that complete coverage is age: composites are typically a day or two old by the time they publish.

Which to trust when

Use a composite for a reliable, complete daily picture: good for scouting a region before a trip, or checking a general pattern. Use a raw pass when freshness or precision matters more than coverage: finding a temperature break where it actually is today rather than where the daily blend says it was. See finding temperature breaks for that exact workflow.

How the map labels each one

Every reading on the map carries its own capture time and satellite name, and clicking a spot with no usable data returns a label that tells you which kind of layer you're looking at: a satellite pass with no reading at a point reads "No usable data," a composite with a gap reads "No data (gap)." Composite data is never labeled as a raw pass, and vice versa. See how to use Chlorofishy for the rest of the map's controls, or pricing for which tier unlocks passes.