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Signal strength variations at 2 GHz for three sea paths in the British Channel Islands: 1
detailed discussion and propagation modelling 2
3
S.D. Gunashekar1, E.M. Warrington1, D.R. Siddle1 and P. Valtr2 4
5
1 Department of Engineering, University of Leicester, Leicester, LE1 7RH, UK. 6
2 Department of Electromagnetic Field, Czech Technical University in Prague, Technická 2, 166 27 7
Prague 6, Czech Republic 8
9
10
Abstract 11
Signal strength measurements at 2 GHz have recently been made on three over-sea paths in the 12
British Channel This paper focuses on explaining the propagation characteristics during periods of 13
normal reception and periods of enhanced signal strength with particular emphasis on a 48.5 km 14
transhorizon path between Jersey and Alderney path. Evaporation ducting and diffraction appear to 15
be the dominant propagation mechanisms at most times. The influence of the evaporation duct 16
during periods of normal propagation has been confirmed by modelling the over-sea propagation 17
conditions using Paulus-Jeske evaporation duct refractivity profiles as input to the parabolic 18
equation method. During periods of enhanced propagation, which occur approximately 8% of the 19
time on the longest path (48.5 km), the presence of additional higher-level ducting/super-refractive 20
structures has been verified and their influence has been modelled with reasonable success. 21
22
1. Introduction 23
Signal strength measurements at 2 GHz have recently been made on three over-sea paths in the 24
British Channel Islands (see Table 1 for transmitter and receiver locations). A summary of these 25
-1-
measurements are presented in the companion to this paper [Siddle et al., 2007], together with a 26
statistical analysis of the received signal strength variations and a comparison with predicted values 27
made using current ITU-R Recommendations. The antenna heights were such that the ends of the 28
two longest links were beyond the optical horizon, and for the shortest link the ends were within the 29
optical horizon for most of the time. A large tidal range is prevalent in the Channel Islands (up to 30
10 m in Guernsey on a spring tide), and consequently the obscuration due to the bulge of the earth 31
varies significantly within the tidal cycle. 32
In order to correlate the varying signal strengths with different weather processes, meteorological 33
data were obtained from a number of sites around the Channel Islands (see Table 2). Hourly, sea-34
level meteorological data were available from the Channel Light Vessel (CLV) anchored in the 35
English Channel to the northwest of all three radio paths. The distance of the CLV to the midpoint 36
of the Jersey-Alderney link is approximately 70 km, and the nominal height at which observations 37
are made at this station is 5.0 m above mean sea level. Higher altitude weather data were obtained 38
from the airports on Jersey, Alderney and Guernsey with heights of 84.0, 88.7, and 102.0 metres 39
above mean sea level respectively. Data from the Maison St. Louis Observatory in St. Helier, 40
Jersey (54.0 m above mean sea level) and from a privately owned weather station in La petit Val, 41
Alderney (10.7 m above mean sea level) were also employed. 42
This paper focuses on explaining the propagation characteristics during periods of normal reception 43
and periods of enhanced signal strength (ESS) with particular emphasis on the 48.5 km transhorizon 44
Jersey to Alderney path (signal strengths that exceed a threshold calculated assuming free space 45
loss along the path are classified as enhanced signals). 46
2. Signal Strength Variations and the Estimated Evaporation Duct Height 47
The correlation between the computed Paulus-Jeske (P-J) evaporation duct heights [Paulus, 1985] 48
and the corresponding hourly signal strengths measured at the Alderney high antenna is shown in 49
Figure 1 together with the ESS threshold and diffraction threshold (assuming mean antenna heights 50
-2-
above sea level for the upper antennas) calculated assuming standard atmospheric conditions. 51
Hourly measurements of air temperature, sea temperature, relative humidity and wind speed made 52
at the CLV were employed in calculating the duct heights according to the P-J formulation. Ideally, 53
the meteorological measurements would have been made close to the midpoint of the propagation 54
paths, however such data were not available and the CLV was the closest available source. Since 55
the CLV was somewhat displaced from the paths of interest, horizontal homogeneity was (by 56
necessity) assumed. 57
To illustrate the effect of the tide, the data have been divided into four parts: cases when the tide 58
height (assumed to be the average of the heights at Jersey and Alderney) between the transmitter 59
and receiver is less than -2 m relative to its mean value, cases when the tide height lies between -60
2 m and 0 m, cases when the tide height lies between 0 m and 2 m, and cases when the tide height 61
between the transmitter and receiver exceeds 2 m. Best-fit lines for each tidal range are also 62
indicated in the figure. The majority of the data lie between the free space and diffraction threshold 63
values indicating that the evaporation duct is able to increase the received signal strength at 64
Alderney well beyond the diffraction level. However, the enhancement in signal strength provided 65
by the evaporation duct is not sufficient to exceed the free space threshold. Additionally, the 66
distribution of data corroborates the observation made in the companion paper [Siddle et al., 2007] 67
that during periods of normal reception, stronger signals are received when tide heights are low and 68
vice versa. 69
At most times during periods of non-ESS, the measured signal strengths increase with duct height, 70
an observation consistent with reports made by various authors [SPAWAR, 2004; Hitney et al., 71
1985; Hitney and Veith, 1990]. Considering only the non-ESS data, the signal strength at the 72
Alderney high antenna increases at the rate of 0.61 dB per metre increase in duct height. For the 73
Guernsey and Sark high antennas, the corresponding values are 0.59 dB/m and 0.25 dB/m 74
respectively. 75
-3-
For the cases of ESS, there is no definite correlation between measured signal strength and 76
calculated evaporation duct height, suggesting either the existence of additional propagation 77
mechanism(s) during these periods, or that under these conditions the estimate of the duct height is 78
incorrect. The inverse relationship between tide height and signal strength is no longer evident, and 79
in general the calculated evaporation duct heights during periods of ESS appear to be less than the 80
duct heights during periods of normal reception (for all valid data, the mean of the P-J evaporation 81
duct height is 8.3 m, reducing to 6.0 m for times when ESS signals are observed at Alderney). 82
It is important to note that the P-J method of estimating evaporation duct heights is an open ocean 83
model [Paulus, 1985; Hitney and Veith, 1990; Babin et al, 1997] that works reasonably well for 84
conditions of atmospheric instability (mostly prevalent in the open ocean) where the air is colder 85
than the sea. During stable periods, when the air temperature exceeds the sea temperature, the P-J 86
method incorporates a temperature correction (on the assumption that an error has been made 87
during measurement) that results in an under-estimation of the evaporation duct height [Paulus, 88
1985]. Whilst it may be true that stable conditions are uncommon in the open ocean [Paulus, 1985; 89
Babin et al, 1997], it is likely that these will occur more often in coastal regions that are particularly 90
prone to land-induced effects such as advection of warm air over a cooler sea surface. This is 91
another reason for the departure from the general trend of higher duct heights corresponding to 92
higher signal strengths during periods of ESS (Figure 1), as these occur primarily when stable 93
atmospheric conditions are prevalent. 94
3. Modeling Periods of Normal Reception with the Parabolic Wave Equation Method 95
With the advent of powerful computers, the computationally intensive parabolic equation (PE) 96
method [Dockery, 1988; Craig and Levy, 1991; Barrios, 1994; Levy, 2000] has become an efficient 97
and practical tool for tropospheric radiowave propagation calculations (see, for example, studies of 98
the effects of tropospheric ducting on the performance of UHF radio links presented by Slingsby 99
[2001] and Sirkova and Mikhalev [2004]). In this section, the propagation conditions during 100
-4-
periods of normal reception in the Channel Islands have been modelled using the PE method. In 101
particular, the split-step parabolic wave equation [Dockery, 1988; Kuttler and Dockery, 1991] that 102
implements impedance-boundary conditions [Dockery and Kuttler, 1996] was utilised for field 103
strength calculations. Predictions for short periods of time (in summer and winter) were also made 104
using the radiowave propagation assessment tool, AREPS [SPAWAR, 2004] that makes use of a 105
hybrid model incorporating the split-step PE method as a sub-model [SPAWAR, 2004]. The results 106
for a few weeks of test cases indicate that the propagation loss values calculated with the PE 107
method and with AREPS are within 1-2 dB of each other. 108
Modified refractivity profiles based on the Paulus-Jeske method [Paulus, 1985] were generated for 109
each hourly reading and utilised as inputs to the PE model. A typical modified refractivity-height 110
profile (for 7 December 2003 at 18:00 UT) illustrating the presence of an evaporation duct is shown 111
in Figure 2 (left frame). For this particular case (air temperature: 7.8°C, sea temperature: 12.9°C, 112
dew point temperature: 2.6°C and wind speed: 14.3 m/s), the evaporation duct height is 14.7 m 113
while the transmitter and receiver heights above sea level are 13.8 m and 11.1 m respectively. For 114
the purpose of illustration, also shown in Figure 2 (right frame) is the height vs. range ray-trace plot 115
for the evaporation duct profile and transmitter specified above (produced in AREPS [SPAWAR, 116
2004]). Trapping of some of the direct and reflected rays between the earth’s surface and the top of 117
the evaporation duct at 14.7 m is evident and consequently propagation occurs for extended ranges 118
within the trapping layer. There is good agreement between the measured signal strength of 119
-86.6 dBm and the predicted signal strength of -88.1 dBm. 120
3.1 Illustrative Examples 121
During a distinctive cold weather period (4-10 December 2003) when normal reception occurs, 122
there is very good agreement between the measured and the PE-predicted signal strengths at the 123
Alderney high antenna (Figure 3). This behaviour was also apparent for the Guernsey and Sark 124
measurements, and for both the high and low antennas. In contrast, for a typical period of signal 125
-5-
enhancement during late summer (12-18 September 2003), there is little correlation between the 126
observations and the PE-predicted values (Figure 4). The predicted signal strengths in this case 127
simply indicate the regular oscillation in received power caused by the tides. 128
3.2 Analysis with Complete Signal Strength Data Set 129
A scatter plot of the measured signal strengths and the PE-predicted signal strengths for all the valid 130
hourly data at the Alderney high antenna is shown in Figure 5 (left frame). The overall correlation 131
for these of data is poor (correlation coefficient = 0.17), however there appears to be a definite 132
correlation between the observed and predicted signal strengths particularly for cases of normal 133
reception. A clearer depiction of this correlation can be seen in the right frame of Figure 5 in which 134
all cases of enhanced signal strength have been removed. The correlation coefficient for these data 135
is 0.45. 136
Every tropospheric duct has a maximum wavelength that it can support, depending upon the 137
geometry and the change in refractivity across the duct. The maximum cut-off wavelength, λmax, 138
provides a general indication of the radio-wave trapping capability of a duct, and is given in 139
Equation 1 [Turton et al, 1988; Brooks et al, 1999]. 140
λmax
= 23
k t δM (1) 141
where t is the duct thickness (m), δM the modified refractivity change across the duct (M-units), 142
and k = 3.77 x 10-3 for a surface-based duct or 5.66 x 10-3 for an elevated duct. 143
It is noteworthy that when only those cases of non-enhanced signal strengths are used in which the 144
corresponding evaporation duct cut-off wavelengths exceed 15 cm, the correlation coefficient 145
between the PE-predicted and measured signal strengths at the Alderney upper antenna increases to 146
0.66. 147
-6-
Thus, even though the cut-off wavelength is simply a rough indication of the trapping capability of 148
an evaporation duct, it can still be used to show that when the likelihood of 2 GHz radio waves 149
getting trapped within a duct is maximized, the evaporation duct does becomes the dominant 150
propagation phenomenon. (For a detailed discussion of the concept of maximum cut-off wavelength 151
for evaporation ducts, the reader is directed to the works of Hall [1979] and Turton et al [1988].) 152
Finally, it is also interesting to note that when the Paulus-Jeske evaporation profiles are used in the 153
PE model, none of the predicted signal strengths exceed the value of the free space threshold. 154
Further evidence of the correspondence between the measured and PE-predicted signal strengths 155
can be obtained from Figures 6 and 7. The cumulative frequency distribution curves for three sets of 156
data with reference to the Alderney high antenna are shown in Figure 6. These data sets are (a) all 157
measured signal strength, (b) only non-enhanced measured signal strength and (c) PE-predicted 158
signal strength (using P-J evaporation duct profiles). The mean hourly signal strengths for the two 159
years of data (for the same three signal strength data sets) are presented in Figure 7. In both figures, 160
the change in shape of the distributions for that of all measured data and for just non-enhanced 161
signal strength data is very significant. The PE-predicted signals provide a much-improved estimate 162
of the measured signal strengths during periods of normal reception. 163
Given that only the effect of the evaporation duct has been accounted for in these cases, this 164
suggests that (a) the evaporation duct is responsible for propagation during periods of normal 165
reception (i.e. cold weather periods) and (b) the evaporation duct refractivity profiles assumed 166
within the PE predictions during periods of enhanced reception are insufficient to model the 167
propagation, at least as it impacts on our paths/antenna heights. The latter conclusion points towards 168
the existence of propagation mechanism(s) other than the evaporation duct which are responsible 169
for the occurrence of enhanced signal strengths and that are not being taken into account in the 170
prediction scheme. 171
-7-
4. Explanation of Enhancements Using Meteorological Data from Higher Levels in the 172
Troposphere 173
4.1 Estimation of Refractivity Lapse Rate 174
Hourly weather data from the meteorological stations listed in Table 2 were closely analysed in 175
order to corroborate the existence of higher-level ducting/super-refractive structures during periods 176
of enhanced signal strength. The refractivity lapse rate, dN/dh (in N-units/km), in approximately the 177
first 100 m of the troposphere was estimated for two years of data. This was achieved by finding the 178
slope of the best-fit line through points on the refractivity vs. height plot for hourly data from the 179
various sites noted above. The mean refractivity gradient was calculated to be approximately 180
-71 N-units/km, showing that on average the conditions in the lowest part of troposphere are very 181
close to being super-refractive. It should be noted that a number of conclusions that are arrived at in 182
this section are based on estimations of the refractivity at different locations. Ideally, co-located 183
refractivity measurements at different altitudes midway between the transmitter and receiver path 184
are required. 185
Monthly curves of the mean value of dN/dh between the earth’s surface and a height of 1 km 186
derived from historical radiosonde data are presented in ITU-R Recommendation P.453 [ITU-R, 187
2003]. For the region around the English Channel, this gradient varies between -40 and -50 N-units 188
in the 1 km layer. The departure from these standard values of dN/dh is to be expected since we are 189
dealing with the lowest 100 m of the troposphere in a marine environment. Further statistics in 190
ITU-R Recommendation P.453 indicate that the refractivity gradient in the lowest 100 m above the 191
surface of the earth is less than -100 N-units/km for small percentages of time. Additionally, more 192
recent data extracted from ITU-R databases [ITU-R, 2003] indicates that the refractivity gradient 193
exceeded for 50% of the time in the lowest 65 m of the region is about -55 N-units/km. 194
Of 8340 valid Alderney high antenna signal strength and dN/dh data, 730 (8.8%) correspond to 195
cases of enhanced signal strength. The occurrence statistics of the four types of refractive conditions 196
-8-
(ducting, super-refraction, normal and sub-refraction) and the corresponding percentages of 197
occurrence of enhanced signal strength at the Alderney high antenna are listed in Table 3. The most 198
important result that may be derived from this table is that 664 out of 730 (91%) cases of signal 199
strength enhancements occur during ducting or super-refractive atmospheric conditions, thus 200
underlining the significance of these non-standard modes of propagation in the context of long-201
range UHF propagation. Also, despite the fact that ducting or super-refraction occurs almost 40% 202
of the time, ESS events are recorded only 8.8% of the time. This would suggest that although 203
ducting and super-refraction are primarily responsible for the occurrence of enhanced signal 204
strengths on transhorizon over-sea paths, they do not necessarily always result in ESS (though the 205
likelihood of ESS reception increases). Nevertheless, these anomalous effects do allow radio signals 206
(enhanced or non-enhanced) to reach distant receivers that under normal atmospheric conditions 207
would not propagate beyond the horizon. 208
As expected, there are very few cases of enhanced signal strengths during periods of sub-refraction. 209
Furthermore, as Figure 8 illustrates, practically all the ducting events occur in the spring and 210
summer months. Thus, by simply utilising the long-term refractivity lapse rate as an indicator, we 211
can get a reasonably clear verification of the different atmospheric conditions encountered in the 212
lowest region of the troposphere during long-range UHF propagation over the sea. 213
4.2 Identification of Potential Higher-Level Trapping Layers in the Troposphere 214
During the spring and summer months, the sea temperature at the CLV is lower than the air 215
temperatures measured at all sites including the CLV, indicative of a stable atmosphere. This 216
confirms that a stable atmosphere correlates well with the occurrence of enhanced signals, and the 217
extent of the stability is not just restricted to the lowest few metres above the surface of the sea. In 218
contrast, during autumn and winter, the average sea temperature well exceeds all the air temperature 219
readings, indicating a highly unstable atmosphere during these periods. 220
-9-
In addition, there are also inversions in modified refractivity taking place (that is, a decrease in M 221
with height instead of the normal increase in M). Inversions in modified refractivity are an 222
indication of potential ducting layers [Hitney et al., 1985]. In particular, these inversions appear to 223
occur between the heights of the Alderney and Guernsey airports during the spring and summer 224
months. 225
In order to identify the reasons for these M-inversions, the monthly occurrence frequency of 226
temperature inversions between Alderney (88.7 m) and Guernsey (102.0 m) and the monthly 227
average of the relative humidity difference between these two heights are shown in the top and 228
bottom frames, respectively of Figure 9. The former parameter has been quantified by determining 229
the rate of incidence of the temperature at the Guernsey Airport altitude exceeding the temperature 230
at the Alderney Airport altitude by more than 1ºC. Under normal circumstances, air temperature and 231
water vapour pressure in the troposphere decrease with altitude. However, a temperature inversion 232
and/or rapid lapse rates in the water vapour pressure between two layers of air can result in the 233
occurrence of very high refractivity lapse rates (i.e. dN/dh ≤ -157 N-units/km or dM/dh ≤ 0 234
M-units/km). Together, or in isolation, these two effects will result in the occurrence of 235
tropospheric ducting layers. 236
A definite seasonal pattern is evident from both plots. The occurrence frequency of temperature 237
inversions taking place between the heights of the Alderney and Guernsey airports increases 238
substantially during summer and spring (March to August) while reaching a minimum in the 239
autumn and winter months (September to February). The difference in relative humidity also rises 240
during the spring and summer months, indicating a faster-than-normal RH lapse rate between the 241
heights of 88.7 m and 102.0 m. Thus, the plots verify that the two key processes that result in 242
ducting in the troposphere (manifested in M-inversions) are taking place. 243
The monthly percentage occurrence of strong M-inversions between the altitudes of the Alderney 244
and Guernsey airports are presented in the bottom frame of Figure 10; that is, the occurrence 245
-10-
frequency of MGuernsey - MAlderney being less than -5 M-units. This translates to an equivalent 246
refractivity lapse rate of approximately -533 N-units/km, indicating extreme ducting conditions. For 247
comparison, the monthly occurrence frequencies of ESS cases on the Jersey-Alderney radio path 248
(both high and low antennas) are also shown in the top frame of Figure 10. Clearly, both plots 249
follow very similar seasonal patterns with the respective occurrence percentages reaching 250
comparable values. 251
As mentioned previously, during the spring and summer months there is a definite change in the 252
physical properties of the air at higher altitudes relative to that at the surface. The CLV M (5.0 m 253
above mean sea level) is normally well below the value of M at the Jersey Airport (i.e. 84.0 m 254
above mean sea level). Figure 11 shows the number of monthly occurrences of M-inversions 255
between the surface (i.e. the CLV) and the altitude of the Jersey Airport. Specifically, the graph 256
illustrates the number of cases per month when the surface modified refractivity exceeds the 257
modified refractivity at an altitude of 84.0 m. Over the two years of measurement, it is estimated 258
that there are 937 cases of such inversions in modified refractivity, of which approximately 40% 259
coincide with the occurrence of enhanced signal strengths at Alderney. The monthly variation in 260
this figure is very similar to the trend exhibited by the monthly ESS occurrence curve presented in 261
top frame of Figure 10, peaking predominantly in the spring and summer months. Almost 61% of 262
these cases of M-inversions occur in the spring and summer periods. If we include September 2003 263
– a month in which a relatively large number of enhanced signals were recorded – the latter figure 264
increases to 83%. A strong correlation therefore exists between the occurrence of ESS signals and 265
very high lapse rates of refractivity taking place aloft in the troposphere throughout the spring/ 266
summer months. 267
Finally, the hourly occurrence frequency of potential trapping layers between the heights of the 268
Guernsey and Alderney airports during periods of enhanced signal strength is depicted in the 269
bottom frame of Figure 12. Once again, for comparison, the diurnal variations in the occurrence of 270
enhanced signals at the Alderney high and low antennas are shown in the top frame (Figure 12). As 271
-11-
with the signal strength, the hourly occurrence frequency of higher-level M-inversions follows the 272
same diurnal trend, with approximately 40% of the inversions occurring between 1500 UT and 273
2000 UT, and comparatively fewer existing in the morning. 274
In the foregoing analysis, due to the lack of meteorological data above an altitude of approximately 275
100 m, the upper limit of these potential ducting layers cannot be specified. Nevertheless, the 276
exceptionally high refractivity lapse rates (resulting in M-inversions) caused by temperature 277
inversions and rapid RH lapse rates between approximately 85.0 m and 100.0 m, provide definitive 278
evidence of the existence of higher-level ducting structures. These higher-altitude ducting layers 279
are most likely resulting in the occurrence of enhanced signal strengths on over-sea UHF paths, 280
primarily during the warm spring and summer periods. 281
4.3 Modeling Periods of Enhanced Signal Strength with the Parabolic Wave Equation Method 282
It has been shown earlier that during periods of normal reception, when the low-level evaporation 283
duct profile was used as input to the parabolic wave equation model, an excellent correlation was 284
achieved between the PE-predicted and measured signal strengths. During periods of enhanced 285
signal strength however, the PE-predictions using the evaporation duct refractivity profile were 286
relatively inaccurate, providing an indication that certain additional higher-level tropospheric 287
phenomena are more dominant at these times. The two preceding sections have focussed on the 288
identification and characterisation of these higher-level ducting/super-refractive layers by utilising 289
refractivity data at different altitudes from nearby weather stations. In this section, an attempt has 290
been made to model the propagation effects during periods of signal strength enhancement, using 291
the limited higher-level refractivity data available to us. 292
Based on the results that have been presented so far indicating the existence of higher-level ducting 293
layers in the troposphere, and in the absence of more detailed meteorological data, refractivity 294
measurements from the various weather stations (listed in Table 2) were combined to provide an 295
-12-
atmospheric profile for the first 100 m to input to the parabolic equation model and AREPS 296
[SPAWAR, 2004]. 297
Figure 13 presents a comparative plot of the PE-predicted signal strength and the measured signal 298
strength at the Alderney high antenna for the same period of enhanced signal strength (12-299
18 September 2003) that was presented in Figure 4. In the case of the higher-level refractivity data 300
simulations (Figure 13), we observe that there is a much better correlation between the measured 301
and predicted signal strengths. Thus, for periods of enhanced signal strength, the correlation 302
between measurements and predictions is better when a higher-level refractivity profile is used than 303
when the low-level evaporation duct profile is used; whereas for periods of normal propagation, the 304
evaporation duct model provides a better correlation. 305
In conclusion it may be said that a seemingly basic scheme that involves the use of refractivity 306
measurements at different altitudes, from sea level up to approximately 100 m, has been applied to 307
the PE-model and AREPS to produce a signal strength profile that agrees reasonably well with the 308
experimental signal strength during phases of enhanced reception. This result provides confirmation 309
of the existence of higher layer ducting stratifications that become dominant (over the low-lying 310
evaporation duct) during periods of ESS propagation over the sea. 311
4.4 Analysis of Upper-Air Radiosonde Data from Nearby Stations 312
Historical as well as current data from nearby radiosonde stations were closely analysed to 313
corroborate the existence of higher-level super-refractive and ducting structures in the English 314
Channel region, particularly when signal strength enhancements are observed at Alderney, 315
Guernsey and Sark. 316
Historical upper-air climatology (contained for example in the AREPS database [SPAWAR, 2004]) 317
from nearby radiosonde stations indicate that surface-based ducts and elevated ducts occur 318
reasonably frequently in the region. Camborne (50.22° N, 5.32° W, altitude: 87 m above mean sea 319
-13-
level) and Brest/Guipavas (48.45° N, 4.42° W, altitude: 103 m above mean sea level) are two such 320
coastal stations in the vicinity of the radio paths in the Channel Islands. In particular, it was noted 321
that surface-based ducts occur more frequently in the months of May to September with less 322
ducting taking place in the autumn and winter months. This occurrence trend of surface-based ducts 323
in this region agrees well with the seasonal pattern of enhanced signal strength incidence along the 324
Channel Island radio links under consideration. Furthermore, it is also interesting to note that the 325
average height of the trapping layers producing the surface-based ducts at Camborne are reasonably 326
close to the approximate height at which trapping layers (caused by temperature inversions and 327
rapid humidity lapse rates) were observed in the upper-air data from various sources in the Channel 328
Islands. 329
High-resolution radiosonde data from two nearby stations were closely analysed for two typical 330
months of normal reception (December 2003) and enhanced signal reception (May 2004). Since 331
there are no radiosonde launch-sites located in the Channel Islands, the closest locations from which 332
high-resolution radiosonde data are available to us are Herstmonceux (50.90° N, 0.32° W, altitude: 333
52 m above mean sea level) and Camborne (50.22° N, 5.32° W, altitude: 87 m above mean sea 334
level), both located very close to the southern coast of UK. Measurements are recorded at 2-second 335
intervals, twice a day (at 1100 UT and 2300 UT) and were obtained from the British Atmospheric 336
Data Centre. 337
The air temperature, pressure and relative humidity (obtained from the air and dew point 338
temperatures) radiosonde measurements from Herstmonceux and Camborne were utilised to 339
produce corresponding values of modified refractivity, M. In order to be sure about the upper 340
extent of these potential ducting structures, weather data was analysed up to approximately twenty 341
height readings. Depending on the case being examined, this roughly corresponds to a maximum 342
altitude of 230-270 m for Herstmonceux and 260-300 m for Camborne. 343
-14-
During December 2003 (when there are no cases of ESS), examination of the radiosonde data 344
reveals that practically all the valid cases have monotonically increasing values of M from the 345
surface value. Very few inversions in modified refractivity are observed, and if at all, are limited to 346
the first two readings (i.e. up to a maximum of 60-75 m for Herstmonceux and 90-100 m for 347
Camborne). There are practically no significant temperature inversions taking place aloft. 348
Hourly ESSs occur at the Alderney high antenna 42% of the time in May 2004. During this month, 349
inspection of the modified refractivity height profiles reveals that there are many more inversions in 350
M compared to December 2003. Furthermore, most of these inversions are accompanied by 351
temperature inversions at the same altitude. Some of the times, a rapid decrease in the relative 352
humidity is also observed. This suggests a correlation between the existence of higher-level 353
trapping layers in the troposphere and the occurrence of ESS events along the over-sea radio links 354
under consideration. 355
It should be noted that despite providing reasonable evidence in support of the existence of higher-356
level trapping and super-refractive structures, the results from the analysis of the high-resolution 357
radiosonde data and the historical upper-air climatology data should be treated with caution: the 358
data that have been studied are from coastal stations that are located some distance away from the 359
over-sea radio paths being investigated; furthermore, since the data are available only twice a day, 360
tangible conclusions about the temporal scope of these higher-level structures cannot be made. 361
Nevertheless, in the absence of more accurate meteorological data, close examination of high-362
resolution radiosonde data from nearby stations does provide some indication of the strong 363
correlation between the occurrence of enhanced signal strengths and the presence of upper-air 364
super-refractive/ducting structures. Furthermore, the higher-level trapping layers are observed 365
reasonably concurrently at different locations around the English Channel region, which strongly 366
indicates (along with the fact that ESSs are observed concurrently at Alderney, Guernsey and Sark) 367
that these are a widespread phenomenon occurring over a large area. 368
-15-
4.5 Analysis of Synoptic Charts 369
Areas of high pressure are often associated with anticyclonic weather [McIntosh and Thom, 1973; 370
McIlveen, 1986] that, in general, are characterised by settled weather and light wind conditions, 371
both of which have been observed in the context of ESS occurrences in the Channel Islands. Dry 372
air from the upper troposphere descends and is heated, sometimes producing an inversion of 373
temperature. Furthermore, anticyclones usually extend over large regions and are slow-moving 374
phenomena. In order to further investigate this, synoptic charts of the region (acquired from the UK 375
Met Office) were closely analysed to identify any distinctive meteorological processes occurring 376
during periods of sustained ESS events. 377
Of the 119 days on which ESS occur, 50 cases of high-pressure centres were noted to be present 378
directly over the English Channel region, and 41 of these 50 events correspond to days on which 379
ESS cases occurred for four hours or longer. Additionally, it was observed that there are 41 days on 380
which high-pressure centres exist over nearby regions in Europe and in the Atlantic. Thus, of all 381
the days on which ESS occurrences are recorded at Alderney, approximately 91 correspond to days 382
(77%) on which high-pressure cells are observed either directly over or close to the Channel Islands 383
region. It is worth mentioning that the presence of high-pressure cells in the region does not always 384
result in the enhancement of signals. In some cases, there is simply a marginal increase in the 385
received power (but not above the free space threshold), while at other times, the anticyclonic 386
weather does not seem to affect the signal at all. 387
It is evident that anticyclonic weather systems (occurring predominantly in the spring and summer 388
months) are a major contributing factor to the occurrence of enhanced signal strengths on over-sea 389
radio paths in the English Channel. It is most likely that the process of subsidence and 390
accompanying advection associated with anticyclones is resulting in the creation of a layer of air at 391
low altitudes within which an inversion in temperature and a strong humidity gradient exists. 392
-16-
Historical data shows that advection ducts frequently form over the English Channel during the 393
summer [Bean and Dutton, 1966]. 394
5. Concluding Remarks 395
This paper describes a series of long-term UHF propagation measurements carried out over three 396
completely over-sea paths in the English Channel ranging from 21.0 km to 48.5 km in length. The 397
measurements and accompanying statistical analyses that have been presented both here and in the 398
companion paper [Siddle et al., 2007] provide a useful addition to the limited statistics related to the 399
low-level propagation of 2 GHz radio waves over long-range sea paths in temperate regions. 400
Evaporation ducting and diffraction appear to be the dominant propagation mechanisms at most 401
times. The influence of the evaporation duct during periods of normal propagation has been 402
confirmed by modelling the over-sea propagation conditions using Paulus-Jeske evaporation duct 403
refractivity profiles (generated using sea surface weather data) as input to the parabolic equation 404
method. 405
Signal strength enhancements have been observed on all three paths subject to investigation, 406
primarily in the late afternoon and evening periods, in the spring and summer months. During 407
periods of enhanced propagation, which occur approximately 8% of the time on the longest path 408
(48.5 km), the presence of additional higher-level ducting/super-refractive structures has been 409
verified and their influence has been modelled with reasonable success. These structures have been 410
characterised by identifying regions of inversions in the estimated modified refractivity profiles and 411
have been shown to be caused by strong lapses of humidity and/or temperature inversions aloft. The 412
higher-level ducting/super-refractive structures follow similar diurnal and seasonal trends as the 413
occurrence of ESS. Finally, analysis of both current and historical data from nearby radiosonde 414
stations also point towards the existence of higher-level trapping structures at comparable altitudes 415
in the region. 416
-17-
Acknowledgements 417
The authors are grateful to Ofcom (formerly the Radiocommunications Agency) for their support of 418
this work, and to Mr. Jon Kay-Mouat (Alderney), St. Peter Port Harbour Authority (Guernsey), 419
Ronez Quarry (Jersey) and Mr. Simon de Carteret (Sark) without whose help, cooperation and 420
agreement it would have been impossible for the measurements to have been made. Additionally, 421
the authors wish to thank Mr. Tim Lillington (Guernsey Airport Meteorological Observatory), Mr. 422
Anthony Pallot (Jersey Airport Meteorological Department) and Mr. Brian Bonnard (Alderney) for 423
providing meteorological data and weather information from the Channel Islands, and Mr. Wayne 424
Patterson (SPAWAR, USA), for his help in using the AREPS software. 425
References 426
Babin, S.M., Young, G.S., and Carton, J.A. (1997), A New Model of the Oceanic Evaporation 427
Duct, Journal of Applied Meteorology, 36(3), 193-204. 428
Barrios, A.E. (1994), A Terrain Parabolic Equation Model for Propagation in the Troposphere, 429
IEEE Transactions on Antennas and Propagation, 42(1), 90-98. 430
Bean, B.R., and E.J. Dutton (1966), Radio Meteorology, U.S. Department of Commerce, National 431
Bureau of Standards Monograph 92. 432
Craig, K.H., and M.F. Levy (1991), Parabolic equation modelling of the effects of multipath and 433
ducting on radar systems, IEE Proceedings – Part F, 138(2), 153-162. 434
Dockery, G.D. (1988), Modeling Electromagnetic Wave Propagation in the Troposphere Using the 435
Parabolic Equation, IEEE Transactions on Antennas and Propagation, 36(10), 1464-1470. 436
Dockery, G.D. and J. R. Kuttler (1996), An Improved-Boundary Algorithm for Fourier Split-Step 437
Solutions of the Parabolic Wave Equation, IEEE Transactions on Antennas and Propagation, 438
44(12), 1592-1599. 439
Hall, M.P.M. (1979), Effects of the Troposphere on Radio Communication, Institution of Electrical 440
Engineers. 441
-18-
Hitney, H.V., J.H. Richter, R.A. Pappert, K.D. Anderson, and G.B. Baumgartner Jr. (1985), 442
Tropospheric Radio Propagation Assessment, Proceedings of the IEEE, 73(2), 265-283. 443
Hitney, H.V., and R. Veith (1990), Statistical Assessment of Evaporation Duct Propagation, IEEE 444
Transactions on Antennas and Propagation, 38(6), 794-799. 445
ITU-R (2003), ITU-R Recommendation P.453, The radio refractive index: its formula and 446
refractivity data, International Telecommunication Union. 447
Kuttler, J.R., and G.D. Dockery (1991), Theoretical description of the parabolic 448
approximation / Fourier split-step method of representing electromagnetic propagation in the 449
troposphere, Radio Science, 26(2), 381-393. 450
Levy, M.F. (2000), Parabolic Equation Methods for Electromagnetic Wave Propagation, IEE 451
Electromagnetic Wave Series 45. 452
McIlveen. J.F.R. (1986), Basic Meteorology – a physical outline, Van Nostrand Reinhold (UK) Co. 453
Ltd. 454
McIntosh, D.H., and A.S. Thom (1973), Essentials of Meteorology, Wykeham Publications 455
(London) Ltd. 456
Paulus, R.A. (1985), Practical Application of an Evaporation Duct Model, Radio Science, 20(4), 457
887-896. 458
Siddle, D.R., E.M. Warrington and S.D. Gunashekar (2007), Signal strength variations at 2 GHz for 459
three sea paths in the British Channel Islands: observations and statistical analysis, Radio 460
Science (ibid). 461
Sirkova, I. and M. Mikhalev (2004), Parabolic-Equation-Based Study of Ducting Effects on 462
Microwave Propagation, Microwave and Optical Technology Letters, 42(5), 390-394. 463
Slingsby, P.L. (1991), Modelling Tropospheric Ducting Effects on VHF/UHF Propagation, IEEE 464
Transactions on Broadcasting, 37(2), 25-34. 465
-19-
Space and Naval Warfare Systems Command (SPAWAR) (2004), Atmospheric Propagation 466
Branch, San Diego, U.S.A., User’s Manual (UM) for Advanced Refractive Effects Prediction 467
System (AREPS). 468
Turton, J.D., D.A. Bennetts and S.F.G. Farmer (1988), “An introduction to radio ducting,” 469
Meteorological Magazine, 117, pp. 245-254. 470
471 472
-20-
472
Table 1: Geographical positions and altitudes (above mean sea level) of the transmitting and 473
receiving antennas. 474 475 476
477
478
479
480
481
482
483
484
Table 2: Geographical positions and altitudes of weather stations in the Channel Islands 485
Latitude
Longitude
Altitude above
mean sea level
Channel Light Vessel 49° 54’ N 02° 54’ W 5.0 m
La Petit Val, Alderney 49° 43’ N 02° 13’ W 10.7 m
Maison St. Louis Observatory,
St. Helier, Jersey
49° 12’ N 02° 06’ W 54.0 m
Jersey Airport 49° 13’ N 02° 12’ W 84.0 m
Alderney Airport 49° 42’ N 02° 13’ W 88.7 m
Guernsey Airport 49° 26’ N 02° 36’ W 102.0 m
486
Table 3: Occurrence statistics of the four major types of refractive conditions and the 487 corresponding percentages of occurrence of enhanced signal strength at the 488 Alderney high antenna (August 2003 to August 2005) 489
Jersey (Transmitter)
Alderney (Receiver)
Guernsey (Receiver)
Sark (Receiver)
Latitude
49° 16’ N
49° 43’ N
49° 27’ N
49° 26’ N
Longitude
02° 10’ W
02° 10’ W
02° 31’ W
02° 21’ W High antenna
17.5 m
13.0 m
14.0 m
13.0 m
Low antenna
14.5 m
10.0 m
10.0 m
10.0 m
Atmospheric
condition
Refractivity gradient, dN/dh (N-units/km)
Modified refractivity gradient, dM/dh (M-units/km)
Number of occurrences
Number of corresponding occurrences of ESS at the
Alderney high antenna
Ducting/Trapping dN/dh ≤ -157 dM/dh ≤ 0 734 391 (53.6%)
Super-refraction -79 ≥ dN/dh > -157 78 ≥ dM/dh > 0 2565 273 (37.4%)
Normal 0 ≥ dN/dh > -79 157 ≥ dM/dh > 78 4602 59 (8.1%)
Sub-refraction dN/dh > 0 dM/dh > 157 439 7 (0.9%)
Total 8340 730 (8.8%)
-21-
490
491 492 493 494 495 496 497 498 499 500 501
502
Figure 1: Scatter plot of the Paulus-Jeske evaporation duct heights and the measured signal strengths at the Alderney high antenna with the data characterised according to four distinct tidal ranges
Mea
sure
d si
gnal
stre
ngth
at t
he A
lder
ney
high
ant
enna
(dB
m)
P-J evaporation duct height (m)
-22-
502 503
504
Figure 2: Sample modified refractivity vs. height profile for an evaporation duct (Paulus-Jeske) on 07 December 2003 at 18:00 UT (using weather data from the Channel Light Vessel) (left frame) and the corresponding height vs. range ray-trace plot with the transmitter placed at 13.8 m (right frame)
Evaporation duct height = 14.7 mTransmitter height = 13.8 m
-23-
504 505
506
Figure 3: Comparison between the hourly measured signal strength and the PE-predicted signal strength (using P-J evaporation duct profiles) at the Alderney high antenna during a period of normal reception in winter (4-10 December 2003)
Free space threshold
Hig
h a
nte
nn
a si
gnal
str
engt
h a
t A
lder
ney
(dB
m)
-24-
506
Figure 4: Comparison between the hourly measured signal strength and the PE-predicted signal strength (using the P-J evaporation duct profile) at the Alderney high antenna during a period of signal enhancement in summer (12-18 September 2003)
Hig
h a
nte
nn
a si
gnal
str
engt
h a
t A
lder
ney
(dB
m)
Free space threshold
-25-
506 507
508
Figure 5: Scatter plots showing the correlation between the measured signal strength and the predicted signal strength (using the PE model with P-J evaporation duct profiles as input) for all data (left frame) and for non-ESS data only (right frame)
-26-
508 509 510
511
Figure 7: Graph depicting the hourly means for three sets of signal strength data at Alderney (high antenna): (a) all measured signals (b) only non-enhanced measured signals and (c) PE-predicted signals (using P-J evaporation duct profiles)
Figure 6: Cumulative frequency distribution curves for three sets of signal strength data at Alderney (high antenna): (a) all measured signals (b) only non-enhanced measured signals and (c) PE-predicted signals (using P-J evaporation duct profiles)
Perc
enta
ge e
xcee
ding
sig
nal s
tren
gth
(%)
-27-
511 512 513
514
Figure 8: Graph illustrating the seasonal distribution of ducting events in the Channel Islands (i.e. dN/dh ≤ -157 N-units/km), using refractivity data from nearby meteorological stations
dN/d
h (N
-uni
ts/k
m)
-28-
514 515
516
Figure 9: Monthly plots of the percentage occurrence of temperature inversions between the heights of the Guernsey (102.0 m) and Alderney (88.7 m) airports (top frame) and the average relative humidity difference between these two heights (bottom frame) from August 2003 to August 2005
Mon
thly
ave
rage
of
(Ald
erne
y R
H –
Gue
rnse
y R
H) (
%)
Month
Month
Occ
urre
nce
frequ
ency
of
(Gue
rnse
y Ta
ir –
Ald
erne
y Ta
ir) >
1°C
(%)
-29-
516 517
518
Figure 10: Monthly plots of the percentage occurrence of enhanced signal strengths at the Alderney high and low antennas (top frame) and the corresponding occurrence frequency of modified refractivity inversions between the heights of the Guernsey (102.0 m) and Alderney (88.7 m) airports (bottom frame) from August 2003 to August 2005
-30-
518 519
520
Figure 11: The number of occurrences of M-inversions per month between the heights of the CLV (5.0 m) and the Jersey airport (84.0 m) (August 2003 to August 2005)
Month
Num
ber o
f occ
urre
nce
of M
-inve
rsio
ns b
etw
een
the
CLV
(5.0
m) a
nd J
erse
y A
irpor
t (84
.0 m
)
-31-
520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571
Figure 12: Graph illustrating the diurnal variation in the occurrence of enhanced signal strengths at the Alderney high and low antennas (top frame) and the corresponding occurrence frequency of modified refractivity inversions between the heights of Guernsey (102.0 m) and Alderney airports (88.7 m)
-32-
572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600
601
Figure 13: Comparison between the hourly measured signal strength and the PE-predicted signal strength (using higher-level refractivity data) at the Alderney high antenna during a period of signal enhancement in summer (12-18 September 2003)
Free space threshold
Recommended