2.1

Detection capabilities using only MWIR bands compared with MWIR and LWIR bands

A main interest in the study performed for ESA has been the potential for replacement of the LWIR band with a second MWIR band. The initial reasons for this interest are related partly to detector options. Microbolometer arrays were initially considered, since they have the advantage that they do not require cooling – this option has been discarded mainly because of low sensitivity: very large optics are required to feed the detectors at short f-numbers. The likely cooled photo-detectors are mercury cadmium telluride (MCT) arrays. The advantages of MWIR over LWIR MCT arrays are:

To assess the viability of a system based on two MWIR bands the brightness-difference detection metric for a MWIR/LWIR sensor: BT_MWIRfire - BT_LWIRfire - (BT_MWIRback - BT_LWIRback), has been compared with an equivalent metric for two MWIR bands, MWIR1 and MWIR2: BT_MWIR1fire - BT_MWIR1fire - (BT_MWIR2back - BT_MWIR2back). Results for the comparison are shown in Figure 1. Figure 1(a) shows the comparison with two MWIR bands that are relatively closely spaced in wavelength (3.5 and 3.9 pm). The curve lies mainly below the 1:1 line, indicating that detection capability using these two MWIR bands is worse than for the LWIR-MWIR combination. Figure 1(b) shows the comparison with two MWIR bands that are spaced further apart: 3.5 and 4.65 μm. In this case the brightness temperature difference (x-axis for the MWIR-LWIR baseline) lies above the 1:1 line for all fire temperatures, up to a BT differences over 20 K. This indicates that the detection ability for the same fire temperature and sub-pixel size using these two MWIR bands can be as good, or better, than if a MWIR and LWIR band were used conventionally. Above a MWIR-LWIR BT difference of 25 K the curves fall below the 1:1 line, but this is already a region where fire detection is becoming much more unambiguous, with strong MWIR signals above the background.

Figure 1:

Simulations of top-of-atmosphere pixel integrated brightness temperature differences (B_BAND1fire - BT_BAND2fire) - (BT_BAND1back - BT_BAND2back) compared between MWIR and LWIR bands (horizontal axis) and two MWIR band (vertical axis). Simulated for subpixel fires covering a range of pixel proportions and with different active fire temperatures (650 -1350 K). A 300 K background temperature and a 1976 US Standard Atmosphere is assumed, with simulations conducted using MODTRAN 5. (a) 3.50 - 3.90 μm BT difference vs. 11.0 - 3.9 μm BT difference, (b) 3.50 - 4.65 μm BT difference vs. 10.0 - 3.9 μm BT difference.

2.2

Measurement of fire radiant power and temperature

Fire radiant power (FRP) is typically measured using the MWIR band of a system with MWIR and LWIR bands, since radiance at around 3.9μm varies almost linearly with FRP over typical fire temperature ranges. This is shown in Wooster et al (2005) ⁴ where the 3.9 μm spectral radiance from fires of varying pixel proportion and varying temperature (650 - 1350 K) are simulated on a 300 K background and the FRP calculated from these radiances (no atmospheric effects are included). To check that MWIR bands at 3.5 μm and/or 4.65 μm can also be used to estimate FRP, the same computation has been performed with the waveband shifted to 3.5 μm and 4.65 μm. Figure 2 shows the strong linear relationship between the FRP’s derived from the two new MWIR wavelength measurements, and the measurement at 3.9 μm. The high coefficient of variation (r²) between both the 3.5μm- and 4.65μm-derived FRP values, and the 3.9μm-derived FRP, indicates that either of these two alternative wavebands could likely be used to derive FRP estimates, after the observed radiances have been atmospherically corrected. It may be preferable to use the 4.65 μm measurement due to the significantly lower solar reflected radiation component within this waveband compared to the 3.5 μm waveband.

Figure 2

Simulations of FRP retrieved from spectral radiances simulated in two MWIR bands

2.3

Discrimination against sun glints using two thermal IR bands

As outlined in 2.1 above, fire detection using BTDs is expected to work with two MWIR bands at least as well as with one MWIR band and one LWIR band. However, this analysis so far ignores effects of solar reflections. The main issue for the MWIR bands is expected to be sun glints: specular surface reflections from water surfaces, and occasionally from glass and metal surfaces of man-made structures and vehicles. (Diffuse bulk reflections can be strong in the visible range, but high diffuse emissivities and low diffuse reflectances are generally expected in thermal IR bands for both fires and background. ^5,6) The MWIR intensities of sun glints can be significant compared with small fires, particularly at the shorter MWIR wavelengths. For example a crude calculation at 3510nm indicates that a sun glint giving a pixel-averaged reflection equivalent to 10% diffuse reflectance (Lambertian polar distribution) would raise BT in this band by 24K at 300K. The same glint would raise the BT in the 4653nm band by only 2.5K at 300K.

Glints from water produce a very wide range of radiances depending on the roughness of the water and the angle of view with respect to sun-specular. Very flat water produces a diffuse-equivalent reflectance of about 100,000% over a half-degree spread. A reflection from flat glass would produce a radiance equivalent to diffuse reflectance of about 400,000%. Effective reflectances are of course reduced for sub-pixel areas, so that a 1m square of flat glass in a 100m square pixel will produce diffuse equivalent reflectance of 40% in the sun glint direction.

Operation only on the dark side of the orbit would of course eliminate solar reflectance effects, but this would be considered a serious limitation on mission value. Using only the BT difference metric with any two thermal bands, typical sun-glint signals in the shorter waveband will often be interpreted as small fires. An additional approach will be required to allow reliable detection of small fires in daylight. Figure 3 illustrates a possible approach and the likely problems for a system limited to two MWIR bands. Here we address a pixel with a brightness temperature of 315K at 4653nm (band 2), in a scene at an average temperature 300K. We calculate the black-body signal anomaly in band 2 implied by the 15K difference in BT, and calculate the signal that we would expect in band 1 (at 3510nm) on different assumptions about the temperature of the anomaly in the pixel: fires at 650K or 1350K, or a sun glint at 6000K. The simplifying assumptions include: (a) glint reflectance and background emissivity are equal in the two MWIR bands and (b) the atmosphere has effectively 100% transmission in both bands. The calculated signals in band 1 are plotted relative to the 300K background signal. However the plots are extended to show the results on a range of assumptions for the true background temperature in the pixel: between -5K and +5K with respect to the assumed 300K scene average.

Figure 3:

Signal expected from the target pixel in band 1 (3510nm) if the scene average temperature is 300K and there is a measured BT of 315K in the target pixel in band 2 (4653nm). Target-pixel signal in band 1 is plotted relative to the 300K scene-average in band 1. The signal in band 1 is calculated on the assumptions that the raised BT in band 2 is produced (a) by a fire at 650K, (b) by a fire at 1350K, (c) by a sun glint. The relative signals are plotted against deltas in the true background temperature of the target pixel with respect to the scene average (i.e at true target-pixel background temperatures from 295K to 305K). Also shown is the relative signal in band 1 with a 15K mean anomaly, plus the same deltas.

As indicated in Figure 3, the increased signal in band 1 due to sun glint is typically 60% above the increase that would be expected from the hottest fires, allowing sun-glint to be suspected. However, the plots show that sun-glint cannot be identified reliably in the presence of significant uncertainty in the true background temperature of the target pixel. In the example shown, the predicted signal in band 1 is the same for sun-glint with a +5K background delta as for a hot fire with a -5K background delta. The same uncertainty range (at 15K delta BT in band 2) also produces a full-range uncertainty in prediction of fire temperatures. It should also be noted (a) that significant errors in relative signal levels will be introduced by mis-registration between sensor images in the two bands (including temporal mis-registration since sun glint can be transient) and (b) errors will be introduced by differences in emissivities glint-reflectances and atmosphere transmission between the two MWIR bands.

Use only of two MWIR bands has therefore been provisionally rejected. A similar approach (comparing excess BT levels in the shorter wave band for a given BT excess in the longer waveband, for fires and sun-glint) has also been applied to a system with a single MWIR band and a LWIR band: results are show in Figure 4. As expected, use of longer wavelengths will provide better immunity to effects of sun glint. However, uncertainties much over ±5K in the true background temperature of target pixels will again affect discrimination between sun glint and the hottest fires. If a LWIR band were an easy addition, it would probably be included. However, to work effectively, it would need similar

Figure 4:

As Figure 3, but with band 1 at 4000nm and band 2 at 10,000nm

GSD to the MWIR band(s). This would require a much larger aperture than a MWIR-only system (typically by a linear factor 2.5 due to the diffraction limit, as noted above). The practical impact is probably that GSDs would be increased in size to limit the sensor size, with impacts on minimum detectable fire radiative power (FRP). The analysis does not make a strong case for inclusion of a LWIR band.

2.4

Use of a visible channel to identify sun glint: minimum detectable radiant power

In the preferred sensor design, a visible channel is likely to be included for several possible functions in addition to discrimination against sun glint, including cloud masking and providing images of areas with fires in progress. Spatial and temporal registration of visible and thermal channels may be considered essential for sun glint rejection, since highly directional glint from small flat areas (typically glass and smooth water) will be seen as rapidly-changing from the satellite (potentially at vignetted intensity or complete missed by the visible channel). However, the minimum angular spread of glint will equal the angular subtense of the sun, so that it will be reasonable to allow an along-track misregistration up to 0.25° (half sun subtense) between instrument MWIR and visible fields – corresponding to 3.5km on ground or about 0.5 s temporal registration error.

Simplistically, the visible channel may be used to discard all pixels that indicate visible diffuse-equivalent (Lambertian) reflection above an absolute level about 120%, that would be produced only or partly by sun glint. A glint at a diffuse-equivalent level of 120% corresponds to a delta brightness temperature of 22K at 300K in a MWIR band at 4653nm. I.e. all sun-glint affected pixels are in theory discarded, from a scene at a uniform 300K temperature, if we also exclude those showing a BT below 322K in the longer MWIR band. (Use of the shorter MWIR band would be less effective.) With margins for background non-uniformity, this probably means that we ignore pixels at BT levels less than about 25K above the average background at 4653nm. This would correspond to a fractional fire area of 0.0046 for a fire at 650K, or 0.00036 for a fire at 1350K. For pixels 100m square, these fractional fire areas would be 45m² at 650K or 3.5m² at 1350K. The equivalent FRPs are 0.45MW at 650K and 0.65MW at 650K.

2.5

Other possible strategies against sun glint

An approach closer to classical methods might use a brightness temperature different (BTD) between two thermal IR bands (together with a visible channel threshold) rather than a simple threshold based on difference-from-background in any single thermal band. This would in principle remove or reduce the effects of background temperature uncertainty (if uncertainties in ground emissivity and atmosphere transmission are ignored). However, if the visible channel fails to discriminate against relatively high levels of glint – for example 120% diffuse equivalent – potential glint errors in the MWIR bands will dominate the uncertainties due to background non-uniformity. It therefore does not seem clear that thresholding against sun glint, based on visible signal plus a BTD between two MWIR bands, would be useful.

Other possible methods to reduce false fire-detections due to sun glint might include (a) pitching the platform to avoid peak sun-glint from water, or ignoring scene areas within a TBD angle of peak sun-glint, (b) detecting rapid changes in signals over an along-track field: this will indicate highly-directional glint, (c) discarding data from areas in which false fire detections have been recorded on previous orbits.

3.1

MWIR detector selection and method of use

Optical design concepts for thermal detection and assessment of fires are driven largely by detector capabilities as noted in 2.1 above. The baseline concept works only in MWIR bands (plus visible for sun-glint identification) partly because available LWIR detectors have fewer array elements. Linear array detectors could be used effectively in a push-broom scanning system, but area arrays tend to be preferred partly because manufacturers have relatively little interest in development of linear arrays for thermal bands. In the baseline design, the MWIR detector is the Selex SuperHawk, which has 1280 columns that will be assigned to spatial resolution, and 1024 rows. The detector element size is 8μm square. The large number of rows be will used in several ways:

3.2

Baseline MWIR optical design

A baseline optical design, using 8 telescopes with one detector, is shown in Figure 5. The beams collected by the 8 telescopes are shown in separate colours. The beam from each telescope is folded at two flat mirrors and one reflecting prism: the 8 beams form parallel strip images on a set of filters that define the required spectral bands. The combined image is then relayed onto the detector by a relay lens. The detector dewar and cooling engine are included at approximately correct scale. Most of the optics – including the fold mirrors and prisms, the relay lens and the detector – are in a common plane. However, the telescope lenses are out of the main optics plane, as shown in Figure 6. The first fold mirror on each telescope axis is angled to generate 8 different view-directions across-track. The 8 fields, each 8.8° degrees wide, nominally abut across-track, producing a 70° total field.

Figure 5:

Fire monitoring optics – view on nadir axis. Optics dimensions excluding structure: 500mm along-track x 500mm across-track.

Figure 6:

Fire monitoring optics – view along-track. Optics dimension on nadir axis, excluding structures: 240mm

Shaded models are shown in Figures 7 and 8. For clarity, a single telescope channel is shown in Figure 7 and the complete optical system in Figure 8.

Figure 7:

Fire monitoring optical systems – shaded view of a single telescope channel

Figure 8:

Fire monitoring optical systems – shaded view of all channels

Fold prisms and filters

Figure 9 shows a section through the fold prisms and filters. Silicon prisms are used, rather than simple mirrors, to reduce the convergence angle of beams in the fold paths: the high refractive index of the prisms reduces vignetting losses in the useful strip-image widths at the filters. Each of the 8 sub-swath image sections will be split between two strip filters, as shown in Figure 9. Each silicon filter is nominally 3mm wide along-track and 60+mm long across-track. The filters are provisionally 10mm thick, and cemented on 10mm x 60+mm faces to provide a robust assembly that can be mounted relatively easily. Band-pass filter coatings will be applied on the inside faces (detector sides) of the substrates, with anti-reflection coatings on outer faces.

Figure 9:

Fold prisms and filters

Dichroic option to register two MWIR bands

In the baseline design, the two MWIR bands will be separated along-track by approximately 2.2mm in the primary image plane. This corresponds to a 0.32° along-track angle, equivalent to a 4.6km separation on ground, and a temporal misregistration of 0.7s. This is not necessarily a significant concern since use of multiple detector rows with asynchronous detector read-out, as outlined above, will allow any fire to be imaged close to the center of an identifiable detector element in each of the two MWIR bands, allowing effective spatial registration between the bands in theory to <10% of the pixel dimensions. Temporal registration is a concern mainly for registration with a visible channel for identification of sun-glint from flat surfaces, but registration with a visible channel is essential only for one of the two MWIR bands (at the longer wavelengths). However, there will be an option if required to use a fairly simple dichroic design to superimpose two MWIR bands. The arrangement, located immediately outside each of the 8 telescopes, is sketched in Figure 10. The dichroic transmits the 4653nm band and reflects the 3150nm band. The dichroic plate is plane-parallel, introducing no angular deflection or aberration. The mirror is angled approximately 0.16° along-track with respect to the dichroic, to correct the 0.32° along-track misregistration of the two MWIR bands. Typically, the dichroic can be designed to allow very low transmission of the 3150nm band, so that only the 4653nm band is on the transmitted path. The dichroic may reflect a small percentage of the 4563nm band; this can be reduced by making the mirror coating a dielectric stack with low reflection in the 4563nm band.

Figure 10:

Additions to each telescope showing (a) uncoated zinc sulphide wedge used to add ghost images (b) option on a dichroic and mirror to superimpose ground images in two MWIR bands.

3.3

Visible channel definition

A visible channel will be included, as outlined in 2.4, to allow identification of false fire detections due to sun glint. A visible channel will also provide context for detected fires and may also be useful in detection of cloud cover. For sun glint detection, the visible channel must be spatially registered with at least one of the IR channels, since sun glint can be transient: the likely preference is registration with the longer MWIR band (at 4653nm). The along-track field imaged by one MWIR channel, in the current design, is approximately 0.25°. Conveniently this is about half of the sun subtense, so that a sun glint detected by a registered visible channel will be imaged close to the center of an MWIR pixel at nearmaximum signal level. Initial thresholding, to detects fires while excluding sun glints, can apply:

There are advantages in providing spatial sampling for the visible band that is substantially finer than that of the MWIR bands. Resampling of over-sampled data in the visible channel can be used to provide good registration with MWIR data and fine spatial sampling in the visible band can provide improved performance particularly in discrimination against small sun glint areas.

3.4

Baseline visible sub-system design

A visible sub-system added to the MWIR system described in 3.2 above will preferably provide a GSD <50m over a 1000km swath width. The likely option is a simple pushbroom imager, using linear CCD arrays. For example two e2v CCD 21-40 arrays with 8μm square elements will provide a total of 24,000 samples across the swath. Provisionally, two arrays will be used with separate lenses. A preliminary design is shown in Figure 11. The focal length of each lens is 167mm, giving a nominal 39m GSD at nadir with 8μm detector elements. The entrance aperture diameters are set at 20mm. Resolution of the optics is diffraction limited over a spectral band from 500nm to 600nm.

Figure 11:

visible channel optics

3.5

Thermal control for MWIR optics and detection system Detector cooling

In principle, a SuperHawk detector could be operated at a temperature of at least 175K: at this level, dark signal would reach only 3% of saturation, and the noise on dark signal would be similar to the estimated read noise (nominally 400 rms electrons). Passive cooling can be considered. However, there is provisional preference for use of an active cooler (typically supplied as part of a unit including the detector), which will allow greater flexibility in operation and can easily cool to lower temperatures. Provisionally, the operating temperature is set at 150K.

3.6

Payload electronics

Payload electronics have not been detailed. Functions of front-end electronics (FEE) will include at least: detector control and drive, and detector signal conditioning and digitisation. An instrument control unit (ICU) will probably be responsible for command, controls, transmission of data to storage, power conditioning and instrument thermal control (monitoring of thermistors and heater control if necessary). Digital data processing will be required to reduce raw sensor data for storage and transmission. The processing may be formally a function of the ICU, but some processing can also be provided within the FEE. As a baseline, the on-board processing will comprise:

Data processing could in principle include identification of cloud pixels and generation of cloud outline maps, using visible and MWIR data to identify extended bright and/or cold areas.

3.7

Radiometric and geometric calibration

Radiometric calibration of the sensor has not been considered in detail. Fire identification and power measurement depend on detection of brightness temperature differences between MWIR bands typically of >10K and brightness temperatures differences with respect to local background. It is unlikely to be useful to measure absolute brightness temperatures to better than a few K. Inter-band relative accuracy to order of 1K is probably desirable. Absolute and relative calibration to these levels will almost certainly be achieved using vicarious calibration in flight, for example over a selected ocean area for a cold level and desert for a warm level. The visible sub-system is required critically to detect sun-glint at levels substantially above a high albedo threshold. Accuracy for albedo measurements better than 10% will probably be adequate. Vicarious calibration will again be expected to suffice, probably using an eclipse image as a dark level and desert as a bright level. Results from vicarious calibration will update calibration data files generated preflight. Pre-flight radiometric calibration will be performed at pixel level, using black body sources for MWIR bands and a calibrated integrating sphere for the visible band. It will cover operation for a range of optics and detector temperatures, and will include characterization the ghost images used to extend the dynamic range in MWIR bands.

Relative spatial radiometric response and dark-level non-uniformity will be measured conventionally in flight, for both MWIR and visible bands, by detection of banding in extended images. Further development is unlikely to include onboard sources outside telescope apertures, but further study may indicate a use for internal “stim lamp” sources to measure response drifts over periods between vicarious calibrations. Conventional methods for measurement of dark level drift over sub-orbital periods will include measurement of signals from masked pixel and possibly from over-scan (dummy-read) pixels.

Pre-flight geometric calibration will include at least:

Correlation of image data between sub-fields and bands will be used to update pre-flight registration data.

4.1

MWIR system summary

Design parameters of the baseline MWIR system, and assumption for radiometric analysis, are summarized in Table 1, with the principle results of radiometric performance analysis. The most powerful fires are expected to saturate the detector, as discussed in 3.2, which leads to inclusion of a ghost-image generator allowing high intensities to be measured.

4.2

Visible sub-system summary

Design parameters of the baseline visible sub-system, are summarized in Table 2, with the principle results of radiometric performance analysis. The detector will not saturate at diffuser-equivalent albedo values up to approximately 500%, and signal to noise ratios will be more than adequate for the glint-detection purpose. As for the MWIR system, a smaller aperture could be justified considering the radiometric requirements; however, diffraction at a much smaller aperture would begin to reduce effective resolution.

Table 2:

Design summary and radiometric analysis results for the visible sub-system

4.3

System performance in fire detection and analysis

4.4

Payload budgets

No structure design or electronics design has been generated so far. Estimated payload budgets are listed in Table 3. The estimates for recorded data are based on the assumption that all data within 2km along- and across-track of a fire pixel are retained. For the MWIR bands, this also includes multiple samples (frames) for each point in the area. For total data recorded per orbit, data in this 4km square area will of course be multiplied by an estimate of the total number of fires seen in an orbit, taking account of clumping of fires or extended fires.

Table 3:

Payload budget estimates