BIBLIOGRAPHY
3.1. RADIO BUOYS
3.1.3. Radio Buoys Tracked by Polar-Orbiting Satellites
In situations in which trajectories of surface currents are to be tracked over long distances in the open ocean regions for long periods of time, the radio buoys are tracked by polar orbiting satellites. Satellite-tracked drifting radio buoys, with operating lifetimes of a year or more, are attractive tools for measuring the spatial structure of sea surface currents. Deployed in large arrays, they offer an unmatched capability of mapping the two-dimensional field of surface/
near-surface currents in the open oceans over long periods of time (D’Asaro, 1992). Freely drifting radio buoys
deployed in the open ocean are tracked by polar orbiting satellites. The satellite technology meets the majority of drifting buoy-tracking needs and has the virtues of conti- nuity over time, remote monitoring, true global coverage, all-weather operation, relatively good immunity to natural and manmade interferences, frequent availability of buoy position fixes, and good location accuracy. Such features also make satellite tracking far more affordable than more conventional techniques. In fact, satellite-tracked radio buoy experiments have helped in locating several large- scale eddies. When the existence of large current paths or current loops with large radii are to be studied, compre- hensive ship surveys prove very costly and often ineffec- tive. Alternatively, satellite-tracked buoys, with an estimated life of about 300 days, provide an ideal tool to study the movement of water bodies. Such studies, initially supported by NASA satellite NIMBUS and later by NOAA satellites, have produced direct Lagrangian current measurements on large time and spatial scales in many regions. The presence of cyclonic and anticyclonic drifting eddies could be readily identified from the spiraling motion of the buoy track. Today NOAA is a leading organization that provides such satellite facilities.
The radio buoys are equipped with transmitters (plat- form transmit terminals) to facilitate their tracking by polar orbiting satellites. Since 1978, the Service Argos, France, offers capabilities for satellite-based position fixing of radio buoys. The footprint of the polar orbiting satellites on the surface of the Earth is w5,000 km in diameter (see Figure 3.4). A satellite can receive signals from any radio buoy located at any point within its footprint. Further, the visibility time of a satellite is only 10e13 min. Special
FIGURE 3.4 Footprint of a polar orbiting satellite centered on its ground track on the Earth’s surface.(Source:ÓCLS 2012, reproduced with kind permission of CLS Service Argos, Toulouse Cedex, France.)
FIGURE 3.3 Diagram illustrating the scheme used for velocity and position determination of a drifting transponder with the aid of two known spatially separated stationary interrogating-sites. (Source: CODAR brochure, CODAR Ocean Sensors Ltd., Los Altos, CA, reproduced with kind permission of CODAR Ocean Sensors Ltd.)
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precautions are therefore needed to identify each drifting buoy and to determine its location. The buoy identification is achieved from its assigned uniqueidentification number, which is transmitted by the radio buoy. Determination of the position of a radio buoy within the 5,000-km-diameter footprint of the satellite, without ambiguity, requires the measurement of the Doppler shift in the frequency received by the satellite.
The platform transmit terminal (PTT) onboard the radio buoy, featuring microcircuitry, transmits at a nominal frequency of w400 MHz. The whole PTT message is transmitted in less than 1 second and includes 160 ms of unmodulated carrier to allow the satellite’s receiver to lock onto the carrier. To maximize the probability of accurate message reception by the satellite, the PTT message is transmitted several times. The probability of message reception is claimed to be 0.9920 for PTT messages repeated three times and 0.9999 for messages repeated six times.
The satellite-borne data collection and location system (DCLS) receives and records all transmissions from the radio buoys, which are located in the visibility zone of the orbiting satellite. The relative motion between the polar orbiting satellite and the radio buoy causes a Doppler shift in the PTT signal received by the satellite. As the satellite passes through its point of closest approach to the PTT, there is a Doppler shift in the carrier frequency received by the satellite. As the satellite approaches the PTT, the received frequency fr is higher than the trans- mitted frequencyft (i.e., the Doppler shift is positive); at the point of closest approach, fr is equal to ft (i.e., the Doppler shift is zero); and as the satellite goes away,fr is less than ft (i.e., the Doppler shift is negative). The Doppler shift is a function of the relative velocity between the satellite and the PTT. If the PTT is stationed below the orbital path of the polar orbiting satellite (which is along a longitude over the Earth), the Doppler shift would remain constant at all times, the only change being a sudden jump from positive to negative as the satellite passed over the PTT. However, if the PTT is a little farther away from the orbital path of the satellite, the closest approach range is larger, and therefore the slope of the Doppler curve will be less steep (Figure 3.5). There is thus a direct correlation between the change of slant range and the shape of the Doppler curve.
Because the frequency transmitted by the PTT and the latitude and longitude of the satellite at every instant are known, measurement of the Doppler shift defines the field of possible positions for a given PTT (a radio buoy in the present case). The field is in the form of a half-cone, with the satellite at its apex, and the satellite velocity vector (V) as the axis of symmetry (Figure 3.6). The Doppler shift is related to the apex half-angle(A)of the cone, the satellite velocity(v)relative to the PTT, and the velocity(c)of the
electromagnetic wave by the expression (Carpiniello and Buell, 1972):
cosA ¼ ðfrftÞc
vft (3.3)
In this expression, ft and fr are the transmitted and the received frequencies, respectively. Different location cones, obtained from successive Doppler measurements in a given satellite pass, intersect the sea surface to yield the two possible positions of the radio buoy. Such positions are symmetrical with respect to the trajectory (longitude) of the satellite ground track. Additional information such as previous position of the buoy, range of possible speeds, and so forth are used to determine which of the two possible positions are realistic. The processor in the satellite calculates the location of that radio buoy, from which at
FIGURE 3.5 Doppler curve corresponding to differing distances of the buoy-borne platform transmit terminal (PTT) from the satellite’s orbital path. (Source: ÓCLS 2012, reproduced with kind permission of CLS Service Argos, Toulouse Cedex, France.)
FIGURE 3.6 Half-cone that defines the field of many possible positions for a PTT.(Source:ÓCLS 2012, reproduced with kind permission of CLS Service Argos, Toulouse Cedex, France.)
least four messages were collected, and of which the first and the last messages were separated by at least 4 min. To minimize the ambiguity in the estimation of the buoy location, we use certain criteria, such as lowest least- squares value, absurdity test on distance covered by the PTT since last estimated location, and so forth. The esti- mated buoy location is rejected by the location estimation algorithm if the number of PTT messages were lower than a preset value or if the PTT was not within the required range of the satellite. The location estimation algorithm assumes the PTT to be stationary. Any movement of the PTT, which occurs in the case of a drifting radio buoy, therefore causes an error in the location estimation unless it is factored into the location estimation as uniform motion from one satellite pass to the next.
The real-time or recorded location information for each radio buoy is accessible to the user from the processing centers via international telex network, switched telephone network, data transmission network, or the Global Tele- communication System (GTS), which is reserved for meteorological and oceanographic data.
Two NOAA satellites are simultaneously in circular mutually orthogonal polar orbits. The orbit altitudes are different (w830 km andw870 km), producing aw1-min difference in the orbital period. Each satellite makesw14 revolutions in a day. As the satellite orbits the Earth, the visibility zone sweeps a swath of w5,000 km in width.
As a result of the Earth’s rotation to the east, this swath shifts 25 west about the polar axis on each revolution (Figure 3.7), corresponding to a distance ofw2,800 km at
the equator. As a result, the satellite orbits provide complete coverage of the Earth’s surface. In polar regions, the PTTs can be deployed on icebergs, the drift of which provides a description of the surface circulation in the region.
Different PTTs (distinguished by their unique identifi- cation numbers) have different repetition periods and transmission frequencies. Furthermore, transmissions by different PTTs are asynchronous. Theoretically, these schemes would enable the onboard DCLS to pick up and sort messages from all the PTTs in the visibility zone.
However, the PTT is ignorant of the three possible states of its message (Sherman, 1992): (1) there is no satellite in reception range, (2) due to bad signal quality, the satellite has rejected the message completely, and (3) the message has been received, but with some undetermined number of bit errors. Further, the PTT receives no acknowledgment from the satellite and does not know whether the message has been successfully received. An analysis by Sherman (1992)revealed that only 6% of the transmitted messages are received by theArgossatellite, with 17% rejected while a satellite was in view and 9% of received messages con- taining at least one error. Inclusion of the satellite’s orbital information in the PTT’s memory and transmission only when the satellite is in its visibility zone are expected to improve the overall efficiency.
A source of error in the sea surface currents, estimated from radio buoys, is the random error in the position fixes.
The root-mean-squared error in position fixes is quoted to be less than 350 m. For moving drifters, additional nearly random errors will be contributed by unresolved high- frequency motions induced by surface and internal gravity waves. Messages received at the ARGOS center in Toulouse, France, are sorted according to the user identification number and coded with the time of recep- tion at the satellite-borne ARGOS DCLS (Bellamy and Rigler, 1986).
Despite many limitations, satellite tracking of radio buoys has revealed many new details of eddies associated with ocean currents. In fact, the existence of gigantic gyres of more than 100 km in diameter, described in the literature asmesoscale eddies(and sometimes asocean storms), has been confirmed after an accidental discovery during remote monitoring of some radio buoys that were trapped for several weeks within the periphery of a drifting gyre.
Further observations have indicated that such gyres are occasionally present in many regions of the oceans.
A conceptual impression of the satellite tracking of a gyre based on messages received from a conglomeration of drifting buoys is given in Figure 3.8. Synoptic surface current measurements by satellite tracking of freely drifting buoys in offshore areas have helped in the past in identi- fying ocean eddies (gyres) as large as 200 km in diameters and in monitoring their movement (Cresswell, 1977;
Grundlingh, 1977).
FIGURE 3.7 Schematic picture illustrating the mechanism of satellite orbit providing complete coverage of the Earth’s surface as a result of the satellite’s visibility zone (comprising a swath ofw5,000 km in width) shifting 25west about the polar axis of the Earth on each of its revolution to the east.(Source:ÓCLS 2012, reproduced with kind permission of CLS Service Argos, Toulouse Cedex, France.)
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Figure 3.9shows a plot of the trajectory of a satellite- tracked drifting buoy deployed off Goa, India.
Starting on 15 March 2011, the Argos user communities were provided the option to choose between two location processing algorithms:
l The algorithm based on the classical least-squares method that has been employed since Argos process- ing began in 1986
l The algorithm based on Kalman filtering
The trajectory of an Argo float in the Pacific calculated with the algorithm based on Kalman filtering and least- squares analysis is given inFigure 3.10.