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The Atlantic Meridional Transect
Spatially Extensive Calibration and Validation of Optical Properties and Remotely-Sensed Measurements of Ocean Colour

James Aiken
Plymouth Marine Laboratory
Prospect Place, Plymouth, PL1 3dH, UK
E-mail: j.aiken@pml.ac.uk 		Stanford B. Hooker
Goddard Space Flight Center
Code 970.2, Greenbelt, MD 20771, USA
E-mail: stan@ardbeg.gsfc.nasa.gov

This article has been commissioned by the IOCCG and has appeared in the backscatter magazine, published by the Alliance of Marine Remote Sensing (AMRS)

Twice a year, the Royal Research Ship (RRS) James Clark Ross (JCR) steams a meridional transect of the Atlantic Ocean between Grimsby (UK) and Stanley (Falkland Islands) with a port call in Montevideo (Uruguay), as part of the annual research activities of the British Antarctic Survey (BAS). In September, the JCR sails from the UK, and the following April it makes the return trip. The ship is operated by the BAS for the Natural Environment Research Council (NERC). The Atlantic Meridional Transect (AMT) Program exploits the passage of the JCR from approximately 50°N to 50°S with a primary objective to investigate physical and biological processes, as well as to measure the meso- to basin-scale bio-optical properties of the Atlantic Ocean. The calibration and validation of remotely sensed observations of ocean color is an inherent objective of these studies: first, by relating in situ measurements of water-leaving radiance to satellite measurements, and second, by measuring the bio-optically active constituents of the water.

To date, there have been four AMT cruises: AMT-1 departed Grimsby on 21 September 1995 and docked at Stanley on 24 October 1995; AMT-2 left Stanley on 18 April 1996 and arrived in Plymouth on 25 May 1996; AMT-3 departed Grimsby on 20 September 1996 and docked in Stanley on 25 October 1996; and AMT-4 started from Stanley on 21 April 1997 and ended in Grimsby on 27 May 1997. The AMT Program is led by the Plymouth Marine Laboratory (PML) in partnership with the Southampton Oceanographic Centre and BAS with contributions from University groups in the UK and Europe. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project has been a major partner with PML in the AMT Program, supplying state-of-the-art optical sensors and logistical support. Figure 1 depicts the AMT-2 cruise track superimposed on the major current systems of the Atlantic Ocean between 50°N to 50°S, plus a set of composite AVHRR sea surface temperature images for the cruise time period.

To exploit the passage of the JCR, the AMT Program employs three sampling strategies:

1) Continuous, underway, surface layer measurements from pumped sea water of temperature (T) and salinity (S), CO2, nutrients, and inherent optical properties at nine wavelengths (Wet Labs AC-9), with discrete measurements (every two hours) of phytoplankton species high performance liquid chromatography (HPLC) pigments;

2) Towed measurements (10-80 m) using the Undulating Oceanographic Recorder (UOR) with sensors for T-S, chlorophyll fluorescence, water transmission (660 nm), plus downwelling irradiance, Ed(), and upwelling radiance, Lu(), at seven SeaWiFS wavelengths;

3) Station measurements, made daily at local solar noon or pre-noon, for conductivity, temperature, and depth (CTD) and water samples to 200m, for plankton, pigments, and productivity determinations; separate casts for multispectral optical properties, Ed() and Lu() with the SeaWiFS Optical Profiling System (SeaOPS) and the SeaWiFS Free-falling Advanced Light Level Sensors (SeaFALLS).

All biogeochemical measurements and all optical measurements adhere to the Joint Global Ocean Flux Study [JGOFS, 1991] and SeaWiFS protocols [Mueller and Austin, 1995], respectively. Whenever possible, new instrumentation and novel technologies have been employed to enhance the data acquisition in both quantity and quality; e.g., autonomous CO2, the UOR, the AC-9, SeaOPS and SeaFALLS, as well as the SeaWiFS Quality Monitor (SQM) which is used to monitor the stability of the radiometers. All of the radiometers used, including spares, were manufactured by Satlantic, Inc. (Halifax, Canada) to ensure redundancy and intercalibration. Figure 2 depicts the main radiometric instruments used in the AMT Program.


Figure 1
Figure 1. The AMT-2 cruise track superimposed on the major current systems of the Atlantic Ocean between 50°N to 50°S, plus a set of composite AVHRR sea surface temperature images for the cruise time period.
The calibration and validation of ocean color satellites and the acquisition of data for the development of remote sensing algorithms, requires sensor and data accuracies of the highest order, generally better than those achieved until now; the same data with comparable accuracies are required for the validation of atmospheric correction algorithms and the validation of the biogeochemical products derived from remotely sensed observations. For SeaWiFS and other ocean color sensors, the goal is to produce water-leaving radiances with a 5% accuracy in clear-water regions [Hooker and Esaias, 1993]. This objective requires field instruments with a calibration and measurement accuracy that is of the order of 1%. Calibrating optical sensors before and after a cruise rarely delivers this goal, but recent developments of the SeaWiFS Project produced the SQM-a self-contained, portable, and stable light with integral optical assemblies-which can be used to check the optical stability of radiometers deployed in situ, throughout the course of a research cruise lasting four weeks or more.

The engineering design and characteristics of the SQM are described by [Shaw et al. 1997] and [Johnson et al. 1997], so only a brief description is given here. The SQM is capable of monitoring the stability of radiance and irradiance sensors to within 1% in the field. Used in conjunction with laboratory calibration sources, the SQM can be used to transfer the calibration to the field equipment. Three flux levels are possible, using two lamp assemblies. The exit aperture of the SQM is large, homogeneous in radiance, and was designed to approximate a lambertian radiator. An internal heater provides operational stability and decreased warm-up intervals. Temperature-controlled silicon photodiodes with colored glass filters are mounted inside the SQM and monitor the stability of the generated light field. The independent monitors normalize the flux of the source, so the actual change in the responsivity of the field sensor can be determined. A change in the responsivity of the field sensor may be distinguished from a change in the reflectance of the front surface of the sensor using three reference devices, or fiducials, that are designed to mimic the reflectivity of the optical surface of the field sensor, but are not functioning detector units. The front surface of the fiducial is protected when not in use and is kept clean during the field experiment. A kinematically designed D-shaped collar is used on both the optical sensors and the fiducials to ensure the devices being tested view the same part of the SQM aperture each time they are used.

The SQM was deployed on the AMT-3 cruise for the first time and demonstrated the following capabilities [Hooker and Aiken, 1997]: 1. The SQM can be used to track the stability of field radiometers at less than the 1% level in terms of the radiometric response of the sensors; 2. The SQM light field is sufficiently stable to model changes in the radiometric detectors; 3. Based on the radiometers used during AMT-3, daily SQM measurements are required to resolve short-term temporal changes of the radiometric detectors; and 4. The field assessment showed that the SQM performance decayed approximately 0.6% during the course of the 36 day deployment.

SeaOPS is composed of an above-water and in-water set of instruments [Robins et al., 1996]. The in-water component is composed of a downward-looking radiance sensor (OCR-200) which measures upwelling radiance, Lu, and an upward-looking irradiance sensor (OCI-200) which measures downwelling irradiance, Ed. The in-water instruments are mounted on a T-shaped frame which is lowered and raised through the water column by a winch; data are collected during the lowering and raising of the frame. The above-water component, an OCI-200, measures the incident solar irradiance at the ocean surface, Ed(0+). The OCI-200 and OCR-200 radiometers have seven channels, which were chosen to correspond with the SeaWiFS instrument wavelengths and bandwidths [Hooker and Esaias, 1993].

The UOR is a measurement platform which is towed approximately 400 m behind a ship [Aiken and Bellan, 1990]. It uses a programmable servo that controls the attitude of a diving plane which causes the vehicle to undulate through a preset depth range, typically 10-80 m, at tow speeds between 10 to 12 kts. At speeds in excess of 6 kts, the servo unit is powered by an alternator, which is driven by a propeller on the rear of the body. During AMT-3, an OCI-200 was fitted to the top of the UOR instrument bay to measure downwelling irradiance, Ed, and an OCR-200 was fitted to the bottom of the instrument bay to measure upwelling radiance, Lu. The OCI-200 and OCR-200 radiometers employed in the UOR and SeaOPS use 16-bit analog-to-digital (A/D) converters and are capable of detecting light over a four decade range.

SeaFALLS is composed of two subsystems: a SeaWiFS Profiling Multichannel Radiometer (SPMR) and a SeaWiFS Multichannel Surface Reference (SMSR). The former measures Ed and Lu as it falls freely through the water column, while the latter measures the incident solar irradiance just below the sea surface, Ed(0-). The profiler receives its power and sends its data via an umbilical cable, while the reference floats just below the surface suspended from a square floating frame [Waters et al., 1990]. Both the profiler and the reference can be deployed far away from the ship, so any ship-induced disturbances to the in situ light field are minimized [Mueller and Austin, 1995]. Since SeaFALLS can be deployed quickly by only two people, the ship can be stopped when light conditions are optimal. SeaFALLS is equipped with 13-channel OCI-1000 and OCR-1000 radiometers, which employ 24-bit A/D converters, and are capable of detecting light over a seven decade range.

The data acquired by these means are used to validate the performance and accuracy of algorithms for the interpretation of SeaWiFS imagery and generate new algorithms where deficiencies are detected. Along with the HPLC pigment measurements, the analysis points to the validation of the Platt-Longhurst hypothesis [Platt et al., 1995; Sathyendranath et al., 1995] of bio-optical provinces with distinct optical properties; AMT measurements show that these align precisely with physical provinces determined by T-S relationships. Significantly, the analysis shows that interprovince bio-optical variability is driven by phytoplankton accessory pigment concentrations, particularly photoprotective carotenoid (PPC) abundance, and ratio to photosynthetic carotenoids (PSC), i.e., PPC/PSC, which is highest in high-light equatorial zones; though this is as expected ecologically, the ratios are province-specific rather than intensity-specific.

With the wide diversity of ecosystems encountered (two hemispheres and two seasons are sampled over a period of 30-35 days), an AMT cruise is like many cruises rolled into one, making it inevitable that new observations and new discoveries will emerge on every transect. For the future, the project will continue to pioneer new sensors and new technology. During AMT-4, a new free-fall radiometric instrument called the Low-Cost NASA Environmental Sampling System (LoCNESS) and a new hyperspectral radiometer called SeaSPEC were both tested. By AMT-5, a Fast Repetition Rate Fluorometer (FRRF) will be deployed in the UOR to provide measurements of photosynthetic parameters in all the provinces of the Atlantic Ocean - data which are necessary to exploit remotely sensed observations of ocean color for estimating global primary production.

Figure 2
Figure 2. A collage of the core optical instruments deployed on the AMT cruises: a) SeaFALLS profiler, b) SeaFALLS reference, c) SeaOPS, d) the UOR, and e) the SQM.


References

Aiken, J., and I. Bellan, "Optical Oceanography: An Assessment of a Towed Method." In: Light and Life in the Sea, P.J. Herring, Ed., Cambridge University Press, 39-57, 1990.

Hooker, S.B., and W.E. Esaias, "An Overview of the SeaWiFS Project." Eos, Trans. AGU, 74, 241-246, 1993.

Hooker, S.B., and J. Aiken, "Calibration Evaluation and Radiometric Testing of Field Radiometers with the SeaWiFS Quality Monitor (SQM)." J. Atmos. and Oceanic Tech., (submitted), 1997.

Joint Global Ocean Flux Study, "JGOFS Core Measurements Protocols." JGOFS Report No. 6, Scientific Committee on Oceanic Research, 40 pp, 1991.

Johnson, B.C., P-S. Shaw, S.B. Hooker, and D. Lynch, "Radiometric and Engineering Performance of the SeaWiFS Quality Monitor (SQM): A Portable Light Source for Field Radiometers." J. Atmos. and Oceanic Tech., (submitted), 1997.

Mueller, J.L., and R.W. Austin, "Ocean Optics Protocols for SeaWiFS Validation, Revision 1." NASA Tech. Memo. 104566, Vol. 25, S.B. Hooker, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 66 pp, 1995.

Platt, T., S. Sathyendranath, and A. Longhurst, "Remote Sensing of Primary Production in the Ocean: Promise and Fulfilment" Phil. Trans. R. Soc. Lond. B348, 191-202. 1995

Robins, D.B., A.J. Bale, G.F. Moore, N.W. Rees, S.B. Hooker, C.P. Gallienne, A.G. Westbrook, E. Marañón, W.H. Spooner, and S.R. Laney, "AMT-1 Cruise Report and Preliminary Results." NASA Tech. Memo. 104566, Vol. 35, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 87 pp, 1996.

Sathyendranath, S., A. Longhurst, C.M. Caverhill, T. Platt, "Regionally and seasonally differentiated primary production in the North Atlantic." Deep-Sea Res., 42(10), 1773-1802, 1995.

Shaw, P-S., B.C. Johnson, S.B. Hooker, and D. Lynch, "The SeaWiFS Quality Monitor-A Portable Field Calibrator Light Source." Proc. SPIE, 2,963, 772-776, 1997.

Waters, K.J., R.C. Smith, and M.R. Lewis, "Avoiding Ship-Induced Light-Field Perturbation in the Determination of Oceanic Optical Properties." Oceanography, 3, 18-21, 1990.



This article appeared in the August 1997 issue of the backscatter magazine, published by the Alliance of Marine Remote Sensing (AMRS).

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