ATSR-1/2 User Guide
Contents
1. When a dual view retrieval is required the procedure is similar except that the 11 and 12um brightness temperatures for both nadir view and forward view must be checked for validity If all four are valid the solar elevation and 3 7um brightness temperatures for the image pixel in both views are inspected If both brightness tem peratures are valid and the solar elevation is negative for both views a 3 channel retrieval is made otherwise a two channel retrieval is performed As before if one of the 11 or 12um brightness temperatures is invalid the forward view SST valid flag is set to false again it is initialized to this value and the retrieved temperature is set to the 11m nadir view brightness temperature In theory SSTs should not be calculated for cloudy pixels If both views forward and nadir are cloud free then clearly a valid dual view retrieval is possible and if the nadir view is cloud free a valid nadir view retrieval is possible However if the nadir view pixel is cloudy or if both views are cloudy a valid retrieval is not possi ble for either the nadir view or dual view cases Nevertheless in SADIST both nadir and dual view retrievals are derived in all cases where the pixel is over the sea and the interpretation is left to the user who of course has available the cloud identifica tion flags in the confidence word FIGURE 3 Schematic of the decision flow in the ATSR SST retrieval algorithm Pixel data Di2. Thermal infrared detectors T records containing the thermal infra red near infra red 12 0um 11 0um 3 7m 1 6um channels which are available from both ATSR 1 and ATSR 2 instruments Visible detectors V records containing the visible near infra red 1 6um 0 87pm 0 65um 0 55um channels which are available from only the ATSR 2 instrument Pixel latitude longitude positions L records containing precise Earth locations for ATSR instrument pixels Pixel X Y coordinate positions X records containing precise pixel locations for ungridded products or sub pixel offsets for gridded products in the across track along track coordinate system defined by the ERS platform trajectory Cloud clearing land flagging results C records containing the detailed results of cloud clearing tests and pixel land flagging It should be noted that not every category is available for each product type so not all product options are always available For each product and for each instrument type ATSR 1 ATSR 2 there is a default product such default products have been chosen to satisfy most product users whilst minimising product size Note also that the ACLOUD and ASST products have no optional contents Since their product sizes are relatively small and the contents are valid for both ATSR 1 and ATSR 2 instruments flexibility provides no benefit ATSR 1 2 User Guide Issue 1 0 7 of 29 Data Products 3 4 3 5 Portabi
3. gt q As fae NYK hy f AA M e ae L 2 f j pean i DHT Descoping Regions in Cycle 33 08 June through 13 July 1998 ESA ESTEC NW ERS 2 35 day repeat orbit 501 71 723 deg First Orbit 16388 Last Orbit 16888 5 5 In general day time data acquired over land or ice are selected for omission although considerable amounts of sea data are also lost A full set of maps indicat ing the range of descoping for the ERS 2 mission is available on the ATSR web site http www atsr rl ac uk Although a region is subject to descoping during a certain period this does not mean it has no coverage for example even if all the day time overpasses are lost there will be night time overpasses Outgassing Contaminants from the satellite continually condense onto the cold surfaces of the focal plane FPA and its detectors This degrades instrument operation both due to signal attenuation and because the changed surface emissivities increase the radia tive load on the cooler Calibration is not affected by this as the calibration reference sources are forward of the field stop and thus subject to the same modification as the Earth view signal Occasionally the focal plane assembly is allowed to warm to vaporise these contaminants These outgassings are conducted several times a year No useful infrared data can be acquired during these times Although continuity of data is maintained for t
4. ATSR 1 2 User Guide Issue 1 0 Data Products 3 3 atures reflectances from all or some of the ATSR 1 ATSR 2 detectors categorised by view channel surface type and cloud presence ACLOUD is a spatially averaged cloud temperature coverage product unchanged from the SADIST 1 ACLOUD product The product contains spatially averaged measures of cloud temperature and abundance ASST is a spatially averaged sea surface temperature product an extension of the SADIST 1 ASST product The product contains spatially averaged sea surface temperatures at ten arcminute and half degree resolution using nadir only and dual view retrieval algorithms In the spatially averaged products generated by SADIST 2 the pixels which contrib ute to such products are taken from gridded and therefore collocated pixel data Optional product contents The approach adopted by SADIST 2 to strike a balance between flexibility and simplicity is to split product contents into several significant categories Each cate gory is represented by a single letter code in product requests and in product file names The combination of codes defines in a concise way the actual product con tents The product content categories are Nadir view only N only those records containing nadir view ATSR data Note that this option is rather different from the others in that its presence indicates the absence of product records those containing ATSR forward view data
5. SST SST polar temperate polar w ABS latitude polar_index temperate_index polar_index The approach used ensures that the retrievals do not show discontinuities at lati tudes equal to one of the values TROPICAL_INDEX TEMPERATE_INDEX or POLAR_INDEX and varies smoothly at points in between as the air mass type changes SST Smoothing The final step in generating the SST images but not the spatially averaged products is to smooth the derived temperature images This step is necessary because although the derived temperatures are estimates of the true SST they are affected by noise to a greater degree than the measured brightness temperatures themselves This follows because the coefficients multiplying the brightness temperatures in equations may exceed unity or combine to yield a net increase in variance The smoothing technique adopted is not to filter the images directly but to work with the difference between the derived SST image and the nadir view llum brightness temperature image If there were no atmosphere the 11um brightness temperature at near normal incidence would be a very good approximation to the SST differing only because the emissivity of the sea surface viewed at normal inci dence differs slightly from unity Thus the difference between the retrieved SST and the nadir view llum brightness temperature is a good measure of the atmos pheric attenuation in the 11um channel and might be expected to show
6. of 6 7 kms across the Earth s surface and an orbital period of about 100 minutes Usually the ERS spacecraft are in Yaw Steering Mode in which the satellite is continually rotated about the yaw axis to compensate for the Earth s rotation Both spacecraft have been positioned to operate with a south bound equator cross ing descending node of around 1030 local solar time and a north bound equator crossing ascending node of 2230 local solar time As the satellite performs a non integer number of orbits per day the orbital tracks do not repeat on a daily basis although local solar time for passing any latitude is essentially invariant The ERS 2 orbit has been established with a 1 day lag over ERS 1 so ATSR 2 views the loca tion that was observed by ATSR 1 on the same orbit the previous day Both platforms have orbit manoeuvring capability and can alter the phasing of the successive ground tracks by making slight adjustments to the spacecraft s altitude Various repeat cycles can be achieved and 3 35 and 168 day repeats have been employed during the two missions Only occasional orbit correction manoeuvres are required to maintain subsatellite track repeatability to within 1km from nomi nal The repeat cycle history of both spacecraft is given in Table 7 on page 21 Note that the 512 km wide swath of ATSR does not result in complete global coverage when the parent satellite is in a 3 day repeat cycle 20 of 29 ATSR 1
7. 2 User Guide Issue 1 0 Data Characteristics TABLE 7 ERS 1 2 orbit repeat cycles Satellite Date range Repeat cycle Orbit phase name Global ATSR ERS 1 31 July 1991 10 December 1991 3 day Commissioning Phase N 10 26 December 1991 ERS 1 orbit manoeuvres 26 December 1991 30 March 1992 3 day Ice Phase N 30 March 1992 14 April 1992 ERS 1 orbit manoeuvres 14 April 1992 17 December 1993 35 day Global Phase Y 17 21 December 1993 ERS 1 orbit manoeuvres 21 December 1993 10 April 1994 3 day Second Ice Phase N 10 April 1994 19 March 1995 168 day Geodetic Phase Y 19 21 March 1995 ERS 1 orbit manoeuvres 21 March 1995 present 35 day Global Phase Tandem Phase Y ERS 2 22nd April 1995 present 35 day Y In June 1996 ESA chose to cease data collection from ERS 1 At this time the sat ellite had exceeded its design lifetime by almost two years The platform was com manded into hibernation mode but remains functional and was re activated to acquire three days of data once every 70 days until December 1997 until an anom aly with the solar array occurred After this ATSR 1 was put into a hibernation mode but has been operated successfully for a 3 day period in May 1999 5 2 ATSR 1 For the ATSR 1 mission with only four channels and 958 useful pixels per scan data rate was not a major issue The four channels were transmitted as shown in Table 8 Summary of data transmission for ATSR 1 on page 21 TABLE 8
8. Summary of data transmission for ATSR 1 Channel Digital resolution Transmitted Notes 10 8um 12 bit Always 12 0um 8 bit Always Transmitted as 11um 12um difference 11 bit accuracy recoverable 1 6um 10 bit Day time Blanking pulse also transmitted 3 7um 10 bit Night time Blanking pulse also transmitted Data from the 1 6 and 3 7m channels is encoded using an exponential method see Zavody et al 1994 for further details The criterion for selecting which of the 1 6 or 3 7um channels is placed in the telemetry is based on the 1 6um reflectance Usually this is above a certain threshold value only in day time however lightning and other bright events can cause 1 6m data to be preferred to 3 7um at night Dur ing the ATSR 1 mission the 1 6um threshold was maintained at 110 counts until ATSR 1 2 User Guide Issue 1 0 21 of 29 Data Characteristics following the failure of the ATSR 1 3 7um channel then the threshold value was lowered in order to keep all the 1 6um data A higher threshold of 150 counts in 1 6um channel was chosen for ATSR 2 opera tion as it was considered that 3 7um data usually discarded in day time remain useful at low levels of sunlight Different threshold settings have been used at different times during the ATSR 2 mission so please contact the ATSR Project Team if more information is required 5 3 ATSR 2 data rates and flexible formats pixel maps For ATSR 2 there is a guaranteed 320kbs data
9. a linear detector response The size of this reduction depends on the temperature and decreases as the detector temperature increases The non linear detector responses are corrected using the measured radiances from the pre flight calibration and characterisation Visible and near infrared channels Visible channel calibration is achieved in a similar way to the infrared channels Careful pre flight calibration and characterisation of the visible channels was per formed and this is supplemented by continuous in flight calibration of the channels with an on board visible calibration system The radiometric offset for the visible channels is determined by viewing the ATSR 2 cold blackbody The signal measured while viewing this target is assumed to be the dark signal for the channels i e the signal observed by a blinded detector 12 of 29 ATSR 1 2 User Guide Issue 1 0 Ground segment data processing 4 2 1 3 4 3 The radiometric gain is determined once each orbit when the instrument s visible calibrator is illuminated by the Sun as the satellite moves away from the South pole At this point sunlight enters the VISCAL baffle through a protective window and is directed onto a Russian opal diffusing plate This plate is seen by the instrument scan mirror as a bright patch at the edge of the hot blackbody during each scan The VISCAL provides a radiance equivalent to a 25 signal from a Lambertian scat terer The per
10. and 2 and also provides a comprehensive ATSR image data service to NERC funded UK scientists It also has a mandate to provide low vol umes of sample products to any new user of ATSR data who applies and to pro vide data for validation purposes Figure 2 shows the flow of ATSR 1 and 2 data from the satellite to the user commu nity FIGURE 2 4 2 4 2 1 4 2 1 1 Schematic showing the distribution chain for ATSR 1 and 2 data a 4 x Data transmitted to i 1 ESA Ground Stations m m Prince Albert S amp S Saf ox ae Les Bye 2 S V 7 ra R Maspalomas Gatineau Kiruna G Raw data sent to processing centres on Exabyte tape Data processed in one of two UK centres which each serve different types of user NERC Funded ESA Funded P Facilit Processing Facility FOege Sing hacityy ESA Pls NERC Funded UK Scientists Commercial Users New Users non NERC Funded UK Scientists Instrument Consortium partners The limitation of both of these facilities is that they can only offer off line ATSR 1 2 product generation services and supply data to the community 7 14 days after its original collection at the earliest Such a delay is too long for many users There fore ESA have developed a pilot ATSR near real time processing system which is now in operation at the Troms Satellite Station TSS By agreement and because of its geographic proximity this station can eavesdrop on the 10 orbits of satellite d
11. channel signals Channel Nominal SCP gain Normalised gain Approx normalised range 1 6m 3 79 20 0 0 21108 0 87um 3 73 20 0 0 21447 0 67um 3 10 20 0 0 25806 0 56um 4 38 20 0 0 18264 Once as here the offsets and gains have been removed and normalised respec tively the only part of the calibration of the visible channels which remains is applying 100 reflectance scaling factors to convert normalised counts into true top of atmosphere reflectances Such scaling factors are the product of RAL s char acterisation of ATSR 2 s VISCAL unit see Visible and near infrared channels on page 12 for details their derivation and application The calibration tables them selves can be found at the following URL http www atsr rl ac uk calibration html Ground segment data processing 4 1 Introduction ATSR 1 and 2 data cannot be received directly from the satellites by users because there is no continuous direct broadcast of data from either ERS 1 or 2 Instead the ATSR data collected each orbit together with the low bit rate data from the other sensors on the platform are stored on an on board tape recorder for subsequent transmission to the ground These stored data are then transmitted to the ground during each orbit when the satellite is within the reception range of one of the desig nated ESA ground stations which are at Kiruna Sweden Maspalomas Canary Islands Gatineau and Prince Albert Canada Kiruna
12. high accuracy blackbody calibration targets and in the case of ATSR 2 and AATSR calibration of the visible and near infrared channels with an on board visible calibration system e use of the multichannel approach to SST retrieval previously demonstrated by the AVHRR instruments e use of the along track scanning technique to provide two views of the surface and thus an improved correction for atmospheric effects ATSR s field of view comprises two 500 km wide curved swaths with 555 pixels across the nadir swath and 371 pixels across the forward swath The nominal instan taneous field of view IFOV pixel size is 1 km at the centre of the nadir swath and 1 5 km x 2 km at the centre of the forward swath see Figure 1 ATSR 1 2 Viewing 1 of 29 Introduction 1 1 Geometry on page 2 Each pixel is the result of a 75 us integration of the signal from the scene This viewing geometry produces 500 km wide high resolution infrared and in the case of ATSR 2 and AATSR visible images of the Earth s surface from which sea surface temperature maps and other geophysical products can be retrieved through ground processing Along Track Scanning Application of the along track scanning technique is the ATSR instrument s most innovative development This works by making two observations of the same point on the Earth s surface through differing amounts of atmosphere the along track view passes through a longe
13. is the main station receiving 10 out of the 14 orbits of data collected each day No real time data is lost during the tape recorder playback because the satellite operates two simultaneous data links meaning that the current payload data can be transmitted as it is collected without affecting the tape recorder dump The real time data and the tape recorder dump are merged together duringprocessing at the ground stations It should be noted that the combination of tape recorder capacity and acquisition time at the ground station is a limitation during some ERS orbits this is explained further in Descoping on page 23 The data received at the ESA stations are then subsequently supplied to the various processing facilities on Exabyte tapes The two centres which process the ATSR 1 and 2 data are in the UK Processing and Archiving Facility UK PAF at the National Remote Sensing Centre NRSC in Farnborough UK and the ATSR Project Team at the Rutherford Appleton Labora tory UK Each of these facilities serves a different set of ATSR data users 1 The UK PAF at NRSC is the official ESA Processing and Archiving Facility PAF for ATSR and therefore supplies image data to the international science community non NERC funded UK scientists and commercial users ATSR 1 2 User Guide Issue 1 0 9 of 29 Ground segment data processing 2 The ATSR Project Team at RAL generates the spatially averaged climate prod ucts from ATSR 1
14. only small spatial variations over distances of a few kilometres Thus if this difference is smoothed the result may be regarded as the correction to be added to the nadir view llum brightness temperature to give the true SST ATSR 1 2 User Guide Issue 1 0 19 of 29 Data Characteristics In practice the difference is averaged over blocks of 3 x 3 pixels the pixels corre sponding to valid retrievals being included in the average with equal weight Thus up to 9 pixels contribute to each average The smoothed difference is then added to the nadir view 11 um brightness temperature to give the final retrieved SST value If no valid pixels contribute to the average or if there is no valid nadir view SST a corrected SST is not calculated and the smoothed SST value is set to 1 The smoothing is carried out separately for the nadir and dual view images The smoothing takes account of cloud flagging that is pixels flagged as cloudy are not included in the average otherwise an increased variance of the smoothed SST in cloudy areas would result 5 0 Data Characteristics The following sub sections describe the characteristics of the ATSR 1 and 2 data and the mission constraints that affect this data and its availability 5 1 ERS Orbit repeat cycles and global coverage Both ERS 1 and ERS 2 are in a near circular retrograde sun synchronous orbit at a mean height of approximately 780 km This orbit results in a sub satellite velocity
15. rate but dependant on the disposition of the Active Microwave instrument AMI 683kbps can be available to ATSR 2 at some times The guaranteed data rate is known as low rate L rate and the higher rate is known as high rate H rate Over sea the AMI is in wind wave mode and acquires substantially more data than when in the wind mode employed over land Thus ATSR usually gets H rate over land which is fortunate as that is where the visible data are most useful The ATSR 2 H rate is only used in day time i e when the sub satellite solar zenith angle gt 10 degrees and when a minimum of 60 seconds of H rate format is availa ble Figure 5 on page 22 shows the global coverage for high rate data during the 35 day repeat cycle 33 from 8th June to 13 July 1998 FIGURE 5 ATSR 2 H rate coverage for ERS 2 cycle 33 from 8th June to 13 July 1998 150 120 90 60 30 0 30 60 90 120 150 60 30 J f l 0 I SaF 30 a 60 wan a on iz Sy eae res aaa has aa To ale ws Ade aol an See TN ATSR 2 High Rate Coverage Cycle 33 08 June through 13 July 1998 ESA ESTEC NW ERS 2 35 day repeat orbit 501 71 723 deg First Orbit 16388 Last Orbit 16888 22 of 29 ATSR 1 2 User Guide Issue 1 0 Data Characteristics Generally the data bit rate per second required by ATSR 2 is given by no pixels scan X bits pixel X no channels X scan rate 6 7 Hz At night
16. scan However the effect on data quality can be mitigated by suitable processing and the RAL ATSR data processing system detects and flags the worst occurrences of this condition In flight monitoring of these jitters revealed a decreasing but persistent problem in 1995 The extra power dissipated in maintaining the scan mirror rotation results in a warming of the scan mechanism At 06 20 UTC on 22nd December 1995 the scan encoder temperature exceeded its switchdown limit causing ATSR 2 to switch into STANDBY mode Prior to this several orbits had been characterised by a high jitter rate although the problem appeared to have been resolved before the switch off Various attempts were made to restart the mechanism It was realised that the scan encoder temperature limit was unduly conservative and this was raised in a software patch which was loaded on 26th June 1996 Continuous operation resumed on Ist July 1996 Subsequent performance has been generally good although a few periods of difficult running have occurred ATSR 1 2 User Guide Issue 1 0 27 of 29 References 6 3 ATSR 1 ATSR 2 comparative performance Performance parameters from the two instruments are summarised in Table 10 ATSR 1 ATSR 2 comparative performance will be expanded in next version on page 28 TABLE 10 ATSR 1 ATSR 2 comparative performance will be expanded in next version Parameter ATSR 1 ATSR 2 Cooler temperature NEAT a
17. 15 June 1999 Issue 1 0 ATSR 1 2 User Guide Edited by Chris Mutlow from contributions by J Murray P Bailey A Birks and D Smith A short guide to the ATSR 1 and 2 instruments and their data products 1 0 The Along Track Scanning Radiometer ATSR instruments are imaging radiome ters that provide images of the Earth s surface from space ATSR 1 was launched in July 1991 and operated until June 1996 ATSR 2 is the current operational instru ment and went into operation in 1995 AATSR will be launched in the year 2000 Data from these instruments are useful for scientific studies of the land surface atmosphere clouds oceans and the cryosphere The purpose of this guide is inform potential data users about the capabilities of the Along Track Scanning Radiometers ATSR 1 and 2 and their data products If you already know about ATSR 1 2 and want find out how to order data please turn straight to Section 2 0 Getting ATSR 1 and 2 Data Products on page 4 of this document Introduction Each ATSR instrument has been designed for exceptional sensitivity and stability of calibration which are achieved through the incorporation of several innovative fea tures in the instrument design e use of low noise infrared detectors cooled to near optimum temperatures i e less than 95 K by a Stirling cycle mechanical cooler e continuous on board radiometric calibration of the infrared channels against two stable
18. 3 62 1997 Zavody A M M R Gorman D J Lee D Eccles C T Mutlow and D T Llewellyn Jones The ATSR data processing scheme developed fro the EODC Int J Remote Sensing 15 827 843 1994 28 of 29 ATSR 1 2 User Guide Issue 1 0 References Zavody A M C T Mutlow and D T Llewellyn Jones A radiative transfer scheme for SST retrieval for the ATSR J Geophys Res 100 937 952 1995 Zavody A M C T Mutlow and D T Llewellyn Jones ATSR Cloud clearing over ocean in the processing of data from the along track scanning radiometer ATSR Accepted for publication by the J of Atmos Ocean Technol 1999 ATSR 1 2 User Guide Issue 1 0 29 of 29
19. ATSR The ATSR 2 and Advanced ATSR AATSR instruments are developments from the original experimental ATSR 1 instrument which in addition to the ATSR 1 s infrared channels carry extra visible channels at 0 55 0 67 and 0 87um for vegeta tion remote sensing The evolution of ATSR 2 was constrained by the requirement to maintain the ATSR 1 precision measurement of global SST The ATSR 2 instrument launched in April 1995 is currently flying as part of the payload of the ESA ERS 2 satellite and AATSR will be launched early next cen tury on ESA s Envisat platform The AATSR instrument represents an orderly devel opment of the ATSR series of instruments The ATSR channels are given in Table 1 ATSR 1 ATSR 2 and AATSR Spectral Channels on page 3 TABLE 1 ATSR 1 ATSR 2 and AATSR Spectral Channels Feature Wavelength Bandwidth ATSR 1 ATSR 2 Detector type AATSR Chlorophyll 0 55um 20nm N Y Si Vegetation Index 0 67um 20nm N Y Si Vegetation Index 0 87um 20nm N Y Si Cloud Clearing 1 65um 0 3um Y Y PV InSb SST retrieval 3 7m 0 3um Y Y PV InSb SST retrieval 10 8um 1 0um Y Y PC CMT SST retrieval 12 0um 1 0um Y Y PC CMT The ATSR 2 instrument for ERS 2 is largely the same as ATSR 1 except for e the inclusion of 3 extra spectral bands in the visible mainly for vegetation moni toring e an on board visible calibration system The AATSR instrument is functionally the same as the ATSR 2 but the structure and some of the ot
20. agram showing the decision flow in the ATSR SST retrieval algorithm Is the pixel over Set result to the sea 11m BT value Are the 114m amp 12um Set result to the BTs valid 11um BT value Is it night time Use 2 channel retrieval scheme 11m amp 12um BTs Is the 3 7um BT valid Data on SST validity and the presence of Use 3 channel cloud are provided in a seperate confidence retrieval scheme word users should check this before using 3 7 11 12um BTs the retrieval This approach has the merit of simplifying the logic of the algorithm slightly A fur ther justification is that if the cloud identification algorithms have flagged a pixel as ATSR 1 2 User Guide Issue 1 0 17 of 29 Ground segment data processing cloudy in error then this approach ensures that the best available SST is still pro vided notwithstanding the error in cloud identification TABLE 5 Definition of the across track band selection scheme Band Number Band Limits km 0 256 to 200 1 200 to 150 2 150 to 100 3 100 to 50 4 50 to 0 5 0 to 50 6 50 to 100 h 100 to 150 8 150 to 200 9 200 to 256 The across track band is identified from the across track co ordinate of the pixel The across track bands are the same as those defined for the cloud clearing algo rithms The bands are numbered form 0 to 9 inclusive and each is 50km wide except for the two extreme bands which are each 56 km wide see Table 5
21. ata downlinked to the Kiruna station each day and processes this data in near real time to deliver ATSR 2 products to real time users The spatially averaged products from this system are now being supplied to the meteorological community and other customers get access to this data and can also order image products from TSS Data Processing Algorithms Calibration Infrared channels The signal in counts from a radiometer channel observing a blackbody target at tem perature Tp is 10 of 29 ATSR 1 2 User Guide Issue 1 0 Ground segment data processing S T GL T So Q 1 where G is the radiometric gain L T is the radiance from a target i e the Planck function integrated over the filter passband and S is the radiometric offset of the channel Radiometric calibration of the instrument consists of determining the linear rela tionship between the radiance and detector counts from each channel The conven tional way of doing this is to allow the instrument to view a zero radiance target such as a cold space view to determine the radiometric offset S i e L Tpp 0 Then having determined Sp the radiometer views a hot calibration target to deter mine the radiometric gain of the channel Then the gain of the system is given by Scold a So G Leola Q 2 In real radiometers there is always some degree of non linearity which if not treated properly in the ground processing al
22. ble and near infrared image data sup plied to users from SADIST 2 instead this must be done explicitly by the user with the calibration tables provided on the ATSR Project Web site http www atsrrl ac uk The visible and near infrared data provided in the SADIST product are in the form of raw uncalibrated telemetry but they have been normalised to lie within a given range The normalisation procedure applied to each of the visible channels including the 1 6um near infrared channel to achieve this is 1 The channel offset has been removed 2 The channel gains have been normalised to account for variations in the signal channel processor SCP gain setting Thus in SADIST 2 products which contain visible channel signals UBT GBT GBROWSE ABT the visible channels have been normalised to SCP gains of 20 That is the signals are those which would have been generated by the instrument if SCP gains of 20 were being used Table 2 on page 9 shows the actual nominal SCP gains used for ATSR 2 and the effective increase in signal due to the normalisa tion procedure Note that since the gains have been commanded to give a full scale uncalibrated count 4095 during the day time peak the approximate normalised 8 of 29 ATSR 1 2 User Guide Issue 1 0 Ground segment data processing maximum in each case represents the signal during the day time peak and not 100 reflectance TABLE 2 4 0 ATSR 2 normalised visible
23. channel signal to noise is well within the design specification How ever long term trend plots reveal oscillations in short wavelength signal intensity This effect is caused by a build up of condensation on a relay lens mounted on the cold focal plane assembly FPA This is accounted for in calibration and has a neg ligible effect on accuracy Degradation of VISCAL optics has been less than 2 0 per year This subject is covered in more detail by Smith et al 1997 and in Section 4 2 1 2 Visible and near infrared channels on page 12 ATSR 2 scan jitter problem Correct positional registration of the 2000 pixels around a scan relies on a steady scan rotation rate 6 7Hz A scan jitter arises when the rotation speed of the scan mirror deviates from this as can happen if the rotation is obstructed by debris Pre flight testing showed that the ATSR 2 scan mirror rotation produced more debris than that of ATSR 1 and this is likely to be the cause of the scan jitter which has been an intermittent feature of ATSR 2 operation However it is not clear whether this is the direct effect of this debris on the bearing stiffness or whether the debris is causing obscuration of the optical sensor that controls the drive to the mechanism Irregular rotation results in a misalignment of data from successive scans In some rare cases the infrared calibration may be compromised if the blackbodies are not viewed at the expected positions in the
24. channels for full 500km swath amp 12 bit digitisation Map 12 Not sent 0 55 0 67 amp 0 87um Full 500 km swath width with 12 bit digitisation Map 13 As ATSR 1 0 55 0 67 amp 0 87um Reduced 180 km swath width with 12 bit digitisation Map 14 As ATSR 1 0 55um Reduced 300 km swath with 8 bits digitisation in nadir 0 67 amp 0 87um Full 500 km swath with 8 bits digitisation in nadir amp alternate interlaced pixels in forward with 8 bits digitisation Headers supplied with each data product include information on which pixel maps were used during data acquisition 5 4 Descoping For certain orbits the tape recorder capacity and the contact time with the ground station is insufficient to download all the data that could be acquired This problem is resolved by descoping such that data from certain parts of the orbit are not transmitted ESA have a document known as the High level Operations Plan HLOP that defines the rules as agreed with the ESA National Delegates that govern the way the descoping operates Figure 6 on page 24 shows the regions selected for descoping during the ERS 2 35 day repeat which took place from 15th May 1995 until 19th June 1995 ATSR 1 2 User Guide Issue 1 0 23 of 29 Data Characteristics FIGURE 6 60 30 30 60 ERS 2 Descoping regions during cycle 33 from 8th June to 13 July 1998 150 120 90 60 30 0 30 60 90 120 150 eo lees Sek op ESF aa Fg
25. e bright ness temperatures from the nadir view only is also catered for It would be possible in theory to derive retrieval coefficients for the case of a forward view image only but this is not done in practice The algorithms using the nadir view only are given by nadir nadir nadir T ao aTi aTi sst i EQ 8 or nadir nadir nadir nadir Tes gt b bi J b Tri bs EQ 9 When both views are used the corresponding equations are dual nadir nadir frwrd frwrd Toi Co cTor i c Tin j eTii i c Tio EQ 10 or dual nadir nadir nadir frwrd frwrd frwrd To d 5 d Tri 35 d T i T d T37 T d Tii z d Tjin ag d Tj sst i EQ 11 respectively If the pixel is over land an SST retrieval is clearly not appropriate In order to pro vide a precision estimate of the land surface temperature it would be necessary to have detailed information about the emissivity of the land surface in the various channels This would present some difficulty given the large spatial variability in the physical characteristics of land surfaces In this case therefore SADIST supplies the Ilum nadir view brightness temperature as the best available estimate of the land surface temperature in the absence of such detailed information The logic of the procedure used within SADIST for deriving a retrieved SST is therefore as follows in Figure 3 on page 17 16 of 29 ATSR 1 2 User Guide Issue 1 0 Ground segment data processing
26. ection 2 2 2 Following such an initial data grant the user will be referred to the appropriate data centre as described below for any future requests NERC funded UK Scientists and Validation Scientists This category includes NERC Staff holders of NERC funded research grants or thematic programme awards and scientists providing validation data These users can obtain their data from the ATSR Project Team by submitting a NERC AT1 form Further information on the procedure and the actual form can be found on the ATSR project web pages at http www atsr rl ac uk or by contacting 4 of 29 ATSR 1 2 User Guide Issue 1 0 Data Products 2 2 3 2 2 4 3 0 Nigel Houghton ATSR Project Rutherford Appleton Laboratory Chilton Didcot Oxon OX11 0QX email n houghton rl ac uk Other Scientists and Commercial users This category of users must obtain their data from ESA through the ERS Help Desk at Esrin in Frascati near Rome The contact details for ATSR data through ESA are ERS Help Desk via Galileo Galilei CP 64 0044 Frascati Italy Phone 39 06 94180 666 Fax 39 06 94180 272 E mail eohelp esrin esa it ESA Browse Service http earthnet esrin esa it These data are processed on behalf of ESA by NRSC Ltd at Farnborough who are the official ESA processing and archiving facility for the ATSR instrument Near Real Time Users In recent weeks a new near real time service for ATSR 2 data products has come into o
27. el X Y across track along track coordinates Gridded products There are three gridded products GBT is a gridded brightness temperature reflectance product an extension of the SADIST 1 BT product The product contains gridded calibrated brightness tem perature or reflectance images from all or some of the ATSR 1 ATSR 2 detectors The product optionally includes pixel latitude longitude positions X Y offsets sub pixel across track along track coordinates and the results of cloud clearing land flagging GBROWSE is a gridded browse product an extension of the SADIST 1 BROWSE product The product contains gridded sub sampled calibrated brightness temper ature or reflectance images from all or some of the ATSR 1 ATSR 2 detectors The product optionally includes the results of cloud clearing land flagging GSST is a gridded sea surface temperature product an extension of the SADIST 1 SST product The product contains gridded sea surface temperature images using both nadir only and dual view retrieval algorithms The product optionally includes pixel latitude longitude positions X Y offsets sub pixel across track along track coordinates and the results of cloud clearing land flagging Spatially averaged products There are three spatially averaged products ABT is a spatially averaged brightness temperature reflectance product a new product for SADIST 2 The product contains spatially averaged brightness temper 6 of 29
28. ene is distorted There is a direct correspondence between the contents of a product record and the contents of an ATSR instrument scan Nadir and forward view pixels in a record correspond to the same scan and are therefore not co located ATSR 1 2 User Guide Issue 1 0 5 of 29 Data Products 3 1 3 2 3 2 1 Gridded products contain 512 x 512 pixel images The correspondence between a pixel and the ATSR instrument scan from which it came has been lost Nadir and forward view pixels are collocated and have been regridded mapped onto a 1 km grid Spatially averaged products have contents derived from up to a whole orbit of raw data which have been spatially averaged to a ten arcminute or half degree res olution Ungridded products There are two ungridded products UCOUNTS is an ungridded detector count product The product contains ungrid ded uncalibrated detector counts from all or some of the ATSR 1 ATSR 2 detec tors Although the product remains ungridded it may optionally contain pixel latitude longitude positions and or pixel X Y across track along track coordi nates UBT is an ungridded brightness temperature reflectance product a new product for SADIST 2 The product contains ungridded calibrated brightness temperatures or reflectances from all or some of the ATSR 1 ATSR 2 detectors Although the prod uct remains ungridded it may optionally contain pixel latitude longitude positions and or pix
29. formance and degradation of the VISCAL is monitored by a photodi ode The gain Gis of the visible channels is therefore given thus S viseal S dark vis L viscal Q 7 where S4arg is the radiometric offset derived from the internal blackbody views Sisea 18 the signal from the ATSR 2 VISCAL unit and L 1s the solar radiance from the VISCAL These calibration data are not used directly within SADIST 2 to calibrate the data from the visible and near infrared channels it is left to the user to do this using the calibration tables provided at the URL http www atsr rl ac uk calibration html These tables are updated on a regular basis using the method described above Users are reminded that some care is required to ensure that up to date calibration information is used as trend plots reveal oscillations in short wavelength signal intensity caused by a build up of condensation on a relay lens mounted on the cold focal plane assembly FPA This is accounted for by the calibration procedure and has a negligible effect on accuracy if the correct set of calibration coefficients are used Care must also be exercised if data from an outgassing period is used as the calibration becomes undefined while an outgassing is underway owing to rapid changes in the condensate film thickness as it evaporates If you are in any doubt over the use of the calibration data please contact the ATSR Project Team at RAL who will be able to ad
30. gorithms results in errors in calibration This is a particular problem if the non linearity changes with time To avoid these problems as far as possible the approach adopted in the ATSR instruments has been to minimise the sensitivity of the calibration to any non linear ity in the radiometer s characteristics This has partly been done by careful design of the signal processing electronics and by careful pre flight determination of the non linearity for beginning of life and end of life conditions on the satellite but mainly through designing the calibration system in such a way that the instrument s on board calibration is optimised over the limited range of temperatures that span the expected range of SST observations ATSR uses two blackbody calibration targets rather than the more usual single hot target and a space view In ATSR one of these targets operates at a temperature cooler than the coldest expected SST and the other one warmer than the hottest expected SST With this arrangement the calibration is most precise over the temperature range covering the normal range of SST The effects of any non linearity in the system are minimised because linearity is only assumed over a small range of measurement space Outside this range the calibration is no worse than using the space view and single calibrator approach but using the ATSR method the precision is concentrated into the portion of the measurement space where the mo
31. hat the test did not indicate the presence of cloud or that the test was not applied because suitable data was lacking 4 3 3 SST retrieval The objective of this procedure is to use the measured infrared brightness tempera ture values to determine for each cloud free pixel over sea the best estimate of the ATSR 1 2 User Guide Issue 1 0 15 of 29 Ground segment data processing Sea Surface Temperature SST of the pixel to form an SST image at 1 km resolu tion The SST is calculated using predetermined coefficients In the current version the coefficients are given for three geographical regions tropical midlatitude and polar A new version is under test which use a global set of coefficients The values of the coefficients also depend to some extent on the viewing angle and so the across track distance of the pixel expressed in terms of band number determines the set of coefficients to be used for a given pixel In the current version five different sets of coefficients are defined for each geographical region to represent the 5 different across track distances and therefore air masses corresponding to the across track bands There are thus 15 sets of coefficients for each case Whenever possible both the nadir view and forward view pixels are used Cloud contamination for the forward view pixels is more likely than for the nadir view due to the larger sampling area in the former hence the possibility of using th
32. he visible channels as the operation of the visible detectors is unaffected by the increased temperature considerable care must be taken in using visible data col lected during an outgassing This is because the condensation also affects the throughput of the visible channels and the sudden loss of the condensation film invalidates the calibration data collected during the previous orbits Hence when outgassings are occurring the calibration of the visible channels is undefined It should be noted that during outgassings the unavailability of infrared data permits more telemetry bandwidth for the visible data and these can be transmitted in full at 12 bit digital resolution 24 of 29 ATSR 1 2 User Guide Issue 1 0 Instrument Performance 6 0 Instrument Performance 6 1 6 1 1 ATSR 1 Performance ATSR 1 performance was generally good and met the pre flight specification although the loss of the 3 7um channel in May 1992 was a major disappointment The performance of the cooler deteriorated several years into the mission neverthe less ATSR 1 succeeded in delivering high quality data for almost five years The instrument is still viable although the power constraints on ERS 1 prevent its rou tine operation ATSR 1 cooler performance After initial cooldown the ATSR 1 cooler reached a cold block temperature of 89 IK From early 1994 it became increasingly difficult for ATSR s on board cooler to maintain
33. her components have been re worked to match the environment of the Envisat platform which is somewhat different to the ERS satellites The major purpose of AATSR is to provide continuity of the crucial sea surface temperature data sets which have been produced by ATSR 1 and ATSR 2 There fore the key scientific parameters which were optimised for ATSR are retained for AATSR Thus details of the scan the optical system the basic spectral bands ther mal calibration system spatial resolution and swath have been kept as close as pos sible to those of the original instrument to ensure continuity The major advantage AATSR has over ATSR 2 is in the telemetry bandwidth avail able on Envisat For ATSR 2 the limited telemetry available on ERS 2 imposed severe limitations on the collection of visible channel data on Envisat there are no ATSR 1 2 User Guide Issue 1 0 3 of 29 Getting ATSR 1 and 2 Data Products 2 0 such restrictions so AATSR can telemeter all the visible channel data it can collect This significantly simplifies the ground processing required for AATSR data as the processor does not need to cope with the wide range of data formats that are possi ble from ATSR 2 Getting ATSR 1 and 2 Data Products 2 1 2 1 1 2 1 2 2 2 2 2 1 The following sub sections describe how the various sections of the user commu nity can order ATSR data sets and the services that are available to support brows ing and se
34. lection prior to placing an order How do I find out what ATSR 1 2 data are available In addition to the information provided on the ATSR Project web pages there are now new services that allow potential ATSR data users to view quick looks of the image data available through the various processing facilities prior to placing an order for the full resolution data sets NERC ATSR Browse Facility To provide easy access to ATSR data the UK NERC has established a Browse Facil ity at RAL which provides on line access to quick looks of the entire ATSR 2 image data set and is starting to be populated with ATSR 1 data set as well Users are strongly encouraged to make use of this facility to establish their data needs before requesting data from either RAL or ESA Access to the facility can be gained directly via a link from the ATSR Project s home page at URL http www atsr rl ac uk ESA Multi Mission Browse Facility ESA also has an ATSR browse service that can be reached at URL http earth net esrin esa it Where do I order ATSR 1 2 data The following sub sections describe where the various sections of the user commu nity can place their order for ATSR 1 2 data products New Users The ATSR Project Team at RAL are able to supply any new user with samples of ATSR 1 2 data to get them started quickly with an ATSR data set appropriate to their needs this service is available through the ATSR Project at RAL at the address in S
35. lity byte ordering and the byte order word In designing the ATSR products it has been attempted to keep the products as port able as possible between operating systems and languages To this end e The products contain no floating point numbers Only ASCII text within prod uct headers and one two and four byte integers are used throughout the prod ucts e The products contain fixed length records This provides portability between record based operating systems such as OpenVMS and stream based operating systems such as UNIX However an intrinsic difference between systems remains Some systems interpret the bytes within integers such that the bytes are given increasing significance whilst others interpret the bytes within integers such that the bytes are given decreasing significance SADIST 2 is a VMS application Since VMS is a little endian system the bytes within two byte words and four byte words are stored in increasing order of significance If SADIST 2 products are to be read on big endian systems the byte ordering must be reversed That is the internal representation must be changed so that the intended value will be retrieved To provide a mechanism whereby the process of byte swapping might be auto mated the first two bytes within each SADIST 2 product header are fixed and can be used to test the byte ordering on the local system Visible channel normalisation No routine calibration is performed on the visi
36. nd forward views separately applied to nadir and forward views separately applied to nadir and forward views separately uses both views uses both views applied to nadir and forward views separately A series of cloud state flags is defined within the SADIST code for each pixel and for the forward and nadir view separately These are listed in Table 4 TABLE 4 Cloud clearing land flagging flag bit settings nadir or forward view bit 0 Co ON Dn FW YP Se N oO Meaning if set Pixel is over land Pixel is cloudy result of all cloud tests Sunglint detected in pixel 1 6um reflectance histogram test shows pixel cloudy day time only 1 6um spatial coherence test shows pixel cloudy day time only llum spatial coherence test shows pixel cloudy 12 um gross cloud test shows pixel cloudy 11 12um thin cirrus test shows pixel cloudy 3 7 12um medium high level test shows pixel cloudy night time only 11 3 7um fog low stratus test shows pixel cloudy night time only 11 12um view difference test shows pixel cloudy 3 7 1lum view difference test shows pixel cloudy night time only 11 12um thermal histogram test shows pixel cloudy These flags are set according to the results of the tests Thus if one of the flags num bered 3 to 12 is set this means that the corresponding test has indicated the presence of cloud If on completion of the cloud clearing sequence any of these flags is not set it may mean either t
37. ome rough running of the scan mechanism Unfortunately this problem precipitated a shutdown of the instrument for the period December 1995 to July 1996 Instrument operations were recovered on the Ist July 1996 and apart from a few short shutdowns of a few days the instrument has remained operational ever since Irregularities in scan mirror lock can be seen as slipped lines of data in some for ward view scenes These are rarely observed in nadir data It is estimated that less than 1 of the ATSR 2 data set is affected by this problem and statistically it has no discernable impact on the climate SST products from the instrument ATSR 2 cooler performance The cooler has maintained a cold tip temperature of 81 1K and with orbital varia tion of only 0 1K much better than ATSR 1 and with a lower cooler drive power ATSR 2 radiometric accuracy and noise characteristics Pre launch calibration showed that the on board targets agreed to within 1OmK of the external reference targets i e at the limit of sensitivity of the system The radi ometric noise NEAT for a scene at 270K were found to be 50mK 21mK and 25mk for the 3 7 11 0 12 0 respectively Signal gain offset control loops threshold settings compression modes and pixel maps have all been optimised to deliver the instrument s best performance 26 of 29 ATSR 1 2 User Guide Issue 1 0 Instrument Performance 6 2 3 6 2 4 Visible channels The visible
38. on page 18 It will be noted that the bands are symmetrical about the ground track The path lengths to pixels in band 4 are for example are identical to those in band 5 and similarly for the other symmetrical pairs so that only 5 sets of coefficients are required The latitude of the pixel is also determined This governs whether the coefficients for the tropical temperate or polar regions are to be used Three zonal limits are defined TROPICAL INDEX TEMPERATE INDEX and POLAR_INDEX Numerical values are given in Table 6 TABLE 6 Latitude Limits Latitude Index Latitude Limits Tropical 12 5 Temperate 37 Polar 70 The latitude and across track band number of the pixel determine the usage of the retrieval coefficients as shown in Figure 4 on page 19 18 of 29 ATSR 1 2 User Guide Issue 1 0 Ground segment data processing FIGURE 4 SADIST scheme for selecting appropriate SST retrieval coefficients based on pixel latitude ABS Pixel Latitude Retrieve using Tropical ABS latitude lt Tropical_index coefficients SSTropical hi Retrieve using Polar ABS latitude gt Polar_index coefficients SST polar SST SST w SST SST tropical tropical emperate Temperate_index gt ABS latitude gt Tropical_index Polar_index gt ABS latitude gt Temperate_index 4 3 4 w ABS latitude tropical_index temperate_index tropical_index SST SST w
39. pec ATSR 1 2 User Guide Issue 1 0 25 of 29 Instrument Performance 6 1 3 6 2 6 2 1 6 2 2 tral response is to depress retrieved SSTs particularly in humid tropical conditions This is being addressed in the reprocessing of the ATSR 1 data ATSR 1 radiometric accuracy and noise characteristics Inevitably the noise associated with the ATSR 1 channels increased with the detec tor temperature The mission requirements specified noise equivalent temperatures NEAT better than 0 05K for a 300K scene for each channel Pre flight testing of ATSR 1 with the detectors at 80K produced NEAT s for the 11 and 12um channels of 44mK and 37mK respectively By March 1995 the NEATs for the 11 and 12m channels were around 60mK and 130mK respectively ATSR 1 black bodies calibration stability Pre flight testing checked for drift in platinum resistance thermometer PRT cali brations and variations in target emissivity caused by a degradation in the black sur face finish Measurements showed residual temperature gradients across the blackbody base to be less than 25mK at conditions of equilibrium ATSR 1 3 7um failure On May 27th 1992 the 3 7um channel failed and SST retrievals from that point on only used the 10 8 and 12um channels See Murray et al 1998 for more informa tion on the effects of this failure ATSR 2 performance summary ATSR performance has been good and within specification with the exception of s
40. peration at the Troms Satellite Station The web address for accessing this service is http 192 111 33 173 ATSRNRT There is direct access the quick look images archives but users have to register with ESA to if they wish to access the full resolution data sets Data Products All ATSR 1 and 2 data from whatever source are processed using SADIST Syn thesis of ATSR Data Into Sea surface Temperatures the Rutherford Appleton Lab oratory s ATSR data processing scheme Z vody et al 1994 Currently two distinct versions of SADIST exist ATSR 1 data were processed with SADIST 1 and ATSR 2 data with SADIST 2 However over the last few months a unified version of the software has become available so in future ATSR 1 data will be re processed using SADIST 2 and be in a common format Major enhancements in the second version of the software include the capability to provide additional the visible channel data and more robust cloud identification and additional product confidence data Full details of the ATSR product set are described in the ATSR product format guides available from the ATSR Project Web Site http www atsr rl ac uk sepa rate guides cover the SADIST 1 and 2 product sets The set of SADIST 2 ATSR 1 2 products comprises three logical groups Ungridded products contain pixels in the ATSR scan geometry i e in the instru ment frame of reference where the curved scans appear as straight lines and the sur face sc
41. r atmospheric path and so is more affected by the atmosphere than the nadir view see Figure 1 ATSR 1 2 Viewing Geometry on page 2 First the ATSR views the surface along the direction of the orbit track at an inci dence angle of 55 as it flies toward the scene Then some 150s later ATSR records a second observation of the scene at an angle close to the nadir By combining the data from these two views a direct measurement of the effect of the atmosphere is obtained which yields an atmospheric correction for the surface data set which is an improvement on that obtained from a single measurement FIGURE 1 ATSR 1 2 Viewing Geometry ATSR Instrument Sub satellite Track Nadir view swath 555 nadir pixels Flight Direction 1 km resolution Forward view swath 371 along track pixels 1 5 km x 2 km resolution 1 2 ATSR 1 ATSR 1 was launched as part of the payload of ESA s ERS 1 satellite on 17th July 1991 and was the test bed for the along track scanning concept It carries infrared 2 of 29 ATSR 1 2 User Guide Issue 1 0 Introduction 1 3 channels at 1 6 3 7 10 8 and 12 0um and has no visible channels Routine ATSR 1 operations stopped when ERS 1 was put into hibernation in June 1996 but the instrument is still capable of operation as even after nearly 7 years of use the signal to noise performance of the detectors is higher than for a typical AVHRR at launch ATSR 2 and A
42. st accurate measurements are required At temperatures outside this range the precision of the observations are not so critical and the larger calibration errors resulting from extrapolation can be tolerated The scheme used for determining the ATSR instrument s calibration parameters from the hot and cold blackbody signals is given below ATSR 1 2 User Guide Issue 1 0 11 of 29 Ground segment data processing 4 2 1 2 The signal from ATSR s cold blackbody is given by Scotia GLeoiat So EQ 3 and the signal from the hot blackbody is given by Shot GLhror t So EQ 4 Hence by subtracting the above equations to eliminate S the radiometric gain G is G Sor Scold Lot Leold EQ 5 and by substituting G back into the equations the offset S is So Shor GL not EQ 6 The infrared focal planes of the ATSR instruments use two different types of infra red detectors 1 the 1 6 and 3 7um channels employ photovoltaic indium antimonide InSb 2 the 10 8 and 12 0um channels use photoconductive cadmium mercury teluride CdHgTe or CMT detectors The response of the InSb detectors is fairly well behaved and linear over the range of temperatures from liquid nitrogen to 310 K The same is not true of the CMT detectors which show a marked non linear behav iour because of Auger recombination This causes a reduction in the measured detector signal at high photon fluxes compared to that predicted assuming
43. t 300K FOV Long term variation 90 110K 1991 1995 81 1 K 1995 1999 Orbital variation 1K 0 1 K 11 0um 60mK March 1995 46mK 1995 12 0um 130mK March 1995 36mK 1995 11 0 amp 12 0um Some non uniformity for CMT Some non uniformity but better than ATSR 1 Better co aligned than ATSR 1 1 6 amp 3 7um Fairly uniform All very uniform Better co aligned than ATSR 1 0 55 0 67 amp 0 87um N A Very uniform 7 0 References Edwards T et al The along track scanning radiometer measurement of sea sur face temperature from ERS 1 J Br Interplanet Soc 43 160 180 1990 Gray P F et al The optical system of the along track scanning radiometer MK II ATSR 2 Proc of ICSO 91 Toulouse 1991 Murray M J M R Allen C T Mutlow A M Zavody T S Jones and T N For rester Actual and Potential information in dual view radiometric observations of sea surface temperature from ATSR J Geophys Res 103 8153 8165 1998 Mutlow C T A M Zavody I J Barton and D T Llewellyn Jones Sea surface temperature measurements by the along track scanning radiometer on the ERS 1 satellite Early results J Geophys Res 99 575 588 1994 Smith D L Read P D and Mutlow C T The Calibration of the Visible Near Infra Red Channels of the Along Track Scanning Radiometer 2 ATSR 2 in Sen sors Systems and Next Generation Satellites Hiroyuki Fujisadsa Editor Pro ceedings of SPIE 3221 5
44. tes of a given pixel are retained in the gridded products This regridding has two effects Pixels which are small and whose Earth locations are therefore very small may be placed within the same 1 x 1 km box in which case the first is overwritten Also some pixels in the regridded image may remain unfilled This unfilling occurs when pixels are large and consequently further apart than 1 km All latitudes provided within SADIST 2 products are geodetic that is they show the angle formed by the intersection between the equatorial plane and the local nor mal at the Earth s surface Cosmetic fill Grid pixels which remain unfilled by the regridding process are filled by copying the nearest real neighbour It can be seen that this process of cosmetic filling has the effect of approximately reconstituting original pixel sizes Filling occurs only where actual pixels are large and therefore widely spaced but have been squeezed into 1 x 1 km boxes Nearest neighbour copying reverses the pixel squeezing and allows pixels to expand to a more representative size Cloud Clearing The process of cloud clearing or the identification of clear pixels is accom plished by applying in turn a series of tests to the brightness temperature data in the 12 11 and 3 7m channels and to the reflectance data in the 1 6um channel The pixel is flagged as cloudy if any one of the tests indicates the presence of cloud Considered in detail
45. the detector temperatures below 95K To reduce mechanical wear and maximise the life of the cooler temperatures were allowed to rise gradu ally reaching 110K in early 1996 The step discontinuities in the cooler cold tip temperature seen in Figure 7 occur either after an instrument outgassing where the heat load on the cooler has been reduced following warming of the FPA to liberate the condensed material trapped on its cold surfaces or due to changes in cooler performance following a modifica tion of the cooler amplitude setting i e and increase of decrease in the cooler power After Day 800 in the figure there are only short bursts of data this period corre sponds to the so called hibernation phase of ERS 1 During this period satellite and ATSR 1 are only active for 3 days in every 70 FIGURE 7 Variation in ATSR 1 Detector Temperatures during the mission showing daily maximum minimum and mean values ip Temperature ulud ululi 500 16 1500 Days since Launch 17 July 91 day 197 Until ERS 1 entered this hibernation phase the overall trend in the temperature of the ATSR 1 cooler cold tip and detectors has been a gradual warming This warm ing affects the response of all the detectors but the main difficulty is in the case of the 12um channel where the shift in response is sufficient to modify the ATSR long wavelength filter cutoff Generally the result of this effective change in s
46. the physics involved is complicated however broadly speak ing the detection of cloudy pixels is based upon identifying deviations caused by the presence of cloud from the properties of and relationships between measured bright ness temperatures expected for clear conditions See Z vody et al 1999 for more details of the ATSR cloud clearing scheme Table 3 below summarises the cloud clearing tests implemented in SADIST All of the tests are of course conditional on the appropriate infrared or 1 6um data being available The 1 6m test operates on daytime data only The tests involving the 3 7um channel on the other hand are only applied to night time data because reflected solar radiation may be significant in this channel during the day Those tests that involve the 11 and 12um channels are applicable to both daytime and 14 of 29 ATSR 1 2 User Guide Issue 1 0 Ground segment data processing night time data Not all of the tests are implemented over land so cloud clearing over land is not as effective as over the ocean TABLE 3 ATSR cloud clearing tests Cloud Test 1 6um histogram test llum spatial coherence test Gross cloud test Thin cirrus test Medium high level cloud test fog low stratus test 11 12um nadir forward test 11 3 7um nadir forward test Infrared histogram test Views used nadir and forward views nadir and forward views applied to nadir and forward views separately applied to nadir a
47. time data from the visible channels are not considered useful and the Night format is employed This completely omits the visible channels and returns 12 bit data for the thermal channels and 11 bit data for the 1 6m channel In day time Flexible format is employed by default This entails the use of pixel maps in which only some of the various possible elements of the visible data are selected to squeeze the data rate into the available bandwidth The operational pixel maps are summarised in Table 9 Summary of pixel maps used for routine ATSR 2 operations on page 23 Note that one choice involves selecting only alternate pix els from the forward view Pixel map 12 is only used during outgassings when no useful infrared data are avail able In general for the early part of the mission pixel map 14 is used for the first nine days of every month and pixel map 13 for the remainder To be precise pixel map 14 is enabled for the first orbit after 1100UTC on the first of the month with the switch to map 13 taking place at the first appearance of daylight on the 10th day However recently the cycle of changes has been modified to fit in with special data acquisitions from GOME see the new pages at the ATSR WWW site or contact the ATSR Project Team at RAL for more information on this TABLE 9 Summary of pixel maps used for routine ATSR 2 operations Pixel Map IR data Visible data H Rate All 12 bit 0 55 0 67 amp 0 87um All
48. vise on the correct procedure and also on the status of the instrument General Points on Calibration It should be noted that although it is not explicitly mentioned elsewhere odd and even pixels from the sensor are calibrated separately as they are obtained from dif ferent integrators In the current version of the ATSR software this may not always be done correctly in jittered scans see Section 6 2 4 on page 27 a fix is availa ble and will be included in a later update Geolocation and regridding ATSR SADIST image products betray no sign of the fact that the ATSR instruments possess a conical scan mechanism which results in the acquisition of nadir and for ward view pixels many hundreds of kilometres apart and which possess a curved geometry It is an important part of the data processing within SADIST 2 to remove such spatial view differences and scan geometry by performing pixel geolocation ATSR 1 2 User Guide Issue 1 0 13 of 29 Ground segment data processing 4 3 2 the derivation of the Earth locations of the acquired pixels and view collocation the process by which assuming the geolocation is sound the nadir and forward views are spatially matched The geolocation proceeds by mapping the acquired pixels onto a km grid whose axes are the ERS satellite ground track and great circles orthogonal to the ground track The resampling is done using a nearest neighbour method and the actual X Y co ordina
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