Line data Source code
1 : //# ATAtmosphere.cc: Model of atmospheric opacity
2 : //# Copyright (C) 2004
3 : //# Associated Universities, Inc. Washington DC, USA.
4 : //#
5 : //# This library is free software; you can redistribute it and/or modify it
6 : //# under the terms of the GNU Library General Public License as published by
7 : //# the Free Software Foundation; either version 2 of the License, or (at your
8 : //# option) any later version.
9 : //#
10 : //# This library is distributed in the hope that it will be useful, but WITHOUT
11 : //# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 : //# FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public
13 : //# License for more details.
14 : //#
15 : //# You should have received a copy of the GNU Library General Public License
16 : //# along with this library; if not, write to the Free Software Foundation,
17 : //# Inc., 675 Massachusetts Ave, Cambridge, MA 02139, USA.
18 : //#
19 : //# Correspondence concerning AIPS++ should be addressed as follows:
20 : //# Internet email: casa-feedback@nrao.edu.
21 : //# Postal address: AIPS++ Project Office
22 : //# National Radio Astronomy Observatory
23 : //# 520 Edgemont Road
24 : //# Charlottesville, VA 22903-2475 USA
25 : //#
26 : //#---------------------------------------------------------------------------
27 :
28 : // own includes
29 : #include "ATAtmosphere.h"
30 :
31 : // casa includes
32 : #include <casacore/casa/Utilities/Assert.h>
33 : #include <casacore/casa/Quanta.h>
34 :
35 : // std includes
36 : //#include <cmath>
37 :
38 : using namespace casacore;
39 : namespace casa{
40 :
41 : /**
42 : * Default Constructor (apart from optional parameters).
43 : * The class set up this way will assume International Standard Atmosphere (ISA) conditions,
44 : * except for humidity. The latter is assumed to be 50%, which seems more realistic for
45 : * Australian telescopes than 0%.
46 : * @param[in] wvScale water vapour scale height (m), default is 1540m to match MIRIAD's model
47 : * @param[in] maxAlt maximum altitude of the model atmosphere (m), plane parallel layers are spread linearly up to
48 : * this height, default is 10000m to match MIRIAD.
49 : * @param[in] nLayers number of plane parallel layers in the model (essentially for a numberical integration),
50 : * default is 50 to match MIRIAD.
51 : **/
52 0 : ATAtmosphere::ATAtmosphere(Double wvScale, Double maxAlt, Int nLayers) :
53 0 : itsHeights(nLayers), itsTemperatures(nLayers),
54 0 : itsDryPressures(nLayers), itsVapourPressures(nLayers),
55 0 : itsGndTemperature(288.), itsPressure(101325.), itsGndHumidity(0.5),
56 0 : itsLapseRate(0.0065), itsWVScale(wvScale), itsMaxAlt(maxAlt), itsObsHeight(200.)
57 : {
58 0 : recomputeAtmosphereModel();
59 0 : }
60 :
61 : /**
62 : * Constructor with explicitly given parameters of the atmosphere
63 : * @param[in] temperature air temperature at the observatory (K)
64 : * @param[in] pressure air pressure at the sea level if the observatory elevation is set
65 : * (default is set to 200m) or at the observatory ground level if the elevation
66 : * is set to 0 (Pascals)
67 : * @param[in] humidity air humidity at the observatory (fraction)
68 : * @param[in] lapseRate temperature lapse rate (K/m), default is 0.0065 K/m to match MIRIAD and ISA
69 : * @param[in] wvScale water vapour scale height (m), default is 1540m to match MIRIAD's model
70 : * @param[in] maxAlt maximum altitude of the model atmosphere (m), plane parallel layers are spread linearly up to
71 : * this height, default is 10000m to match MIRIAD.
72 : * @param[in] nLayers number of plane parallel layers in the model (essentially for a numberical integration),
73 : * default is 50 to match MIRIAD.
74 : **/
75 0 : ATAtmosphere::ATAtmosphere(Double temperature, Double pressure, Double humidity, Double lapseRate,
76 0 : Double wvScale, Double maxAlt, Int nLayers) :
77 0 : itsHeights(nLayers), itsTemperatures(nLayers),
78 0 : itsDryPressures(nLayers), itsVapourPressures(nLayers),
79 0 : itsGndTemperature(temperature), itsPressure(pressure), itsGndHumidity(humidity),
80 0 : itsLapseRate(lapseRate), itsWVScale(wvScale), itsMaxAlt(maxAlt), itsObsHeight(200.)
81 : {
82 0 : recomputeAtmosphereModel();
83 0 : }
84 :
85 : /**
86 : * Set the new weather station data, recompute the model
87 : * @param[in] temperature air temperature at the observatory (K)
88 : * @param[in] pressure air pressure at the sea level if the observatory elevation is set to non-zero value
89 : * (default is set to 200m) or at the observatory ground level if the elevation
90 : * is set to 0 (Pascals)
91 : * @param[in] humidity air humidity at the observatory (fraction)
92 : **/
93 0 : void ATAtmosphere::setWeather(Double temperature, Double pressure, Double humidity)
94 : {
95 0 : itsGndTemperature = temperature;
96 0 : itsPressure = pressure;
97 0 : itsGndHumidity = humidity;
98 0 : recomputeAtmosphereModel();
99 0 : }
100 :
101 : /**
102 : * Set the elevation of the observatory (height above mean sea level)
103 : * It affects only interpretation of the pressure supplied as part of the weather data, if this value
104 : * is non-zero, the pressure (e.g. in setWeather or constructor) is that at mean sea level. If the
105 : * observatory elevation is set to zero, regardless on real elevation, the pressure is that at the
106 : * observatory ground level.
107 : *
108 : * By default, 200m is assumed and the pressure should be a mean sea level pressure..
109 : * @param[in] elev elevation in metres
110 : **/
111 0 : void ATAtmosphere::setObservatoryElevation(Double elev)
112 : {
113 0 : itsObsHeight = elev;
114 0 : recomputeAtmosphereModel();
115 0 : }
116 :
117 :
118 : /**
119 : * Build the atmosphere model based on exponential fall-off, ideal gas and hydrostatic
120 : * equilibrium. The model parameters are taken from the data members of this class.
121 : **/
122 0 : void ATAtmosphere::recomputeAtmosphereModel()
123 : {
124 0 : AlwaysAssert(itsGndTemperature > 0, AipsError);
125 0 : AlwaysAssert(itsPressure > 0., AipsError);
126 0 : AlwaysAssert((itsGndHumidity >= 0.) && (itsGndHumidity<=1.), AipsError);
127 0 : AlwaysAssert(itsMaxAlt > 0., AipsError);
128 0 : AlwaysAssert(itsWVScale > 0., AipsError);
129 :
130 0 : const Double heightStep = itsMaxAlt/Double(nLayers());
131 : // molar mass of the air
132 0 : const Double M = 28.96e-3;
133 : // free-fall acceleration
134 0 : const Double g = 9.81;
135 :
136 0 : const Double wvGndSaturationPressure = wvSaturationPressure(itsGndTemperature);
137 0 : const Double gndPressure = itsPressure*exp(-M*g/(QC::R( ).get().getValue()*itsGndTemperature)*
138 0 : (itsObsHeight+0.5*itsLapseRate*itsObsHeight*itsObsHeight/itsGndTemperature));
139 0 : for (Int layer = 0; layer < nLayers(); ++layer) {
140 0 : const Double height = Double(layer)*heightStep;
141 0 : itsHeights[layer] = height;
142 0 : itsTemperatures[layer] = itsGndTemperature/(1.+itsLapseRate*height/itsGndTemperature);
143 0 : const Double pressure = gndPressure * exp(-M*g/(QC::R( ).get().getValue()*itsGndTemperature)*
144 0 : (height+0.5*itsLapseRate*height*height/itsGndTemperature));
145 0 : itsVapourPressures[layer] = casacore::min(itsGndHumidity*exp(-height/itsWVScale)*wvGndSaturationPressure,
146 0 : wvSaturationPressure(itsTemperatures[layer]));
147 0 : itsDryPressures[layer] = pressure - itsVapourPressures[layer];
148 : //std::cout<<"layer="<<layer<<": H="<<itsHeights[layer]<<" T="<<itsTemperatures[layer]<<
149 : // " Pvap="<<itsVapourPressures[layer]<<" Pdry="<<itsDryPressures[layer]<<endl;
150 : }
151 0 : }
152 :
153 : /**
154 : * Obtain the number of model layers, do consistency check that everything is
155 : * resized accordingly
156 : * @retrun number of model layers
157 : **/
158 0 : Int ATAtmosphere::nLayers() const
159 : {
160 0 : const Int result = itsHeights.size();
161 0 : DebugAssert(result > 2, AipsError);
162 0 : DebugAssert(itsTemperatures.size() == result, AipsError);
163 0 : DebugAssert(itsDryPressures.size() == result, AipsError);
164 0 : DebugAssert(itsVapourPressures.size() == result, AipsError);
165 0 : return result;
166 : }
167 :
168 : /**
169 : * Determine the saturation pressure of water vapour for the given temperature.
170 : *
171 : * Reference:
172 : * Waters, Refraction effects in the neutral atmosphere. Methods of
173 : * Experimental Physics, vol 12B, p 186-200 (1976).
174 : *
175 : * @param[in] temperature temperature in K
176 : * @return vapour saturation pressure (Pascals)
177 : **/
178 0 : Double ATAtmosphere::wvSaturationPressure(Double temperature)
179 : {
180 0 : if (temperature <= 215.) {
181 0 : return 0.;
182 : }
183 0 : const Double theta = 300.0/temperature;
184 0 : return 1e5/(41.51/std::pow(theta,5)*std::pow(10.,9.834*theta-10.0));
185 : }
186 :
187 : /**
188 : * Compute the complex refractivity of the dry components of the atmosphere
189 : * (oxygen lines) at the given frequency.
190 : * @param[in] freq frequency (Hz)
191 : * @param[in] temperature air temperature (K)
192 : * @param[in] pDry partial pressure of dry components (Pascals)
193 : * @param[in] pVapour partial pressure of water vapour (Pascals)
194 : * @return complex refractivity
195 : *
196 : * Reference:
197 : * Liebe, An updated model for millimeter wave propogation in moist air,
198 : * Radio Science, 20, 1069-1089 (1985).
199 : **/
200 0 : DComplex ATAtmosphere::dryRefractivity(Double freq, Double temperature,
201 : Double pDry, Double pVapour)
202 : {
203 : // the number of parameters per atmospheric line and the number of lines taken into account
204 0 : const Int nLineParams = 7;
205 0 : const Int nLines = 48;
206 : // actual tabulated values
207 0 : const Double lines[nLines][nLineParams] =
208 : {{49.452379, 0.12E-6, 11.830, 8.40E-3, 0.0, 5.60E-3, 1.7},
209 : {49.962257, 0.34E-6, 10.720, 8.50E-3, 0.0, 5.60E-3, 1.7},
210 : {50.474238, 0.94E-6, 9.690, 8.60E-3, 0.0, 5.60E-3, 1.7},
211 : {50.987748, 2.46E-6, 8.690, 8.70E-3, 0.0, 5.50E-3, 1.7},
212 : {51.503350, 6.08E-6, 7.740, 8.90E-3, 0.0, 5.60E-3, 1.8},
213 : {52.021409, 14.14E-6, 6.840, 9.20E-3, 0.0, 5.50E-3, 1.8},
214 : {52.542393, 31.02E-6, 6.000, 9.40E-3, 0.0, 5.70E-3, 1.8},
215 : {53.066906, 64.10E-6, 5.220, 9.70E-3, 0.0, 5.30E-3, 1.9},
216 : {53.595748, 124.70E-6, 4.480, 10.00E-3, 0.0, 5.40E-3, 1.8},
217 : {54.129999, 228.00E-6, 3.810, 10.20E-3, 0.0, 4.80E-3, 2.0},
218 : {54.671157, 391.80E-6, 3.190, 10.50E-3, 0.0, 4.80E-3, 1.9},
219 : {55.221365, 631.60E-6, 2.620, 10.79E-3, 0.0, 4.17E-3, 2.1},
220 : {55.783800, 953.50E-6, 2.115, 11.10E-3, 0.0, 3.75E-3, 2.1},
221 : {56.264777, 548.90E-6, 0.010, 16.46E-3, 0.0, 7.74E-3, 0.9},
222 : {56.363387, 1344.00E-6, 1.655, 11.44E-3, 0.0, 2.97E-3, 2.3},
223 : {56.968180, 1763.00E-6, 1.255, 11.81E-3, 0.0, 2.12E-3, 2.5},
224 : {57.612481, 2141.00E-6, 0.910, 12.21E-3, 0.0, 0.94E-3, 3.7},
225 : {58.323874, 2386.00E-6, 0.621, 12.66E-3, 0.0, -0.55E-3, -3.1},
226 : {58.446589, 1457.00E-6, 0.079, 14.49E-3, 0.0, 5.97E-3, 0.8},
227 : {59.164204, 2404.00E-6, 0.386, 13.19E-3, 0.0, -2.44E-3, 0.1},
228 : {59.590982, 2112.00E-6, 0.207, 13.60E-3, 0.0, 3.44E-3, 0.5},
229 : {60.306057, 2124.00E-6, 0.207, 13.82E-3, 0.0, -4.13E-3, 0.7},
230 : {60.434775, 2461.00E-6, 0.386, 12.97E-3, 0.0, 1.32E-3, -1.0},
231 : {61.150558, 2504.00E-6, 0.621, 12.48E-3, 0.0, -0.36E-3, 5.8},
232 : {61.800152, 2298.00E-6, 0.910, 12.07E-3, 0.0, -1.59E-3, 2.9},
233 : {62.411212, 1933.00E-6, 1.255, 11.71E-3, 0.0, -2.66E-3, 2.3},
234 : {62.486253, 1517.00E-6, 0.078, 14.68E-3, 0.0, -4.77E-3, 0.9},
235 : {62.997974, 1503.00E-6, 1.660, 11.39E-3, 0.0, -3.34E-3, 2.2},
236 : {63.568515, 1087.00E-6, 2.110, 11.08E-3, 0.0, -4.17E-3, 2.0},
237 : {64.127764, 733.50E-6, 2.620, 10.78E-3, 0.0, -4.48E-3, 2.0},
238 : {64.678900, 463.50E-6, 3.190, 10.50E-3, 0.0, -5.10E-3, 1.8},
239 : {65.224067, 274.80E-6, 3.810, 10.20E-3, 0.0, -5.10E-3, 1.9},
240 : {65.764769, 153.00E-6, 4.480, 10.00E-3, 0.0, -5.70E-3, 1.8},
241 : {66.302088, 80.09E-6, 5.220, 9.70E-3, 0.0, -5.50E-3, 1.8},
242 : {66.836827, 39.46E-6, 6.000, 9.40E-3, 0.0, -5.90E-3, 1.7},
243 : {67.369595, 18.32E-6, 6.840, 9.20E-3, 0.0, -5.60E-3, 1.8},
244 : {67.900862, 8.01E-6, 7.740, 8.90E-3, 0.0, -5.80E-3, 1.7},
245 : {68.431001, 3.30E-6, 8.690, 8.70E-3, 0.0, -5.70E-3, 1.7},
246 : {68.960306, 1.28E-6, 9.690, 8.60E-3, 0.0, -5.60E-3, 1.7},
247 : {69.489021, 0.47E-6, 10.720, 8.50E-3, 0.0, -5.60E-3, 1.7},
248 : {70.017342, 0.16E-6, 11.830, 8.40E-3, 0.0, -5.60E-3, 1.7},
249 : {118.750341, 945.00E-6, 0.000, 15.92E-3, 0.0, -0.44E-3, 0.9},
250 : {368.498350, 67.90E-6, 0.020, 19.20E-3, 0.6, 0.00E00, 1.0},
251 : {424.763120, 638.00E-6, 0.011, 19.16E-3, 0.6, 0.00E00, 1.0},
252 : {487.249370, 235.00E-6, 0.011, 19.20E-3, 0.6, 0.00E00, 1.0},
253 : {715.393150, 99.60E-6, 0.089, 18.10E-3, 0.6, 0.00E00, 1.0},
254 : {773.838730, 671.00E-6, 0.079, 18.10E-3, 0.6, 0.00E00, 1.0},
255 : {834.145330, 180.00E-6, 0.079, 18.10E-3, 0.6, 0.00E00, 1.0}};
256 :
257 : // convert to the units of Liebe
258 0 : const Double theta = 300./temperature;
259 0 : const Double kPaPVap = 0.001*pVapour;
260 0 : const Double kPaPDry = 0.001*pDry;
261 0 : const Double fGHz = freq * 1e-9;
262 :
263 : // some coefficients
264 0 : const Double ap = 1.4e-10*(1-1.2e-5*std::pow(fGHz,1.5));
265 0 : const Double gamma0 = 5.6e-3*(kPaPDry + 1.1*kPaPVap)*std::pow(theta,0.8);
266 : // initial refractivity
267 0 : DComplex result(2.588*kPaPDry*theta +
268 0 : 3.07e-4*(1.0/(1.0+std::pow(fGHz/gamma0,2))-1)*kPaPDry*theta*theta,
269 0 : (2*3.07e-4/(gamma0*(1+std::pow(fGHz/gamma0,2))*(1+std::pow(fGHz/60,2))) +
270 0 : ap*kPaPDry*std::pow(theta,2.5))*fGHz*kPaPDry*theta*theta);
271 :
272 : // sum the contributions of all the lines
273 0 : for (Int l = 0; l < nLines; ++l) {
274 0 : const Double S = lines[l][1]*kPaPDry*std::pow(theta,3)*exp(lines[l][2]*(1.-theta));
275 0 : const Double gamma = lines[l][3]*(kPaPDry*std::pow(theta,0.8-lines[l][4]) + 1.1*kPaPVap*theta);
276 0 : const Double delta = lines[l][5]*kPaPDry*std::pow(theta,lines[l][6]);
277 0 : const Double x = (lines[l][0]-fGHz)*(lines[l][0]-fGHz) + gamma*gamma;
278 0 : const Double y = (lines[l][0]+fGHz)*(lines[l][0]+fGHz) + gamma*gamma;
279 0 : const Double z = (lines[l][0]+gamma*gamma/lines[l][0]);
280 0 : result += DComplex (S*( (z-fGHz)/x + (z+fGHz)/y - 2./lines[l][0] +
281 0 : delta*(1/x-1/y)*gamma*fGHz/lines[l][0]),
282 0 : S*( (1/x+1/y)*gamma*fGHz/lines[l][0] -
283 0 : delta*((lines[l][0]-fGHz)/x + (lines[l][0]+fGHz)/y)*fGHz/lines[l][0]));
284 : }
285 :
286 0 : return result;
287 : }
288 :
289 : /**
290 : * Compute the complex refractivity of the water vapour monomers
291 : * at the given frequency.
292 : * @param[in] freq frequency (Hz)
293 : * @param[in] temperature air temperature (K)
294 : * @param[in] pDry partial pressure of dry components (Pascals)
295 : * @param[in] pVapour partial pressure of water vapour (Pascals)
296 : * @return complex refractivity
297 : *
298 : * Reference:
299 : * Liebe, An updated model for millimeter wave propogation in moist air,
300 : * Radio Science, 20, 1069-1089 (1985).
301 : **/
302 0 : DComplex ATAtmosphere::vapourRefractivity(Double freq, Double temperature,
303 : Double pDry, Double pVapour)
304 : {
305 : // the number of parameters per atmospheric line and the number of lines taken into account
306 0 : const Int nLineParams = 4;
307 0 : const Int nLines = 30;
308 : // actual tabulated values
309 0 : const Double lines[nLines][nLineParams] =
310 : {{22.235080, 0.1090, 2.143, 27.84E-3},
311 : {67.813960, 0.0011, 8.730, 27.60E-3},
312 : {119.995940, 0.0007, 8.347, 27.00E-3},
313 : {183.310117, 2.3000, 0.653, 28.35E-3},
314 : {321.225644, 0.0464, 6.156, 21.40E-3},
315 : {325.152919, 1.5400, 1.515, 27.00E-3},
316 : {336.187000, 0.0010, 9.802, 26.50E-3},
317 : {380.197372, 11.9000, 1.018, 27.60E-3},
318 : {390.134508, 0.0044, 7.318, 19.00E-3},
319 : {437.346667, 0.0637, 5.015, 13.70E-3},
320 : {439.150812, 0.9210, 3.561, 16.40E-3},
321 : {443.018295, 0.1940, 5.015, 14.40E-3},
322 : {448.001075, 10.6000, 1.370, 23.80E-3},
323 : {470.888947, 0.3300, 3.561, 18.20E-3},
324 : {474.689127, 1.2800, 2.342, 19.80E-3},
325 : {488.491133, 0.2530, 2.814, 24.90E-3},
326 : {503.568532, 0.0374, 6.693, 11.50E-3},
327 : {504.482692, 0.0125, 6.693, 11.90E-3},
328 : {556.936002, 510.000, 0.114, 30.00E-3},
329 : {620.700807, 5.0900, 2.150, 22.30E-3},
330 : {658.006500, 0.2740, 7.767, 30.00E-3},
331 : {752.033227, 250.000, 0.336, 28.60E-3},
332 : {841.073593, 0.0130, 8.113, 14.10E-3},
333 : {859.865000, 0.1330, 7.989, 28.60E-3},
334 : {899.407000, 0.0550, 7.845, 28.60E-3},
335 : {902.555000, 0.0380, 8.360, 26.40E-3},
336 : {906.205524, 0.1830, 5.039, 23.40E-3},
337 : {916.171582, 8.5600, 1.369, 25.30E-3},
338 : {970.315022, 9.1600, 1.842, 24.00E-3},
339 : {987.926764, 138.000, 0.178, 28.60E-3}};
340 :
341 : // convert to the units of Liebe
342 0 : const Double theta = 300./temperature;
343 0 : const Double kPaPVap = 0.001*pVapour;
344 0 : const Double kPaPDry = 0.001*pDry;
345 0 : const Double fGHz = freq * 1e-9;
346 :
347 : // initial refractivity
348 0 : DComplex result(2.39*kPaPVap*theta + 41.6*kPaPVap*theta*theta +
349 0 : 6.47e-6*std::pow(fGHz,2.05)*kPaPVap*std::pow(theta,2.4),
350 0 : (0.915*1.40e-6*kPaPDry + 5.41e-5*kPaPVap*theta*theta*theta)*
351 0 : fGHz*kPaPVap*std::pow(theta,2.5));
352 :
353 : // sum contributions of all the lines
354 0 : for (Int l = 0; l < nLines; ++l) {
355 0 : const Double S = lines[l][1]*kPaPVap*std::pow(theta,3.5)*exp(lines[l][2]*(1.-theta));
356 0 : const Double gamma = lines[l][3]*(kPaPDry*std::pow(theta,0.8) + 4.80*kPaPVap*theta);
357 0 : const Double x = (lines[l][0]-fGHz)*(lines[l][0]-fGHz) + gamma*gamma;
358 0 : const Double y = (lines[l][0]+fGHz)*(lines[l][0]+fGHz) + gamma*gamma;
359 0 : const Double z = (lines[l][0]+gamma*gamma/lines[l][0]);
360 0 : result += DComplex(S*((z-fGHz)/x + (z+fGHz)/y - 2./lines[l][0]),
361 0 : S*((1./x+1./y)*gamma*fGHz/lines[l][0]));
362 : }
363 :
364 0 : return result;
365 : }
366 :
367 : /**
368 : * Calculate zenith opacity at the given frequency. This is a simplified version
369 : * of the routine implemented in MIRIAD, which calculates just zenith opacity and
370 : * nothing else. Note, that if the opacity is high, 1/sin(el) law is not correct
371 : * even in the plane parallel case due to refraction.
372 : * @param[in] freq frequency of interest in Hz
373 : * @return zenith opacity (nepers, i.e. dimensionless)
374 : **/
375 0 : Double ATAtmosphere::zenithOpacity(Double freq) const
376 : {
377 : // essentially a numerical integration with the Trapezium method
378 0 : Double tau = 0.;
379 0 : for (Int layer = Int(nLayers()) - 1; layer>=0; --layer) {
380 0 : Double dH = 0.;
381 0 : if (layer == 0) {
382 0 : dH = 0.5*(itsHeights[1]-itsHeights[0]);
383 0 : } else if (layer + 1 == int(nLayers())) {
384 0 : dH = 0.5*(itsHeights[nLayers()-1]-itsHeights[nLayers()-2]);
385 : } else {
386 0 : dH = 0.5*(itsHeights[layer+1]-itsHeights[layer-1]);
387 : }
388 : // imaginary part of the total complex refractivity
389 0 : const Double nImag = 1e-6*std::imag(dryRefractivity(freq,itsTemperatures[layer],itsDryPressures[layer],
390 0 : itsVapourPressures[layer])+vapourRefractivity(freq,itsTemperatures[layer],itsDryPressures[layer],
391 0 : itsVapourPressures[layer]));
392 0 : tau += dH*4.*casacore::C::pi/QC::c( ).get().getValue()*freq*nImag;
393 : }
394 0 : return tau;
395 : }
396 :
397 : /**
398 : * Calculate zenith opacity for the range of frequencies. Same as zenithOpacity, but
399 : * for a vector of frequencies.
400 : * @param[in] freqs vector of frequencies in Hz
401 : * @return vector of zenith opacities, one value per frequency (nepers, i.e. dimensionless)
402 : **/
403 0 : Vector<Double> ATAtmosphere::zenithOpacities(const Vector<Double> &freqs) const
404 : {
405 0 : Vector<Double> result(freqs.size());
406 0 : for (uInt ch = 0; ch<freqs.size(); ++ch) {
407 0 : result[ch] = zenithOpacity(freqs[ch]);
408 : }
409 0 : return result;
410 0 : }
411 :
412 : /**
413 : * Calculate opacity at the given frequency and elevation. This is a simplified
414 : * version of the routine implemented in MIRIAD, which calculates just the opacity and
415 : * nothing else. In contract to zenithOpacity, this method takes into account refraction
416 : * and is more accurate than if one assumes 1/sin(el) factor.
417 : * @param[in] freq frequency of interest in Hz
418 : * @param[in] el elevation in radians
419 : * @return zenith opacity (nepers, i.e. dimensionless)
420 : **/
421 0 : Double ATAtmosphere::opacity(Double freq, Double el) const
422 : {
423 : // essentially a numerical integration with the Trapezium method
424 0 : Double tau = 0.;
425 0 : const Double sineEl = sin(el);
426 0 : for (Int layer = Int(nLayers()) - 1; layer>=0; --layer) {
427 0 : Double dH = 0.;
428 0 : if (layer == 0) {
429 0 : dH = 0.5*(itsHeights[1]-itsHeights[0]);
430 0 : } else if (layer + 1 == int(nLayers())) {
431 0 : dH = 0.5*(itsHeights[nLayers()-1]-itsHeights[nLayers()-2]);
432 : } else {
433 0 : dH = 0.5*(itsHeights[layer+1]-itsHeights[layer-1]);
434 : }
435 : // total complex refractivity
436 0 : const DComplex n = dryRefractivity(freq,itsTemperatures[layer],itsDryPressures[layer],
437 0 : itsVapourPressures[layer]) +
438 0 : vapourRefractivity(freq,itsTemperatures[layer],itsDryPressures[layer],
439 0 : itsVapourPressures[layer]);
440 : // real and imaginary part of the total complex refractivity scaled appropriately
441 0 : const Double nImag = 1e-6*std::imag(n);
442 0 : const Double nReal = 1. + 1e-6*std::real(n);
443 : // length increment
444 0 : const Double dL = dH*nReal/sqrt(nReal*nReal+sineEl*sineEl-1.);
445 0 : tau += dL*4.*casacore::C::pi/QC::c( ).get().getValue()*freq*nImag;
446 : }
447 0 : return tau;
448 : }
449 :
450 : /**
451 : * Calculate opacities for the range of frequencies at the given elevation. Same as
452 : * opacity, but for a vector of frequencies.
453 : * @param[in] freqs vector of frequencies in Hz
454 : * @param[in] el elevation in radians
455 : * @return vector of opacities, one value per frequency (nepers, i.e. dimensionless)
456 : **/
457 0 : Vector<Double> ATAtmosphere::opacities(const Vector<Double> &freqs, Double el) const
458 : {
459 0 : Vector<Double> result(freqs.size());
460 0 : for (uInt ch = 0; ch<freqs.size(); ++ch) {
461 0 : result[ch] = opacity(freqs[ch],el);
462 : }
463 0 : return result;
464 0 : }
465 :
466 : } //#end casa namespace
467 :
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