A methodology for gas flaring detection and characterisation using SLSTR

Gas flaring is a disposal process widely used in the oil extraction and processing industry. It consists in the burning of unwanted gas at the tip of a stack. We have successfully adapted the VIIRS Nightfire algorithm for the detection and characterisation of gas flares to SLSTR observations on-board the Sentinel-3 satellites. A hot event at temperatures typical of a gas flare will produce a local maximum in the night-time readings of the shortwave and mid-infrared (SWIR and MIR) channels of SLSTR. The SWIR band centered at 1.61 μm is closest to the expected spectral radiance maximum and serves as the primary detection band. The hot source is characterised in terms of temperature and area by fitting the sum of two Planck curves, one for the hot source and another for the background, to the radiances from all the available SWIR, MIR and thermal infra-red channels of SLSTR. The flaring radiative power is calculated from the gas flare temperature and area. Our algorithm differs from the original VIIRS Nightfire algorithm in three key aspects: (1) It uses a granule-based contextual thresholding to detect hot pixels, being independent of the number of hot sources present and their intensity. (2) It analyses entire clusters of hot source detections instead of individual pixels. This is arguably a more comprehensive use of the available information. (3) The co-registration errors between hot source clusters in the different spectral bands are calculated and corrected. This also contributes to the SLSTR instrument validation. Cross-comparisons of the new gas flare characterisation with temporally close observations by the higher resolution German FireBIRD TET-1 small satellite and with the Nightfire product based on VIIRS on-board the Suomi-NPP satellite show general agreement for an individual flaring site in Siberia and for several flaring regions around the world. Small systematic differences to VIIRS Nightfire are nevertheless apparent. The retrieved flaring radiative power can be used to calculate flared gas volumes when calibrated against reported flared gas volumes. The estimated flared gas volume can be further combined with published emission factors in order to compute emissions of atmospheric trace constituents like carbon dioxide and black carbon.


Introduction
Gas flaring (GF) is part of the upstream oil and gas industry processes as a means of disposing of unwanted natural gas through high temperature oxidation at the tip of a stack.Flaring produces greenhouse gases (GHG) and black carbon as the main by-products of the combustion.In terms of the global GHG budget, the importance of flaring is diminutive [1].However, regarding the short-lived climate forcer black carbon, GF may be of regional prime importance.GF has been identified as the main input of black carbon in boreal regions [2,3], with implications for the albedo of snow-covered surfaces, the earth's radiative balance [4,5] and the Arctic amplification phenomenon [6], being therefore of global relevance [7].Again in the Arctic region, GF's contribution to the NO 2 concentrations is important, and has been increasing in the past decade [8].GF also impacts the local environment [9]: noise [10,11], a visual pollution [12,13], heat stress [12,14] and the emission of further air pollutants like polycyclic aromatic hydrocarbons, volatile organic compounds and acid rain precursors [15][16][17][18].
The information on flared volumes and emissions is sparse and methodologically inconsistent due to technical difficulties and the lenient reporting requirements and guidelines of some jurisdictions.Remote sensing offers the possibility of a globally consistent and independent monitoring of flaring.The identification of flaring in the night-time data of visual and near infra-red (Vis-NIR) spectral range [19,20] allowed for the first semi-automatic monitoring of flares from space [21][22][23].With the advent of vegetation fire detection products based on mid and thermal infrared bands (MIR and TIR), flaring was highlighted as a main source of false alarms [24,25].This feature was exploited to study flaring [26,27] and later the additional evaluation of short-wave infrared (SWIR) bands allowed for a more accurate detection [28][29][30][31][32], being at present the state-of-the-art.
Casadio et al. [29] considered the radiances at four wavelengths (in the SWIR, MIR and TIR bands of (A)ATSR) to be a linear combination of black body radiances from two areas with different temperatures within the satellite pixel footprint (actively flaming and background).Elvidge et al. [30] developed the Nightfire algorithm in which a Planck curve is fitted to the night-time Vis-IR measurements of the Visible Infrared Imaging Radiometer Suite instrument (VIIRS, on-board the Suomi-National Polar Partnership) and considered an emission scaling factor to estimate the flare size.The latter methodology was further developed into a dual Planck curve fitting (one Planck curve fitted to the background signal and one fitted to the flare signal) to the observations in five bands (NIR, SWIR and MIR) of VIIRS [32] and used to retrieve information on the global distribution and characteristics of gas flaring.Its results are made public by NOAA's National Centre for Environmental Information as daily global fields.
The BIRD algorithm was developed to apply the bi-specral method [33] to MIR and TIR data from the Hot Spot Recognition System (HSRS) instrument on board the bi-spectral Infrared Detection (BIRD) Experimental Small Satellite (2001--04) [34][35][36].It was designed to retrieve effective temperature, effective area and effective radiative power of sub-pixel hot sources, namely fires [37,38].The methodology was later ported to the successor FireBIRD mission, with a similar sensor in IR and a modified Vis-NIR payload on board of the TET-1 (Technologieerprobungsträger-1) spacecraft [39].
The burnt gas volume in billion cubic metres (BCM) and emissions to the atmosphere can be estimated from satellite observations of gas flaring.This estimation uses a conversion factor that scales observed radiative energy release to BCM together with emission factors that convert BCM into the amount of different chemical smoke species.The emission factors have been measured in-situ and reported in the literature.The conversion factor has been calculated as fraction of reported BCM and the observed radiative energy in entire countries.[32].
The Sea and Land Surface Temperature Radiometer (SLSTR) instrument on board ESA's Sentinel-3 features night-time observations in two SWIR (S5: 1.61 µm and S6: 2.25 µm), one MIR (S7: 3.74 µm) and two TIR bands (S8: 10.85 µm and S9: 12.0 µm).The instrument also measures in two fire-dedicated bands (F1: 3.74 µm and F2:10.85 µm) with the same central wavelength and band width as S7 and S8, but extended dynamical ranges to prevent saturation over active fires.[40] The distribution of the SLSTR spectral channels with SWIR, MIR and 2 channels in TIR should allow for the detection and characterisation of GF via the SWIR detection and the dual Planck curve fitting methodologies.A dedicated SLSTR gas flare product would nicely fill some of the observation gaps of the VIIRS Nightfire product since the instruments are on satellites with different overpass times.Furthermore, the long-term commitment of the EU Copernicus programme, which funds the Sentinel satellites, would warrant data availability well into the 2030s.
In this paper, we present a new algorithm for gas flare detection and characterization from SLSTR observations and apply it to actual SLSTR data.Section 2 presents the data used.Section 3 describes the developed methodology in detail.In Section 4 we present the results of the application of the newly developed algorithm at the regional level in four regions of interest (West Africa, The North and Caspian Seas and the Persian Gulf) and evaluate our methodology against the VIIRS Nightfire product.The performance of our algorithm is further evaluated against VIIRS Nightfire and retrievals based on HSRS on-board the German small satellite TET-1 at the level of a single gas flaring site on the Yamal peninsula, Northern Siberia.Finally, in Section 5 we present our conclusions.

Data
SLSTR Level 1b version 2 products obtained from the ESA's Sentinel Expert User's Hub in January 2017 were used for this work.The downloaded products were selected using a geographic criterion (four regions of interest: West Africa, the Caspian and the North Seas and the Persian Gulf) and a time of day criteria (only night-time acquisitions).The products were sampled in the second half of 2016.For the SWIR bands (S5 and S6), the data available in the product are top of the atmosphere (TOA) radiances, while for the MIR (S7 and F1) and TIR (S8, S9 and F2) bands, the available data are brightness temperatures.The latter were converted back to TOA radiances using lookup tables provided by the European Space Agency.
FireBird TET-1 level-2 co-registered data of TET-1 night time acquisition mode (only MIR and TIR bands at 170 m spatial resolution) were obtained for near coincident or temporally close SLSTR and TET-1 observations during polar night conditions in 2016/17 over the Yamal peninsula and other areas in Northern Siberia.Due to solar light contamination, only data North of about 70 • latitude could be used, which excluded the other regions.Probably due to the extremely cold background, the standard level-2 fire processor of FireBird [39] failed to identify valid background pixels, and thus did not detect any hot clusters.The co-registered data were therefore reprocessed with the BIRD night-time algorithm [38] which was adapted to the spatial resolution of TET-1 and the cold background.The algorithm for fire detection and characterisation output includes an estimate of the fire area and its uncertainty, the fire temperature and its uncertainty, and the fire radiative power [38] which were used for comparison with the temporally close SLSTR retrievals.
Table 1 summarizes the specifications of the used sensors, while Table 2 summarizes the data used.

Methodology
A general flowchart of the developed methodology is shown in Figure 1.The individual processing steps are described in the following sections.

Detection
The SLSTR SWIR channels S5 and S6 capture the peak radiation of typical gas flaring temperatures around 1500 K as given by the Planck equation (Figure 2).Remote detection of gas flares with space-borne SWIR observations has been using a priori fixed thresholds on one or several bands [28,29,31] or determining the threshold based on the pixel's surroundings, i.e. contextual thresholding [30,32,41].
In the present work, we use contextual thresholding by identifying all pixels that stand out of the continuum of background values by more than one digitization step size of the respective satellite band.All pixels with a radiance larger or equal to the threshold radiance B threshold λ are detections and processed further: where } is the ordered set of the 1000 largest radiances in the granule (typically SWIR bands: 2400 × 3000 pixels per granule, MIR and TIR bands: 1200 × 1500 pixels per granule), and s f λ is the scale factor for the product and that band, which is calculated from the smallest interval between records due to the digitization of the analogue radiometric signal.
Several approaches were tested, the selected one was the most robust, identifying hot pixels in different contexts.This approach captures hot pixels when there is a gradual or an abrupt increase (upper and lower plots of Figure 3, respectively).While not fixed, and therefore adaptable to product-specific conditions, this contextual methodology is not statistical and therefore not directly sensitive to the number of hot events sampled and their intensity.
At the end of this step, a collection of i hot pixels are registered, for each of the four SWIR and MIR bands, with the following information: • hot pixel location (x i , y i ) as pixel index pair

Clustering
The clustering of contiguous hot pixels is necessary because the signal of a single gas flare may influence more than one pixel [27,42].It may also be the case that a single gas flare is detected in two or more adjacent pixels.[31,41] Indeed, a flaring facility may comprise arrays of many flares with sizes of up to several hundred metres, while individual stations are several kilometres apart.
A cluster is defined as a set of hot pixels which are adjacent spacially.A cluster may comprise only one pixel if none of its adjacent pixels are hot.
A background area is defined as extending two pixels beyond the cluster's limits in any direction, including diagonally.For each single pixel i of the n cluster's pixels, the radiance B λ,i and area A λ,i are registered.Likewise, for each single pixel j of the m background pixels, B λ,j and A λ,j are registered.
For each cluster, the following quantities are then computed: the average radiance in [Wm −2 µm −1 sr −1 ]: the average background radiance in [Wm −2 µm −1 sr −1 ]: the background radiance standard deviation in [Wm −2 µm −1 sr −1 ]: (4) the cluster area in [m 2 ]: the cluster x-position as across-track index: the cluster y-position as along-track index: where n is the number of hot pixels in the cluster and m is the number of pixels in the hot cluster's background.

Misregistration characterisation
A hot event at temperatures typical of a gas flare will produce a local maximum in the SWIR and MIR channels (see Figure 2), which will be translated into a cluster in those bands.The MIR fire channel F7 of SLSTR has been designed with a different footprint size and position than the corresponding SWIR channels.Because of these differences the clusters in the different bands cannot be spatially superimposed.
The S5 cluster location is used as reference because it is the band closest to the maximum energy output expected for a gas flare (Figure 2).The along-track and across-track distances between a cluster in the reference SWIR band S5 and the closest cluster in the remaining SWIR and MIR bands (S6, S7 and F1) are being parameterised as functions of the across-track position using second order polynomials.Its parameters are determined by fitting 500 random products from the dataset.The confidence intervals for the distances are assumed to be bound by second order polynomials that are vertically shifted such that 10% of the data points are below, resp.above (Figure 4).

Misregistration correction: building multi-band clusters
Multi-band clusters C f laring are subsequently constructed.They consist of the single-band clusters C λ with λ in S5, S6, S7 and F1 that observe the same hot source.The SWIR cluster C S5 is used as reference.Then those cluster from the remaining bands (C λ with λ in S6, S7 and F1) that are closest to after correction of the misregistration are added.In doing so, only clusters in the confidence intervals are considered: where C λ is any of the λ-band clusters (λ in S6, S7 and F1), C f laring λ is the λ-band cluster within the multi-band cluster C f laring (λ in S6, S7 and F1), For the TIR bands S8, S9 and F2, the flaring high-temperature event is not expected to impact the radiance (Figure 2).The average radiance for each TIR band 2 pixels around the reference S5 cluster position is then associated to the multi-band cluster.

Radiance corrections
The SWIR radiances in S5 and S6 exhibit a systematic overestimation of 11 and 20%, respectively.[43] Following recommendations by ESA, all values in the SWIR bands were corrected accordingly.This correction needs to be verified and possibly updated for future versions of SLSTR products.
After building the multi-band clusters, the MIR S7 channel is checked for saturation.If the S7 radiance is above the saturation value (0.56 Wm −2 µm −1 sr −1 , corresponding to a brightness temperature of 306K) in any of the hot pixels within the cluster, then it is discarded.If the S7 cluster is discarded or not present, the F1 cluster is used.The F1 band is a fire dedicated channel of SLSTR which measures at the same wavelength as the S7 channel, but with a larger dynamic range and lower sensitivity.Only F1 clusters where all of the pixels have a brightness temperature within the linearity range 300-480K are considered.This way, sensitivity is maximized without compromising detection.

Super Cluster definition
The areas of the individual clusters in the SWIR and MIR bands within a multi-band cluster may vary.To overcome this, hypothetical super clusters, with perfect coregistration and identical footprint  areas in all bands, are built.The largest of the individual band clusters' areas A λ is chosen as area A cluster of the super cluster (Figure 5).
For each band's cluster C f laring λ within a multi-band cluster C f laring , the observed cluster radiance B obs λ is calculated as area-weighted average of the observed radiances of the cluster and the background: It is assumed that the background radiance B bg λ is constant in the vicinity of the cluster.For the TIR bands (S8, S9 and F2) the weighting is not necessary and the average registered radiance is used.

Planck curve fitting
In order to determine the temperature and the area of the flaring event, the sum of two Planck curves is fitted to the radiance data of the multiband cluster, as established by Elvidge et al. [32].The two Planck curves represent the two contributors for the IR radiance measured by the sensor at night, i.e. the flaring event and the background, each weighted by its respective relative area:

Radiative power
The radiative power (RP) of the hot source may then be computed using the Stefan-Boltzmann equation and assuming a perfect emissivity (in W): where σ SB is the Stefan-Boltzmann constant (5.670373 × 10 −8 Wm −2 K −4 ).
The respective uncertainty is computed by propagation of errors.

Regional study: 4 flaring regions
The described algorithm was applied to observation in the four regions of West Africa, the North sea, the Caspian sea and the Persian gulf, cf.Table 2.

Temperature and area retrievals
Figure 7 shows the hot spot detections of SLSTR with temperatures in the range [500 K, 5000 K], which we consider to be valid detections.The methodology successfully captures well-known flaring locations such as the North Sea oil fields, the mouths of the Congo and Niger rivers, the Persian Gulf and the Tigris and Euphrates rivers valleys.However, a large number of hot spots were detected outside of known flaring regions, e.g. in the North Sea, or in the plateau of central Angola and the Southern D. R. Congo.
Figure 8 shows the temperature distributions in nine specific areas: four areas with know gas flaring, one area with known biomass burning area and four areas without any know gas flaring activity.The retrieved temperatures for the detections in the non-flaring areas is clearly lower than for the flaring areas (see Figure 8).The retrieved temperatures for the biomass burning area is intermediate.Observing fire temperature on a global scale, even only at night-time, is complementary to the widely used burnt area and fire radiative power observations and has great potential for reducing errors in the current fire emission datasets.
Gas flare areas seen from space could be dependent on the viewing angle.However, Figure 9 shows that the retrieved hot spot area is not dependent on the across track index, and therefore the viewing angle.

Persistence
Other hot or bright events that can be observed from space in the IR part of the spectrum at night include wildfires, auroras, industry (e.g.steel mills) and heavily lit boats.Noise and an increased exposition of the sensor to radiation due to the South Atlantic Anomaly may also generate spurious hot spots.[28,32] It is difficult to discriminate between these events based only on the radiance.An analysis of the persistence of the signal at the location of a given valid detection has been proposed to filter out noise and ephemeral phenomena [28,32].
Although not suitable for a thorough analysis due to the short sampling period, the persistent observations (at least 3 detections within a spatial accuracy of ±0.02 degrees in longitude and latitude over the sampling period) mainly correspond to locations within known flaring regions (see Figure 10).In future works where this methodology will be used on data from longer periods, this threshold might need to be raised or otherwise adapted.For known interferences, persistent or semi-persistent hot sources such as volcanoes, the use of a mask will also be useful.

Selection of persistent observations and radiative power computations
In order to filter low-quality determinations, cloud cover and overfitting were considered.Clouds may interfere with the amount of radiation measured by the SLSTR instrument, in turn interfering with the temperature, area and radiative power retrievals.However, simply discarding cloudy pixels could incur a large omission error since pixels containing gas flares are frequently marked as clouds, namely as isolated cloudy patches (e.g. for the VIIRS instrument [32] ).For this reason, we analyse the cloudiness of the background, as defined in 3.2, and discard only observations with less than 3 cloud-free background pixels.
In order to avoid too much dependence on the TIR channels, which are not subject to the thresholding and clustering process and thus virtually always present, we discard persistent observations for which no S6 or S7/F1 cluster were detected.
Figure 11 shows the distributions of the retrieved temperature, area and radiative power for detections that have been filtered with persistence, cloud cover and S6/S7/F7 availability criteria described above.We will label these "quality persistent" below.The distributions of the quality persistent observations fall within the expected range for gas flares.The retrieved temperatures show a clearly unimodal distribution approximately centered at 1600K for the Persian Gulf and the Caspian Sea test regions.For the North Sea test region, the distribution is bi-modal, with modes around 1000K  and 1600K.There were very few quality persistent observations for the West Africa test region due to unfavourable cloud conditions.

Comparison with VIIRS Nightfire
The SLSTR valid detections at persistent locations were compared to the VIIRS Nightfire data for a same time period and a roughly similar area.The VIIRS Nightfire results were subject to the same spatial persistency analysis as the SLSTR data (3 times within a spatial accuracy of 0.02 degrees in longitude and latitude).VIIRS detects more gas flaring locations than SLSTR, which can be traced back to its wider swath (3040 km against 1420 km), and thus a shorter revisit time.Another reason for the larger number of detections by VIIRS is that the Nightfire algorithm processes single pixels, selected as the local maxima of pixels above the threshold, the method here aggregates contiguous hot pixels into clusters.Despite these limitations, all but one persistent locations detected by SLSTR were also detected by VIIRS.
The clustering of contiguous hot pixels used in the present method also explains why the VIIRS Nightfire temperature retrievals tend to peak at higher values than the SLSTR ones (Figure 12).Selecting only pixels which are local maxima, as the VIIRS Nightfire algorithm does, will produce detections with higher radiances and mostly higher temperatures will be retrieved.with very high temporal co-location between SLSTR and HSRS.The detections are clustered around 4 distinct locations (see Figure 13) and are summarized in Table 4 Due to clustering in the SLSTR and HSRS data analysis, these sensors may detect one cluster in a situation, in which the VIIRS product lists two or more flares in very close proximity.For SLSTR, the observing conditions were very close to twilight (the night-only mask was not considered in order to have the opportunity to observe the same flaring site by the three sensors), and thus not ideal.
Figure 14 shows the temperature, area and radiative power determinations at Locations 2 and 3 for the 3 instruments.
At Location 2, the single SLSTR observation agrees well with the determinations based on VIIRS and HSRS for the temperature.At Location 3, the temperature determinations based on SLSTR are lower than those based on VIIRS but in the same range as those based on HSRS.The gas flare temperature depends on the flared gas composition and the completeness of combustion, which depends on flare design and meteorological conditions [44][45][46][47].Thus the flaring characteristics may vary in time.The methodological differences may also explain lower temperatures in the SLSTR product as explained above.The agreement between SLSTR and HSRS is noteworthy since HSRS uses a bi-spectral method based on the MIR and TIR, the latter of which is prone to low signal/noise ratios for hot spots.
While the retrieved areas from TET-1 are larger than those from SLSTR, area determinations based on VIIRS and SLSTR are close for both locations except the highly uncertain SLSTR observation on 2/1/2017.The shape of the flame, and thus its area as observed by a satellite, is dependent on the flow rate and the meteorological conditions [45,48] and therefore also variable in time as its temperature.
Despite the variability seen between sensors due to inherent variability of flares and methodological differences, the radiative power determinations, the variable to which the emissions computations are linked, compare well between sensors.

Conclusions
We have adapted the VIIRS Nightfire algorithm [32] for the detection and characterisation of gas flares using the SLSTR instrument on-board the Copernicus Sentinel-3 satellite [40].The algorithm is based on fitting Planck curves that represent the hot source and the cool background to all the available satellite observations in the short-wave, mid and thermal infrared spectral range.This approach relies on radiometrically calibrated radiance data as input.
The main difference to the original methodology is that we analyse the integrated radiances of contiguous clusters of hot pixels instead of the maximal radiances detected in the clusters.We expect this representation to be more realistic and accurate for large arrays of gas flares.The presented algorithm furthermore tolerates misregistration and variable footprint areas between different spectral channels.The misregistration itself is quantified in a post-processing step.Finally, a new hot spot detection algorithm is employed and spurious cloud masking of just the hot source is ignored.
The concept of monitoring gas flares with SLSTR using the new algorithm has been tested using Sentinel-3A observations in four regions of interest: North Sea, Caspian Sea, Persian Gulf and Gulf of Guinea.Our methodology detects gas flaring activity at locations coincident with oil extraction sites and comparisons to the established VIIRS Nightfire product show that the location of the detections is largely consistent.The characterisation of the gas flares in terms of temperature, area and radiative power is also similar.Thus we conclude that night-time gas flaring can be monitored and characterised with the Sentinel-3 satellites.The retrieved gas flare temperatures are slightly lower, areas are slightly larger and radiative power is slightly larger than in the VIIRS Nightfire product.This is expected from the algorithmic difference and thought to more realistically represent large arrays of multiple gas flares.

Preprints
Our methodology was also tested with a short time series of observations over an individual flaring site at Bovanenkovo on the Yamal peninsula in Siberia.Here, we compare near-coincidental overpasses of Sentinel-3A (SLSTR), TET-1 (HSRS) and Suomi-NPP (VIIRS).In terms of temperature, both SLSTR and HSRS show similar results, while VIIRS retrievals deliver slightly higher temperatures.Areas derived from SLSTR data are similar to those from VIIRS, and smaller than those from HSRS.Taking into consideration the inherent variability of the operation of gas flares, we consider those values, as well as the derived radiative power values, from the three instruments to be in good agreement.
The presented algorithm is suitable for the generation of an operational global gas flaring product from the series of Copernicus Sentinel-3 satellites.Based on the derived FRP of the gas flares, the product may also include estimates of the flared gas volume (BCM) and the emissions of black carbon and other smoke constituents.
Including the gas flaring emissions into the Copernicus Atmosphere Monitoring Service (CAMS ) [49,50] will be necessary to simulate realistic black carbon deposition rates in the Arctic and quantify the corresponding albedo reduction and climate effects.This could be achieved by including black carbon emissions from a new Sentinel-3 gas flaring product or by extending the Global Fire Assimilation System (GFAS, [51]) of CAMS to ingest and interpret also FRP from gas flaring as reported by VIIRS Nightfire and a potential future "SLSTR Nightfire" product.
Estimates of the temperatures of vegetation fires are also provided, albeit only at night-time.This is a new observational capability.It is complementary to the burnt area and fire radiative products, which are currently being used for vegetation fire emission estimation, and carries a high potential for reducing some of the large uncertainties in these estimates.

Figure 1 .Figure 2 .
Figure 1.General algorithm flowchart for the detection of gas flares and their characterisation: temperature (T), area (A) and radiative power (RP).The dotted line represents the one-time misregistration determination.The parameterization was then used when building the multi-band cluster.n cloud− f ree bg > 3 represents at least three cloud-free pixels in the background.> 4λ means there is radiance data at least 4 of the 5 wavelengths considered by the algorithm.

Figure 3 .
Figure 3. Four examples of thresholding for the S5 band.The horizontal line marks the threshold value.The threshold is set contextually as the lowest radiance value whose difference to the closest inferior radiance value is larger than the scale factor.

1 2
is the distance between C λ and the parameterised misregistration position for the band λ, x S5 and y S5 are the across track and along track position of the S5 reference cluster C f laring S5 , x λ and y λ are the across track and along track position of C λ , x avg λ , x min λ and x max λ are the average, minimum and maximum distances in the across track axis given by the misregistration parameterisation as a function of the reference cluster across track position x S5 , y avg λ , y min λ and y max λ are the average, minimum and maximum distances in the along track axis given by the misregistration parameterisation as a function of the reference cluster across track position y S5 .

Figure 4 .Figure 5 .
Figure 4. Parameterizations of the distances between a cluster in S5 and the closest cluster in S6, S7 and F1 as a function of the scan index.The solid line shows the best fit (second order polynomial).The dashed lines are parallel to the best fit so that 10% of the points are above resp.below.They represent the confidence interval.

Figure 6 .
Figure 6.An example of the temperature and area retrieval by dual Planck curve fitting.

PreprintsFigure 7 .Figure 8 .
Figure 7. Maps of the valid detections within the test regions.The rectangles represent the 9 study areas (red for flaring, green for biomass burning, blue for non-flaring)

Table 1 .
Comparison between the used sensors.

Table 2 .
Data used in this study.The number of VIIRS products was derived from the VIIRS Nightfire dataset from NOAA, corresponding to the products which exhibited detections co-located in space and time.

Table 3 .
Summary of the detections based on SLSTR and spatial comparison with VIIRS Nightfire.
N. Sea C. Sea P. Gulf W.

Table 4 .
Summary of the detections at the Bovanenkovo gas and condensates field in the Yamal Peninsula, Siberia.