MODIS: the long record

As with VIIRS, a thermal detection: brightness temperature of a short-wave and a long-wave channel in kelvin, plus fire radiative power in megawatts.

One difference lies in the confidence value. VIIRS grades it as low, nominal or high; MODIS supplies an integer from 0 to 100. samoEKO maps it onto the same three grades, below 30 low, 30 to 80 nominal, above 80 high and keeps the raw value. Without that harmonisation the MODIS values would silently disappear from any joint analysis.

One kilometre at nadir. MODIS has no equivalent of the VIIRS aggregation scheme: its pixels grow from one kilometre beneath the satellite to almost five kilometres at the edge of the scanned strip. Beyond about 900 kilometres either side of the ground track the kilometre figure is only nominal, such an overpass is marked as an edge pass in the forecast. The ground footprint carried with each detection measures between 1.00 and 1.08 kilometres.

The detection threshold lies at roughly 100 square metres, fifty times what VIIRS sees at night. For fires below 50 hectares MODIS detects 10 to 35 per cent, VIIRS 15 to 70 per cent.

In figures from the working area: in one week of the high season there were 176 MODIS detections against roughly 870 per VIIRS satellite. For the fine-grained fire situation along the Adriatic, MODIS is not a substitute but a supplement.

For Aqua the date is set: the NASA mission page gives September 2026 as the life expectancy and autumn 2026 as the start of passivation, Aqua has flown without orbit corrections since December 2021 and is drifting to ever later overpass times. Terra follows in February 2027.

In the samoEKO architecture MODIS is therefore optional from the outset, and it is not in the running fetch plan at all: 0.0 per cent of the 2026 season detections came from MODIS (as of 4 August 2026). The numbers are carried by VIIRS at about 90 and Sentinel-3 at about 10 per cent.

That is a deliberate decision against the more convenient design. A system that builds an expiring source into its foundations has an expiry date that nobody in operations has in their calendar. So that such a loss can never pass as a decline in fire activity, samoEKO shows below the season curve and in the observation status which spacecraft contributes how much.

Two reasons. First, the independent second opinion: a detection seen by two different sensor families is more robust than a single one, particularly near known industrial heat sources.

Second, the archive. MODIS collection 6.1 reaches back to 1 November 2000, VIIRS only to January 2012 (Suomi-NPP), April 2018 (NOAA-20) and January 2024 (NOAA-21). Anyone needing a time series spanning two decades, for a trend statement, for a thesis, has only this one source.