Wallops-2000

Project

The Wallops-2000 Experiment (April/May 2000) consisted of EM propagation loss measurements along radial paths away from the coast in conjunction with near-surface meteorological measurements (Stapleton et al. 2001). Vertical refractivity gradients can create anomalous EM propagation (e.g., ducting, superrefraction, subrefraction). The mesoscale topography of this coastal region, and strong sea surface temperature gradient between cool shelf waters and the warm Gulf Stream, produce complex boundary layer (BL) structures that often generate pronounced vertical refractivity gradients and alter EM propagation. This multidisciplinary collaboration addresses the intricate coastal dynamics that alter EM propagation and impact Naval operations. High-resolution forecasts, using the NRL Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS^Ò), have permitted us to quantify the impact of atmospheric coastal circulations and BL evolution on EM propagation.

Results

Figure 1 depicts the study area and describes the measurements made during Wallops-2000. Figure 2 reveals the rapid development of a surface-based duct captured aboard an instrumented helicopter on 4 May 2000 and the corresponding M profile forecast by COAMPS. Here the temporal trend in the refractivity profile is consistent with observations but the duct strength is under-forecast. Assessing the model’s fidelity in forecasting refractive structures yields greater confidence in the spatially inhomogeneous fields used for predicting propagation factor, which measures the difference in received signal power vs. standard propagation and is routinely used in Naval operations to assess signal ranges and detection. Figure 3 compares the directly measured propagation factor for a C Band Radar to that obtained from COAMPS fields showing differences of less than ~10 dB within 16 nautical miles of the coast. In this case, we can see over the horizon for slightly super-refractive propagation, but COAMPS under-predicts how far.

References

¹J. Stapleton, D. Shanklin, V. Wiss, T. Nguyen, and

E. Burgess, “Radar propagation modeling

assessment using measured refractivity and

directly sensed propagation ground truth”,

NSWCDD/TR-01/132, 49 pp., 2001.

Figure 1. Wallops-2000 Field Program study area and measurements.

Figure 2. Vertical profiles ~12 km from shore of modified refractivity (M) measured by helicopter (colored symbols) and forecast by COAMPS (black lines) between 12 and 20 UTC 4 May 2000. Observed M profile at ~12 UTC has a weak duct (~2 M-units, top at 100 m) that lowers, strengthens and becomes surface based by ~18 UTC (~8.5 M-units, top at 80 m), continuing to lower and strengthen at ~21 UTC (12 M-units, top at 70 m). The model has the same overall trend but has a weaker and higher duct (2 M-units, top at 150 m at 18 UTC; ~6 M-units, top at 85 m at 20 UTC).

Figure 3. Example of a radar cross-section of propagation factor for a C Band (5.1 GHz) radar on 28 April 2000 using the TEMPER model (courtesy of Rob Marshall, NSWC Dahlgen). Propagation factor (PF) directly measured at a shore-based receiver from a transmitter aboard a boat moving along a radial away from the coast (a), PF obtained from COAMPS fields along the same path (b) and the difference (c). The propagation factor gives the difference between the signal power at the location and the signal power at the location when standard propagation is considered. Both observed and modeled propagation factor indicate slightly super-refractive conditions with no ducting, but COAMPS under-estimates by ~10 dB at 15 nautical mi how far over the horizon detection ranges are enhanced.