Teletransmission : why do we need satellite transmission for Argo floats ?
Most of the floats use the Argos System of satellites to recover data. Argos is a satellite-based system which collects, processes and disseminates environmental data from fixed and mobile platforms worldwide. What makes Argos unique is the ability to geographically locate the source of the data anywhere on the Earth utilizing the Doppler effect.
Argos was established in 1978 and since that time, it has provided data to environmental research and protection communities that, in many cases, was otherwise unobtainable. Argos is a key component of many global research programs including : TOGA, TOPP, WOCE, ARGO and others.
The Argos System is the only global positioning system and data collection satellite dedicated to studying and protecting the global environment.
The one-way communication : Argos-2
The functioning for Argo floats can be described in 5 steps as follow :
1) - The platforms send regular messages to the satellite,
2) - The polar orbiting satellites receive their messages
3) - The antennas receive information from satellites
4) - Treatment centers receive the data and distribute them to users
5) - Worldwide, Argos users receive their data directly to the office or on site, depending on their personal choice (Internet, email, fax, or cd-rom). Once data is received, it may be shared with the scientific community or government that uses data as tools for monitoring and management of resources and the environment.
The first Argos-2 system was a one way of communication with the platforms, which needed Argo floats to spend 6 to 12 hours at surface to transmit around 100 levels of only CTD sensors.
The two-way communication system : Argos-3 and Iridium
But new requirements appeared these last years for profiling float applications :
- For Argo deployments, the performances of these instruments could be affected in marginal seas. Thus, it is critical to reduce the transmission time at surface, in order :
- to lower the risk of thefts, trawling or impacts in these highly trafficked seas,
- to delay the time of beaching on the shores,
- to have better estimates of subsurface currents.
- For coastal applications, the reduction of time spent at surface facilitates the realisation of successive profiles at the samelocation in order to delay beaching, and delays the bio-fouling development,
- The modification of mission parameters during operation is now required to monitor specific events.
The new Argos-3 and Iridium satellite transmissions respond to this necessity of a two-way communication with the platforms.
1) - The Argos-3 System
With Argos-3 system , we can :
- send more data, thanks to an interactive data collection and an high data rate link (around 1000 CTD levels transmitted),
- Reduce surface time of the float (less than 40 min at surface) thanks to a possibility to Rendez-Vous with satellite, a better transmission efficiency, and a faster data transfer,
- improve transmission efficiency thanks to a two-way communication allowing commands sent to your float from your desktop, and re-programmation of the float during its mission.
2)- The Iridium System
A few of Argo floats are using the newer Iridium satellite constellation. The Iridium satellite constellation is a large group of satellites providing voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface.
The Iridium system offers significant advantages associated with the much faster data transfer. Since an Iridium float spends only 3 minutes at the sea surface, the opportunity to observe surface currents by tracking the movements of the floats is lost but the trajectories of the floats become more representative of the flow at their parking depth.
To summarize ...
The enhancement targets of developing innovative two-way of communication for Argo floats are both to limit drift and fouling by tuning the float mission, and to save the power in order to increase float lifetime.