Microwave and Millimeterwave Propagation within

the Marine Boundary Layer

Helmut Essen, Hans-Hellmuth Fuchs

Research Institute for High Frequency Physics and Radar Techniques (FGAN-FHR), Neuenahrer Str. 20, D-53343 Wachtberg (Germany)

Email: h.h.fuchs@fgan.de

Short Abstract—A series of measurement campaigns were initiated at the Baltic Sea to validate existing radar propagation models under various atmospheric conditions in the marine environment. To assess the radar propagation within different layers simultaneously at X-, Ka- and W–band measurements were performed, using the experimental three frequency radar MEMPHIS operating against point targets at different heights above sea, carried on a naval vessel, which moved on outbound and inbound courses. The paper describes the experimental approach and gives representative results.

Keywords- millimeterwave radar, propagation, refraction, duct, model)

I. INTRODUCTION

Within the marine boundary layer radar propagation is influenced by the atmospheric conditions, the sea surface and the transmission geometry. At classical radar bands up to 18 GHz multipath propagation at low sensor level above the sea imposes severe problems. Therefore it is of considerable interest to investigate the propagation characteristics at alternative frequency bands. Millimeterwave radar sensors are much less influenced by multipath effects, as the sea surface is much rougher in comparison to the radar wavelength than at lower frequencies; however refraction and turbulence are also of importance at millimeterwave frequencies. Measurements have been conducted during recent years to validate existing radar propagation models under various atmospheric conditions. To assess the radar propagation within different layers simultaneously at X-, Ka- and W–band measurements were performed, using the experimental three frequency radar MEMPHIS operating against point targets at different heights above sea, carried on a ship, which moved on outbound and inbound courses.

For the time being, only little data exist with the full information content needed for a thorough validation of radar propagation models, that means that radar data exist for

different frequency bands, to validate the modelling of the frequency characteristics, that data exist for different geometries at equal environmental conditions, to validate the influence of geometry and that input data for all relevant environmental parameters exist for the set of radar data. During the measurements discussed in the following a complete environmental characterization was performed [1] by the FWG, Kiel.

II. EXPERIMENTAL CONDITIONS

The experiments were mainly conducted at the Baltic Sea, but at different seasons of the year. Some data were gathered over sea close to Singapore. The radar MEMPHIS [2], operating simultaneously at X-Band, Ka-Band and W-Band, was located at a height of about 20 m, thus representing a geometry with possibly strong influences of multipath and ducting. During the experiments a vessel was moving on outbound and successively on inbound courses. For the radar measurements trihedral reflectors acting as point targets were mounted at different heights and displaced by range, two looking to the bow and two looking to the stern of the ship. All reflectors had the same size and a nominal RCS of 48.5 dBsm at 94 GHz. The vessel was additionally equipped with a GPS unit to determine its location which was transferred via a data link to the data acquisition of MEMPHIS were it was recorded together with the radar data. The GPS information transferred by radio link was also used for tracking of the target.

The environmental characterization during the trials covered a description of the sea and sea/air interface using sensors mounted on free drifting buoys, on a research vessel and using radio probes on balloons, which were launched on a regular schedule.

During the measurement period different meteorological conditions could be covered. Fig. 1 compares the refractivity conditions of the atmosphere for RF and electro-optical (EO) propagation by means of a plot of air/sea temperature

difference (ASTD) versus air/sea vapour pressure difference (ASVPD). The example exhibits solely superrefractive conditions. The varying ASTD and the negative ASVPD changes the layer structure above the sea surface, particularly the height of the evaporation duct.

III. MEASUREMENTS

The boat runs were conducted over ranges close the horizon, which means over more than 25 km. The time series of calibrated range profiles showing the two reference reflectors on board the ships was plotted in pseudo-color representation. Fig. 2 shows an example for an inbound run at 35 GHz.

The traces for both reflectors, separated by range, can clearly be distinguished. Using the profiles range diagrams for each reflector were deduced by tracking of the respective trace. In order to evaluate and study the propagation effects the range dependant free space attenuation and the nominal RCS of the corner reflectors were subtracted resulting in two-way propagation factors for each corner reflector. Fig 3a, b, c gives the respective example of an inbound run.

All diagrams show a pronounced signal fading due to multipath propagation with fade depths up to 30 dB for 9.4 GHz and 35 GHz.

The resulting fading characteristic by the interference pattern depends on the transmission geometry, namely the sensor and target height, and on the used wavelength. Increasing roughness of the reflecting sea surface diminish the depth of the fading minima due to an enhanced diffuse incoherent scattering process. This effect is more pronounced at the shorter millimeter waves than at X-band, e.g. at 94 GHz.

The amplitude in the maxima of the interference pattern of the X-band and Ka-band signal does not vary much with range confirming superrefractive conditions. At 94 GHz additional atmospheric attenuation effects degrade the signal up to 12 dB with range but the maximum amplitude of the propagation factor does not fall below -10 dB near the geometrical horizon.

Varying refractive conditions in the atmosphere change the periodicity of the multipath interference pattern. Notable is the degree of displacement associated with shorter wavelength. Different refractive conditions can replace a minimum by a maximum. As an example Fig. 4 compares the propagation

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Figure 3. Range diagrams for inbound run 9.4 GHz (a), 35 GHz (b) and

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Figure 1. ASVPD versus ASTD as extracted from the FWG data for one

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factors of two runs measured at 35 GHz on two different days. In order to ease the comparison the measured data were averaged over 1.5 min. The meteorological situation on the first day (March 25) was characterized by an onshore air-flow with a characteristic height of the evaporation duct of about 6 m while on the second day (March 27) durin off-shore air-flow the duct height dropped down to < 1 m till an upcoming fog in the night removed any layered structure. As shown clearly in the diagrams this has an impact on the range performance of radar systems. Depending on the path geometry a multipath maximum can shift the maximum range of the radar system beyond the visible horizon.

IV. MODELLING OF PROPAGATION

CHARACTERISTICS

For the prediction of the range characteristics of radar cross section two programs, PIRAM [5] and TERPEM [3] had been used. To calculate the propagation factors for a given transmission geometry it is necessary to know the environmental parameters, namely the profiles of temperature, humidity and wind speed. Appropriate measurements were accomplished by the free drifting buoys [2]. Based on this data sets PIRAM computed profiles of the modified refractivity M(h) and duct heights applying standard solution methods (Charnock-Smith). These profiles and the radar transmission geometry have been used in TERPEM to calculate coverage diagrams and 2-way propagation factor vs. range characteristics based on a hybrid model using parabolic equation techniques and ray optics. Fig. 5 shows the simulation at 35 GHz for two cases. Both sample cases represent superrefractive conditions but with different impact on the propagation. The first one on 27 March looks like the coverage diagram of a standard atmosphere with no layer structure. At that time high humidity, low ASTD and upcoming fog removed the existing evaporation duct. The maxima of the propagation factor vary less than 5 dB with range up to the horizon and fall off beyond this limit with about 2 dB/km.

Fig. 6 and 7 shows the comparison between measured and calculated propagation characteristics of both runs. The curves fit the experimental results very well independent on frequency and height.

It may, however, be noted that the calculated multipath minima especially at 94 GHz are considerably overemphasized. The latter is due to a lack in precision for the description of the roughness of the sea surface at millimeterwaves. Surface roughness is modeled by modifying the Fresnel reflection coefficients by a roughness reduction factor and correlating the

Figure 6. Comparison of propagation factor at 9.4 GHz, 35 GHz and

94 GHz between simulation and measurement, run on 27 March.

Figure 5. Coverage diagram and extracted 2-way path loss diagrams at 35 GHz for two corner reflector heights. Input buoy data from 27 March (a)

and 03 April (b)

Figure 4. Comparison of propagation factor for run I1 and I3 at 35 GHz

a: corner reflector height 5.6 m, b: corner reflector height 12.9 m

rms wave height with wind speed. For 35 GHz the depth of the minima is modeled best, which may lead to the assumption that the roughness model used here is most appropriate for this frequency.

V. RESULTS

The comparison of experimental results and simulations at millimeterwaves show that the parabolic equation model TERPEM gives a good estimation for the propagation factor over a wide range of frequencies, ranging from 10 GHz to 94 GHz. This was already found during earlier experiments [4]. A general result of the radar experiments is, that for

maximum range also millimeterwave radar should be taken into account. Detection of ships is possible for ranges up to and over the horizon in the majority of environmental conditions in the Atlantic and Mediterranean and specifically in the Baltic Sea. Using the 35 GHz frequency band, rain is less important, which at 94 GHz is a major concern, if the extend of the rain cell is big enough. The 94 GHz band exhibits another advantage: For an object like a naval vessel, where scatterers are distributed over different heights, the related interference patterns could be so dense that the resulting overall signal fading is very low. The proof of this effect especially at 94 GHz is due to current research. The experiments in tropical environment, namely in Singapore, however, showed, that the propagation conditions in that type of environment can differ considerably.

REFERENCES

[1] S. Boehmsdorff and H. Essen, "MEMPHIS, an experimental platform

for mm-wave radar," DGON Intl. Conf. On Radar, 1998, Munich, pp. 405 – 411.

[2] T.K. Scholz and J. Förster, "Environmental characterization of the marine boundary layer for electromagnetic wave propagation“, SPIE 11th Intl. Symposium on remote sensing, RS 06 Optics in Atmospheric Propagation and adaptive systems VII, Maspalomas, 2004.

[3] TEPEM, Version 5.1, Signal Science Ltd, 1994 – 98.

[4] H. Essen, H.-H. Fuchs, "Millimeterwave propagation within Boundary Layers over Sea, Comparison of Modelling and Experimental Data“, SPIE 9th Intl. Symposium on Remote Sensing, Conf. 4884-12, Agia Pelagia, 2002.

[5] J. Claverie and Y. Hurtaud, "Propagation transhorizon en atmosphère marine - Modélisation et nouveaux résultats expérimentaux", 49th AGARG EPP Symposium, Télédétection du milieu de propagation, 19, Izmir, 1991..

Figure 7. Comparison of propagation factor at 9.4 GHz, 35 GHz and 94 GHz between simulation and measurement, run on 1 April.