Methods Of Retrieving Tropospheric Ducts Above Ocean Surface Using Radar Sea Clutter And GPS Signals

A marine tropospheric duct is an anomalous refractivity structure that often occursin the low-altitude troposphere on the ocean surface. This structure can bend a radiowave to the earth surface with a curvature larger than the earth’s curvature, resulting inducting propagation within an atmospheric layer with a certain thickness. Thisanomalous propagation mechanism has a serious effect on the performances of manyradio systems working in the marine atmosphere environment, such as radar, and mobilecommunication systems. Sensing the tropospheric duct environment in advance, we canuse some certain methods to compensate or modify the effects of the duct trappingstructure on the radio systems, and also can make the radar’s over-the-horizon detectionof low-altitude military targets by using the anomalous propagation mechanismsinduced by the tropospheric duct. Therefore, it is meaningful to detect or retrieve thetypical parameters of the marine tropospheric duct in real-time.This dissertation studies the propagation characteristics of the radar andlow-elevation GPS signals in tropospheric duct environments over the actual roughocean surface at first, then systemically studies the inversion methods for the marinetropospheric duct from radar sea clutters and ocean-scattered low elevation GPS signals.The main contents and innovations are as follows:1. The mechanism of formation, types, and regional statistics of the marinetropospheric duct are described in detail. The propagation mechanisms and theconditions to form ducting propagation of the electromagnetic wave within thetropospheric duct environment are analyzed. The parabolic equation (PE) andray-tracing theories used to model the radio wave propagation are described andimplemented on the computer, and they are used to analysis the propagationcharacteristics of radar and ocean-scattered GPS signals in the ducting environment.2. The propagation of the electromagnetic wave in a tropospheric duct on therough ocean surface with or without islands is modeled, then the effects of the oceansurface roughness and the island size on the propagation characteristics of radar waveare analyzed in detail. In order to quantify the effects of the horizontal variation of therefractivity profile on the radio wave propagation, the propagation characteristics ofradar wave in range-independent and range-dependent tropospheric ducts are calculatedand compared. A new GPS PE initial field model which takes ocean surface roughness,impedance, and earth’s curvature into account is proposed to model the low elevationGPS signal propagation in marine tropospheric ducts. 3. On the basis of forward modeling the radar sea clutter propagation in marinetropospheric duct environments, the model of retrieving the marine tropospheric ductfrom radar sea clutter is presented theoretically. A new four-parameter modifiedrefractivity profile (M-profile) model for the evaporation duct is proposed. It has theability to more accurately match and retrieve real-world evaporation duct M-profiles.Besides, in order to obtain not only the tropospheric duct M-profile estimations but alsothe uncertainties in these estimates, we establish a Bayesian inversion model for theduct parameters. Through this model the one-dimensional and multi-dimensionalmarginal posterior probability distributions of the duct parameters are calculated, andthe inversion uncertainty estimation is realized.4. The factors that affect the inversion of marine tropospheric duct from radar seaclutters are analyzed and discussed in detail. Accordingly, two methods for improvingthe inversion quality are proposed.(1) An adaptive objective function model which canbe used in the inversion problem is proposed. In this new model, the contributions of themeasured radar sea clutter power from different distances on the objective function areweighted differently according to the radar system parameters and clutter measuringranges, thus the duct inversion precision is improved;(2) An inversion method whichuses the changes in radar sea clutter power versus receiving height as input is proposedto retrieve the marine tropospheric duct. Moreover, we established a metric model usedfor quantitatively evaluate the performance of the radar parameters which are employedin the tropospheric duct inversion. On this basis, the optimal radar parameters used forthe tropospheric duct inversion in the sea areas around China are explored.5. The range-dependent tropospheric duct M-profile is modeled using theprincipal component analysis method. On this basis, the inversion model of therange-dependent tropospheric duct from radar sea clutters is established andimplemented. Further, a Bayesian posterior probability estimation model of thetropospheric duct parameters which incorporates the historical information of thenumerically predicted regional tropospheric duct is proposed. By using this model, theinversion of the regional tropospheric duct is realized successfully. Besides, because ofthe incorporation of the historically predicted duct information, the inversion precisionis improved.6. The ocean surface scattering properties of low-elevation GPS signals arecalculated and analyzed, on this basis, the propagation model of the ocean-scatteredlow-elevation GPS signals within marine tropospheric duct environment is established and evaluated. Using this propagation model as the forward model, a passive inversionmodel of the tropospheric duct is proposed, which uses the ocean-scattered GPS signalsfrom multiple low-elevation GPS satellites as the inversion inputs. This model realizesthe passive inversion of partially regional tropospheric duct successfully.