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Vol. 1 No. 1 (2000) pp.3-35
A Review of Volcano Surveillance Applications Using the ATSR Instrument Series
M.J.Wooster§✩ and D.A. Rothery‡
§
Department of Geography, King's College London, Strand, London, WC2R 2LS, UK.
‡
D.A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7
6AA, UK. ✩Corresponding author e-mail: [email protected]
ABSTRACT
The Along Track Scanning Radiometer (ATSR) has operated almost continuously since 1991
onboard the polar-orbiting ERS-1 and ERS-2 satellites. The instrument was originally
designed to accurately record sea surface temperatures for climate change applications.
However, the multi-spectral infrared radiance data that it acquires have also been used to
study thermal signals at a number of active terrestrial volcanoes, with eruption styles ranging
from large effusive lava flows to persistent fumarolic degassing from summit domes. The
data have been used to identify potential thermal precursors to eruptive events, to develop
and parameterise models of volcano processes involving substantial heat loss or heat transfer,
and to monitor the onset and development of new eruptive activity. This paper reviews the
volcanological application of the ATSR instrument series, providing a brief historical
background to the work and summarising the particular technical features of the instrument
pertinent to making such volcanic observations. The recent development of a freely
accessible, internet-based near real-time data processing and supply service for global ATSR
data makes it possible for interested researchers and those involved in volcano surveillance to
make use of ATSR data in their own studies. Here we supply the background and technical
details to aid this process.
KEYWORDS : Volcano Monitoring, Thermal, Infrared, ATSR
1.0 A BRIEF HISTORY OF THERMAL VOLCANO SURVEILLANCE
1.1 Background
Thermal observation is one of many techniques used for volcano surveillance and for the
monitoring of ongoing volcanic activity (McGuire et al., 1995). Most thermal monitoring
programs are aimed at (i) detecting surface or near-surface thermal manifestations of internal
changes in state of the volcano, for example changes in fumarole temperature brought about
by magma ascent (e.g. Suwa and Tanaka, 1959; Connor et al., 1993), or (ii) parameterising
and validating numerical models of volcanic activity, such as the flow processes of erupted
lava (e.g. Dragoni, 1989; Rothery and Pieri, 1993). In the 1960s and 1970s remote sensing at
infrared wavelengths was identified as a suitable method of reducing the time, risk and
logistical difficulty involved in making in situ temperature measurements of volcanic
phenomena (Moxham, 1971). The theoretical basis of remote temperature measurement is
Planck’s Radiation Law, which governs the relationship between the absolute temperature (T)
of a body and the emitted power per unit area per unit wavelength. In thermal remote sensing
we usually use a version of the Planck Function adapted to give values of spectral radiance
(L) in W/m²/sr/µm, these being one of the common working units in the subject:
L(λ,T) = 2hc²λ-5[exp(hc/λkT)-1]-1 x 10-6
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(equation 1)
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Vol. 1 No. 1 (2000) pp.3-35
λ is wavelength (m),
T is temperature (K),
L is spectral radiance (W/m²/sr/µm),
h is Planck’s constant (6.6 x 10-34 Js),
k is the Boltzmann’s constant (1.38 x 10-23 J/k),
c is the velocity of light in a vacuum (3 x 108 m/s)
A graph of Equation 1 (Figure 1) indicates that emitted spectral radiance increases with
temperature over the entire wavelength range, and also demonstrates that the hotter the
emitting body the shorter the wavelength of peak thermal emittance (Wein’s law). Thus
exceptionally hot bodies will emit significant amounts of radiation at even visible
wavelengths, which explains the red-orange glow of newly erupted magma. By carefully
choosing the infrared regions in which to observe, the entire range of ambient to magmatic
surface temperatures can be covered. Moxham (1971) and Oppenheimer and Rothery (1989)
detail the use of hand-held infrared temperature measurement devices operating on this
principle, potentially reducing hazards to operators and easing logistics by allowing
measurements to be made from distances of several kilometres (e.g. Lange and Avent, 1975).
Success with field-based instruments led to the volcanological use of airborne scanners
working at similar infrared wavelengths, these being capable of producing imagery that could
clearly delineate volcanic surface thermal anomalies (e.g. Moxham, 1970; Ehara et al., 1975;
Perry and Crick, 1976). Overflights by suitably equipped aircraft were, however, fairly rare
at active volcanoes and infrared observations made from Earth orbiting satellites were
suggested as a suitable data source with which to adapt the methods to a wider context
(Moxham, 1971; Francis, 1979). The repetitive and inclusive nature of satellite observations
was seen as a great benefit for volcano monitoring, allowing the collection of large timeseries datasets and the documentation of activity occurring in otherwise unobserved areas.
Furthermore since only the cost of imagery need be paid, and not the actual cost of the
satellite operation, obtaining data from satellites has been considerably cheaper than
mounting dedicated airborne surveys.
Figure 1 Blackbody spectral radiances according to Planck’s Radiation Law, shown
over ultra-violet, visible, near IR, shortwave/middle IR and thermal-IR wavelengths.
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The first spaceborne thermal observations of volcanic activity were made in 1964 by the
Nimbus 1 meteorological satellite, the first spacecraft to provide thermal imagery of the
Earth. Nimbus 1 possessed a middle infrared channel centred at 3.8 µm. Figure 1 indicates
that this waveband is very sensitive to high temperature phenomena. Despite the low 6 km
spatial resolution of the measurements, Gawarecki et al. (1965) were able to use these data to
demonstrate that the active Kilauea volcano on the Big Island of Hawaii provided a higher
thermal radiance than its quiescent neighbour, Mauna Loa. This study first showed the
special utility of nighttime thermal imagery which, because of the absence of solar reflected
radiation, generally allows greater thermal contrast of anomalies than on data recorded during
daylight conditions. The formation of the new volcanic island of Surtsey was studied using
Nimbus II middle infrared data (Friedman and Williams, 1968), and US National Oceanic
and Atmospheric Administration (NOAA) meteorological satellites provided Very High
Resolution Radiometer (VHRR) thermal infrared data (10.5 - 12.5 µm) that were used to
detect new volcanic eruptions in the Galapagos Islands in 1977 and 1979 (Simkin and
Kreuger, 1977; SEAN, 1979; Rothery et al., 1988). The devastating 1980 eruption of Mount
St Helens provided further evidence of the role that thermal remote sensing could play in
volcano monitoring. Scientists were alerted to the possibility of a forthcoming eruption by
changes in a variety of geophysical phenomena, with the result that a high intensity
monitoring campaign was instigated at the volcano (Lipman et al., 1981). This campaign
included data acquisition by airborne thermal infrared scanners that revealed hotspots along
the flank of the main stratocone. A flank collapse in this area on 18 May 1980 led to a
catastrophic explosive eruption of great magnitude, with instruments onboard the GOES
geostationary meteorological satellite obtaining an unprecedented view of the spreading
eruption cloud (Short, 1982). Later thermal observations by the orbiting Heat Capacity
Mapping Mission (HCMM) (10.5 - 12.5 µm, 600 m spatial resolution) were able to observe
the early emplacement of a lava dome within the newly created amphitheatre (Short, 1982).
Following on from these early successes there was much interest in the potential utility of
data from the improved Advanced Very High Resolution Radiometer (AVHRR), first
operated in 1978 onboard the TIROS-N satellite. As with the VHRR, the AVHRR still had a
nominal spatial resolution of just over 1 km, but had an increased spectral range, including
channels in both the middle and thermal infrared (Table 1). Of particular interest was the
incorporation of the 3.7 µm middle infrared waveband to the design since, as temperatures
rise, thermal emittance increases much more rapidly at this wavelength than at the longer
AVHRR thermal infrared wavelengths (Figure 2). Thus it is easier to detect surface thermal
anomalies using the 3.7 µm observations than when using the longer thermal infrared
wavelengths. Gawarecki et al. (1965) had already made volcanological use of such middle
infrared data, albeit using data from Nimbus I at a much lower spatial resolution.
Channel
1
2
3
4
5*
Wavelength Range (µm)
0.55 - 0.90
0.58 - 0.68*
0.73 - 1.10
3.55 - 3.93
10.5 - 11.5
11.5 - 12.5*
Spectral Region
Visible
Red*
Near Infrared
Middle Infrared
Thermal Infrared
Thermal Infrared*
Table 1. The channels of the AVHRR instrument, carried onboard the NOAA Polar Orbiting
Environmental Satellites (NOAA/POES) from 1978 to the present. * indicates a channel
available on the AVHRR version 2 only, operational from 1981 onwards.
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Figure 2 Relationship between spectral radiance and blackbody temperature for
electromagnetic radiation at 1.6, 3.7 and 11 µm wavelengths.
A further benefit of the 3.7 µm data was first pointed out by Dozier (1981), who highlighted
the possibility of using AVHRR 3.7 µm data to detect high temperature Earth surface features
of subpixel resolution. Since high temperature sources emit so strongly at 3.7 µm, a hot
source covering a small fraction of the 1 km AVHRR field of view can cause a much more
significant rise in 3.7 µm channel pixel radiance (or brightness temperature) than it can at
11 µm (Figure 3). Dozier (1981) further suggested that comparison of the 3.7 µm and 11 µm
data could be used to confirm the subpixel nature of the heat source since, if the entire ground
surface were at a uniform temperature, the atmospherically corrected 3.7 µm and 11 µm
brightness temperatures should be almost identical. Significant differences between the 3.7
and 11 µm brightness temperatures meant that the body acting as the high temperature source
was likely to be of subpixel size. Using this technique Weisnet and D’Aguanno (1982)
studied AVHRR data of the Mount Erebus lava lake in Antartica, whilst Scorer (1986; 1987)
made repeated AVHRR observations of hotspots at Mount Etna (Sicily). Oppenheimer
(1989) attempted a more wide-ranging and quantitative study, utilising imagery of the
Erebus, Stromboli, Vulcano and Etna volcanoes. At that time he concluded that the utility of
AVHRR data for the quantitative thermal monitoring of volcanoes was rather limited, being
severely hampered by the large pixel size compared to the generally small thermal anomalies
present on the volcanic surfaces. Specifically Oppenheimer (1989) found that it was
relatively easy to detect an active volcanic hotspot with AVHRR but that variations in the
pixel brightness temperature could not be unambiguously attributed to volcanic changes.
Furthermore the AVHRR 3.7 µm channel was often plagued by noise problems, and hotspot
signals were often ‘smeared’ over many pixels due to the large pixel overlap associated with
off-nadir parts of the AVHRR scan (Oppenheimer, 1989; Breaker, 1990).
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Figure 3 AVHRR 3.7 and 11 µm channel pixel brightness temperatures for a modelled
dual thermal component situation. The modelled pixel has a background temperature
of 0 °C and contains a subpixel-sized hotspot at 500 °C. The whole-pixel brightness
temperature is plotted for situations where the hotspot varies from nothing to 1/1000th
of the pixel area. As the hotspot size increases, the 3.7 µm brightness temperature
increases sharply whilst the 11 µm brightness temperature increases only slowly. Thus
the 3.7 µm measurement is seen to be much more sensitive to the presence of subpixel
sized hotspots than are the longer wavelength observations.
Around the time of Oppenheimer’s (1989) study, the usefulness of high spatial resolution
data from the Landsat Thematic Mapper (TM) instrument was becoming evident to the
volcanological community. TM operates in six wavebands covering the visible, near infrared
and shortwave infrared region at 30 m spatial resolution, with an additional thermal infrared
channel operating at 120 m spatial resolution (Engel and Weinstein, 1983). Francis and
Rothery (1987) and Rothery et al. (1988) first analysed TM data of Lascar Volcano (Chile)
and found that certain summit pixels were anomalously radiant in both the thermal and
shortwave infrared wavebands, suggesting the presence of a very high temperature source
within Lascar’s active crater. They also found a similar signature in TM data of Erta ‘Ale
volcano (Ethiopia), where Le Guern et al. (1979) had documented the presence of a long
duration active lava lake. These early TM-based studies led to a whole new area of still
ongoing research, using the data for the measurement of volcanic surface temperatures and
relating these measurements to parameters such as mass and energy fluxes. These high
spatial resolution studies have been comprehensively reviewed by Flynn et al. (2000) and
Francis and Rothery (2000) and need not be covered here. However, one result of this work
was a reappraisal of the usefulness of lower spatial resolution data such as that provided by
AVHRR. Though extremely powerful as a measurement tool, the main problem when using
TM data to study or monitor active volcanoes was the relative paucity, high cost and long
delivery times of the imagery. Though TM views each of Earth’s volcanoes twice every 16
days (once by day and once by night), cloud cover and the fact that not all data are returned to
ground stations or archived (particularly the most useful nighttime data) means that many
volcanological studies were forced to rely on only one or two images. Since information on
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the longer-term changes in temperature and thermally emitted radiance at active volcanoes is
often more valuable than absolute values taken at any particular time, the relative difficulty
and cost associated with compiling time-series from Landsat TM proved a hindrance.
However, low-cost data from the AVHRR and other similar lower spatial resolution sensors
continued to be available on a daily or near-daily basis, potentially allowing detailed timeseries of the volcanic radiance history to be easily and cheaply compiled. Thus in the 1990s
data from these low spatial resolution sensors was re-examined as a method of extending the
more spatially and spectrally detailed, but temporally limited, observations made by Landsat
TM. Furthermore, since low spatial resolution data are generally made available for use soon
after being received by the ground station, they offer the possibility of near-real time
monitoring.
The new developments made using AVHRR were largely driven by A. Harris and co-workers
at the Open University (UK) who, in a series of papers, indicated how these data could be
used for identification of newly active lavas (e.g. Harris et al., 1995a), the monitoring of
effusive lava flows (e.g. Harris et al., 1995b; 1997a) and for observations of persistent
volcanic activity (e.g. Harris et al., 1999). Data from the AVHRR still suffered from the
problems of 3.7 µm sensor noise and saturation (Setzer and Verstraete, 1994; Harris et al.,
1995c) but their low cost and high temporal resolution led to their relatively wide use.
Because of this, volcanological developments in the use of data from the AVHRR mirrored
those in the wider Earth observation arena in the way that more research has been carried out
using this spaceborne sensor than any other, even though the AVHRR is far from ideal in
many respects (Cracknell, 1997). The 1990s developments in the use of the AVHRR for
volcano monitoring have been comprehensively demonstrated and reviewed by, for example,
Harris et al. (1997b), Oppenheimer (1999) and Dehn et al. (2000) and so will not be covered
in detail here. However, concurrent with the development of AVHRR-based volcanological
applications was the investigation of another Earth observation data source somewhat similar
to AVHRR but having some additional novel and potentially advantageous characteristics.
This data source was the Along Track Scanning Radiometer (ATSR), versions of which flew
on the European Space Agency (ESA) remote sensing satellites from 1991 onwards. The
ATSR instrument series has similar capabilities to the AVHRR, having the same spatial
resolution and similar infrared wavebands, but is more advanced in a number of important
areas including spectral range, radiometric resolution and data calibration (Delderfield et al.,
1985). This review concentrates on developments in the volcanological use of the ATSR
instrument series, the spur being that the global, full resolution ATSR data are now made
freely available in near-real time over the Internet by the European Space Agency/European
Space Research Institute (ESA/ESRIN). Whilst data from the new generation of high spatial
resolution remote sensing instruments (e.g. Landsat ETM+ and Terra ASTER) is typically
being made available more rapidly and at much lower cost than previously was the case, there
are still many instances where near-daily imaging or where data availability in near real-time
can make valuable contributions. The development of the ATSR NRT service allows
interested users, including those in the volcanological community, to access such data for
their own research or monitoring activities.
2.0 THE ALONG TRACK SCANNING RADIOMETER
2.1 The ATSR Instrument
The ATSR instrument series was designed by a consortium led by the UK Rutherford
Appleton Laboratory (RAL) to meet the exacting requirements of the World Climate
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Research Program (WCRP) in measuring long-term global sea surface temperature (SST) to
better than 0.5 °C accuracy (Houghton and Morel, 1983). Reflecting this primary mission the
original instrument (ATSR-1) was equipped with three wavebands in the middle-to-thermal
infrared (3.7, 11 and 12 µm) and one shortwave infrared (1.6 µm) waveband which was
designed to aid daytime cloud detection. The ATSR thermal infrared wavebands are very
similar to those of the AVHRR, which is also used to routinely measure global SST but to an
accuracy somewhat lower than that required by the WCRP (Kidwell, 1995). In order to meet
the WCRP SST accuracy requirement a number of novel features were included in the ATSR
design (Delderfield et al., 1985), including (i) a conical scanning mechanism that allows the
instrument to view the Earth’s surface through two different atmospheric path lengths and
thus improve the atmospheric corrections, (ii) a Stirling cycle cooler that maintains the
infrared focal plane assembly at around 80 K in order to minimise detector noise, and (iii) a
precision onboard blackbody calibration system which is extremely well characterised and
continually monitored in orbit. ATSR-1 was launched onboard the ERS-1 satellite via an
Ariane 4 rocket on 17 July 1991 and operated reliably until 1995, apart from the early failure
of the 3.7 µm channel in May 1992. By that time the second generation ATSR was already
being devised, with the same primary design as ATSR-1 but with the addition of three new
channels operating at visible and near-infrared wavelengths to improve land surface
monitoring (Stricker et al., 1995). ATSR-2 was launched onboard ERS-2 on 21 April 1995
and is expected to operate until at least 2002. The new Envisat satellite, expected for launch
in 2001, will carry the Advanced Along Track Scanning Radiometer (AATSR), whose data
will be effectively identical to that of ATSR-2. In this way a full resolution, global archive of
ATSR 1.6, 11 and 12 µm data will be available for an almost continuous 15-year period, with
3.7 µm data available for the latter 10 years, allowing detailed long-term time-series
observations of to be made of active volcanoes. The only major glitch with the ATSR-2
dataset so far has been a temporary sensor problem that halted data collection between
December 1995 and July 1996. With the development of the ESA/ESRIN near real-time
ATSR service at http://192.111.33.173/ATSRNRT/ the current ATSR-2 data for any location
are easily available to compare with the previous radiance history that may be extracted from
the long-term archive.
As outlined in Table 2, ATSR records data predominantly at infrared wavelengths, though
ATSR-2 and AATSR also possess two visible channels at green and red wavelengths. In
order to provide a sufficiently accurate atmospheric correction for the required SST
determination the sensor views the same location on the ground through two different
atmospheric path lengths (Figure 4). A particular location is first imaged during the forward
view scan at a zenith angle of 53 - 55 °. Then 137 seconds later, when the satellite is over
that same location, the nadir view will provide another image through a shorter atmospheric
path length at a zenith angle of between 0 and 22° depending on the exact position of the
ground location within the nadir view scan. The spatial resolution of the observations is 1 x 1
km in the centre of the nadir view scan, increasing to 2.8 x 1.6 km in the centre of the
forward view (Mason, 1991). In the standard ERS 35 day repeat cycle orbit, Earth locations
are typically images by ATSR once by night and once by day every 3 days. Figure 5 shows
the location of all the volcanic activity observed by ATSR that is included in this review.
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Channel
1*
2*
3*
4
5
6
7
Wavelength Range (µm)
0.54 – 0.56
0.64 – 0.67
0.80 – 0.87
1.57 – 1.65
3.48 – 3.90
10.51 – 11.32
11.60 – 12.507
Spectral Region
Visible
Visible
Near Infrared
Shortwave Infrared
Middle Infrared
Thermal Infrared
Thermal Infrared
Table 2. The channels of the ATSR instrument, carried onboard the ERS series of satellites
from 1991 to the present. * indicates a channel available on ATSR-2 and AATSR only,
operational from 1995 and 2001 (planned) onwards respectively.
Figure 4 The viewing configuration of the ATSR instrument, mounted on board the
polar orbiting ERS spacecraft. Earth surface data are obtained during both forwardand nadir-views, with the instrument detectors viewing the internal calibration targets
between each forward- and nadir-view scan.
Figure 5 Location of the active volcanoes included in this review. 1: Etna (Sicily), 2:
Galeras (Columbia), 3: Metis Shoal (Tonga Islands), 4: Fernandina (Galapagos), 5:
Lascar (Chile), 6: Unzen (Japan). Shaded elevation data are courtesy of the US NOAA
Global Land One-km Base Elevation (GLOBE) Project.
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ATSR data are usually provided in so called Gridded Brightness Temperature (GBT) format,
where data from both views have been calibrated into a measure corresponding either to topof-atmosphere pseudo-reflectance (channels 1 – 4) or brightness temperature (channels 5 - 7)
and all pixels are co-located and resampled to a 1km latitude-longitude grid. Figure 6 shows
an extract from am ATSR GBT data product of Sicily, including Mount Etna Volcano, whilst
Figure 7 shows a detailed comparison of co-located forward view and nadir view data.
Archived ATSR GBT data files typically contain data from all channels and both views,
along with additional information such as the geographic centre of each pixel. The file size is
approximately 10 MB per GBT image product. Detailed information on the design and data
characteristics of the ATSR instrument series can be found in, for example, Delderfield et al.
(1985), Tinker et al. (1985), Prata et al. (1990) and Stricker et al., (1995), whilst
documentation on the GBT data format can be found on the ESA ATSR
[http://earth.esa.int/eeo4.80] and Rutherford Appleton Laboratory ATSR project
[http://www.atsr.rl.ac.uk/] web sites.
Figure 6 ATSR 1.6 µm channel image data extracted from an ATSR GBT product of
Sicily taken 31 December 1991. The forward- and nadir-view images appear almost
identical since data from both scenes have been geometrically resampled to the same
512 x 512 pixel grid. Detailed examination of the two views would, however, reveal
differences in image clarity since the original forward view sampling is less dense and
the pixel size somewhat greater than the nadir view.
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Figure 7 Subscenes extracted from the 1.6 µm (a) forward-view and (b) nadir-view of
an ATSR GBT data product of the Oman coastal region. The data from each view are
co-located but the lower resolution of the forward view observations is immediately
apparent. Data were obtained at 6:48 (forward view) and 6:50 GMT (nadir view) on
3 July 1992.
2.2 ATSR Near Real Time Service
The ATSR Near Real Time (NRT) service [http://192.111.33.173/ATSRNRT/] was
developed by ESA/ESRIN to provide free rapid online access to the ATSR image products
that are generated for the Earth on a daily basis. The database has an hourly update
frequency and at full acquisition capacity the number of products found on the service is
about 1000 per day, with data stored online for around 5 days after acquisition. Potential
users of the service are encouraged to read the introductory online documentation
[http://192.111.33.173/ATSRNRT/help/helpGuide.html] provided by ESA/ESRIN, who they
may
then
contact
for
the
appropriate
username
and
password
[http://192.111.33.173:80/ATSRNRT/data_policy.html] required to make use the service. It
is important to note that each GBT data file made available online via the NRT service is
actually split into ten mini-products so that only data useful to the particular user in question
need be downloaded (thus minimising data transfer times). In this way scientists interested in
infrared volcano monitoring may choose to download only the mini-GBT product containing
the nadir-view, thermal channel data for example. The mini products have a second
extension appended to the main GBT data product name to denote their respective contents.
Thus the original ATSR GBT format product of a ‘Test’ location dated 17 December 1998
might be called:
test-de-9812170649-20060-981217-2av300.gbt-tvlxc
- GBT full data product
whereas on the NRT service it would be split into ten mini-products made available online:
test-de-9812170649-20060-981217-2av300.gbt-tvlxc.H – GBT Product Header
test-de-9812170649-20060-981217-2av300.gbt-tvlxc.T – Nadir View Thermal Channel Data
test-de-9812170649-20060-981217-2av300.gbt-tvlxc.V – Nadir View Visible Channel Data
test-de-9812170649-20060-981217-2av300.gbt-tvlxc.Tf - Forward View Thermal Channel
Data etc
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These GBT mini-products can be read into standard image processing software by specifying
the structure of each particular file, which is outlined on the FTP data download pages
[http://192.111.33.173:80/ATSRNRT/help/options.html] of the NRT service. For example
the Nadir View Thermal Channel mini-product contains 4 bands (1.6, 3.7, 11 and 12 µm) of
512 x 512 unsigned integer data, with each pixel recording the brightness temperature x 100.
This equates to a total file size of 2097152 bytes. If desired the full set of mini-product files
can be downloaded and re-combined to reproduce the original full GBT product. Full details
of the NRT GBT mini-product data format are provided in the PDF document ‘ATSR
Quickguide’ (Buongiorno, A., 1999) which can be downloaded on the NRT service help page
[http://192.111.33.173/ATSRNRT/help.html] and are also outlined on the FTP data download
pages [http://192.111.33.173:80/ATSRNRT/help/options.html] of the NRT service.
3.0 Advantages for volcanic observations
One of the primary advantages of ATSR for volcano monitoring is that it possesses a
shortwave infrared channel that records during nighttime observations. Shortwave infrared
data taken by the Landsat TM have been shown to be highly valuable for volcanic thermal
studies (e.g. Flynn et al., 2000) but ATSR is the first instrument series offering nighttime
shortwave infrared data taken at low spatial resolution. Because of its shorter wavelength,
the 1.6 µm channel is capable of making unsaturated spectral radiance measurements over a
much wider range of high temperature targets than the AVHRR or ATSR 3.7 µm channel
(Figure 8). This is very important for quantitative studies. However unlike the 30m
observations of Landsat TM, the large 1km pixel size of ATSR means that daytime 1.6 µm
thermal flux observations of volcanic phenomena are usually swamped by the solar reflected
signal, so volcanic thermal studies at this wavelength are best carried out via nighttime
observations. A further advantage of nighttime data is that any spectral radiance recorded at
1.6 µm is solely derived from volcanic hotspots since, unlike at 3.7 µm, 1.6 µm emission
from bodies cooler than a few hundred degrees Celsius is minimal (Figure 1). In fact the
version of the AVHRR instrument on the latest NOAA satellites, the 3rd version of the
instrument, also possess a 1.6 µm channel. However, unfortunately for volcanic observations
operation of this channel is apparently alternated day/night with that of the 3.7 µm channel,
meaning 1.6 µm nighttime data and 3.7 µm daytime data are now unavailable from this
instrument (Cracknell, 1997). In fact ATSR-1 also could not operate the 1.6 µm and 3.7 µm
channels concurrently. However, rather than a simple day/night switch ATSR-1 used a
radiance threshold so that observations having a sufficiently large 1.6 µm signal were always
recorded in that channel, whilst at other times the 3.7 µm signal was recorded. Generally this
means that ATSR-1 made nighttime measurements in the 3.7 µm channel unless a hot target
was visible that gave a signal at 1.6 µm (such a target may have saturated the 3.7 µm channel
anyway). In May 1992 the 3.7 µm detector failed so the 1.6 µm signal was recorded
continuously after that. For ATSR-2 the sensor design was updated so that 1.6 and 3.7 µm
data can be recorded concurrently.
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Figure 8 The sensor saturation envelope for observations of subpixel sized hotspots at
3.7 µm (AVHRR and ATSR sensors) and 1.6 µm (ATSR sensor only). Observations at
1.6 µm are unsaturated over a much wider range of hotspot temperature/size
combinations than observations at 3.7 µm. For this modelled example the ambient
background temperature has been set to 10 °C. Increases in background temperature
make 3.7 µm observations more prone to saturation (since thermal emission from the
background will be increased) but will not affect the situation at 1.6 µm since this
shorter wavelength is insensitive to such low temperature surfaces.
The vast majority of volcanic studies using the ATSR have concentrated on use of the nadirview 1.6 µm data, often in combination with data from one or more of the other infrared
channels in order to maximise the sensitivity to the wide range of surface temperature
conditions found on active volcanoes. Due to the extreme viewing angles the forward view
data are of a lower spatial resolution and the volcanic phenomena of interest are more likely
to be shielded from view by cliffs or crater walls (Mounginis-Mark et al., 1994). However, a
study by Wooster et al. (1998a) indicates that in unshielded situations the forward view and
nadir view observations provide the same information provided data from all thermally
anomalous pixels is correctly combined. Thus certain aspects of the geometry of observed hot
spots can be assessed using by comparing forward looking and nadir data sets. An additional
advantage of the dual view nature of the data is in the stereoscopic study of eruption clouds in
order to determine their height. However, all the studies described in this review concentrate
on ATSR nadir view data only, which Conway et al. (1995) show to provide a more tightly
focused field-of-view than that provided by AVHRR. This helps alleviate the ‘smearing’ of
hotspots that has sometimes been associated with AVHRR observations as mentioned in
Oppenheimer (1989), the improvement being a function of the particular viewing mechanism
employed by the ATSR instrument, as outlined by Prata et al. (1990) and Wooster et al.
(1998a).
As an example of ATSR data of an active volcano, Figure 9 shows a subset centred on Mount
Etna, and taken from the same ATSR GBT product shown in Figure 6. Images from the
ATSR short, middle and longwave infrared channels are shown. The scene was recorded
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during the early stages of the 1991-1993 Etna lava flow, which formed a 7.6 km long flow
field extending down the Valle del Bove on the volcano’s eastern flank (Calvari et al., 1994).
It is clear that the different wavelength channels respond in varying ways to the subpixel high
temperature features. For example, the active lava flow is visible as a bright linear feature in
all channels, but the summit craters (just north of the lava flow) are visible only at the shorter
3.7µm and 1.6 µm wavelengths because these are much more sensitive to subpixel hotspots
(Figure 1 and 2). The relative spatial coarseness of the ATSR measurements is apparent when
compared with a daytime TM image recorded two days later (Figure 10). It is apparent that
TM is far better for mapping and recognising features on the volcano. However, the low
cost, frequent imaging capability and lack of 1.6 µm channel saturation provided by low
spatial resolution data such as ATSR combine to make them of great value to the
investigation and monitoring of active volcanoes.
Figure 9 Calibrated, geo-referenced TM band 5 daytime imagery of the Etna region,
obtained on 2 January 1992 during phase 2 of the 1991 - 1993 eruption. Certain
surfaces on the active lava flow within the Valle del Bove of the volcano are hot enough
to emit significant amounts of radiation within this waveband. These show up as highly
radiant (bright) pixels. Outlined areas correspond to the regions affected by the lava
flow and summit crater activity shown in the ATSR data of Figure 8. The linear traces
downscan of the hottest areas of the flow field are due to sensor saturation effects. It
should be noted that the later version of the Landsat Thematic Mapper (Landsat 7
ETM+) does not suffer from these downscan saturation effects.
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Figure 10 Calibrated, geo-referenced TM band 5 daytime imagery of the Etna region,
obtained on 2 January 1992 during phase 2 of the 1991 - 1993 eruption. Certain
surfaces on the active lava flow within the Valle del Bove of the volcano are hot enough
to emit significant amounts of radiation within this waveband. These show up as highly
radiant (bright) pixels. Outlined areas correspond to the regions affected by the lava
flow and summit crater activity shown in the ATSR data of Figure 9.
4.0 EXAMPLES OF ATSR VOLCANO SURVEILLANCE APPLICATIONS
4.1 Identifying new effusive eruptions
The first test of whether ATSR data could identify a new effusive eruption were made at
Galeras Volcano, Colombia (Wooster et al., 1998b). In 1989 Galeras had seen no significant
eruptive activity for 12 years and 4000 people lived within the high hazard zone that had
previously been affected by short (<5 km) pyroclastic flows during the latest major eruption
in 1977. In February 1989 fumarolic and ash emissions began at Galeras and, sometime in
October 1991, a lava dome with a diameter of around 100 m emerged within the active crater,
being first noticed during an aircraft overflight (Smithsonian Institution, 1991). ATSR data
were investigated to see whether they could have provided an early warning of this new dome
emplacement. Nighttime imaging occurred every three days but the cloudy conditions
characterising this region gave only six nighttime, cloud-free ATSR scenes of the volcano
during October and November 1991. In all but one scene the volcano was characterised by a
significant 3.7 µm radiance anomaly when compared to the immediately surrounding pixels
(Figure 11). However on the 12 October scene the sensor recorded a significant 1.6 µm
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signal, meaning the 3.7 µm measurement was not available (see Figure 9 caption). This
suggests that the first appearance of fresh lava within the active crater lay sometime between
30 September and 12 October 1991, consistent with the first aerial observations of the new
dome. Interestingly the magnitude of the 12 October 1.6 µm radiance anomaly appears lower
than the 3.7 µm anomaly recorded on the immediately subsequent date, even though certain
surfaces would be at their maximum temperatures at or soon after emplacement. This is
explained by reference to Figure 2, which indicates that for temperatures below ~ 950 °C
more energy is emitted at 3.7 µm than at 1.6 µm and also that significant emittance at 1.6 µm
ceases at around 400 °C but continues to much lower temperatures at 3.7 µm. Combined
with the fact that the hottest surfaces of the volcano (> 950 °C) will likely be very much
smaller than the areas at lower temperatures, this provides the explanation for why the 1.6 µm
signal is lower than the former 3.7 µm signal, despite the supposed greater activity soon after
emplacement. After 12 October, data recording reverted back to the 3.7 µm channel,
presumably because more of the lava surface had become cooled or because incandescent
cracks had become fewer or narrower. Continued cooling is evident from the subsequent
3.7 µm radiance decreases. On 16 July 1992 an explosive eruption, presumably related to the
preceding dome development, destroyed 90 % of the dome and ejected blocks of 1-m
diameter up to 1.3 km from the volcano.
Figure 11 The 3.7 µm and 1.6 µm spectral radiance anomaly at Galeras Volcano,
Columbia. The best fit 2nd order polynomial to the 3.7 µm data illustrates the general
trend. The switch-on date of the ATSR 1.6 µm channel by high radiance levels is close
to the first sighting date of the new summit lava dome.
At the same time as the Galeras study a further examination of ATSRs capacity to detect new
lava dome eruptions was made under a very different setting, namely a subsea volcanic
location. This work is reported here for the first time. The target was Metis Shoal, a
normally submerged volcano located in the Tonga Island group. Eight previous eruptions of
Metis Shoal are known to have occurred since 1851, with temporary islands being created on
at least three of these occasions (Smithsonian Institution, 1995). The 1995 eruption was
reported to have commenced on 6 June 1995, with the activity first breaching the surface on
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12 June. By the end of the eruption in late June a 50 m high lava dome of ~ 280 m diameter
had been formed. Nine ATSR-1 images were available between the eruption start and end,
though only six were cloud free at the volcanic location (6, 11 and 28 June). Due to the May
1992 failure of the ATSR-1 3.7 µm channel only the 1.6 and 11 µm data were available for
these observations. Analysis showed no 1.6 µm anomaly evident, presumably because the
aquatic nature of the activity rapidly cooled the dome surface. However on 11 June a large
6.5 K anomaly was present in the 11 µm channel at the volcanic location, with lesser
surrounding anomalies presumably related to heating of the neighbouring surface waters. By
28 June, at the end of the eruption, only a small ~1K anomaly was detectable at the location
of the volcano, indicating the end of the eruptive episode (Smithsonian Institution, 1995).
Again ATSR data had proved its ability to detect and monitor signs of renewed volcanic
activity and, in March 1999, similar methods were used by the European Space Agency to
identify new effusive lava flows at Mount Cameroon, West Africa
(http://earth.esa.int/cameroon.html).
4.2 Monitoring effusive lava flow eruptions
Once an eruption has been identified, ongoing monitoring can provide information on the
waxing or waning of activity, and quantitative radiance measurements maybe useful in
parameterising numerical models of the processes involved. ATSR were first used to
examine effusive volcanic activity variations at Fernandina volcano on the Galápagos
archipelago, where the remote and uninhabited nature of the island often prevents detailed
study of eruptions in-progress. Fernandina erupted most recently between January and April
1995 and Wooster and Rothery (1997a) used ATSR data to document the development of the
lava flow field (Figure 12). As outlined in that study the 1.6 µm and 11 µm channels are
primarily responsive to different components of the lava flow surface, namely the exposed
highest temperature material and the much cooler lava flow crust respectively. Using
assumptions about the likely emitting temperature of these surfaces Wooster and Rothery
(1997a) were able to convert the observed spectral radiances into estimates of the area
covered by these two lava flow surface components. The indeterminacy of the solutions was
addressed by performing the calculations for a realistic range of surface temperatures,
allowing maximum and minimum areas to be determined (Figure 13). The uncertainty of the
derived estimates is large, but encouragingly the maximum estimated area matched with the
true lava flow area determined via analysis of SPOT high-resolution images.
In addition to Fernandina, the other basaltic volcano known to have been studied using ATSR
is Mount Etna, Sicily. Indeed the relatively frequent eruptions of this volcano have formed
the target for a large number of thermal remote sensing studies (e.g. Pieri et al., 1990; Harris
et al., 1997a,b). Cooling of Etna’s large 1991-1993 lava flow was the subject of a paper by
Wooster et al. (1997) who used ATSR data such as that shown in Figure 9 to parameterise
cooling models of the 8 km long flow field. These models suggested that radiative and
conductive heat loss processes provided comparable energy losses and were most efficient
during the first three months of the eruption (Figure 14). During this early period open lava
channels radiated strongly and horizontal spreading of the lava rapidly increased the area of
magma in contact with new ground (Calvari et al., 1994; Harris et al., 1997a; Wooster et al.,
1997). Results from the cooling models were used to estimate that by the end of the eruption,
the lava had lost less than 30 % of the energy it possessed by virtue of its initial high
temperature, suggesting the flow will take more than a decade to cool.
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Figure 12 Left: 11 µm ATSR data recorded on 8 February 1995, masked to show
thermally anomalous (lava flow-field) pixels only and superimposed on an outline of
Fernandina. Right: 1.6 µm (upper row) and 11 µm (lower row) data from two of the
eight available ATSR scenes of the 1995 eruption. The geographical area covered is
that box outlined in the Figure on the left. Large changes in the 1.6 µm signal reflect
the rapid formation of sub-surface lava tubes (which prevent the sensor from viewing
the highest temperature material) whilst the more slowly varying 11 µm signal reflects
cooling of the lava flow surface crust.
Figure 13 The area of (a) exposed hot core and (b) cooling lava crust derived from the
eight nighttime, cloud-free ATSR scenes of the 1995 Fernandina eruption, showing the
minimum and maximum values permitted by parameterising the retrieval model with a
range of permitted lava flow temperatures. The best-fit 2nd order polynomials to the
data are also shown.
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ATSR data of the same 1991-1993 Etna eruption were used by Harris et al. (2000a) to
parameterise a model that attempts to determine magma effusion rates from similarly
produced estimates of lava flow heat loss. Temporal variations in the estimated effusion rates
compared well with those produced using more traditional ground-based observations and
Wright et al (2000) suggest reasons for the strong level of agreement when considering the
relatively poor resolution of the data. The more recent eruptive activity of Etna has also been
studied with ATSR, with Rothery et al. (in press) comparing the mean 1.6 µm and 11 µm
thermal anomalies from the 1996 – 1999 activity within the summit crater vents to the same
figures determined for the 1991 – 1993 extra-crater lava flow (Figure 15). They found that
the intra- and extra- crater activity plotted in different regions of the 1.6 µm / 11 µm feature
space, thus providing a simple method for discriminating new summit or flank lava flows
from less significant summit crater activity. The method was tested using data from February
– March 1999, when new extra-crater lava flows that developed over this period were
successfully discerned (Rothery et al., in press).
Figure 14 (a) Radiative and (b) conductive power losses during Phases I - V of the 1991 1993 Mount Etna lava flow as estimated by Wooster et al. (1997), with Phase intervals
defined by Calvari et al. (1994). Radiative power loss from the surface of the flow were
calculated from analysis of ATSR-1 nighttime imagery, with the upper and lower curves
giving the maximum and minimum solutions obtained using the supposed potential
range of temperatures for the lava flow thermal components. Power loss due to
conduction through the base of the flow was calculated according to Pitts and Sissom
(1977), with the time-invariant lava flow basal temperature estimated using the
procedures of Turcotte and Schubert (1982).
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Figure 15 Statistics from ATSR pixels at Mount Etna identified as thermally
anomalous during the December 1991- March 1993 extrusive lava flow and the July
1996- January 1999 period when activity was confined within the summit vents. The
statistics shown are the mean 11µm brightness temperature and the mean 1.6 µm
spectral radiance for all anomalous pixels, with dates for each of the 1991-1993 data
points shown. The lava flow data points fall outside the field of vent activity, and it is
notable that this is true even for data recorded during the first month of the flow
(December 1991).
4.3 Long-term monitoring of active lava domes
Certain active volcanoes exhibit some form of almost persistent lava dome activity for years
or even decades. However, the activity styles or magnitudes may vary greatly and so remote
monitoring of these sites can be of value. The volcanic location that has been subject to the
most intensive, long-term scrutiny with satellite thermal remote sensing is Lascar Volcano in
the Chilean Andes. Lascar’s activity has been targeted by a series of thermal remote sensing
campaigns because the characteristic lava dome and fumarole activity provides a suitably
high temperature target that is rarely obscured by cloud due to the volcano’s arid location
(e.g. Francis and Rothery, 1987; Rothery et al., 1988; Glaze et al., 1989). In the most
extensive Landsat TM study Oppenheimer et al. (1993) showed that Lascar’s largest
eruptions appear to be preceded by falls in the level of thermal radiation emitted from
fumarolic vents on the active lava dome. Wooster and Rothery (1997b) showed that this
finding could be replicated using more frequent but lower spatial resolution ATSR data,
allowing the monitoring period of Oppenheimer et al. (1993) to be extended and enhanced.
Using inferences drawn from these and other data, Matthews et al. (1997) developed a model
for explaining Lascar’s eruptive cycles. They proposed that during each cycle a lava dome is
extruded in the active crater, accompanied by vigorous degassing through high temperature
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fumaroles on and around the dome. The fumaroles are the primary heat source observed via
thermal remote sensing and the degassing results in a steady decrease in the bulk volume of
the conduit magma, ultimately causing subsidence of the dome back into the vent and
constriction of the fracture system. This can impede further gas release and so a fall in the
SWIR signal will be observed while inside the conduit gas pressure increases. Ultimately the
pressure build-up may result in a large explosive eruption, such as occurred in September
1986, February 1990, April and December 1993 and, most recently, July 2000 (Smithsonian
Institution, 2000). Regular monitoring of Lascar has continued with ATSR data and Wooster
(2001) has shown that the July 2000 eruption was once again preceded by a significant fall in
thermal radiance one month previous to the eruption and which observed in both the 1.6 and
3.7 µm channels (Figure 16).
Figure 16 The full ATSR time-series of 1.6 µm and 3.7 µm spectral radiant flux for
Lascar Volcano, determined via the ATSR-1 (1992-1995) and ATSR-2 (1995 – 2000)
instruments. The ATSR-2 records include concurrent 1.6 µm (main plot) and 3.7 µm
(inset) radiance anomaly measurements. The 3.7 µm anomaly was calculated as the
difference between the observed 3.7 µm signal and the 3.7 µm signal predicted using the
simultaneously recorded 11 µm brightness temperature. This provides a robust
measure of the high-temperature (volcanic) component of the observed 3.7 µm signal
since it minimises the effect of background temperature variations. No ATSR data is
available between January and July 1996 due to a temporary sensor malfunction.
Arrows indicate the major explosive eruptions with ash columns reaching elevations of
8 km or greater.
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However, the same ATSR time-series of Lascar show that a major fall in radiance also
occurred in August 1998, when the 1.6 and 3.7 µm signal levels continued at low levels for 1
year, but which was not followed by a significant explosive eruption until the July 2000 event
(Figure 16). The reason for the differing activity styles is not clear and this finding indicates
that unambiguous interpretation of apparent eruption thermal precursors remains difficult,
even at this well studied location. Nevertheless, it certainly appears that low spatial
resolution data such as those obtained from ATSR can be used to identify periods when
Lascar, and presumably other similarly acting volcanoes, should be further examined.
A further extension of this principle is the use of ATSR and similar resolution data to aid in
the interpretation of GOES geostationary satellite imagery, which provides 15 minute
temporal monitoring of volcanic hotspots but at very low spatial resolution (5 km). At Lascar
GOES is just capable of detecting the summit thermal anomaly, but inter-comparison of
variations in these data with those from the higher spatial resolution ATSR instrument
provide increased confidence in the interpretations of the GOES real-time data stream (Harris
et al., 2000b). The time-series shown in Figure 16, and discussed more fully in Wooster
(2001) also highlights the complementarity of the ATSR-2 1.6 µm and 3.7 µm measurements.
Trends in the 1.6 µm and 3.7 µm thermal fluxes show a significant positive correlation (r² =
0.57, n =166) but 3.7 µm anomalies are capable of being tracked even when the 1.6 µm
signal is absent. This is due to the previously highlighted lower temperature sensitivity of the
3.7 µm channel (Figure 2). Conversely, however, in the most active periods the 3.7 µm
signal is often saturated, whereas this effect never hampers the 1.6 µm channel and so these
shorter wavelength data provide the most reliable quantitative tracking of the volcanic
radiance during these high activity periods.
Another lava dome that has been subject to analysis using ATSR is the 1991-1995 dome at
Japan’s Unzen Volcano (Wooster and Kaneko, 1998). In this case detailed geophysical
datasets allowed the ATSR signals to be compared to other eruption parameters.
Comparisons of ATSR data with the estimated rate of magma supply to the growing edifice
showed significant correlation during the two discrete phases of the eruption (Figure 17).
This provided further evidence of the potential quantitative value of low spatial resolution
satellite thermal remote sensing data were there to be an absence of such detailed groundbased datasets. Furthermore, at Unzen temporal tracking of the 1.6 µm signal provided easy
identification of a second magma supply pulse to the growing dome in February 1993, after a
year-long decrease in the supply rate during the first phase of lava dome growth (Wooster
and Kaneko, 1998). Such identification is important since the periods of increase magma
supply correspond to the times of major pyroclastic flow at this volcano, these flows being
largely driven by collapses of the growing dome front (Sato et al., 1992).
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Figure 17 The phase 1 and 2 relationships between the shortwave infrared (1.6 µm)
spectral radiance of the ATSR pixel corresponding to the Mount Unzen lava dome and
the lava effusion rate estimated by interpolation of the data collected by the Joint
University Research Group (JURG) and the Geographical Survey Institute (GSI) of
Japan.
5.0 CONCLUSIONS AND FUTURE PROSPECTS
Like every other spaceborne thermal remote sensing instrument that has been used to observe
active volcanoes, ATSR was never designed for this purpose. However, its characteristics of
excellent data calibration, simultaneously operating multiple shortwave-to-thermal infrared
wavelengths, and its relatively high temporal frequency have combined to make it a
potentially powerful tool to assist volcanological remote sensing research and volcano
surveillance. Nevertheless ATSR is not ideal for this purpose and the data it provides are
usefully augmented by higher spatial resolution datasets where available, or by suitable
geostationary satellite imagery if a very high temporal frequency is required. This review
had provided a summary of the volcanological applications of the ATSR instrument so far,
and has outlined certain of the technical points required to make best use of the imagery. As
mentioned in Section 2, it is now possible for interested researchers and those involved in
volcano monitoring to obtain ATSR data in near real-time over the internet. We would
encourage interested parties to avail themselves of this opportunity and note that internally
ESA/ESRIN have already tested a prototype near real-time monitoring system based on
ATSR infrared radiance data (VOMIR – Volcano Monitoring by Infrared), routinely
examining signals from 140 volcanoes world-wide. We look forward to further automated
developments in thermal volcano monitoring with the launch of the Envisat satellite carrying
the Advanced ATSR instrument in summer 2001
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ACKNOWLEDGEMENTS
This paper is derived from a presentation made by M Wooster at the workshop ‘Real Time
Infrared Volcano Monitoring from Space: Principals, Tools and Issues’, part of the IAVCEI
2000 General Assembly held in Bali (Indonesia) in July 2000. The Royal Society are
thanked for generously funding the attendance of M Wooster at this workshop through their
Conference Grants programme. M Wooster is part-supported by the NERC Earth
Observation Science Initiative. All ATSR data reproduced in this manuscript are courtesy
NERC/RAL/ESA/BNSC. The authors thank the two anonymous reviewers for their useful
suggestions in improving the clarity of this manuscript.
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