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Our preliminary analysis of a small, lightweight FireSat payload shows that the mission is feasible but challenging. Several refinements and iterations on the design have the potential to result in a viable and cost-effective payload concept To illustrate the end point of such a process, we turn our attention to MODIS, a large instrument and a mature design with a fire detection mission.

9.6.2 MODIS—A Real FireSat Example

A detailed design for a spacecraft sensor that can automatically detect fires already exists. The MODIS instrument (MODerate-resolution Imaging Spectroradiometer) on the Terra spacecraft has been designed for a comprehensive range of scientific investigations into Earth's atmosphere, oceans, and land use—much more challenging than fundamental requirements for the FireSat mission (therefore, MODIS may be over-designed for the FireSat mission). However, the MODIS instrument represents a mature design and a sophisticated, space-based fire detection system. The features and considerations that drove the MODIS fire detection sensor and data processing algorithms offer an opportunity to inform our broader discussion of FireSat throughout this book.

The development of the MODIS sensor for Terra traces its roots to the GOES and NOAA spacecraft, and it represents at least a decade of research and design to improve the performance of the Advanced Very High Resolution Radiometer (AVHRR) flown on die NOAA series of spacecraft The MODIS sensor on Terra is a whiskbroom, electro-optical system. Table 9-18 lists its technical characteristics and specifications. The MODIS instrument includes specific design features to capitalize on the physics of thermal detection of fires. MODIS fire products include detecting the incidence of fire, its location, emitted energy, its ratio of flaring to smoldering, and the area burned (bum scar detection). These products are important for understanding the influence of burning biomass on many atmospheric processes as well as direct and indirect effects on terrestrial ecosystems [Kaufinan and Justice, 1996]. The key innovations for the fire detection algorithms include distinguishing the flaring and smoldering parts of the fire and the automatic algorithms for reporting the progress of fires.

TABLE 9-18. MODIS Instrument Characteristics. [Herring, 1997.]

Orbit

705 km, 10:30 a.m. descending node, Sun synchronous

Scan Rate

20.3 rpm cross track

Swath Dimensions

2,330 km (across track) by 10 km (along track)

Telescope

17.78 cm diam off-axis, afocal (collimated, with Intermediate field stop)

Size

1.0 x 1.6 x 1.0 m

Mass

274 kg

Power

162.5 W (avg for one orbit), 168.5 W (peak)

Design Life

6 years

Quantization

12 bits

Data Rate

6.2 Mbps (avg), 10.8 Mbps (day), 2.5 Mbps (night)

Spectral Range

0.4-14.4 pm

Spectral Coverage

±55deg, 2,330 km swath (contiguous scans at nadir at equator)

Spatial Resolution

250 m (2 bands), 500 m (5 bands), 1,000 m (29 bands) at nadir

Duty Cycle

100%

The algorithm developed for MODIS fire detection data products employs two of the 500 m resolution bands, one at 4 pm and the other at 11 pm. The algorithm is an extension of the methods developed using AVHRR. A summary of the steps in the fire processing algorithm follows [Kaufman and Justice, 1996]. (See also Sec. 163.)

Initialization. The algorithm eliminates pixels with potential problems due to clouds or extreme viewing angles. It corrects apparent temperature readings for atmospheric absorption (including water vapor), and estimates die background temperature for pixels containing fire.

Fire detection. The algorithm defines fire pixels based on thresholds and temperature differences between readings in the two spectral bands.

Correction. It eliminates potential false positive readings due to sun glint and consolidates fire readings from adjacent pixels to eliminate redundant reports.

Total emitted energy. It estimates the total energy based on measurements in the 4 pm channel.

Smoldering or flaming stage. It estimates the nature of the fire, namely, smoldering, flaming, or a combination of both.

The MODIS payload illustrates many of the design features of an automated fire detection system. In the context of the RreSal mission, this example provides a point design that has finalized a series of trade-offs in size, weight, power, resolution, and data rate.

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