J1.3 MEASURED ANOMALOUS RADAR PROPAGATION AND OCEAN BACKSCATTER IN THE VIRGINIA COASTAL REGION

Janet K. Stapleton, V. Russell Wiss, Robert E. Marshall,Naval Surface Warfare Center, Dahlgren, VA

1. INTRODUCTION

In the spring of 2000, the Office of NavalResearch (ONR) sponsored experiments tocharacterize the effects of low altitude refraction onthe propagation of microwave signals. In specificthese experiments were focused on theperformance of shipboard Navy radars in terms ofthe signal levels returned from targets as well asclutter sources, Stapleton (2001). The location ofthe experiments off the coast of Wallops Island VA,stressed the impact of the land / sea interface in thephenomena observed. These littoral settings are ofincreasing importance to the Navy’s mission andhave proven to be challenging for the performanceof Navy radars. The experiments showed that thisenvironment is also very dynamic in both the spatialand temporal dimensions. Microwave emittersoperating in similar geometries and elevationangles including radar, communications, andtelemetry systems would be subject to thephenomena observed.

2. EXPERIMENT SCENARIOS

The experiments involved the sensing ofmeteorological parameters known to driverefractivity, direct measurements of pathloss atmicrowave frequencies in the S, C, and X bands,and S band radar returns from sea surface clutter.These measurements were conductedsimultaneously to the degree possible, since oneaspect of the experiment was to gain insight intothe temporal variability of refractivity and its effecton the radar pathloss and clutter backscatter.

2.1. Refractivity Measurements

Refractivity was derived from a variety ofmeteorological sources including the JohnsHopkins Applied Physics Laboratory helicoptersensing system, which allowed the collection ofrange and height dependent profiles oftemperature, humidity, and pressure via a sawtoothflight pattern. Bulk measurements of temperature,humidity, pressure, and wind speed were collectedthroughout the experiments at several locations_________________________________________

* Corresponding author address: Janet K.Stapleton, Naval Surface Warfare Center,Photonics Systems and Technology Branch T44,Dahlgren, VA 22448;e-mail stapletonjk@nswc.navy.mil

including the shoreline and at buoy and boat sitesover the water. The bulk measurements allowedthe estimation of evaporation duct profiles, whichare known to be prevalent in maritimeenvironments. On some occasions including thosewhen the helicopter measurements wereunavailable, rocket sondes were used to collectrefractivity profile data over the water at fixed boatlocations.

2.2. Pathloss Measurements

During these experiments, a unique approachwas taken to measuring pathloss via a set ofvertically scanning CW transmitters onboard a boatand a complimentary set of receivers at the highwater line on shore. These measurements weremade simultaneously at S, C and X bands as theboat traversed outbound from 6 km to 56 km fromshore. Using careful calibration for both receiversand transmitters, the pathloss measured wasconverted to propagation factor relative tofreespace spreading loss and coverage diagrams ofthe same were be created. The propagationcoverage diagrams were produced for the rangesmentioned from 1 to 10 m above the surface basedon a receiver at 20 to 30 m above mean waterlevel, depending on the frequency selected. Thisrepresentation of the propagation factor allowed foreasy comparison with propagation model outputsbased on the refractivity data collected in the sameregion and over the same time periods. Anexample of one such coverage diagram comparisonchart appears in Figure 2.2-1 below. (The originalfigure was color-coded. This representation is ingrayscale for purposes of this publication.)

Figure 2.2-1 Measured and Modeled X bandPropagation Coverage Diagrams

2.3. Radar Sea Clutter Measurements

Radar sea surface clutter measurements werealso collected simultaneously with themeteorological, refractivity and pathloss data.Previous observations of sea surface backscatterhad shown a tight coupling of surface backscatterlevels as well as the spatial relationships (rangesand azimuths of intense returns) with the existenceof non-standard or anomalous propagation.Therefore the hope was to capture the change insea surface return along with the associatedchange in meteorology and measured propagationlosses. The sea surface backscatter was collectedusing both the SPANDAR atmospheric researchradar and the AN/SPY-1A both resident at WallopsIsland VA. The two systems had differentlimitations in the spatial and temporal rates for datacollection. The SPANDAR data was collected over180 deg azimuth and out to 270 km range, at anupdate rate of 15 min. The AN/SPY-1A data wascollected over a 60 deg azimuth sector and toapproximately 55 km range, with an update rate of3 min. The time sequences of clutter return levelsover these sectors allowed display of the cluttervariation in range and azimuth as a function of thetime between images. In this way the temporalscale of the changes in the coupled sea surface /propagation environment could be observed up tothe sampling interval.

3. OBSERVATIONS

The following sections show examples of thevarious phenomena observed during theexperiments. The TEMPER (TroposphericElectroMagnetic Parabolic Equation Routine)developed by Johns Hopkins Applied Physics, wasused to model the expected propagation loss basedon meteorological refractivity measurements,Dockery (1988). This model accepts sequences ofrange dependent refractivity data as input, as wellas single profile data. Figure 2.2-1 includes aTEMPER output coverage diagram in the lowerpanel. This result was based on a single rocketsonde refractivity profile and an estimate of theevaporation duct profile using boat meteorologicalmeasurements.

3.1. Spatial Variability

Quantifying the spatial variation of therefractivity, propagation and sea surface return wasa goal of the experiment. The dynamic nature ofthe coastal environment, as well as the practicallogistic limitations of the measurement systemswere limiting factors in achieving this goal. Theradar clutter returns showed some of the bestillustrations of the spatial variations, where largedifferences in character were observed both withazimuth angle and range. The refractivity profilesequences from the helicopter and the repetitive

measurements of pathloss versus range alsoprovided insight; however these measurementswere made using relatively slow platforms(compared to the speed of a microwave signal); aboat at 10 m/s and a helicopter at 30 m/s. In somecases the total time to traverse the 50 km path wasover one hour, and the observed temporal changesin clutter and measured pathloss occurred withinthe measurement period. In this way the temporaland spatial variations observed were not completelyseparable. Figure 3.1-1 shows the refractivityversus range as sensed by the helicoptermeasurement system. The change in the ductheight versus range is reflected by the change inthe vertical position of the negatively slopingsection of the profiles. Figure 3.1-2 shows theresultant measured and modeled propagationcoverage diagrams for this event at S band. Noticethe large convergence zone shown by the higherpropagation factor values beyond 20 nmi range. Atthis location, the S band radars also produced astrong increase in sea clutter backscatter level.

Figure 3.1-1 Helicopter Modified RefractivityProfiles versus Range

Figure 3.1-2 Measured and Modeled S BandPropagation Coverage Diagrams

3.2. Temporal Variability

The temporal variations of the propagationenvironment were also of keen interest during theexperiments. This information would help definethe update rates required for either test oroperational Navy assessments of the environment.Again subject to the limitations in the update ratesof the measurement systems, we were able todetermine that under some conditions, theenvironment was quite stationary with the observedchanges in refractivity, pathloss, and clutter beinggradual and of smaller magnitude. However duringsome events, large variations were evident at thesampling rate of our sensors. Hence, we could notdetermine the true scale of these fluctuations, butonly that they were at least at the measurementrate. Figures 3.2-1 through 3.2-4 show the timesequence of two pathloss measurement events andthe refractivity sensed during these same periods.

Figure 3.2-1 Helicopter Modified Refractivityversus Range 13:18Z

Figure 3.2-2 Measured and ModeledPropagation Coverage Diagram 13:17Z

Figure 3.2-3 Helicopter Modified Refractivityversus Range 15:40Z

Figure 3.2-4 Measured and Modeled S BandPropagation Coverage Diagram 15:58Z

4. IMPLICATIONS

These experiments produced a wealth of data,which is still being analyzed and understood as tothe sources of the variations observed, and theimplications for various types of systems employinglow angle microwave emitters. The results haveshown that the Mid-Atlantic coastal region providesa challenging and dynamic environment in both thespatial and temporal domains. Changes on theorder of 25 dB in one way propagation loss wereobserved in less than 10 minutes time, and overless than 1 meter vertically and 1 km range. For aradar this change would equate to a 50 dB two waysignal level change and subsequent error inassumed radar cross section for an observedscatterer. Clearly the temporal and spatial scale ofthese variations also challenges those beset withthe task of collecting sufficient ground truth orcalibration data for determining propagationadjusted radar return power levels.

5. REFERENCES

Dockery, G. D., 1988: Modeling ElectromagneticWave Propagation in the Troposphere Usingthe Parabolic Equation. IEEE Trans. AntennaPropag. 36(10), 1464-1470.

Stapleton, J. K., 2001: Radar PropagationModeling Assessment Using MeasuredRefractivity and Directly Sensed PropagationGround Truth – Wallops Island, VA 2000.NSWCDD/TR-01/132.