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
UNCLASSIFIEDSECURITY CLASSIFICATION OF THIS PAGE
REPORT DOCUMEMlATION PAGEla REPORT SECURITY CLASSIFICATION l b RESTRICTIVE MARKINGSUNCLASSIFIED
2a SECURITY CLASIFICATION AUTHORITY 3 DISTRIBUTION/AVAILABILITY OF REPORT
2b DECLASSIFICATION/DOWNGRADING SCHEDULEAprvdfrpbireas;itiuioi nlme.
4 PERFORMING ORGANIZATION REPORT NUMBER(S) 5 MONITORING ORGANIZATION REPORT NUMBER(S)
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
Ba NAME OF FUNDING/SPONSORING B b OFFICE SYMBOL 9 PROCUREMENT INSTRUMENT IDENTIFICATION NUMBERORGANIZATION j(if applicable)
Bc ADDRESS (City, State, and ZIP Code) 10 SOURCE OF FUNDING NUMBERSProgram Element Nv Prz.,(1 Nc, Tlni No wor, unitAccession
Number
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.
20 DISTRIBUTION/AVAI LABILITY OF ABSTRACT 21 ABSTRACT SECURITY CLASSIFICATION
El NCtASSIFIEOONLIMIE0 E SAME AS REPORI i1ir 0 ,0SrI'ls UNCLASSIFIED
22a NAME OF RESPONSIBLE INDIVIDUAL 22b TELEPHONE (include Area cd)2 2c OFFICE SYMBOLK. L. Davidson (418.646-2309 IMRDD FORM 1473,84 MAR 83 APR edition inay be used until exhausted SECURITY CLASSIFICATION OF THIS PAGE
All other editionms are obsolete UNCLASSIFIED
i
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
INITIAL DISTRIBUTION LIST
No. CopiesDefense Technical Information Center 2Cameron StationAlexandria, VA 22304-6145
2 Library, Code 52 2Naval Postgraduate SchoolMonterey, CA 93943-5000
3 Professor K.L. Davidson, Code MR/DS 5Naval Postgraduate SchoolMonterey, CA 93943-5100
4 Professor F.R. Williams, Code MR/WFNaval Postgraduate SchoolMonterey, CA 93943-5100
5 Roger HelveyGeophysics DivisionPacific Missile Test CenterPoint Mugu, CA 93042
6 Sr. Almirante 3Comandante Armada NacionalComando Armada NacionalC.A.N., Avenida El DoradoBogoti D.E. Colombia
7 Sr. Contralmirante 2Director Escuela Naval Almirante PadillaEscuela Naval, ManzanilloCartagena, Colombia
8 Sr ContralmiranteAgregado NavalEmbassy of ColombiaOffice of the Naval Attache2118 Leroy PlaceWashington D.C. 20008
62
9 Sr. Capitan de FragataDirector C.I.O.H.Escuela Naval, ManzanilloCartagena, Colombia
10 Teniente de Navfo 2Mauricio Gaviria Maldonado.
p Apartado Aereo # 20152Cartagena, Colombia
66