Guidelines for the delayed mode scientific correction of glider data. WP 5 , Task 5.7, D5.15. Version 4.1.

Abstract

Gliders are a rapidly maturing class of marine observing vehicles that offer long duration, some operate for several months at a time, autonomous ocean profiling in all weather conditions and sea states, to depths typically up to 1000 m. Gliders operate through using the profiling float principle of controlling buoyancy by pumping oil between reservoirs internal and external to their pressure hull; but unlike profiling floats they balance their buoyancy against lift on a pair of short wings, controlling their centre of gravity and therefore pitch and roll through the movement of weight, conveniently their battery packs, within the pressure hull. In this way glider vehicles glide through the water column, with horizontal and vertical speed components of around 25 cm s-1.

Glider vehicles are typically just over one metre in length and as standard they will have a payload bay equipped with the generic triplet combination of conductivity, temperature and pressure sensors. In addition many will be specified with oxygen sensors and/or a suite of fluorescence and optical backscatter sensors. More unusually they have now been equipped with passive acoustic monitoring equipment and even a vessel mounted ADCP. Some gliders have been externally fitted with UV absorption nitrate sensors, and turbulence/microstructure instruments.

Gliders constitute an essential component of coastal observing systems for a number of reasons. Although flying gliders still has a significant manual component, albeit remote, requiring well trained glider pilots, and deployment and recovery, they are highly cost effective compared to ship based operations. Although slower moving than a research vessel, gliders are capable of acquiring data at a higher temporal and spatial resolution than was previously economically practical, and are able to operate even in rough sea states. The spatial and temporal resolutions of coastal data and their quality are of crucial importance to adequately respond to scientific and societal challenges.

In order to generate data of high scientific quality, calibration/correction has to be applied in two steps after a glider mission. While the first calibration is done routinely using the manufacturer’s software, the second is referred to as delayed mode scientific correction. The first calibration is generally applied in real-time and includes a set of specific calibration expressions depending on the sensor type and model with the last manufacturer's calibration coefficients. Whilst instrument manufacturers have significantly improved laboratory calibrations and instrument stability, the effectiveness of gliders as an instrument platform is still limited by the ability to ensure the observations are in-field corrected to a world class standard. This second stage correction requires a careful comparison with measurements acquired by other platforms and instruments in the same region during a sensibly common period. This report focuses on the standards and methods of operation to achieve this.