Calhoun: The NPS Institutional Archive

Theses and Dissertations Thesis Collection

1990-09

Assessment of the effects of refractive conditions on

electronic warfare in central America

Gaviria Maldonado, Mauricio.

Monterey, California. Naval Postgraduate School

http://hdl.handle.net/10945/34892

NAVAL POSTGRADUATE SCHOOLMonterey, California

AD-A240 523

$1D rATe

.-ECT ESEP 13 1991 U

D GIRA N

THESIS

ASSESSMENT OF THE EFFECTS OF REFRACTIVECONDITIONS ON ELECTRONIC WARFARE

IN CENTRAL AMERICA

by

Mauricio Gaviria Maldonado

September, 1990

Thesis Advisor: Kenneth L. Davidson

Approved for public release; distribution is unlimited.

91-10461

ot 9 12 066

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6a NAME OF PERFORMING ORGANIZATION 6b OFFICE SYMBOL 7a NAME OF MONITORING ORGAN17A rIONNaval Postgraduate School (if applicable) Naval Postgraduate School

6c ADDRESS (City, State, and ZIP Code) 7b ADDRESS (City, State, and ZIP Code)Monterey, CA 93943-5000 Monterey. CA 93943-5000

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11 TITLE (Include Security Classification)ASSESSMENT OF THlE EFFECTS Ol REFRACTIVE CONDITIONS ON ELECTRONIC WARFARE IN CENTRAL AMERICA

12 PERSONAL AUTHOR(S) MAURICIO GAVIRIA MALDONADO

13a TYPE OF REPORT 13 b TIME COVERED 114 DATE OF REPORT (year, month, day) 15S PAGE COUNTMastersThesis IFrom To September 1990 I 74~16 SUPPLEMENYARY NOTATIONThe views exuressed in this thesis are those of the uthor and do not reflect the official policy or position of the Department of Defense or the U.S.Government17 COSAT I CODES 18 SUBJECT TERMS (continue on reverse if necessary and identify by block number)

FIELD I GROUP SUBGROUP Refractive Conditions, Proagation, Central America, Radar, Microwave.

19 ABSTRACT (continue on reverse if necessary and identify by block number)

This thesis presents a study of the at mnospheric ref rtLLIvity conditions in the oceanic. area around Central America and a description of thepossible propagation ellet on sur cillince bysteinb 111e barso. prin~iples of atmoblpherit. refraltion are presented along with the techniques usedtou determnine the ot.currente 4f atnwspherii. dut and to charatterize the type uf refracti% e profiles occuring at a station. Radiosonde data collectedby coastal stations in the area were used to establish refractive conditions.

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Approved for public release; distribution is unlimited.

Assessment of the effects of Refractive Conditions

on Electronic Warfare in Central America

by

Mauricio Gaviria Maldonado

Lieutenant, Colombian Navy

Submitted in partial fulfillment

of the requirements for the degree of

MASTER OF SCIENCE IN SYSTEMS ENGINEERING

(Elecironic Warfare)

from the,

NAVAL POSTGRADUATS L

September 1990' . ,

Author:

1iciV /viria Maldonado

Approved by:

Kenneth. L. Davidson, Thesis Advisor

Scott H. Hershqy, Second Reader

V Joseph Stenberg, Cairman

Electronic Warfare Academic Group

ii

ABSTRACT

This thesis presents a study of the atmospheric refractivity conditions

in the oceanic area around Central America and a description of the possible

propagation effects on surveillance systems. The basi principles of atmospheric

refraction are presented along with the techniques used to determine the

occurrence of atmospheric ducts and to characterize the type of refractive profiles

occurring at a station. Radiosonde data collected by coastal stations in the area

were used to establish refractive conditions.

a!

Acce i o:i Fi,

Uj.:, C.:''.:d I

J. ..... ... ..

By, ;ibu ...... ......... ........

Ava;I arc, !orDi Speal

ii , -,i

TABLE OF CONTENTS

I. INTRODUJ:ION ...................... 1

A. PURPOSE......................................... I

B. BACKGROUND......................................1I

II. ATMOSPHER<IC REFRACTION ............................... 3

A. WAVE PROPAGATION............................... 3

B. ATMOSPHERIC REFRACTIVITY ......................... 5

C. TYPES OF REFRACTIVITY PROFILES ..................... 9

1. Standard Atmosphere .............................. 9

2. Superrefractivity and Trapping ...................... 11

D. METEOROLOGICAL PHENOMENA LEADING TO TRAPPING 15

a. Radiation................................... 15

b. Subsidence.................................. 16

c. Land-sea winds .............................. 16

III. CENTRAL AMERICA CLIMATOLOGY ........................ 18

A. CLIMATOLOGY .................................... 18

B. SYNOPTIC DESCRIPTION ............................ 19

iv

IV. DATA ANALYSIS AND RESULTS ........................... 20

A. RADIOSONDE DATA ............................... 20

1. Geographical Distribution of Data ................... 20

2. Distribution of Data Over Time ..................... 21

B. ANALYSIS OF REFRACTIVE CONDITIONS ............... 23

1. Procedure of Analysis ............................. 23

2. Formulas and Algorithms ......................... 24

3. Results by Stations ............................... 26

a. San Andres Island, Station 80001 ................. 26

b. Howard A.F.B., Panama, Station 78806 ............ 30

c. Belize Airport, Belize, Station 78583 ............... 34

d. Kingston, Jamaica, Station 78397 ................. 37

e. Roberts Field, Gran Cayman, Station 78384 ......... 40

V. PROPAGATION EFFECTS ON SYSTEMS ....................... 43

A. EFFECTS ON RADARS ............................... 43

1. Rain Effects ..................................... 44

2. Duct Effects ... ................................ 45

a. Coverage DJiagram with Surface Based Duct ........ 45

b. Propagation Loss ............................. 47

B. EFFECTS ON ESM .................................. 52

V

VI. CONCLUSIONS ..................... 54

APPENDIXA .............................................. 56

LIST OF REFERENCES...................................... 60

INITIAL DISTRIBUTION LIST ................................. 62

vi

LIST OF FIGURES

Figure 1. Troposcatter Propagation .............................. 4

Figure 2. Ray Geometry for Standard Refraction ..................... 8

Figure 3. Modified Refractivity Profile, Mean Tropical Sounding and Low

A ltitude Detail . ......................................... 10

Figure 4. Propagation of Radar Energy in a Surface Based Duct ........ 12

Figure 5. Refractive Profiles of the Different Duct Types .............. 13

Figure 6. Minimum Trapping Frequency for a Surface Based Duct ....... 15

Figure 7. Sea-Wind Inversion Layer .............................. 17

Figure 8. Geographical Location of the Sounding Stations ............. 20

Figure 9. Radiosonde Data, 1988, San Andres, Colombia, Station 80001... 28

Figure 10. Radiosonde Data, 1989, San Andres, Colombia, Station 80001. 28

Figure 11. Refractivity Profile, San Andres Island, Station 80001,

25 Jan 1988 . ........................................... 29

Figure 12. Refractivity Profile, San Andres Island, Station 80001,

3 Jun. 1989 . ............................................ 29

Figure 13. Refractivity Profile, San Andres Island, Station 80001,

14 M ar. 1989 . ......................................... 30

Figure 14. Radiosonde Data, 1988, Howard AFB., Panama, Station 78806. 32

Figure 15. Radiosonde Data, 1989, Howard AFB, Panama, Station 78806. . 32

vii

Figure 16. Refractivity Profile, Howard AFB, Panama, Station 80001,

25 M ar. 1989 . ......................................... 33

Figure 17. Refractivity Profile, Howard AFB, Panama, Station 78806,

13 A pr. 1988 ............................................ 33Figure 18. Radiosonde Data, 1988, Belize Airport, Station 78583 . ....... 35

Figure 19. Radiosonde Data, 1989, Belize Airport, Station 78583 ......... 35

Figure 20. Refractivity Profile, Belize, Station 78583, 14 Mar. 1989 ...... 36

Figure 21. Refractivity Profile, Belize, Station 78583, 14 Mar. 1989 ....... 36

Figure 22. Radiosonde Datav Po8, Kingston Jamaica, Station 78397 ...... 38

Figure 23. Radiosonde Data, 1989, Kingston, Jamaica, Station 78397..... 38

Figure 24. Refractivity Profile, Kingston, Jamaica, Station 78397,

5 Jan. 1988 . ........................................... 39

Figure 25. Refractivity Profile, Kingston, Jamaica, Station 78397,

10 Dec. 1989 . .......................................... 39

Figure 26. Radiosonde Data, 1988, Gran Cayman Island, Station 78384. 41

Figure 27. Radiosonde Data, 1989, Gran Cayman Island, Station 78384. 41

Figure 28. Refractivity Profile, Gran Cayman, Station 78384,

14 M ar. 1989 . .............. ........................... 42

Figure 29. Refractivity Profile, Gran Cayman, Station 78384, 13 Dic. 1988. 42

Figure 30. EREPS Ray Tracing for San Andres Island, Station 80001,

14 M ar. 1989 . .......................................... 46

Figure 31. EREPS Ray Tracing of a Standard Refractive Atmosphere ... 47

Viii

Figure 32. Propagati °n Loss Plot for the Duct found on San Andres Island,

Station 80001 on 3 Jun. 1989 ................................ 49

Figure 33. Evaporation Duct Summary, San Andres isLand, Station 80001. 50

Figure 34. Evaporation Duct Height, World Average ................. 51

Figure 35. Extended Coverage Due to Evaporation Duct .............. 52

ix

I. INTRODUCTION

A. PURPOSE

Modern warfare tactics rely heavily on the performance of radar and

communications systems, both ground based and mobile. Although new

technology has made significant hardware improvements that have extended

range detection (power increases, antenna design, digital signal processing, e.g.),

the atmospheric effects on these systems still play an important role in their

performance.

The study of refraction effects has been concentrated in those areas where

the U.S. military has spent a lot of time, such as the Persian Gulf, the California

coast and the Mediterranean sea, or areas that present obvious abnormal

refraction conditions. The purpose of this work is to establish overall refractive

conditions, deternine the availability of data for the region and evaluate the role

of the near surface refraction effects on the perfer-aance of surveillance and ESM

(Electronic Support Measures) systems for the Central America region.

B. BACKGROUND

Many studies have been performed in different areas of the world which

have resulted in well documented anomalous propagation, especially in the

Mediterranean, Indian and Northern Pacific oceans. However, an in depth study

1

of refractive conditions does not appear to exist for the Central America's two

ocean influence region.

A general meteorological description of the area exists and is given by the

Forecasters Handbook for Central America and Adjacent Waters (Naval

Environmental Prediction Facility, Technical Report TR 89-09,1989). However, the

description has not considered the refraction phenomena of the region and their

effects on radio-wave propagation in depth.

Military operations in open ocean and coastal environments depend on

accurate atmospheric data to estimate systems performance and have to rely on

the existence of sufficient knowledge of the environment. Unfortunately, data has

been and continues to be insufficient to meet most planning as well as operational

needs. The purpose of this thesis is to provide atmospheric information and

analysis of the Central America region to aid military commands.

There are computer based models that have been in use for several years

to assess the effects of the various atmospheric conditions on the performance of

specific systems. One model is EREPS (Engineers's Refractive Effects Prediction

System), similar to IREPS (Integrated Refractive Effects Prediction System), which

is used by the U.S. Navy for operational propagation assessment. Such

microcomputers models provide a convenient way to investigate the effects of

particular atmospheric conditions on the performance of systems.

2

II. ATMOSPHERIC REFRACTION

A. WAVE PROPAGATION

The propagation of the electromagnetic energy is strongly affected by the

atmosphere in several ways: refraction, absorption and noise generation

[Ref. 11 as well as by other phenomena. The latter includes scintillation and

fading, due to atmospheric inhomogeneities, and turbulence and optical

interference, due to multi-path propagation.

There are at least four well differentiated propagation categories: ground

wave, line of sight, ionospheric and scatter propagation. For low frequency ranges

(up to perhaps 1500 KHz) ground wave propagation is the main propagation

mechanism. The ground wave is radiation directly affected by the surface of the

earth at the interface. In general, it is vertically polarized and very stable over

time. For higher frequencies, the ionospheric propagation becomes important up

to a critical frequency near the end of the HF range. Solar induced ionization of

the upper atmosphere is the dominant factor in the propagation. There are large

daily and long term variations in the virtual height and strer-th of the reflecting

layers which affect the frequencies that can propagate and the absorption levels

between any given points. For frequencies above HF the propagation mechanism

is mainly space wave or line of sight (LOS) propagation. Here, the main loss

3

factor in the propagation is due to the spherical spreading of the wavefront that

decreases the signal strength as the inverse of the square of the distance.

The fourth category is scatter propagation that occurs when inhomogeneities

(in the troposphere) or ions (in the ionosphere) scatter and reradiate the incident

energy in all directions. A small amount of this energy will eventually reach the

receiver located beyond the LOS path. This mechanism can lead to reliable, stable

microwave links as long as enough power is used to overcome the losses.

Scattering Volume

E\art/

Figure 1. Troposcatter Propagation.

This thesis will focus on microwave and, hence, space wave propagation.

When microwaves propagate through the atmosphere they are affected mainly by

the lower atmosphere, troposphere, between sea level and approximately 15,000

meters. This ic the part of the atmosphere that presents the strongest changes in

pressure, temperature and humidity that determine the refractivity. The most

severe effect, due to refractive variation, occurs when a layer of air with a

4

negative refractivity gradient bends the rays back toward the earth trapping the

energy inside a duct.

B. ATMOSPHERIC REFRACTIVITY

Deviations from rectilinear wave propagation are determined by the

variations in the index of refraction of the atmosphere. The main variation of the

latter is in the vertical direction and is due to the pressure, humidity, and

temperature changing with height. Refraction occurs because of speed variations

along the path of the wave that are described by the index of refraction.

The radio and radar waves propagate through the lower atmosphere at a

speed, v, lower than the free-space speed as determined by the electromagnetic

properties of the medium:

Cv-i

where:

c = speed of light

= magnetic permeability

k = specific inductive capacity

The ratio of free-space speed to the actual phase propagation speed in the

medium is defined as index of refraction, n, with a typical value for air of 1.0003

at sea level.

5

The Equation for atmospheric refractivity is obtained from the Debye's

theory of the dielectric constant of gases [Ref. 3]:

-1 4i A 1 p 2 1F-+2 37M 3 KT 1+i(OT

where:

= Dielectric constant = n2

A = Avogadro's numberK = Boltzman's constantM = Molecular weightp= Electric dipole momentT= Relaxation time

= Frequency of exiting fieldx= Polarizationp DensityT Temperature

At frequencies below approximately 60 GHz., the relative response time of

the molecules with respect to the frequency makes the product c *T very small

with respect to 1. Since s = n2 and from the equation of state p=P

[Ref. 2], the expression then becomes:

(n-1)=2ic A P 1*[a+ 2MRKT 3SKT

The pressure P is the sum of the water vapor pressure, e, and the dry air

pressure, Pd. The molecular weight, M, is also the sum of the dry air, Md, and

6

water vapor M,, components. With contribution of these two components

identified separately, the index of refraction for moist air is:

n - C1 (P-e) C1 e , 1,

Md T dM T w T

where:CI==-x - C2= p--. -

Assigning values to the constants, leads to the refractivity being conveniently

expressed with the following equation [Ref. 3]:

N = (1-n)*10 6 = 77.6*P+3.73*10*e(1T T2

where:N = Refractivityn = Index of refractionP = Pressure, mbarT = Absolute temperature, Kelvine = Partial pressure of water vapor, mbar

The physical composition of the air through the entire troposphere is almost

constant as far as the oxygen, nitrogen and other gases are concerned. The

refractivity depends, then, mainly on the temperature and water vapor contained

in the air. The contribution of the water-vapor term to the refractivity is relatively

small for cold air because then the saturated vapor pressure is small. But for

warm air the term becomes important and the water-vapor content has a strong

influence on the value of N.

7

There are no special absorption effects up to 60 GHz., where the oxygen

molecule shows its first resonance [Ref. 4], well above the frequencies of

most surveillance radars.

The water content of the air decreases rapidly with height, as does pressure,

while the temperature decreases slowly to the tropopause where it stabilizes.

These changes in the meteorological variables lead to refractivity decreasing with

height, under normal conditions (Standard Atmosphere), at a rate of 39.37 N units

per Km. A decrease of N with height means that as height increases, the wave

propagates faster, closer to the free space speed, causing the rays to bend

downward following Snell's law as shown in Figure 2.

Refracted Ray

-- -- -- ---- - - -- - - - - - - - - - - - - - - - - -

Earth -'

Figure 2. Ray Geometry for Standard Refraction.

To use radar for tactical purposes, the exact knowledge of the path followed

by the waves and their speed are needed to compute position of the target from

the timing and angle measured. Since radar systems base their position

8

calculations on U.S. or ICAN "Standard Atmosphere" at any given time, when the

atmospheric conditions differ, the calculated position of target may be wrong and

it becomes necessary to include refraction effects in the interpretation of radar

products.

The abnormal (anomalous) propagation of electromagnetic waves is called

subrefraction, super-refraction or trapping, according to the extent and direction

of the deviation of the rays from standard.

C. TYPES OF REFRACTIVITY PROFILES

1. Standard Atmosphere

The International Civil Aviation Organization (ICAO) has defined a

standard sounding for the atmosphere at non-tropical latitudes that begins with

a surface (sea-level) temperature of 15 Celsius. Clearly, it is not applicable to

tropical latitudes where the sea-level temperature is usually 26 Celsius and,

therefore, a warmer and, importantly, a more humid Marine Atmospheric

Boundary Layer (MABL) is expected. Jordan (1958) [Ref. 5] has formulated

a Mean Tropical Atmospheric Sounding that is useful for calculating a base

profile of refractivity at tropical latitudes (see Figure 3).

9

io.11.5

E._ Z o 1.0/ (I

,E

2/

//

0..,, 6,-1.0/

206 700 200 1 700 200 300 400

Refractivity M Refractivity M

Figure 3. Modified Refractivity Profile, Mean Tropical Soundingand Low Altitude Detail.

The low altitude detail in Figure 3 shows the refractive profile in the lower

atmosphere for buth the ICAN standard atmosphere and that of the mean tropical

sounding by Jordan [Ref. 5 1. The refractivity gradient at the low altitude is

greater for the tropical atmosphere which would lead to greater radio/radar

horizon.

The distance, d, to the radio horizon is related to the refractivity gradient

by:

d = /2*k*a,*ht

where, h,, is the transmitter height and k is the ratio of the effective earth's

radius, ae , to the actual radius of the earth, a:

10

II

a. +

The ratio, k, was calculated for the first kilometer in the mean tropical

sounding as k = 1.722 where for the ICAN standard atmosphere its value is 1.333.

The radio/radar horizon is proportional to ;1/2 for a given transmitter height.

Therefore, on the average, the Jordan tropical sounding has 14% larger radio

horizon.

2. Superrefractivity and Trapping

The atmospheric conditions in which superrefractivity and trapping of

microwave energy occurs always involve stratification of the atmosphere into

layers of different index of refraction. Trapping layers and resulting ducts can be

surface based or eklvated. Elevated and surface based trapping layers can be

detected by radiosonde temperature, pressure and humidity measurements and

by direct measurement of the refractive profile with an airborne refractometer.

Superrefractive layers are formed when there is a layer in which the

temperature increases with height (the temperature decreases with altitude with

a lapse rate of 9.8 Kelvin per Km in a well mixed atmosphere) and/or the water

vapor sharply decreases with height. Superrefractive layers become trapping

layers when the refractivity gradient causes the propagating wave to bend

downward with a radius of curvature smaller than the radius of curvature of the

11

earth. The energy is reflected at the surface again, (surface based duct), or

refracted back by the base layer, (elevated duct), trapping it inside the duct as

depicted in Figure 4.

......... ...... ::::':r::,.*: : :: .*..:::: .. ""..X ......:" ": ..

Layer.:- . ......

Earth

Figure 4. Propagation of Radar Energy in a Surface Based Duct.

Figure 5 illustrates how the different duct types relate to the various

refractive profiles.

12

Surface Based Duct Elevated Duct

z - -

DJ~t

- - - -- - - - -- -- -- - - -

harfm Law E~dOAMMd OM

M M

Surface Based Duct Evaporation Duct

z z

I 5

Evaporation Duct

r/4 ---- -- -- -- -M M

Figure 5. Refractive Profiles of the Different Duct Types.

The evaporation duct is due to an abrupt change in relative humidity at the

water-air interface, from 100 % to approximately 85 % in the first few meters. The

13

evaporation duct is almost always present over watei. Its thickness varies as a

result of the daily temperature changes and intensity of the surface wind. The

evaporation duct height can be computed from bulk meteorological measurements

(sea surface temperature, air temperature and wind) by using semi-empirical

models like the model described by Paulus [Ref. 6]. For a given duct

thickness, d, the minimum frequency, F.,., that can propagate within the duct

is given by [Ref. 4]:

C

2.5 J

The minimum frequency propagated in the surface based duct -s a function

of the duct thickness can be seen in Figure 6.

14

--I- .- -L .-- L ---- _

. .. .L . . .L _.....L ----- L ----- L ----- L ----- L _

2 T - - -r ----- r ----- r ..... r ----- r ----- r ----- r-" \ i i i i i I i

I I I I i I iI

-- -- ----- . .. L . .. L ----- L ----- L ----- L---: -.L -I / i I i I

/ i i- - i I

10 .----

----- ...---- r.-- ----- r - t

T ... . ..... . ..... .. .. . 'e' ..... A d ..... i ..... 4'' 210 40 80 80 100 120 140

Duct Thickness (meters)

Figure 6. Minimum Trapping Frequency for a Surface BasedDuct.

D. METEOROLOGICAL PHENOMENA LEADTNG TO TRAPPING

Several meteorological conditions modify the distribution of temperature and

water vapor in the lower atmosphere. Variations in temperature, pressure, or

both, determine as described before, the atmospheric refractive gradient. Air flow

associated with the motion of air masses, radiation, subsidence and land-sea

winds, define the vertical temperature and moisture profile of the atmosphere.

a. Radiation

Radiative cooling of the surface during clear nights -is a very

common cause of temperature inversion leading to superrefractive conditions over

15

land. At sea, the ocean acts as a mass;, - heat reservoir so its surface temperature

remains nearly constant and the effect is diminished.

b. Subsidence

Subsidence, the slow, large scale sinking of the air, brings dry air

from aloft and raises the temperature of the air by adiabatic compression. This

creates inversion layers that are common on large tropical and subtropical ocean

areas. Subsidence occurs within subtropical high pressure systems.

c. Land-sea winds

The difference in temperature and humidities between the land,

heated by the sun, and the adjacent sea surface usually causes the warmer, dry

air over land to rise and subside over the adjacent cooler and more moist marine

air. The colder moist air from the sea will replace the raising air as depicted in

Figure 7. The warm, dry air will form the upper boundary of an inversion layer

in which ducting can occur.

16

4-u

Warm dry air

Cold moist air

Figure 7. Sea-Wind Inversion Layer.

17

III. CENTRAL AMERICA CLIMATOLOGY

A. CLIMATOLOGY

The meteorology of Central America is significantly influenced by the

Intertropical Convergence Zone (ITCZ) that lies on the pacific side most of the

year [Ref. 7]. The large scale air flow along the ITCZ is the product of the

northeasterly trade winds meeting the southeasterly trade winds. Since they are

air masses of different origin, their temperatures and water vapor content are

quite different making the ITCZ a zone of very high cloudiness and precipitation

that can be seen all year around in the satellite imagery. Rainfall occurs primarily

between June and September and by December with drier periods in between.

The general air flow direction along the ITCZ is west-to-east during most of

the year. It is deviated by the Andean ranges at the western coast of Sc.*th

America, which have heights up to 5000 meters. The Caribbean side is mostly

affected by the flow of trade winds that flow in harmony with the general

Caribbean regime. Here the dominant phenomena are the tropical disturbances.

Sea surface temperatures are high in both Pacific and Caribbean areas, with small

diurnal and annual changes [Ref. 8], which in turn cause very constant

surface air temperatures.

18

B. SYNOPTIC DESCRIPTION

The Pacific and Caribbean sides of Central America are subject to different

regimes. The Pacific presents a general surface air flow following the ITCZ

parallel to the equator that ends at the western coast of South America where is

deviated by the high Andes mountains. The Caribbean is farther north and is less

affected by the ITCZ.

Local phenomena, such as like sea-breeze and land-sea winds are known to

cause strong inversion layers in coastal areas. The breezes are frequent and strong

enough to dominate the general circulation. The complete synoptic description of

the area can be found in the Forecasters Handbook for Central America and

Adjacent Waters [Ref. 7].

19

IV. DATA ANALYSIS AND RESULTS

A. RADIOSONDE DATA

1. Geographical Distribution of Data

For the purpose of this study, we have considered data taken during

1988 and 1989 by the stations shown in Figure 8. These are listed in Table 1.

78384 <Z78583 738 78397 E

Figure 8. Geographical Location of the Sounding Stations.

20

Table 1

Stations Used in the Analysis

Station Lat. Long. Elev.

78384 Roberts Field, Gran Cayman Island 18-19N 81-20W 3 m

78397 Kingston, Jamaica 18-04N 76-50W 1 m

78583 Belize International Airport, Belize 17-36N 88-18W 5 m

78806 Howard A.F.B., Panama 8-58N 79-35W 66 m

80001 San Andres Island, Colombia 12-34N 81-41W 2 m

There is only one station located on the Pacific side of Central America

(station 78806 in Howard A.F.B., Panama) in the area of this study. Two of the

five stations located in the Caribbean are in the Greater Antilles (stations 78384,

78397 in Jamaica and Gran Cayman), one is in an open ocean island (station

80001, San Andres) and one is a coastal station in Belize (station 78583).

2. Distribution of Data Over Time

Stations are supposed to launch radiosondes twice a day (stations 78806,

78583, 78397, 78384) and daily in the case of station 80001 [Ref. 9]. Due

to different circumstances they generally do not. The resultant distribution of

soundings is presented according to stations in Table 2.

21

z - m e *

00 in

CD 0

00 OD = 0 tN

00

\ 0\ in 0\

0

0 :3 0o m~ N

0 0 0

W 00 -N

Z 4)

0

\0 0- nN C' C14

0

0

0c0 N N

0 0 - ('1 '0o

ca 0 N 10c

-o 0

C: 0 0 N

02

22

B. ANALYSIS OF REFRACTIVE CONDITIONS

1. Procedure of Analysis

The Fleet Numerical Oceanography Center, Monterey, CA, provided

significan' and mandatory level radiosonde data on 9-track tape for 1988 and

1989. The tape files were read and transferred to a PC-type microcomputer for

processing. Pressure, temperature and dew-point depression from significant

levels were used to calculate refractivity and height for display as a refractivity

profile and to compute the refractivity gradient to identify anomalous conditions.

Since ducts occur at various altitudes, with a broad strength range, it is

difficult to appropriately describe with statistical measures the refractive profile.

Therefore, each refractive profile in which ducting was detected was displayed

and visually evaluated with the purpose of stating a typical ducting profile for

each station. EREPS was used to graphically show the effects of these profiles on

a typical radar system performance.

The ray tracing subprogram of EREPS (RAYS) was used to visualize effects

of the typical refractive anomalies detected in the soundings. The propagation

model (PROP) [Ref. 10] was used to assess the performance of typical

systems under the ducting conditions detected. Finally, the evaporation duct

climtatology from EREPS is used to visualize the expected effect of the evaporation

duct in surveillance radar and ESM systems.

23

2. Formulas and Algorithms

Formulae used in the programs to identify and plot ducts are presented

here and the general procedure followed to datcrmine the occurrence of ducts in

the soundings.

Modified Refractivity is calculated from pressure, temperature, partial

pressure of water vapor and height with the following equation:

77.6*P 3.73*105*e hTa Ta2 6.371

where:

P = Pressure in millibarsTa = Temperature in Kelvine = Partial pressure of water vapor in millibarsh = Height in meters

Partial water vapor pressure was calculated from the saturated vapor

pressure at a the dew point temperature. The saturated water vapor pressure for

a given temperature is calculated using Lowe's polynomial [Ref. 111:

Es = A6 *T6 +A5 *T5 +A4*7 +A3*T3 +A2 *72 +A*T+A 0

where:A0 = 6.107799961000000A = 0.443651852100000A2 = 0.014289458050000A3 = 0.000265064847100A4 = 0.000003031240396A5 = 2.0340809481 * 10-A6 = 6.136820928999999 * 10-11

T = Temperature in Celsius

24

The Height is calculated from the initial launch altitude and pressure for the

first level, and from the preceding level for the successive heights using the

relationship for geopotential height [Ref. 12], as expressed in the

following equation:

h = ho +14.63*(Vto+Vt)*Log( PO)P,

where:h0 = Initial Height, metersVto = Virtual Temperature at Initial Height, KelvinP0 = Pressure at Initial Height, mbarVt1 = Virtual Temperature at Actual Height, KelvinP1 = Pressure at Actual Height, mbar

Virtual temperature is computed from the temperature, pressure and water

vapor pressure with the equation:

Vt = Ta+ (0.3794017*Ta*e)(P-e)

where:Ta = Absolute temperature (Kelvin)P = Pressuree = Vapor pressure.

The trapping layers are identified by computing the refractive gradient

aM/dZ and plotting the refractive profile, modified refractivity, M, versus height,

25

for the data sets that have negative gradient to visually evaluate the resulting

trapping layers.

3. Results by Stations

a. San Andres Island, Station 80001

A total of 442 soundings where processed for this station. Trapping

layers were classified as being surface based or not, and if not, as being above or

below 700 mb. Trapping layers were found in 156 soundings, and are distributed

as follows according to occurrence being at the surface or, if not, as being above

or below 700 mb:

Year 1988 1989 Total

Soundings 161 281 442

Surface based trapping layers 15 16 31

Elevated trapping layers (total) 41 84 125

Elevated trapping layers above 700 mb. 10 20 30

Total 56 100 156

The refractive profiles that showed trapping layers were visually evaluated

and it was found that trapping layers were stratified much like the profile in

Figure 11. Many profiles showed more than one trapping layer and therefore have

multiple ducts. Only the lower trapping layer per sounding was used for the

26

above statistics. The trapping layer height was about 2000 meters and below. The

thickness and intensity of trapping layers were less than or equal to the profiles

shown. The local launch time for this station is 06:30 (12:00 UT). Therefore the

observations arc representative of a stable, non mixed MABL in which

stratification is enhanced.

Soundings revealing trapping layers had many data errors. Nineteen of the

156 layers identified by the program were due to errors in the data. These were

identified visually by checking the suspect part of the data for robustness. Surface

based trapping layers were found to occur in 7 % of the soundings. Because

surface trapping layers could have been caused by erroneous launch level values,

they were checked closely. The total duct occurrence found is 31 %, after

excluding the errors. For this station, Ortenburguer and Lawson [Ref. 13]

has found an occurrence of surface based ducts of 20 % and an occurrence of

elevated ducts of 35%.

27

80001 -Radiosonde Data 198830

25cff

. 20

10-"

5 - .....I.-... .10"

1 2 3 4 5 6 7 8 9 10 11 12

MonthFigure 9. Radiosonde Data, 1988, San Andres, Colombia, Station80001.

80001 -Radiosonde Data 1989

30

25 ......................................

0 ........................

l .......... ............. ...........

1 2 3 4 5 6 7 8 9 10 11 12

MonthFigure 10. Radiosonde Data, 1989, San Andres, Colombia, Station80001.

28

Refractivity " units Date Observation: 8M12512 D291 48 691 I N1 12 M

S 11? UTT in1812.8 25.91 2.91

H 1MI6.6 25.6 3.28a 949.6 21.68 6.401 4 938.6 21.48 6A.6g 956.8 19.48 6.91t 050.6 15.0 2.A6h 818.6 13.A6 6.66

3 ?9.6 11.2 2.91X 777.0 14.28 16.98i 752.6 14.46 2.1 766.6 9.0 11.Ao 686.6 6.48 8 m 2 663.6 4.M 19.9

54.8 7.68 28.1t 611.8 5.96 22.86e 551.6 .66 26.89

1 515.6 -4.76 19.9566.6 -5.30 19.91476.6 -6.96 18.9416.6 -18.36 16.91

Figure 11. Refractivity Profile, San Andres Island,Station 80001, 25 Jan 1988.

letractivit9 N units Date Observation: 8966312 S26 486 569 666 1666 1291

5 . " " PPP ITT 11611.6 27.2n 2.91

H IM6.6 26.26 6.661 977.0 25.26 8.911 4 956.6 16.60 6.919 756.6 11.40 0.46t 731.6 9.48 6.90i 717.8 12.20 6.66

3 706.8 1o69 16.91X 695.6 16.6" 16.911 652.6 7.26 4.81 585.6 6.66 1.20o 561.6 -2.36 2.91* 2 545.0 -3.38 2.46S5.37.6 -3.16 8.91

t 566.8 -5.96 18.6.a 482.6 -5.36 16.8r 1 467. -7.36 2.90s 452.6 -9.36 18.91

445.6 -18.16 14.80466.6 -14.76 19.88

Figure 12. Refractivity Profile, San Andres Island,Station 80001, 3 Jun. 1989.

29

latzractivitg I units Date Obasruation: 05631412 D2N 46 fin6O 1W6 12K

5 PPP ITT lTD1815.8 24.66 4.48

H 186.6 23.48 4.48e 940.9 19.99 8.4614 9190 16.28 6.48

896. 16.28 6.60t 9560 12.40 2.66h 9,20. 16.66 4.A9

3 92.6 12.28 16.99K 773.6 13.80 18.98

766. 9.28 16.66! 613.9 6.66 16.99a 554.0 -1.58 16.9.2 6.6 -4.98 16.66* 456.8 -7.98 16.0t 466.6 -15.58 12.98e 34B6. -24.36 12.99

1 366. -32.36 12.663 265.6 -39.56 16.80

26.6 -43.36 6.66266.6 -55.36 6.66

Figure 13. Refractivity Profile, San Andres Island,Station 80001, 14 Mar. 1989.

b. Howard A.F.B., Panama, Station 78806

For this station 766 soundings where processed, trapping layers

were found in 266 soundings distributed as follows:

Year 1988 1989 Total

Soundings 417 359 776

Surface based trapping layers 45 45 90

Elevated trapping layers (total) 107 69 176

Elevated trapping layers above 700 mb. 25 16 41

Total 152 114 266

30

The soundings that showed trapping layers were visually evaluated. The

occurrence of trapping layers was observed centered at around 2500 meters and

the associated ducts tend to be strong and thick as seen in the profile of Figure 16.

The strength of the duct refers to the M difference across the duct while thickness

refers to the vertical separation of the duct top and base. Many refractive profiles

contain more than one trapping layer and associated duct. Local launch times are

06:41 and 18:41 (12:00 UT and 00:00 UT). Data from these times could have

greater ducting occurrence than mid-day time data due to lesser convection

generally occurring before sunrise and after sunset.

Visual inspection of the first 5000 meters in the profiles that showed

trapping layers, revealed fifteen data sets in error of the total 266 soundings with

trapping layers found by the program. This reduces the duct occurrence found

from 34.7 % to 32.3 %. The surface based inversion layer occurred in 11.6 % of

the soundings. As was described above, a surface based inversion layer could

arise from erroneous surface values. This possibility was checked very closely for

this case because the percent occurrence of surface based trapping layers was

above 10%. For this station, Ortenburguer and Lawson [Ref. 13 1 has found an

occurrence of surface based ducts of 5 % and an occurrence of elevated ducts of

29%.

31

78806 - Radiosonde Data 1 9 8840

_ rm-35....

Cl3 30

~25..__20 -

15 •

10

--5.

01 2 3 4 5 6 7 8 9 10 11 12

Month

Figure 14. Radiosonde Data, 1988, Howard AFB., Panama, Station78806.

78806 - Radiosonde Data 19 8960

w-

40 . ..... ........ . ....... . ..................

0 . .. ......................

20 . . .**........... ..

10" 11 2 3 4 5 6 7 8 9 10 11 12

MonthFigure 15. Radiosonde Data, 1989, Howard AFB, Panama, Station78806.

32

Pofractivity I units Date Obsrvation: 89632512 D28 46588 M8 9 18 129

s . ?PP TT! TT1984.9 22.6 6.40

H 997.8 23.89 1.28* 978.8 23.28 1.681 4 827.9 14.08 6.9 816.0 14.28 8.90t 063.0 13.49 6.48h ?4.9 12.68 6.48

3 792.9 13.29 16.86x 769.9 12.48 1.980i 748.8 12.48 28.881 719.8 9.28 8.89o 699.8 9.68 6.9* 2 688.9 8.29 12.80e 595.9 1.28 36.60t 579.9 -6.99 12.8a 551.8 -2.78 39.9r 1 542.9 -3.5 14.98a 462.9 -9.78 36.98

448.8 -9.98 38.98332.8 -29.58 12.98

Figure 16. Refractivity Profile, Howard AFB,Panama, Station 80001, 25 Mar. 1989.

Wfractiuitg " units Date Obsrvation: 88418M S-V8 488 68 998 1988 128N

? PPP ITT TID1092.8 31.40 1.60

H %5.0 29.88 6.0a 952.0 25.60 4.801 4 839.8 17.28 9 .489 818.8 15.28 8.88t 772.8 13.98 2.90h 747.8 14.88 14.80

3 726.8 14.90 39.9x 595.8 3.68 18.89i 588. 2.88 16.9815?8.8 2.98 0.88o 563.8 2.28 36.98

2 2 447.0 -11.38 36.98. _432.9 -13.18 9.9

t 3908 -15.58 36.98a 381.8 -17.38 38.90r 1 312.9 -29.58 36.Ms 30.8 -31.98 0.89

262.8 -39.58 1.98193.8 -r.98 0.98

Figure 17. Refractivity Profile, Howard AFB,Panama, Station 78806, 13 Apr. 1988.

33

c. Belize Airport, Belize, Station 78583

A total of 615 soundings where processed for this station. Trapping

layers and associated ducts were found in 188 soundings and are distributed as

follows:

Year 1988 1989 Total

Soundings 397 218 615

Surface based trapping layers 10 7 17

Elevated trapping layers (total) 108 63 171

Elevated trapping layers 700 mb. 36 13 49

Total 118 70 188

The elevated trapping layers occurred consistently at an altitude of about

1700 to 2000 meters. There where few multiple duct occurrences and the few

surface based ducts where not very strong. Figure 20 and Figure 21 show typical

M profiles. The inversion layer is stronger and thicker in the 12:00 UT sounding.

Two errors were detected in the data of the soundings found with ducts.

The percentage of trapping layer occurrence was 30.2 %. Surface based inversion

layers o.-curred in 2.8 % of the soundings. This station is not included in the work

by Ortenburguer and Lawson [Ref. 13].

34

78 5 83 -Radiosonde Data 19 988

30,

10 -

0-1 2 3 4 5 6 7 8 9 10 11 12

MonthFigure 18. Radiosonde Data, 1988, Belize Airport, Station 78583.

78583 - Radiosonde Data 19 8940 ....... . ................................................................. .* .. ..

cl) 35 .... ... ................. . .... ... . ..............................

15. . . ...... . ............................... ..........

15 . U..... *..........1'.......... f H -

1 2 3 4 5 67 8 9 10 1112

MonthFigure 19. Radiosonde Data, 1989, Belize Airport, Station 78583.

35

Rafractivitg " units Dat Observation: 99031466 S2N1 4N9 6W1 119 1281

5 PpP TT! TTDI1S.6 2S.48 4.40

H I818.6 23.29 6.98e 919.6 16.21 9.981 4 997.8 17.w 8.w9

873.6 15.68 14.98t 736.6 9.0 38.Ah 781.6 9.98 38.

3 426.9 -13.79 38.X 338.0 -27.38 381 279.0 -39.38 12.8

2W1.8 -55.79 8.81o 181.9 -77.99 8.81.2 290 -6.3 8.8

t0

e11

S

Figure 20. Refractivity Profile, Belize, Station78583, 14 Mar. 1989.

flefractivitY, , units Date Observation: 8931412 D2N 4a0 600 m 19 9 12W1

. . . PPP TTT Td1813.0 21.88 1.68

H 166.9 23.20 3.68e 9390. 17.80 9.481 4 828.9 18.88 1.29S981.9 9.99 4.99t 790.9 11.29 14.99h 779.0 13.29 30.0

3 6390 3.00 30.8X 619.9 3.60 3.99i 536. -1.99 38.991 356.9 -24.79 39.99o 272.9 -39.30 12.00.2 299.9 -56.19 .99* 159.9 -63.18 S.99t 146.0 -62.30 9.6e 199.0 -75.96 9.89r1 199.9 -7.39 8.99S

Figure 21. Refractivity Profile, Belize, Station78583, 14 Mar. 1989.

36

d. Kingston, Jamaica, Station 78397

A total of 1182 soundings were processed for this station. Trapping

layers and associated ducts were found in 248 soundings and are distributed as

follows:

Year 1988 1989 Total

Soundings 659 523 1182

Surface based trapping layers 24 16 40

Elevated trapping layers (total) 123 85 208

Elevated trapping layers above 700 mb. 25 27 52

Total 147 101 248

Visual evaluation of the 147 refractive profiles that showed trapping layers

revealed nineteen erroneous data sets. The trapping layer is located generally at

2000 to 2500 meters. Most ducts are neither strong nor thick. The profiles shown

in Figure 24 and Figure 25 are representative of the strongest trapping layers seen.

The frequency of occurrence of trapping layers was 19.4 %. Surface based

trapping layers occurred in 3.4 % of the soundings. For this station, Ortenburguer

and Lawson [Ref. 13] has found an occurrence of surface based ducts of 20 %

and an occurrence of elevated ducts of 33 %.

37

7839 7 -Radiosonde Data 198870 -IIM

60

SS30--- . . ... .

20- --

10 *-----

0.1 2 3 4 5 6 7 8 9 10 11 12

MonthFigure 22. Radiosonde Data, 1988, Kingston Jamaica, Station 78397.

78397 - Radiosonde Data 1989

70 60 ....... ... . .................. .. . ... ..... I

Cl 0 ......... .................................... ..... .............. ... .... ....

40........................... ........... . . ............

S30 ............ -...fl 1 1.I ..........@20- . .. . .. .....

10. ....... .. . .. 11...... -----.--

0- nIR Rl~ -

1 2 3 4 5 6 7 8 9 10 11 12

MonthFigure 23. Radiosonde Data, 1989, Kingston, Jamaica, Station 78397.

38

Refractivity M units Date Obar tion: 8661656 D26 46 666 NP, 1N@ 126

- PPP YT TrS Up TT TTD

1814.8 28.48 6.,AH 976.6 25.66 6.66e 929.8 21.46 GA61 4 960.8 29.26 3,60g 956.6 16.A6 8,48t 829.8 14.68 2,40h 799.6 14.66 16,K

3 749.8 12.66 19.MX 766.8 7.46 2.66i 693.6 6.A6 28.66I 663.0 6.46 3.66S4668 -16.16 38,66

,2 279.9 6.66 1.26266.6 -56.16 86

ta

rl

Figure 24. Refractivity Profile, Kingston, Jamaica,Station 78397, 5 Jan. 1988.

Refractivity " units Date Observatlon: 891210 D290 466 B6 E 6O166 126

5 PPP TT TTD1613.6 27.96 4.890

H 856.6 16.40 4.86e 830.0 15.99 6.60i 4 909.8 13.98 1.60g 792.0 15.6 26.06t 29.6 4.60 12,6

595. 6.66 26,63 568.0 1.60 39.66

x 566.8 -6.10 18.66i 461.6 -9.10 19.901 460.0 -18.70 16.06o 347.0 -24.76 12,0.2 390.0 -34.30 6,6o 284.0 -34.50 196t 261.0 -39.56 19.6o 117.9 -77.98 6.66r 1 166.6 -79.38 0.06s 363.0 -21.7 6.06

Figure 25. Refractivity Profile, Kingston, Jamaica,Station 78397, 10 Dec. 1989.

39

e. Roberts Field, Gran Caynan, Station 78384

For this station, 1148 soundings where processed and 381

soundings were found to contain trapping layers distributed as follows:

Year 1988 1989 Total

Soundings 618 530 1148

Surface based trapping layers 19 18 37

Elevated trapping layers (total) 194 150 344

Elevated trapping layeis above 700 mb. 47 25 72

Total 213 168 381

The v:,sual evaluation of the refractive profiles that contained trapping layers

revealed that the elevated trapping layers where thick and strong and were

grouped mainly near 1500 or near 3000 meters.

The occurrence of trapping layers was found to be 33.2 % and the

occurrence of surface based trapping layers was 3.2 %.

40

78384 - Radiosonde Data 198870- _ _ --

8 0

-4- 40 . .. . .....

30.

20

10

01 2 3 4 5 6 7 8 9 10 11 12

MonthFigure 26. Radiosonde Data, 1988, Gran Cayman Island, Station78384.

78384 - Radiosonde Data 1 98970

.....a

4- 40. ........... ............ . . ..... ....

ii-- 40 .. . ..... . . . . . .."°"'''°° °'

S30 -... .............

~20"10 . ......... ..

0"1 2 3 4 5 6 7 8 9 10 11 12

MonthFigure 27. Radiosonde Data, 1989, Gran Cayman Island, Station78384.

41

hafractivitg II units Data Obirvation: 9961412 D2N6 40 INSW 12K

5 .1..??" ITT ?7D1116.6 21.28 3.610lE .S 22.08 4.9

* 97. 12.98 6.961 4 86 12AU 1.28

8 44.8 12.9 16AEt 584.6 -9.98 38A

Ii381.6 -21.79 3.A3274.8 -39.98 12.9

x 299.8 -S.31 0.Ei INA -77.19 N.E

.2

t

e

Figure 28. Refractivity Profile, Gran Cayman, Station 78384, 14 Mar. 1989.

lhfiactivitgj N units DWe Observation: 99121312 D299 48 686 W9 1999 12M9

1914.9 29.29 8.49H 1999.9 24.29 2.N9* 1996. 24.46 2.961 4 911.9 17.66 6.40

1 9689 15.41 0.099 37.9 1S.4 14A9

h 719.9 9.40 16.993 702.8 198.29 38.A

K 649.6 7.40 39.91 461.9 -11.18 19.A

1366.8 -34.76 14.9o274.6 -39.96 9.89

a2 154.9 -71.58 I.9a 131.9 -Q8.36 6.9t 199.9 -77.19 9.9e 199.9 -77.19 BA9r I

Figure 29. Refractivity Profile, Gran Cayman,Station 78384, 13 Dic. 1988.

42

V. PROPAGATION EFFECTS ON SYSTEMS

A. EFFECTS ON RADARS

The radar equation that conveniently describes the performance of a radar

in terms of transmitted power, antenna gain and effective aperture, radar cross

section of target and receiver sensitivity falls short when used to describe actual

performance of systems.

"In practice, however the simple radar equation does not predict therange performance of actual radar equipments to a satisfactory degree ofaccuracy. The predicted values of radar range are usually optimistic. Insome cases the actual range might be only half that predicted." [Ref. 4]

The radar equation describes the free space range and assumes "...three-

dimensionality and isotropy of space" [Ref. 1]. However, the atmosphere is in

no way isotropic and the three-dimensionality is violated as soon as ducting or

other anisotropic ?henomena are encountered. Superrefraction and ducting can

direct energy in a particular direction that otherwise would spread

isotropically producing extended coverage, sometimes to ranges well beyond

free-space ranges.

Extended range is not the only effect expected. Due to the reciprocal path,

there is a significant increase in the clutter and noise received that can

seriously degrade the overall performance of a surveillance system. Double

return echoes are possible and a target at greater than the maximum

43

appear as an in-range target. The distance at which the signals can be intercepted

will be augmented and the scenario becomes much more complex.

The surface-to-surface propagation over the ocean is affected in the area due

to the moist MABL. The radio horizon (Page 10) is slightly extended on the mean

over what would be expected over subtropical waters and somewhat larger than

the world average.

1. Rain Effects

Rain is the most common and an important phenomena in tropical areas

as nearly all tropical disturbances carry rain. Absorption and scattering of the

radar energy by rain is a very strong loss mechanism. The difficulties involved

in accurately measuring the characteristics and the rate of rainfall prevent the

models from being effectively used to forecast the effect of rain on the radar

coverage. MTI (Moving Target Indicator) or doppler radars show less rain clutter

effects; but, since the energy is scattered in the rain volume, the effective power

that reaches the target and the receiver is strongly reduced and, therefore, so is

the detection range. The rain scatter is frequency dependent [Ref. 141 so

low frequency radars can be used if see-through capability is needed.

The transient nature of rain, even in high precipitation areas like the Pacific

ocean off the western coast of Colombia, suggest the selective use of sensors as

the best way to obtain radar coverage during rain.

44

The transient nature nf rain, even in high precipitation areas like the

Pacific ocean off the western coast of Colombia, suggest the selective use of

sensors as the best way to obtain radar coverage during rain.

2. Duct Effects

The range of radar coverage can be extended by surface based ducts

associated with elevated trapping layers or by evaporation ducts. The

evaporation duct is a subset of ducts where the duct base is at the surface. In

the a' sence of a surface based duct, the evaporation duct is tbci dominant

propagation mechanism for surface-to-surface detection over water, provided

the radar antenna and the target are inside the duct and a frequency

sufficiently high is used (above the cutoff frequency).

a. Coverage Diagram with Surface Based Duct

Due to the large number of different ducting profiles observed

at the stations analyzed, the effects 'a surface based duct on a particular

system ducts will be illustrated using a sounding at station 80001 on March 14

1989. The refractive profile obtained from the sounding is depicted in

Figure 13. A strong trapping layer is clearly visible from the surface up to 100

meters, causing a surface based duct. At around 3000 meters there is another

thin duct. As stated earlier fr this station, many soundings in which a duct is

found revealed multiple ducts.

45

inside the duct. For comparison purposes, a standard refraction raytracing is

shown in Figure 31.

# OF RAYS soHIM AMC deg -8.1MAX ANG deg 0.6

988 REFLECTED RAYS VSOND Ha 2

PROFILEP(mbs) THP(C) H(x)

/ 58 16.6 95480-?88 14 9

?31.8 9.4 66717.0 12.2 SGWO8. 18.8 47

288- 695.9 18.6 3RaarHoe652.9 7.2 76

585.8 9.8 92'.Surface Based Duct 561.9 -2.3 96

I8 - 54S.9 -3.3 838a 1 214 36 48 68 537.9 -3.1 51

WIE imi 599.9 -5.9 42

Figure 30. EREPS Ray Tracing for San Andres Island,Station 80001, 14 Mar. 1989.

46

18- ANT HT 1s

HI NAG dg -8.1MAX AMC de .6

9N- REFLECTED PAYS V$O11DR HT N 0

PROFILE,, P(mbs) THP(C) H(W)

688- 1813.2 5.8 820O.8 -U .86

408-

2N-

RANGE Ml

Figure 31. EREPS Ray Tracing of a Standard RefractiveAtmosphere.

b. Propagation Loss

The ray tracing alone does not give enough information on the

detection capability of a radar under specific refractive conditions. The system

itself needs to be taken into account. Receiver characteristics as well "S antenna

gain, transmitter power, target size, thresholds and other factors need to be

considered to assess detection capability. Table 3 lists the parameters necessary

to estimate loss. The end application of the radar will ultimately dictate the

probability of detection and false alarm rate to be included in the model.

Here, for the purpose of visualizing the effects of ducting in a given system,

a "generic" surveillance radar is used with a target of small radar cross section

47

(RCS) and a Swerling model 1 [Ref. 4] to account for the slow fluctuation of RCS

typical of a low tonnage vessel at sea.

Table 3

Parameters Used to Estimate Propagation Loss

Frequency 3 GHz.

Polarization Horizontal

Radar height 10 meters

Target height 15 meters

Antenna type Cosec-Sq.

Vertical BW. 10 degrees

Elevation Angle 0 degrees

Peak power 500 Kw.

Pulse width 1.0 uSec.

Antenna gain 32 dBi.

System losses 8.4 dB

Receiver noise figure 5 dB

Pulses integrated 10

RCS target 10 sqm.

Probability of detection .8

Probability of false alarm 108

Swerling care 1

The effects of the duct, shown in Figure 32, are tactically important. There

is a significant skip zone in which there is no detection. The detection range is

extended beyond the free-space range after the first reflection at 30 nm up to 55

48

nm. This an additional coverage of 6000 squared nautical miles to the 346 nm.2

of the unextended coverage. If a force is aware of the presence of the duct, it can

be used on purpose to get extended surface surveillance. Otherwise, the force can

give away valuable intelligence by ignoring the existence of the duct.

FREQ fh 3800POIARIZATION HORRADR HT a Is.RGT HI n 15

118- ANY TYPE CSC-SQ-,., VER N deg 10

ELEV ANG deg I... . . . EUDH

HO -- D HT n aH a

X 1.72"SUBS 339

170- ADS HUM g/0" 14.4UIND SP kts 1e

200-

FREE SPACE --- -238 RADAR1 DETECTION

a 1e 20 38 48 0 THREH0LD -------RNGE n~i FS RANGE mi 46.5

Figure 32. Propagation Loss Plot for the Duct found on SanAndres Island, Station 80001 on 3 Jun. 1989.

The Evaporation duct exists nearly all the time. The summary data provided

by EREPS calculated from 15 years of observations shows that an evaporation

duct height of more than six meters 94.5 % of the time (Figure 33) and 16 or more

meters high 53.1 % of the time. These are high values compared with the world

49

average (Figure 34). The evaporation duct is an important propagation

mechanism, particularly for higher frequencies. Tactical effects of the

evaporation ducts are often neglected and underestimated. Since the duct is

present most of the time and is both frequency and height dependent, small

vessels with their radar located inside the duct, operating in the X band, may

have earlier detection of conventional platforms with higher antennas and

lower frequency radars.

50

UD HT xOCCUR 0 5 16 15 29 25 _________

90 to2. 1.1SUFCDUTUNR210O 4. 1.1 1 .. SURFACE 03S: MS 4S410o 6,N 3.3 V610O 8.P 3.8 A .LATITUDE: 161T 0 MI81TO1Is. 5.8 P ,LONGITUDE: 861T 0 WV1610O12. 8 .5 0V AUDHT: 16.6.

*1210O14.m 19.9 R AVGUWIND SP: 1Z.5 ITS1410O16.i 12.4 A SAMPLE SIZE: 99260ODS16TO19t 12.8 T181029.8 11.4 1 UPPER AIR OHS: PS Boos2022.x 9.5 0221TO24.P 7.1 N SAN ANDRES, COLOMBIA2410O26.% 4.82610O29.m 3.9 D2910O39.m 1.9 U LATITUDE: 12.58NM38 TO 32 a :1.1 C LONGITUDE: 81.18 V32 TO 34 .t I.7 T SDD OCCURRENCE: 22.63410O36,i 1.4 AVG SOD H: 33.t3610O38, 6.2 H AVG NS2J8S: 3843BTO44, 6.1 N AVG XI: 1.67

>49 . 91 1 __ SAMPLE SIZE: 249?

Figure 33. Evaporation Duct Summary, San Andres Island,Station 80001.

EUD HT OCCUR 9 5 18 15 29 25 1 ANA AY

----------- ~ ~ ---- SUFAEDU T U89TO 2 5.4210O 4. 5.9 E .SURFACE OHlS: WORLD AVG410O 6. 7.2 U610 8. 8.2 A81010,o 9.2 PAGEDH: 13.10012,Z 19.4 0 AGIDH. 1.

1210O14.x 19.9 R AUG WIND SP: 13.6 ITS:1410O16.P 19.5 A SAMPLE SIZE: 292 SQUARS161018. 9.2 1181029.8 7.5 I UPPER AIR OHS:28022, 5.8 022 10 24 , 4.1 N World Average of 38124 T0 26 . 2.7 Radiosonde Stations2610O28, 1.7 D281039.8 1.9 U3910O32. 9.5 C32 T0 34 . .3 TSOD OCCURRENtCE: 8.0 x3410O36. 6.2 AVG SOD HT: 93610O38, 9. H AVG NSJ8S: 3393810O46, 808 AUG X: 1.45

>49, .1____

Figure 34. Evaporation Duct Height, World Average.

51

The range of the same radar in the presence of an evaporation duct of 16

meters is extended from 10 nmi. to 15 nmi., an increase in coverage from 314

square nmi to 706 square nmi. The increase in area coverage (225 %) and the

relative permanence of the duct (expected 53% of the time at station 80001)

make it an important propagation mechanism. The effects are shown in

Figure 35.

88 FREQ Hz 3008POLARIZATION HORRADR HT m 19TRGT HT m 1s

118 ANT TVPE CSC-SQVER B deg 19ELEU AIG deg 0

140 - ----- . ............. - - ..... EUD HT P 16F'ree-Space Range SBD HT m 9

NSUIBS 339178-( il '"t ADS MU y/03 14.4

" WIND SP kts 19

zoo- Without Evap Duct

FREE SPACE - - - -23 RADAR DETECTION

S I8 28 3 4 58 THRESHOLD -------RANGE To FS RANGE nai 46.5

Figure 35. Extended Coverage Due to Evaporation Duct.

B. EFFECTS ON ESM

Anomalous propagation effects on ESM can be dramatic. Anomalous

propagation can result in signal strength that are orders of magnitude bigger

or smaller than the free-space signal, rendering relative signal strengths useless

52

as a rough distance measure; and the attenuation effects are not readily

observed in the displays due to the passive nature of the system.

Today's expected operating environment is so dense that digital

processing techniques are always used in conjunction with ESM and radar

warning receivers (RWR) to sort and extract the various parameters of a signal.

Sensitive ESM receivers suitable for electronic intelligence gathering (ELINT)

will eventually operate in such a high densities that the processing cannot cope

with the task. The probability of intercept for a given signal will be greatly

reduced.

The saturation of the processing can result from any mechanism that

effectively brings more signals into the receiver. Altitude, in an airborne

receivet, and ducts, for both surface and air borne systems, effectively extend

the radius of coverage for a given sensitivity. Assuming a given number of

emitters per unit area, the number of emitters received will follow the square

of the range covered, resulting in a substantial increase of the density of the

environment. What could be seen as an advantage of getting extended

coverage can very well result in a lower probability of intercept for all signals.

53

VI. CONCLUSIONS

The atmospheric effects on VHF/UHF/Microwave propagation in the

tropical waters surrounding Central America are significant. There is a high

probability of ducting and extended radar horizon is expected most of the

time.

Statistical and historical data are based on the soundings taken by the

WMO station network. These soundings are used for the synoptic scale

weather analysis and are not intended for local refractivity study. Data

collection is seriously flawed when the times, instruments and collection

procedures are not precise and consistent enough to draw conclusions on the

more dynamic refractivity behavior. There is no substitute for real time,

tactically planned soundings and data collection.

The data available for the area are therefore just a sample taken at the

few stations shown and at particular times in which ducting is more probable.

The analysis of the results of refractivity studies using these data should be

done using extreme caution. The overall tropical environment must be

evaluated for consistency. Routine software checks are not enough when

dealing with atmospheric data. Knowledge of the physics involved in the

atmospheric behavior is necessary for military personnel due to the operational

consequences of incorrectly assessing refractive conditions.

54

Further studies are needed to validate the interpretation of the refractive

data available for the area against measurements or case observations. There

are few seasonal variations in the temperature and wind in the tropical area

and therefore correlation with satellite imagery may be useful in determining

the extent of anomalous refractive conditions.

There is the need for increasing the awareness of the military on the

effects of propagation on the systems and tactics to be used, as well as the

particular climatology of the operating theater in Central America. The study

of the environment and its effects on radio propagation should be included in

the undergraduate education and intensified in the graduate education to

provide a wide base of knowledge and understanding of systems capabilities.

The only radiosonde data av4,Jable for the Pacific Ocean comes from the

Howard Air Force Base, Panama Canal Zone, which, as seen in the map in

Figure 8, cannot be very representative of the conditions in Central America's

Pacific Ocean area. Not enough data is available to even characterize the

refractive phenomena that may occur there. Alternative sources such as

satellite imagery and remote sensing of the atmosphere could be used to

obtain the data that current environmental models use.

55

APPENDIX A.

The programs to manipulate the radiosonde data were written in Quick-

Basic(R) version 4.5 and are too big to list, the routines used to obtain

modified refractivity and height and the general subprogram used to identify

and plot ducts, are presented here.

P Pressure in millibarTa Temperature in Kelvine Partial pressure of water vapor in millibarH Height in meters

FUNCTION RefractM (p, Ta, e, H)

M = 77.6 * p / Ta + 3.73 * 10 ^ 5 * e / (Ta * Ta)H / 6.371

RefractM = M

END FUNCTION

Function to find the saturated vapor pressure (millibar)using Lowe's polynomialTemperatures in Celsius

FUNCTION Satvp (Temp)

DEFDBL A

AO = 6.107799961#Al = .4436518521#A2 = .01428945805#A3 = .0002650648471#A4 = .000003031240396#

56

A5 = 2.0340809481D-08A6 = 6.136820928999999D-11

Svp = (((Temp * A6) + A5) * Temp + A4)Svp = ((Svp * Temp + A3) * Temp + A2) * Temp + Al

Svp = Svp * Temp + AOSatvp = Svp

END FUNCTION

HO Initial Height, metersVt0 Virtual Temperature at Initial Height, KelvinP0 Pressure at Initial Height , mbarVtl Virtual Temperature at Actual Height, KelvinP1 Pressure at Actual Height , mbar

FUNCTION Height (H0, VtO, Vtl, P0, P1)Height = HO + 14.63 * (VtO + Vtl) * LOG(PO / Pl)

END FUNCTION

Ta Absolute temperature (Kelvin)P1 Pressure

Ee Vapor pressure environ.

FUNCTION Tv (Ta, P1, Ee)

Tv = Ta + (.3794017# * Ta * Ee) / (P1 - Ee)

END FUNCTION

SUB Process (Alt$, Layer$, Intensity, DuctHeight, NrData%)

Layer$ =

57

Tmp$Intensity = 0LayerHeight = 0

PO = ppp(0)

HO = VAL(Alt$)H(O) =HO

FOR i 0 TO NrData% - 1

Td = TTT(i) - TTD(i)

Ee = Satvp(Td)Ta = AbsoluteTemp(TTT(i))

P1 = ppp(i)Vtl = Tv(Ta, P1, Ee)

IF i = 1 THENvtO = Vtl

END IFH(i) = Height(HO, VtO, Vtl, P0, P1)M(i) = RefractM(P1, Ta, Ee, H(i))

HO = H(i)PO = P1

vtO = Vtl

IF H(i + 1) - H(i) = 0 THEN

Duct$ = II

clMdZ(i) = 0

GOTO OnZeroEND IFclMdZ(i) = (M(i + 1) - M(i)) /(H(i +1) -H(i))

OnZero:IF dMdZ(i) < 0 THEN

SELECT CASE ppp(i)

CASE IS > 700Layer$ = ID

LayerHeight = H (i)IF i = 0 THEN

Layer$ = 'IS"

END IFCASE IS < 700

IF Layer$ = ~"THENLayer$ =Id

LayerHeight = H (i)END IF

58

END SELECTEND IFIF dMdZ(i) < Intensity THEN

Intensity = dMdZ(i)

END IFNEXT i

* 'Plot the Layers found below 700 mbars.

IF Layer$ = I'D" OR Layer$ = 'IS" THENPlot M0, H0, NrData%, Layer$

END IF

END SUB

59

LIST OF REFERENCES

1. Blake, L.V., 1980: Radar Range-Performance Analysis, LexintonBooks.

2. M.Neiburguer, J.G.Edinger and W.D.Bonner, Understanding Our AtmosphericEnvironment, W.H.Freeman and Co., San Francisco.

3. W.,J.,Shaw, Meteorology For EW, Class Notes, 1988.

4. Skolnik M. 1. 1980, Introduction to Radar Systems, McGraw-Hill.

5. Jordan, C.,L., 1958: Mean Soundings for the West Indies area, Journal ofMeteorology, Vol. 15, p.9 1-9 7 .

b. Paulus,R.A. Specification for Evaporation Duct Height Calculations, TechnicalDocument 1596, Naval Ocean Systems Center, 1989.

7. F.R.Williams and others, Forecasters Handbook for Central America and AdjacentWaters, Technical Report TR 89-08, Naval Environmental Prediction ResearchFacility, 1989.

8. USAF Environmental Technical Applications Center, TN-89/003,TheCaribbean Basin - A Climatological Study, K.R.Walters, A.G.Korik andM.J.Vojtesak, 1989.

9. Basic Synoptic N.ktworks of Observing Stations, WMO Secretariat, GenevaSwitzerland, 1981.

10. Patterson,W.,L., and others, Engineer's Refractive Effects Prediction System(EREPS) Revision 2.0, Technical Document 1342, Naval Ocean SystemsCenter, 1990.

11. P.R.,Lowe, "An Approximating Polynomial for the Computation of theSaturation vapor Pressure", Journal of Applied Meteorology, 1977, Vol. 16,p:100-103.

12. Hess,S.L. Introduction to Theoretical Meteorology, Holt, Rinehart and Wiston,New York, 1959.

13. L.N. Ortenburguer, and S.B.Lawson, Radiosonde Data Analysis IV, GTEGovernment Systems Division, 1985.

60

14. Eaves, J.L., 1987, Principles of Modem Radar, Van Nostrand ReinholdCompany, New York.

61

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