AATSR VALIDATION CAMPAIGNS USING THE ISAR RADIOMETER SYSTEM. ESA AOID9081/2

Craig J. Donlon(1), Ian S. Robinson(2)

(1) Joint Research Centre of the European Commission, Institute of Environment and Sustainability, Inland and Marine Waters Unit, Ispra, I-21020, Italy. Email: Craig.Donlon@jrc.it (2) School of Ocean and Earth Science, Southampton Oceanography Centre, University of Southampton, Waterfront Campus, Southampton, England. Email: Ian.s.robinson@soc.soton.ac.uk

ABSRTACT This paper describes the first deployments of the Infrared Sea surface Temperature Autonomous Radiometer (ISAR) system to validate the ENVISAT AATSR and ERS0-2 ATSR in accordance with ESA AO9081 and AO9082. The ISAR has been developed to provide accurate and reliable measurements of the thermal sea surface skin temperature (SSTskin) to an accuracy of better than ±0.1 K that can operate autonomously for extended periods when deployed from a ship of opportunity (SOO). The ISAR system has been specifically designed to address the severe contamination problems that prevent measurement that are often encountered during continuous operation in the marine environment. It consists of a self calibrating infrared radiometer that uses two precision reference black body cavities, an on board GPS system, pitch and roll sensors and an optical rain gauge. The ISAR instrument has been successfully deployed during the ENVISAT Commissioning Phase on two different ships of opportunity operating in the English Channel and Bay of Biscay area. We present ISAR SSTskin observations collected during deployments aboard the Brittany Ferries M/V Val De Loire and P&O Ferries Pride of Bilbao during May-June 2002 and in October 2002. We will use these data to validate both the AATSR and the ERS-2 ATSR instruments once these satellite data become available. 1. INTRODUCTION Independent validation of satellite SST data streams using contemporaneous in situ observations provides a method to test instrument performance, atmospheric correction strategies and geophysical data accuracy. It is an important (but often overlooked) satellite mission component. When satellite instruments are thought to be capable of providing sea surface skin temperature (SSTskin) observations with an accuracy of better than 0.3 K (e.g., ENVISAT Advance Along Track Scanning Radiometer (AATSR) and ERS-2 Along Track Scanning Radiometer (ATSR/2)), it is the quality of the in situ measurement that is compared to the satellite data that is the critical factor within the validation exercise. Without accurate and complete in situ data, it is impossible to demonstrate the accuracy of AATSR and ATSR/2 SSTskin products. Innovative design coupled with technological advances, has over the years produced several ship-based radiometers capable of target measurements accurate to better than 0.075 K. While these instruments continue to provide high quality SSTskin data sets, they are not capable of truly autonomous deployment because they have no means to automatically protect their optical components from the harsh marine environment. Instead, they rely on an instrument operator to manually close and cover them in poor weather. Within a sustained validation program, the requirement for an operator incurs a considerable cost overhead, presents logistical problems (e.g., berth availability) and, also exposes instrument operators to unnecessary risk when securing instruments often located in dangerous areas (such as the ship fore-mast), often in deteriorating weather. Furthermore, only a small number of in situ radiometers exist that are deployed on research ships for satellite SSTskin validation experiments as they are not suitable for long-term deployments at sea. Consequently, the available SSTskin validation data are limited in number and spatial and temporal extent. More importantly, the regular collection of a comprehensive global in situ SSTskin data set has remained an elusive goal for the agencies and scientists tasked with the validation of satellite SSTskin data. There is a clear need for a new generation of automatic in situ infrared radiometers capable of sustained long-term operation that to provide adequate satellite SSTskin validation measurements. The Infrared Sea surface Temperature Autonomous Radiometer (ISAR) system has been developed to collect high quality in situ observations of SSTskin that can be used to validate the SSTskin observations made by the AATSR and ATSR/2. The ISAR design allows the instrument to be deployed on commercial ships operating regular transects without an operator to care for the instrument maximizing the number of contemporaneous matchups between the satellite and in situ

__________________________________________________________________________________________________________Proc. of Envisat Validation Workshop, Frascati, Italy, 9 – 13 December 2002 (ESA SP-531, August 2003)

observations while at sea. ISAR has been deployed on two ships during the ENVISAT Commissioning Phase to obtain contemporaneous SSTskin observations for the validation of AATSR and AATSR/2; the M/V Val De Loire operating from Portsmouth (UK) to Santander (Spain) and the M/V Pride of Bilbao operating from Portsmouth (UK) to Santander. This work has been completed in accordance with the “AATSR Statement of Work for DETR Components of AATSR Core Validation” (PO-SW-GAD-AT-000) as described in work package VAL6000 of the “AATSR Validation Activities Proposal to DETR” (UL-AV-01). This paper describes current progress and status of the ESA AO9081/2 validation project as of December 2002. We expect that the project will continue throughout 2003 as part of our continued effort to validate the AATSR SSTskin data products. 2. AO9081/2 PROJECT STRUCTURE Fig.1 presents a schematic diagram of the AO9081/2 project structure. Two major project components are identified: Data Collection (DC) and Data Analysis (DA). DC considers the construction, development, maintenance, calibration and software engineering required to build, maintain calibrate and deploy the ISAR system and ancillary instrumentation as a distinct set of activities. A second set of activities manages the deployment of these instruments and operational data collection including liaison with Brittany Ferries and P&O Ferries. Activities falling under the heading of DA concern the management and analysis of all data within the project including the processing of L0 in situ instrument data to provide geophysical data products, the archive of all data within the project, conducting validation data match-ups with AATSR data and the submission of results to the NILU web site. The purpose of Fig.1 is to underscore the wide scope of AO9081/2 in situ validation activities; it is a considerable undertaking demanding sizeable investment to collect suitable in situ measurements that can be used with confidence to validate AATSR and ATSR/2 data. The AO9081/2 project is managed by Mr. Gary Fisher based at the Southampton Oceanography Center, overseen by Dr. C. Donlon (European Commission Joint Research Centre Italy and, Prof. I. Robinson, Southampton Oceanography Centre). Team members include personnel from Brookhaven National Laboratories (Mike Reynolds and Ray Edwards, USA) and the Rutherford Appleton Laboratories (Tim Nightingale UK). Other project team members are drawn from the University of Leicester and the University of Southampton.

Fig 1. Schematic diagram indicating the ISAR project structure in relation to ESA AO9081/2.

AO-9081/2 activities began in January 2000 following acceptance of the initial ISAR design. Key project milestones to date include:

• January 2000: Completed design, construction and testing of a prototype ISAR system. • October 2000: Deployment of ISAR-01 at Southampton Oceanography Centre for a 4-week autonomous shore side

test. This experiment was successful and demonstrated the self-preserving concept of the ISAR system. • May 2001: A revised design for the ISAR system (version 5C) was completed based on the results of the 4-week

shore side test and consultation with Brookhaven National Laboratories (US project partner) • April 2001: A prototype operational software on-board data acquisition package (v1.0) was competed and tested. • May 2001: ISAR-01 participated in the Miami radiometer inter-calibration experiment ISAR-01 demonstrated a

comparable accuracy and rmse. deviation to other radiometer systems (accurate to 0.1 K with an rms. Deviation of ±0.1 K).

• July 2001: Two more ISAR systems (ISAR-02/3) were commissioned. • August 2001: A new ISAR-5C electronics design was completed providing dedicated functionality based on the

prototype system and subsequent laboratory and field experiments. • September 2001: The University of Leicester (UoL) contract to Southampton Oceanography Centre and Statement

of Work (see UL-AV-01) was finalized (funding for this work). • September 2001: Purchase of equipment under contract UL-AV-01 commenced. • March 2002: A workshop to integrate and test the new ISAR instruments was held at Southampton Oceanography

Centre. ISAR-01, ISAR-02 and ISAR-03 were constructed and tested for functionality (considerable further work was required prior to deployment).

• March 2002: A fully revised ISAR-5C operational software package was released designed specifically for the M/V Val de Loire deployments..

• May 2002: ISAR-01 was installed on M/V Val de Loire and commenced operational measurement of skin SST for the validation of ENVISAT AATSR.

• June 2002: ISAR operations were transferred from the M/V Val de Loire to the P&O Ferries vessel Pride of Bilbao.

3. THE ISAR IN SITU RADIOMETER SYSTEM The ISAR is a compact (570 x 220 mm) precision, self-calibrating, infrared radiometer capable of measuring in situ SSTskin accurate to ±0.1 K rmse. The ISAR uses two precision calibration black body (BB) cavities to maintain the radiance calibration of a solid-state infrared detector having a spectral window of 9.6-11.5µm. All ISAR target views are made using a single route optical path via a protective scan drum arrangement that may be programmed to any target view to positioned over a range of 160°. In this way, views of the atmosphere can be made at regular intervals providing the necessary measurements required to correct sea view radiance data for sky radiance reflection due to the non-unity value of sea water emissivity. This design is preferential to the alternative use of two separate detector systems having independent optical paths viewing the sea surface and atmosphere at the same time because:

1. A single optical pathway ensures an identical spectral measurement for sea and sky measurements 2. The system is cost effective and robust requiring only a single detector, optical path and calibration system. 3. This solution allows for an extremely compact instrument.

For any optical instrument intended for use in the harsh marine environment, adequate environmental protection is critical. Rain, seawater spray, and high humidity can destroy a poorly protected instrument rapidly and components such as electrical connections and fore-optics should be resistant to these effects. In situ radiometer windows are particularly important in this context. They should not significantly reduce the incoming signal or render it noisy and, be strong enough to resist mechanical, thermal and chemical degradation. When considering the design of an automatic infrared radiometer, it is critical that any optical surface does not become wet otherwise the radiometer will simply determine the temperature of the wetted water surface on the wet component nearest to the detector. Adopting the practical design principle that “ISAR will get wet aboard a ship” and without an operator to cover and protect the instrument in bad weather, the design remit was

to provide a system that would ensure complete optical component protection while allowing rain or sea spray to run off the instrument case before the instrument could be completely closed against the environment. To address this requirement, the ISAR system uses an intelligent shutter arrangement to completely seal the instrument from the environment during bad weather.

Fig.2 A typical time series recorded by the ISAR rain sensor (ORG signal, black dots), ISAR storm shutter (Red triangles

0=closed, 1=open) and, ISAR scan drum aperture position (blue crosses). The BB1 position is at 285° (the “safe” position) and BB2 position is at 330°.

A shutter mechanism, triggered by an optical rain sensor, automatically completely seals the ISAR from the external environment during bad weather. The shutter mechanism is triggered by an extremely sensitive optical rain gauge. The shutter is capable of complete instrument closure within 8s of a rain signal and a typical example of the ISAR system in operation is provided in Fig.2. In this figure the signal measured by the ISAR rain gauge is shown as a black line and when this exceeds a user-defined threshold, the ISAR shutter closes (shown in red). The blue crosses indicate the various target views made by the ISAR system during this particular deployment. Note that the ISAR views a blackbody cavity in closed mode. Five different sea views (angles < 100°) and associated sky views (100° < angles < 200°) were obtained over a three-day period. A rain signal of < 0.05 corresponds to about 1-10 water drops per 30 second period. Once the rain gauge signal falls below the threshold value, following a user programmable period of no rain/spray, the shutter automatically opens and measurements continue. While no system is capable of providing 100% protection from the sometimes-extreme conditions encountered at sea (e.g., as a ship bow “digs in” and throws huge amounts of sea water into the air), experience shows that the ISAR system is a good working solution. 4. CALIBRATION OF THE ISAR RADIOMETER Prior to every deployment and also after each deployment, the ISAR radiometer internal calibration system accuracy is checked using an external independent black body radiance source. Fig.3 shows a typical calibration run, in this case prior to a deployment o the Pride of Bilbao, using a CASOTS black body reference. These experiments also help to identify any problems when reassembling the ISAR instrument or after any major changes in the optical system have been necessary. Fig.3(b) clearly demonstrates that ISAR is able to provide SSTskin observations to an accuracy of +/- 0.1k rmse.

Fig.3. Calibration of ISAR-002 using the SOC CASOTS blackbody reference cavity together with a NIST traceable Hart Scientific 1504 bridge (Serial:A1B256) with Thermometrics ES255 (Serial:203). Laboratory room temperature was 299.5+/-2 K throughout the calibration. (A) Overview of calibration showing HART data as a red line and ISAR ungridded (i.e., 1s) samples. (A) Difference between the Hart thermometer and ISAR retrieved CASOTS blackbody temperature. Data are gridded into 2 min bins identical to those used during normal operations. Data from the first ½ hour of the calibration have been ignored because the ISAR internal calibration blackbody temperatures had not attained operation stability. 5. ISAR INSTRUMENT SOFTWARE DEVELOPMENT Considerable time and effort has been given to the refinement and testing of both the operational data logging software code that resides on the ISAR internal computer, the serial data logging software that resides on the ISAR base computer and the post processing code base that is used to process the ISAR data to geophysical values. Sets of UNIX scripts have been written to log data to a Linux base computer, which fully automates the management of the many ISAR data files that can

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be collected, which is time-consuming. This is important as the support team have only a short period of time to collect data from the ISAR and other systems because Ferry ships do not remain in port for more than a few hours at a time. A real-time graphical user interface showing the major parameters logged by the ISAR system has also been written. The interface provides the operator with a configurable display that has full auto-scaling features. This type of interface greatly facilitates problem solving and enables an operator to check the status of the ISAR system at a glance. An example data capture plot obtained from the Val De Loire deployment is provided in Fig. 4 below. In this example, the ISAR is operating normally for the initial part of the data set as can be seen from the scan drum position that changes as each target measurement is made. However, the rain gauge signal increases and the ISAR shutter closes and scan drum position moves to view only one blackbody cavity.

Fig. 4. Example real-time data display of the ISAR-5C system using the isar-rtp software.

Finally, a dedicated data processing software package called isarospp has been written in the IDL language to process all ISAR data. This package unpacks all raw ISAR data, calibrates each on-board sensor L0 data stream, and derives calibrated radiances for each ISAR target view. 6. Installation of ISAR aboard the M/V Val de Loire The ISAR radiometer system has been installed on the Starboard side of the M/V Val De Loire as shown in Fig. 5. A special ISAR mounting bracket has been designed and constructed to fit onto the ship bridge wing area. In this position the radiometer has a clear view of the sea surface undisturbed by the ship bow wave. The ISAR views the sea surface at an angle of 25° from nadir thereby limiting variability of the emissivity of sea water (due to sea state changes and ISAR view geometrical changes as the ship moves) and also minimises direct reflections from the ship hull. The ISAR data logging computer and power supply are installed in the ships bridge via cables led through the ships superstructure. Measurements began on April 30th 2002 and continued until June 26th 2002 aboard the M/V Val de Loire.

Fig. 5. Installation of the ISAR-5C radiometer aboard the M/V Val de Loire. Clearly seen in the right hand panel is the

optical rain gauge used to control the ISAR shutter system. 7. INSTALLATION ON THE PRIDE OF BILBAO In August 2002, all ISAR operations were transferred from the M/V Val de Loire to the P&O Ferry M/V Pride of Bilbao operating from Portsmouth. The transition was an involved process requiring the use of external contractors to install new dedicated cables within the ship, the design, construction and installation of a new mounting bracket for the ISAR instrument, installation of a new power supply unit and recalibration of the ISAR instrument. ISAR measurements commenced aboard the Pride of Bilbao on the 16th August 2002. Fig. 6 shows a series of photographs describing the ISAR installation on the Pride of Bilbao. These identify the ISAR rain-gauge and provide an indication of the small instrument size.

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Fig.6. (A) Photograph of ISAR-001 mounted on the Pride of Bilbao using the mounting bracket specifically designed for the ship showing the clear view of the sea surface. Note that the ISAR shutter is in the closed position. (B) Photograph showing the support straps used to secure the ISAR mounting bracket in place, the optical rain gauge, junction box (lower left) and cables. (C) Detail showing the installation mount of the Optical rain gauge. (D) SOC technicians during the installation of ISAR-001 aboard the Pride of Bilbao. Note the compact size of the ISAR instrument. 8. RESULTS The ISAR instrument has successfully measured SSTskin temperature for a sustained period of over 50 days providing excellent opportunities for validation of the ENVISAT AATSR and ERS-2 ATSR instruments. A map of the Val de Loire ship track for the period April 30th - June 26th 2002 is shown in Fig 7. The ISAR instrument has functioned in an autonomous mode without any supervision throughout this period. Also marked on this plot as small dots are the location of all the calibrated samples obtained by the ISAR in non-raining conditions. The large colored dots depict AATSR validation opportunities (an AATSR/ATSR/2 "validation indicator"), scaled as “no chance of validation”, “a fair chance” and “a good chance of obtaining useful AATSR and ATSR/2 validation data” within the local satellite overpass time periods of 09:30-

11:30 and 21:30-23:30. This type of simple analysis allows the ISAR team to filter the extensive in situ data sets we have obtained to find good match up and focus further data analysis to these high priority periods (including the order of AATSR and ATSR/2 L1b data streams) The criteria used to derive the validation indicator are:

• Good: If a clear sky prevailed (sky brightness temperature (BTsky) <200 K) • Fair: if broken cloud conditions prevailed (200 K < sky brightness temperature < 260 K) • No chance: Cloudy sky conditions (sky brightness temperature > 260 K)

Further information regarding the possibility of extensive clouds is also manifest in the variability of the SkyBT signal.

Fig.7. Map of the ship track followed by the M/V Val de Loire during Sday 120-168 (30th April - 17th June ) 2002. Also marked are indicators of opportunities for AATSR and ATSR/2 validation.

Fig.8. Time series of SSTskin data obtained using an ISAR radiometer during the period Sday 120-168 (30th April – 17th June ) 2002. Also marked are indicators of opportunities for AATSR validation.

Fig.8 plots a time series of SSTskin temperature derived from ISAR observations with the superimposed AATSR validation indicator. A total of 37 "good" opportunities for AATSR validation are apparent in this figure with considerably more "fair" opportunities within this data set. Note that the final number of actual matchups between ISAR and AATSR will depend on the availability and local cloud contamination of AATSR data but at present, AATSR and ATSR/2 data are unavailable for this period and a validation study is not yet possible although we expect to obtain AATSR and ATSR/2 data shortly. The large number of AATSR and ATSR/2 validation opportunities constitute a considerable success for the ISAR project. At least 6 days of data were successfully collected during a second ISAR deployment aboard the M/V Pride of Bilbao between the 22nd October and 1st November 2002. A summary of these data is shown in Fig.9 which plots the ISAR shutter position, scan drum position and rain gauge signal. Note the intense rain signals where the ISAR closes all operations. Also note that there are many times when the ISAR is making measurements in between rain events albeit in some cases for rather small periods of time. As an indication of what the rain gauge signal refers to, the final period of rain for Sday 300 was during extremely heavy storm conditions in which several parts of the ship superstructure were damaged. Fig.10 shows the location and magnitude of successfully calculated SSTskin data retrieved during this deployment. Also plotted in this figure is the validation indicator described previously demonstrating that while there were several opportunities for validation of AATSR and ATSR/2 data, there are no suitable validation data due to persistent cloudy conditions. However, until the satellite data themselves are checked for cloud, it is difficult to confirm this assumption.

Fig.9. Summary of ISAR measurements aboard the M/V Pride of Bilbao (22nd October 2002 – 1st November 2002) showing the scan drum position, shutter position and rain gauge signal.

Fig.10. SSTskin observations returned from the ISAR-002 instrument during the period 22nd October 2002 – 1st November

2002. (a) Geographic location of successfully retrieved SSTskin data together with an indicator of AATSR validation potential. (b) ISAR-5C SSTskin observations.

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9. FUTURE WORK The ISAR system is currently operating aboard the M/V Pride of Bilbao and will remain in this configuration throughout 2003 (funding permitting). In January 2003, the ISAR system will be fully interfaced to the Southampton University Oceanography Centre FerryBox experiment that measures in situ SSTdepth, Salinity and fluorescence. The FerryBox data are reported back to base in real-time via a dedicated satellite link. ISAR SSTskin data will be ingested by this system and made available on the FerryBox web site (http://www.soc.soton.ac.uk/ferrybox/index.html). We will use ISAR data to validate AATSR and other satellite sensors that are used to derive SSTskin measurements.

Fig.11 Cruise track of the M/V Falstaff that will be used to obtain SSTskin observations in 2003-2004 using an ISAR radiometer system

In addition, we expect to make a significant contribution to a USA National Ocean Partnership Program (NOPP) project that will deploy an ISAR radiometer aboard the M/V Falstaff that operates along a repeat transect from Copenhagen, New York, and in the Gulf of Mexico. The Falstaff is a large car transport ship and will provide an ideal platform for the deployment of an ISAR system as shown in Fig.11. Finally, we expect to participate in a detailed scientific experiment operating on the Aqua Alta Tower in the North Adriatic in June 2003. During this experiment in situ infrared and visible radiometer systems will be operated together with microwave scatterometers and traditional in situ sensors. The aim of the experiment is to assess the effect of surface roughness on the accuracy of radiometer measurements. 10. CONCLUSIONS A new autonomous infrared radiometer system, called the ISAR, has been developed for use on commercial ships without an operator. The ISAR system has been deployed on two different ships operating in the English Channel and Bay of Biscay during the ENVISAT commissioning phase. In situ observations of SSTskin have been collected during these deployments specifically for the validation of AATSR and ATSR/2 instruments. These data have been processed to provide SSTskin measurements. At present (December 2002) the ISAR team are waiting for AATSR and ATSR/2 satellite data to be delivered before ISAR data can be matched with the satellite data and a validation analysis performed. ISAR validation activities will be sustained (subject to funding) throughout the 2003-2004 period and we expect to extend our operations within partnership with other international institutes using ships that make repeated trans-Atlantic voyages in 2003-2004. Finally, we expect that a commercial version of the ISAR system will be available in mid 2003.