Sensors

CTD
  • Measurement accuracy :

Provor floats are fitted with Sea-bird SBE41CP and APEX floats with the SBE41 CTD sensors (http://www.seabird.com/products/profilers.htm). Sea water is pumped through the SBE41 at a rate of 40 ml/sec for 2.5 seconds during which the measurements of T, C and P are made. The SBE 41CP(continuous rofiling) is pumped at a rate of 30 ml/sec flow continuously during the profile.

On all floats the CTD sensor unit is placed at the top of the float and is controlled by the main CPU of the float. The CTD is used :

  • to get pressure measurements for controlling float behaviour, using specific "fast pressure" command. It consumes approx. 100 milliwatts.
  • to get triplets (Pressure, temperature and salinity) mainly for ascent, but also for descent and drift at parking depth. It consumes approx 300mW.

In all cases the pumping of the CTD sensor is turned off between 0 and 10m (adjustable) before the float reaches the surface. This is to avoid contamination (and hence degradation) of the sensor by pollutants at the sea surface.

The specifications for both SBE41 and SBE41CP are:

Pressure

Salinity

Temperature

range

0-2500 dBar

2 to 42 PSU **

-2°C to +35°C

initial accuracy

2,4 dBar

3 mPSU

2 m°C

resolution

0.1 dBar

1mPSU

1 m°C

drift

< 5 dBar / 5 year *

10 mPSU / 5 year

2 m°C / 5 year

transmitted range

0-2500 dbar

10 to 42 psu

-2 to 30°C

transmitted resolution

1 dbar

0.001 psu 15 bits

0.001 °C

* Pressure offset is compensated at each surfacing

** stop pumping at 5 dBar

Since Argo floats are not designed to be recovered at the end of their mission, calibration of the conductivity (salinity) sensors is achieved by comparison of salinity data with recent ship-based measurements and by comparisons between nearby floats. The method is adopted by all float operators and is documented by Owens and Wong, 2009, and is also kept under constant review by the Argo Data Management Team and the Argo Steering Team.
The measurement of salinity is fundamentally dependent on the accuracy of the pressure and temperature measurements. Pressure offsets in all float models can be adjusted by measuring atmospheric pressure when the float is at the surface. Temperature measurements have to be assumed to adhere to the manufacturer’s or float operator’s pre-deployment calibration.

  • CTD acquisition :

a) Provor measurements and sampling method :
During descent, if enabled by the user, measurements are done just after sinking detection (typical threshold 8 dbars). The CTD pump is then running continuously until parking depth is reached. During ascent, measurements are made from leaving start profile pressure until surface. The CTD pump is running continuously until 5 dbar is reached, avoiding to pump any dirty film at sea surface. The main CPU picks PTS samples every 10s, while the CTD delivers one sample every 2s. At first descent, measurements are always done, enabling CTD on ship comparison. At parking depth, the CTD pump is put on every time a measurement is programmed (min 1 hour). A profile from 2000m to surface spends 6.5 KJ , approx. 30% of the total power consumption of the float.
b) Provor data reduction :
The profile is composed of 2 area named bottom area and surface area. Each of them is split into slices, enabling the user to program the number of transmitting points. PTS samples acquired into each slice are averaged in order to reduce amount of data to transmit : the result is a triplet approximately adjusted on mean pressure of the slice. On floats fitted with iridium transmission, standard deviation is also processed.

Dissolved Oxygen

The measurement of dissolved oxygen from profiling floats adds greatly to our understanding of both physical and biogeochemical process and thus has been the focus of considerable technological development effort. To date 291 floats carrying dissolved oxygen sensors have been deployed of which 191 remain active. Most of these are in the Pacific, but a few are in the Southern Ocean and the tropical and subpolar Atlantic.

Two different sensor types are used today on Argo floats, Seabird electrochemical sensor and Aanderaa Optode 3830. Each of these has pros and cons; while for response times and initial accuracy the Seabird sensors are thought to be superior, it is the long-term stability, the ability to measure in low O2 concentrations, and the robustness against biofouling that speaks for the Optode.

Provor - Existing technology on PROVORDO: 3830 Aanderaa optode

3830 Aanderaa optode
  • The absolute sensor accuracy is today it is ±5 % or ±8 µM (whichever is greater). Factory settings and calibration is sometimes erroneous.
  • 90 % response time is today around 40 s, that is too much.
  • The position on the float is not ideal (lower end cap) when profiling on the ascent and atmospheric measurements are not possible when the float is “parked” at the surface. It could also be a difficulty to compare measurements with CTD, eg. in presence of high temperature gradients .
  • Measurement is done every CTD sample (10s). The optode is switched on during 2s, and off 8s. Extra power for optode lead to consume ~8% of total float energy, so lifetime capabilities of Provor float could be maintained beyond 3 years. The cost of an optode is around 4000€.
Ice Sensing

Initially Argo was intended to observe the deep ice-free ocean in real time. However, during recent years, the ice-resilience of Argo floats has been increased. Today four different types of ice compatible floats are in use:

  • Two commercial float types: NEMO (by Optimare Sensorsysteme AG, Bremerhaven, Germany) and APEX (by Webb Research Corporation, East Falmouth, Massachusetts, USA)
  • and two non-commercial float types manufactured at the University of Washington, USA (Steven Riser), and at the Hole Oceanographic Institution, USA (Peter Winsor).

So far, NEMO floats are ice compatible within in the Southern Ocean only. Ice compatibility is achieved by NEMO floats through three modules.
1) Ice Sensing Algorithm (ISA) :
The algorithm aborts the float’s ascent to the sea-surface when ice is expected at the surface. ISA improved float endurance in ice-covered seas significantly from less than 40% to 80% percent.
2) Interim Storage (iStore) :
With some areas being ice covered for significant periods of time, substantial numbers of profiles will be aborted and thus not transmitted immediately, although these profiles had been measured by the float. Hence, it is desirable to save these data until they can safely be transmitted at a later date. To overcome these difficulties NEMO-floats are able to facilitate the interim storage of ISA-aborted profiles. The data of the aborted profiles are transmitted during the subsequent summer season when ice coverage – and hence risk of damage – is minimal, even when extended surface periods are needed to transmit the larger data volume.
3) Subsurface RAFOS navigation :
To optimally utilize interim stored profiles, the profile/float location (under the sea ice) must be known to an acceptable level of accuracy. Use of travel time measurements of frequency modulated underwater sound signals allows retrospective tracking of floats by means of the RAFOS (Ranging And Fixing Of Sound) technology with an accuracy of a few kilometers. A RAFOS array for subsurface positioning of Argo-floats was installed within the Weddell Sea during the past years, consisting of a set of 10 moored sound sources. First results prove the usefulness of RAFOS positioning even under sea-ice.

Biogeochemical

Measurements of biogeochemical parameters was made from oceanographic vessels in the past. With the coming of new small sensors with relatively low power requirements, new measurements are possible on floats.

  • "ProvCarbon" float have a Wetlabs transmissometer CROVER (carbon-related properties) and an oxygen sensor (Aanderaa optode).
  • "ProvBio" float is equiped with Wetlabs and S-Atlantics sensors: a transmissometer, a radiometer (3 wavelenghts), an ECO3 (fluorescence, CDOM, backscattering).

These floats can performed 3 cycles per day. They use Iridium communication to transmit more data in less time and programming of the mission can be modified by downlink capability.


In 2001 a WetLabs precision fluorometer (designated FLSS) was integrated to an APEX float as part of a NASA-funded effort, by University of Washington, University of Maine and Oregon State University. Ongoing development since then has led to multiple deployments of WetLabs combined fluorometer and turbidity sensors (designated FLNTU and recently FLBB) usually in combination with dissolved oxygen and CTD measurements. Other optical sensors carried by APEX floats include WetLabs CROVER transmissometer and SeaPoint turbidity sensor.

Acoustic Sensors

The technology of acoustic sensors for zooplankton measurements from Argo floats is a new one. A large number of autonomous instrument platforms (drifters, floaters, AUVs, gliders, moored rigs etc.) for measuring hydrographical properties (salinity, temperature, oxygen etc.) exist today. When it comes to acoustic systems for measuring biological parameters, the assortment of platforms are much more limited. The main reasons for this are the high power consumption of most existing systems, large physical size and extensive need for storing and processing of the initial sensor data. Systems that do exist, has in general a very limited operation time or are physically large, enabling them to facilitate huge batteries.

The Argo floats are deployed throughout all the oceans of the world. Scientists might want to investigate different issues in dissimilar oceans. Even within European waters the ecosystems are diverse. To facilitate the possibility to perform acoustic investigations with different scientific aims, it is important that the acoustic equipment is built up in a modular way.

Acoustic properties
Based on the aim of the acoustic sampling and the zooplankton species composition of the investigating area, echo sounder frequencies have to be selected. It would be advisable to sample the zooplankton with more than one frequency to reduce the inherent vast ambiguity of the data.

The most applied method for size estimation of zooplankton from acoustic data is based on measuring the volume backscattering coefficient at all frequencies and put these data into an acoustic-mathematical model and running the size estimation through an inversion method. The main requirement in the inversion process is to explore the most nonlinear regions versus frequency of the scattering models of the prevailing zooplankton, e.g. the transition region between Rayleigh scattering and geometric scattering.

Biology

Different species of zooplankton play an important role in the ecosystems of the various oceans of the world. The size span of zooplankton species is large. In some European waters, small copepods of only a few millimetres are vital components in the food chain. In Antarctic waters, krill with a maximum size of approx. 60 mm is regarded as key species of the entire ecosystem. The acoustic challenges are very different between the smaller and the larger zooplankton species. This is the case both from a technical point of view as well as from a biological view.