An important part of the AMT4SentinelFRM project is the validation of the SLSTR (Sea and Land Surface Temperature Radiometer) sensor on Sentinel-3. There are a number of key instruments on board the cruise that capture this crucial data. Werenfrid Wimmer, a current cruise participant explains what these instruments are and their relevance to this process.
The Sea Surface Temperature (SST) part of the AMT4SentinelFRM has one main instrument, ISAR, and a number of ancillary instruments to help understand and interpret the data.
The Infrared Sea surface temperature Autonomous Radiometer (ISAR) is a single channel self-calibrating radiometer and measures in the 9.6 to 11.5 micrometre part of the infrared spectrum. This part of the spectrum is, together with a few other wavelengths, used by SLSTR on Sentinel-3 to estimate SST. ISAR instruments have been used in the past to validate AATSR (Advanced Along Track Scanning Radiometer, SLSTR’s predecessor) and more recently AVHRR on Metop A and B as well as many of the GHRSST SST products. ISAR measurements are traceable to SI-standards and each measurement has its own uncertainty attached.
The suite of ancillary instruments include the instruments listed below and allow to characterise the air-sea fluxes which is and important part of interpreting the SST measured by SLSTR:
- Kipp and Zonen CM11 pyranometer
- Eppley Precision Infrared Radiometer, PIR
- Gill Windmaster Anemometer
- Vaisala HMP243 Air temperature and humidity sensor
- Seabrid SBE48 Hull temperature sensor
- Vaisala RS92 Radiosondes
The CM11 measures the incoming solar radiation in the visible and near infrared spectrum (310 – 2200 nanometre) and gives an indication how much solar energy is available to be absorbed by the ocean.
The PIR is a broadband infrared radiometer (4 – 50 micrometre) used to measure the downward infrared radiation as emitted by the atmosphere. The CM11 and the PIR are mounted on gimbals to ensure that they are always measuring radiation from straight above.
The Gill Windmaster is an ultrasonic anemometer which measures the wind in all three axis and provides high accuracy wind speed and direction information. This measurement together with the CM11 are used as an indicator for diurnal warming events, which is important when SST from various depths is compared.
The HMP243 provides air temperature and humidity data which are need for the calculation of air sea fluxes, as well as showing moisture content in the atmosphere.
The SB48 is a hull mounted thermometer, which is attached on the inside of the ships hull below the water line and measures the SST at depth through the hull. This will be used together with the ships engine intake measurements as an SST depth measurement which are frequently used for validation. However such measurements have a higher bias and lower accuracy than the ISAR instrument when used for SST validation.
The radiosondes are launched once a day attached to a helium balloon, and provide lower atmosphere (up to 30 km) profiles of temperature, humidity, wind speed and direction. This information is used for the validation atmospheric models, which are used for SLSTR atmospheric correction.
We also take bucket SST measurements and meterological observations every six hours.