Operational oceanography has been defined in the previous section as the result of the cooperative effort of many independent systems. To provide the operational oceanography services the following elements are compulsory:
• Radar altimeters flying at low inclination with an orbit optimised to provide precise, tide-free reference measurements of ocean dynamic topography anomalies
• Radar altimeters flying at high inclination with an orbit optimised to provide a dense network of measurements to fill-in the measurement gaps of the reference altimeters flying at low inclination
• Navigation satellites, like GPS, to provide the absolute position of the oceano-graphic satellites, allowing conversion of the distance from the satellite to the water surface into a measurement of the anomaly of the sea level with respect to the Geoid.
• Supplementary calculations to obtain a very precise orbit determination of the flying path of the radar altimetry satellite after supplementary processing of the radar altimetry satellite data with the GPS satellite data.
• Atmospheric pressure and wind velocity measurements to provide the dynamic interaction between the atmosphere, the ocean circulation and the ocean level— lower atmospheric pressure corresponds to a raise in level of the ocean surface.
• Models of the ionosphere and of the water vapour contents of the atmosphere to correct for the radar signal delays imparted by the total electron content and the water vapor.
• Data from different missions and in-situ data to allow inter-calibration and refinement of the measurements
• Dynamic ocean circulation models that integrate all the information and produce the predictions
• Value adders that will augment the information content of the ocean predictions, for example with data on: geography, local population or economic value at risk
• Data distribution networks for relaying the predictions to the users
Conventional radar altimeters send electromagnetic pulses downwards during flight along their orbital path; that means, they produce a track of measurements of received echoes across the Earth's surface. To be able to produce a dense network
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Fig. 4 Meshing of the orbital tracks of the accurate reference low inclination radar altimeters (black) and the dense complementary coverage of high inclination radar altimeters (blue) (Dorandeau et al. 2006). Both tracks combine to provide a dense set of accurate measurement
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Fig. 4 Meshing of the orbital tracks of the accurate reference low inclination radar altimeters (black) and the dense complementary coverage of high inclination radar altimeters (blue) (Dorandeau et al. 2006). Both tracks combine to provide a dense set of accurate measurement of range measurements adequate to feed the dynamic ocean circulation models, it is necessary to fly a constellation of satellites. The orbits (Fig. 4) of the two types of radar altimeters mentioned above—low inclination and high inclination—is chosen for the best possible synergy between them. The best combinations include one 'reference' low inclination polar-orbiting satellite providing the overall frame and several high inclination polar-orbiting satellites providing enough data to fill adequately the dynamic ocean circulation models
GPS satellites are needed to provide an absolute reference frame to the distance measurements provided by the radar altimeter. This combination allows conversion of a relative distance measurement into an absolute sea-level anomaly measurement with which to derive ocean currents. Sea-level anomalies are required to an accuracy of a few centimeters. This requires that the reference frame provided by the GPS satellites must be at the centimeter level. This value is much more demanding than the requirements on GPS positioning precision. Space dynamics specialists have developed special tools that allow them to extract the necessary supplementary accuracy from the GPS data to determine the orbit traveled by the radar altimetry satellites to the required centimeter level precision. This activity is called Precise Orbit Determination and requires complex supplementary calculations that take time over a duration corresponding to an orbit arc or number of successive satellite orbits. The longer the time used, the higher the prediction: precisions of the order of 10 cm in minutes and of the order of 1 cm in a matter of several hours (Montenbruck 2006).
The derivation of the sea-level anomaly data also requires the creation of reliable communication links to receive the necessary information on the atmosphere and ionosphere to be able to compensate for the geophysical features that would generate sources of error. It is necessary to compensate for the delay in the traveling of the radar signals—produced by ions or water vapor—and for the changes on the natural position of the water level—produced by atmospheric pressure or winds.
The cooperation between independent space agencies is also necessary to establish long-term intercomparable data sets that are compulsory to analyse long term trends in sea-level rise.
The nature of operational oceanography requires the establishment of a complex network of long-term cooperative relations. This network requires mutual help between fully independent actors that could at any moment withhold cooperation. On the other hand, the overall result of the cooperation is in the benefits of all the actors. Actually this same scenario has happened before in the development and firm establishment of operational meteorology as a highly successful multi-national endeavour. In the other hand, the users of operational oceanography will need time, money and confidence to build the know-how necessary for the optimal utilisation of the data provided by the satellites. Users will also exercise caution until they see commitments to ensuring robust, high-quality satellite datastreams. That means the firm establishment of operational oceanography requires the long-term commitment by some key actors to provide confidence to everybody on the future long term availability of such a complex system of systems.
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