1998 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.

IEEE Transactions on Antennas and Propagation
Volume 46 Number 7, July 1998

Radar Performance During Propagation Fades in the Mid-Atlantic Region

Page 1056.

Abstract:

Periodically through the year, an Atlantic coast vessel traffic control radar, located near the entrance to the Delaware Bay, observes a reduction in detection range from 37 (20 nmi) to about 17 km (9 nmi). Sometimes ships can be seen visually from the radar tower before they can be observed on the radar screen. The reduction in the radar detection range usually lasts several hours and occurs often when fog is present. An investigation of this phenomenon, referred to as a radar deep fade (RDF), was undertaken to find out what causes the fade and to determine the best method of increasing the radar detection range when one occurs. The investigation indicated that these RDF's arose during the presence of a propagation condition known as subrefraction, which is an increase in the atmospheric index of refraction with altitude above the ocean surface at microwave frequencies. This increase in the index of refraction with altitude results in the effective radius of the earth being much smaller than the usual 4/3 earth radius that is observed for radars during standard atmospheric conditions. During subrefractive conditions, the effective earth's radius can be between its true radius and one-half of its true radius. As a result, the radar horizon is shortened and the radar detection range is reduced. These RDF's will occur during propagation conditions known as sustained deep fades (SDF's), which arise when subrefraction lasts for more than two hours and causes one-way losses exceeding 20 dB relative to free-space for line-of-sight (LOS) geometries. Subrefraction is caused by the movement of subtropical moist air over the cold ocean surface. The recommended solution for increasing the radar range in the presence of subrefractive conditions is to place the radar on a higher tower than the present 19-m-high tower, one that is perhaps 60-m high. This would increase the radar range by 50% when the earth's effective radius is half of normal. This increase in radar height also usually increases radar detection performance when skip zones associated with surface-based ducts occur or when evaporation ducts (EVD's) are present.

References

1. Lloyd's Shipping Economist, Rates Prices, vol. 17, no. 7, pp. 27-31, July 1995.
2. M. I. Skolnik, Introduction to Radar Systems.New York: McGraw-Hill, 1980.
3. C. P. Hattan, "Evaporation-duct effects on the detection of low-altitude targets," Naval Ocean Syst. Ctr., San Diego, CA, Tech. Rep. 1201, Jan. 1988.
4. H. W. Ko, J. W. Sari, and J. P. Skura, "Anomalous microwave propagation through atmospheric ducts," Johns Hopkins APL Tech. Dig., vol. 4, no. 1, pp. 12-26, 1983.
5. W. L. Patterson, C. P. Hattan, G. E. Lindem, R. A. Paulus, H. V. Hitney, K. D. Anderson, and A. E. Barrios, "Engineer's Refractive Effects Prediction System (EREPS), Surface Duct Survey (SDS) database," NRAD San Diego, CA, Tech. Rep. 2648, May 1994.
6. J. Goldhirsh, G. D. Dockery, and B. H. Musiani, "Three years of C-band signal measurements for overwater, line-of-sight links in the mid-Atlantic coast--Part I: Fade statistics," Radio Sci., vol. 29, no. 6, pp. 1421-1431, Nov. 1994.
7. J. Goldhirsh, G. D. Dockery, and J. H. Meyer, "Three years of C-band signal measurements for overwater, line-of-sight links in the mid-Atlantic coast--Part II: Meteorological aspects of sustained deep fades," Radio Sci., vol. 29, no. 6, pp. 1433-1447, Nov. 1994.
8. W. L. Patterson, "Climatology of marine atmospheric effects," Naval Ocean Syst. Ctr., San Diego, CA, Tech. Rep. 573, Dec. 1986.
9. J. Goldhirsh, G. D. Dockery, and J. H. Meyer, "Meteorology associated with sustained deep fades experienced on two over-water, line-of-sight, C-band links in the mid-Atlantic coast," APL/JHU, Laurel, MD, Tech. Rep. S1R-93U-019, Aug. 1993.
10. J. Goldhirsh, APL/JHU, Laurel, MD, personal communication, Jan. 1996.
11. G. D. Dockery, "Modeling electromagnetic wave propagation in the troposphere using the parabolic equation," IEEE Trans. Antennas Propagat., vol. 36, pp. 1464-1470, Oct. 1988.
12. J. R. Rowland and S. M. Babin, "Fine-scale measurements of microwave refractivity profiles with helicopter and low-cost rocket probes," Johns Hopkins APL Tech. Dig., vol. 8, no. 4, pp. 413-417, 1987.
13. C. P. Hattan, "Specification for a surface search radar-detection-range model," Naval Ocean Syst. Ctr., San Diego, CA, Tech. Rep. 1930, Sept. 1990.
14. W. C. Morchin, Airborne Early Warning Radars.Norwood, MA: Artech House, 1990.
15. L. V. Blake, Radar Range-Performance Analysis.Norwood, MA: Artech House, 1986.
16. E. Brookner, Radar Technology.Norwood, MA: Artech House, 1977, p. 222.
17. J. Goldhirsh, G. D. Dockery, and B. H. Musiani, "Three years of C-band fade statistics for over-water, line of sight links in the mid-Atlantic coast," APL/JHU, Laurel, MD, Tech. Rep. S1R-93U-018, July 1993.