Use of ground based radar in hydrography
Contents
Use of ground based radar in hydrography
1. Introduction
Why ground based radar? Radar remote sensing in Earth observation means not only the global and regional survey of geophysical parameter by satellite- or airborne radar but it means as well the local observation by ground based radar techniques. The information provided by ground based instruments mounted at fixed coastal or offshore stations don’t suffer under the episodic character of satellite radar products and thus fill the gap providing the potential for repeated or even permanent observation. The restriction of the limited insight of the ground based radar can be overcome by the use of ship borne radar techniques to extend the observation area along the ship track to a regional scale.
2. Principals of microwave radar application in hydrography
In general radar remote sensing of the sea surface provides a broad variety of observations, which have been discussed intensively over the past decades. While single radar images give the intensity distribution of the backscattered radar power attained from a kind of instantaneously frozen surface, the surface dynamic can be observed by tracking features in subsequent radar measurements. The scattering mechanism is known as Braggscatter that interprets the backwards directed part of the radar power induced by constructive interference of the electromagnetic wave with the structures of the entire radar footprint (see figure 1). The spatial and time variability of the radar signature underlie the interaction of the sea surface with wind, waves, currents and the hydrosphere modulating as well radar intensity as well as radar frequency Doppler shifts. A profound overview on the use of radar in general and in its use in geophysics is given by SKOLNIK, (1990).
By acquiring radar intensity or radar Doppler shift an image of the related processes may be composed on a regional to local scale. By knowing the physics behind these modulations the steering hydrodynamic processes may be assessed. In this article we will focus on the use of ground based X-band radar as tool for hydrographic observations. In coastal management the regional and local survey by ground based radar provides a valuable completion to the in-situ observations. Information on local wind, wave and current conditions as well as indirect parameters like the local bathymetry, and even the coverage by ice, spills or slicks may be deduced routinely from radar products. Although the microwave signal does not enter into the water, as almost 100% are reflected or scattered directly at the surface, it is possible to deduce additional information on the hydrodynamic from below the surface. For example the propagation of waves in shallow water allows deducing the local water depths out of the local wave propagation. Basing on this relation it is possible to compose maps of the water depth (bathymetry), which can be used in monitoring the sand transport (see: DiSC). Another example is the extrapolation of surface features down to the sea bottom as it is used by the operational model VOGELZANG et al: (1997).
Acquisition of radar cross section
The amount of radar signal intensity that is detected at the receiving antenna is called radar cross section. The amplitude and phase of the cross section are instantaneously and locally modulated in time and space by hydrodynamic effects. The most important influence to the radar cross section of an ocean surface element is the mean amplitude of the surface roughness, which is instantaneously steered by the local wind impact. The radars under discussion here are operated synchronously with a precise navigation to compose the cross section values into a geo-coded radar map. While a radar pulse sweeps along a radial line, the received radar cross section is decomposed into a series of radial bins. The radar return is allocated to corresponding scattering surface elements by measuring the microwave runtime and the antenna azimuth. The focusing in azimuth depends on the length of the antenna and the in range on the duration of the transmitted radar signal (see: Figure 1). Typical values for the range resolution are 5 m x 5 m or 10 m x 10 m and for the antenna opening angle about 1°. Whereas the return of each individual radial beam is acquired during nanoseconds the acquisition of the total image takes some seconds or even minutes depending on the acquisition mode. The image composition under the use of a rotating antenna takes about 2 seconds and the product is a matrix in polar coordinates (range and azimuth).
Acquisition of radar Doppler shift
For the detection of Doppler shifts incurred to the microwave signal by the motion of the surface elements an acquisition time of at least some 100 milliseconds is necessary. During that time the antenna must be kept directed towards the individual surface elements. The longer the Doppler frequency information is integrated the more increases its significance. As the frequency shift is sensitive on the instantaneous local wind friction the knowledge on the wind impact is crucial. We demonstrate an application of the Doppler measurements in chapter 4.
3. Bathymetric survey and current field observation using local gravity wave dispersion
Special challenge in coastal protection management is provided by the management of sandy coastlines. Here men interact with the dynamic processes of the ocean with the shore line and undertake measures for their stabilization by measures like “beach nourishment” [see: theme 5]. The interference of men with nature must be attended by intense observations. The increase of sand to the coastal system means the survey of the sand’s residence on the one side but on the other side the observation of the forces causing erosion, transport and deposition of sand. For this it is not enough to acquire time series of physical parameters at single observation points. It longs for area mapping observations to identify the response within the system and to assign the process to their origins, either occasional forcing by storms or to continuous forcing as e.g.: by the tidal cycle. We will point out in the following the methods of radar hydrography allowing area covering and continuous survey of the forcing as well as the response of the bathymetry. The area of investigation is List West at the north end of the Island Sylt in the German bight.
In the current paragraph we presented the assessment of the bathymetry and the current field by imaging the wave backscatter using marine radar. Time series of these images are inversed locally where the linear wave theory is valid and the surface wave dispersion holds. The algorithm used is known as “Dispersive Surface Classificator” and the commercial acronym is DiSC. The method has been developed at the Radar Hydrography Department of GKSS (Link to KOR homepage) and is licensed as commercial product by Vision 2 Technology GmbH (Link to V2T homepage), partner of the Geesthachter Innovations- und Technologie-Zentrum (GITZ). The method is based on the linear wave theory that allows by tracking of wave crest in space and time to determine the parameters water depth and current vector. The dispersion relation of sea-surface waves is derived from the Eulerian equations of motion, the continuity equation and the dynamic and kinematic boundary conditions at the sea surface and the sea floor. A detailed description of the derivation of the dispersion relation is given in [SENET et al. 2001]. The analysis of the image sequences of the inhomogeneous wave field provides a set of physical parameters on a local spatial scale. The basic idea of the method is that in shallow waters the waves vary their propagation (phase speed and wave number) locally and thus impresses the local bathymetry into the image series. The same mechanism acts via the local current that may be assessed by the DiSC as well.
The water depth and the current vector are free parameters influencing the shape of the dispersion shell in the wavenumber -frequency domain (see figure 2). It must be mentioned here that for this application the radar has not be calibrated, as the wave height is not influencing the wave dispersion. From the three dimensional radar observation the three dimensional power spectrum in wavenumber -frequency domain is calculated by Fourier Transformation. Next the shape of the actual effective dispersion is deduced by fitting the detected power values to a plane approximating the dispersion. Deviations from the undisturbed dispersion are used to determine the local water depth and current vector (Senet et al., 2007). The local results are composed to spatial hydrographic-parameter maps.
The result of the DiSC is the instantaneous local depth, the bathymetry, and the estimation of the current field. Figure 2a and 2b illustrate the bathymetries averaged over a tidal cycle each before and after storm (wind conditions 8-9 Bft.). The two maps have common reference, corrected by gauge measurements.
The difference of the sediment volume between the beginning and end of the storm, which under conditions could be considered as the sediment net increase of the area of investigation, is estimated in 50.000m3, with ±20% of accuracy for each grid cell, if we consider the accuracy of the method ±0.25 m per cell as it is given in the (SENET & SEEMANN 2002). In general the assumption of the constant difference between two periods, of the mean sea level is not strong enough and decreases the accuracy of the calculation.
In Fig. 4, is illustrated an example of the current field in the same area as above, the wind conditions are approximately 5 Beaufort and the date of the data acquisition is the 12th July of 2001. The falsely too high current values close to the shore and over the shoal at the north west corner of the observation area indicate grid cells for which the linear dispersion is not valid.
4. Mapping by Radar Doppler Current Profiler (RDCP)
The radar Doppler shift from the sea surface can be used for the detection of the sea surface currents. With a coherent radar system it is possible to measure the scatterers speed directly. The ground based coherent X-band radar system can be mounted either onshore or onboard a ship to observe sea surface scatterer velocities with high resolution. The antenna view direction and data acquisition is computer controlled; adjusting the antenna to fixed azimuthal angles while acquiring data is possible. Three different operating modi are possible and necessary for different applications: rotating antenna (also with different rotating speeds), stepping (changing view directions by set observing time) and the third mode is to work with fix view angle all the time. At a shore based permanent radar station the stepping mode is used for current and small scaled feature measurements like convergences/divergences or small eddies. Using the fixed view direction and a second radar antenna a full current vector can be achieved by moving ship and the covered area increases versus a shore set up. This method by scanning of horizontal current profiles we called Radar Doppler Current Profile (RDCP). The transmitters are synchronized to acquire the two orthogonal components of the sea surface current during a single ships passing.
The Figure above shows the plan view of a ship with a mounted RDCP. In addition to the radars a precise navigation is needed to track the accurate position and the instantaneous radar view directions. ADCP measurements are acquired to validate the surface current observations and to extrapolate the current field in the vertical direction. To avoid interferences the alternating operation is controlled by the master radar (1) that interlaces every second pulse to the slave radar (2). By this each radar transmits and receives 1000 coherent pulses per second. The scan angle between the two antenna directions is 90°, with one antenna looking 45° ahead and the second looking 45° aft. By the Doppler relation we calculate the radial velocities from the backscattered signal for each range bin (length ~ 7.5m). Integrating the radar observations over a second the radial speed results in an accuracy of 1.5 cm/s . These values have to be corrected by the local impact of the actual wind friction and ship movement. An automatic quality control rejects routinely faulty data. For offshore application a motion sensor will be integrated in the system to correct the instantaneous antenna movements due to wave impact. The post processing procedure is to compose the full surface current vector by merging the two components into a geo-coded grid with the grid mash up to 10 m.
The figures above 6. a) and b) show the speed and direction of the surface current field. An eddy with a diameter of about one nautical mile shows the outflow directed westward on the northern side of the gully and a counter current directed eastward on the south side. This is overlaid by a current modulation due to the change in the cross section over the under sea sand dunes, which interact with the ebb current in a way that we observe acceleration over the crest of the dunes and slowing down where the cross section is widening. For the comparison with the satellite SAR data it is of high interest to see that the directionality within the current field is in interaction with the dunes as well. It is evident that the current direction rotates into the main gully direction (clockwise), where the current speeds up above narrowing cross sections. The direction rotates anti clockwise in cross gully orientation slowing down currents above widening cross sections.
5. References
Alpers, W., and K. Hasselmann (1982) Spectral signal to clutter and thermal noise properties of ocean wave imaging synthetic aperture radars. Source: Int. J. Remote Sensing, 3, 423-446
Braun, N, F. Ziemer, and A. Bezuglov (2007) Sea-Surface Current Features Observed by Doppler Radar. Source: accepted by IEEE Transactions on Geoscience and Remote Sensing.
CHOWDHURY, M. (2007) Assessment of water flow and the impact on sediment motion in a tidal channel of north Sylt basing on radar observation. Source: Coastal Research Laboratory, Christian Alberchts University of Kiel.
Cysewski, M. (2003) Radarscanning in der Hydrographie. Source: Diplomarbeit GKSS 2003/26. Geesthacht.
Flampouris, S. (2006) Investigation of correlations between radar deduced bathymetries due to the outer impact of a storm in the area “Salzsand” . Source: M.Sc. Thesis; GKSS 2006/16. Geesthacht.
Nieto, J.C. and C. G. Soares (2000) Analysis of directional wave fileds unsing X-band navigation radar. Source: Elsevier Science Coastal Engineering V. 40, pp 375-391
Plant, W.J., Keller, W.C. & K. Hayes (2005) Measurement of river surface currents with coherent microwave systems. Source: IEEE Trans. Geosci. Remote Sens., 43, pp. 1242-1257.
Senet, C.M., Seemann, J. & F. Ziemer (2001) The Near-Surface Current Velocity Determined from Image Sequences of the Sea Surface. Source: IEEE Transactions on Geoscience and Remote Sensing, Vol. 39, No. 3, pp. 492-505.
SENET, C.M., SEEMANN, J., FLAMPOURIS, S. & ZIEMER, F. (2007, submitted): Determination of Bathymetric and Current Maps by the Method DiSC Based on the Analysis of Nautical X–Band Radar-Image Sequences of the Sea Surface. Source:
Vogelzang, J., Wensink, G.J., Calkoen, C.J. & M.W.E. Van der Kooij (1997) Mapping submarine sandwaves with multi-band imaging radar 2. Experimental results and model comparison. Source: Journal of Geophysical Research, 102, pp. 1183-1192
Ziemer, F. and Cysewski, M. (2006) High Resolution Sea Surface Maps Produced by Scanning with Ground Based Doppler Radar. Source: eProceeding IGARSS 2006, IEEE Denver, Colorado 31 July – 04 August 2006.
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