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comparison: docs/depolarization/depolarization.rst

docs/depolarization/depolarization.rst

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1 1. Particle Linear Depolarization Ratio Implementation
2 ======================================================
3
4 The most important improvement included in the SCC v4.0 is the implementation of a new optical product which is the particle linear depolarization ratio.
5
6 1.1 Background
7 --------------
8
9 The calculation of the volume linear depolarization ratio profile (*VLDR*) and particle linear depolarization ratio profile (*PLDR*) needs two different steps:
10
11 #. the calibration of the polarization sensitive lidar channels;
12 #. the calculation of the *VLDR* or *PLDR* itself.
13
14 The SCC allows the user to make both the above points. In particular the calibration step is made by a completely new module called **scc\_calibrator** which computes the *apparent calibration factor* :math:`\beta^*` out of the pre-processed data provided by the standard **ELPP** (Earlinet Lidar Pre-Processor) module and it records it in the SCC database (SCC\_DB). Once logged into the SCC\_DB this factor can be used whenever it is necessary.
15
16 The raw lidar calibration measurements should be put in a NetCDF file which has the same structure as the “standard” raw SCC NetCDF input file (for more details see sections 2 and 3.2).
17
18 New signal types have been introduced to take into account special channel configurations used for calibration purposes.
19
20 Moreover new product types for both calibration and *PLDR* calculation have been defined. As, in principle, it is possible to calculate the *PLDR* only when the aerosol backscatter coefficient profile is available the following new products have been defined:
21
22 #. *Linear polarization calibration (factor* h) *(product\_type\_id=6);*
23 #. *Raman backscatter and linear depolarization ratio
24 (product\_type\_id=7);*
25 #. *Elastic backscatter and linear depolarization ratio
26 (product\_type\_id=8).*
27
28 The first product in the above list is used only for calibration while the other two are used for the calculation of *PLDR*. Basically, in most of the cases, the products 2 and 3 are equivalent to the corresponding backscatter product types with the exception that also the following new variables are available:
29
30 ::
31
32 double VolumeDepol(Length) ;
33 double ErrorVolumeDepol(Length) ;
34 ErrorVolumeDepol:long\_name = "absolute error of VolumeDepol" ;
35 double ParticleDepol(Length) ;
36 double ErrorParticleDepol(Length) ;
37 ErrorParticleDepol:long\_name = "absolute error of ParticleDepol" ;
38
39 1.2 Polarization calibration
40 ----------------------------
41
42 An important point is the definition of reliable *PLDR* calibration procedures. Within EARLINET the following calibration procedures are currently used:
43
44 a) Rayleigh calibration;
45 b) +45 calibration method, or D90 calibration method (made by +45 and -45 measurements);
46 c) 3 signals (total, cross and parallel).
47
48 It is well known that method a) could produce easily large errors on *PLDR* which cannot be controlled. For this reason only the methods b) and c) can be used to provide reliable polarization calibrations and so only those methods will be implemented in the SCC.
49
50 For what it concerns the method c) it, basically, requires to solve the equation:
51
52 .. math::
53 \alpha_s P_s + \alpha_p P_p = P
54
55 in two different of atmospheric layers with considerably different *VLDR*. So to calibrate in this way the implementation of automatic layer identification in the SCC is required. As at moment this feature is not yet available within the SCC **ONLY** the method b) is considered.
56
57 1.3 SCC procedure to calculate the PLDRP
58 ----------------------------------------
59
60 According to what mentioned before the SCC calculates the *PLDR* through the following steps:
61
62 #. The user needs to create a new system configuration in the SCC\_DB including only lidar channels used for the calibration. One (or more) *Linear polarization calibration (product\_type\_id=6)* product should be associated to this new configuration (see section 3.2 for more details);
63
64 #. This new system configuration should contain only the polarization channels in the configuration used for the calibration (for example rotated in the polarization plane of +45 degrees). A channel in calibration measurement configuration should have a **DIFFERENT** channel ID from the channel ID corresponding to the same channel in standard measurement configuration. For example, if a system has two polarization channels which in standard measurement configuration correspond to the channel ID=1 and 2 respectively, the same physical channels under calibration measurement configuration should correspond to different channel IDs (let's say ID=3 and 4 for the +45 degrees polarization rotated channels and ID=5 and 6 for the -45 degrees polarization rotated ones in case D90 calibration method is used). Moreover, the polarization channels should be labeled correctly using the new signal types available (*+45elPT, +45elPR, -45elPT, -45elPR, +45elPTnr, +45elPTfr, +45elPRnr, +45elPRfr, -45elPTnr, -45elPTfr, -45elPRnr, -45elPRfr).* For more details see section 3.2;
65
66 #. In SCC v4.0 the polarization channels are **NOT** labeled on the base of their polarization state (as it was done in the SCC v3.11) but **ALWAYS** as transmitted and reflected channels. So the channels that in SCC v3.11 were labeled as *elCP, elCPnr, elCPfr, elPP, elPPnr elPPfr* will be labeled in SCC v4.0 as *elPR, elPRnr elPRfr elPT, elPTnr elPTfr* where the letter *T* stands from transmitted and the letter *R* for reflected.
67
68 .. warning:: In switching from the SCC v3.11 to SCC v4.0 the following modifications have been made on **ALL** channels of **ALL** registered configurations:
69 *elPP→elPR*
70
71 *elCP→elPT*
72
73 *elPPnr→elPRnr*
74
75 *elPPfr→ elPRfr*
76
77 *elCPnr→ elPTnr*
78
79 *elCPfr→ elPTfr*
80
81 Please be sure these modifications reflect to your actual lidar setup(cross channels are transmitted and parallel channels are reflected);
82
83 4. The user needs to submit a file (same format as raw SCC input file) containing the raw data for the lidar channels defined at the point 1 (see section 3.2 for more details);
84 #. The file at point 2 is pre-processed by **ELPP** module which applies the standard pre-processing procedures applied to “standard” lidar data;
85 #. The pre-processed files are then processed by the new modules **scc\_calibrator** which calculates :math:`\eta^*` *the apparent calibration factor* and logs it into the SCC\_DB;
86 #. The user needs to create a new system configuration in the SCC\_DB (which should be different from the one used for the calibration) and associate it the new product *Raman backscatter and linear depolarization ratio product\_type\_id=7)* or *Elastic backscatter and linear depolarization ratio (product\_type\_id=8).* Alternatively the calculation of those products can be added to an already existing lidar configuration as long as it is different from the calibration one;
87 #. The product defined at point 5 should be linked to the product containing the polarization calibration (defined at point 1) in a way that the *apparent calibration factor* can be selected from the SCC\_DB (see section 3.3 and in particular figure 3.4);
88 #. The user needs to submit another SCC raw data file containing the “standard” measurements;
89 #. Finally **ELPP** and **ELDA** will produce a b-file containing backscatter coefficient profile and *PLDR*. In particular this calculation is made in two different steps: from the pre-processed lidar polarization signals, and taking into account the *apparent calibration factor* and the *calibration factor correction K* (defined as option of *Linear polarization calibration* product\ *)* written into the SCC\_DB, an “apparent” *VLDR* :math:`\delta^*` is calculated. Even if :math:`\delta^*` is a calibrated quantity it can be still affected by possible systematic errors due to not perfect optics or alignment of the system;
90
91 #. To take into account these errors a corrected *VLDR* (:math:`\delta`) is calculated using the *polarization cross-talk correction parameters* *G* and *H* calculated on the base of Müller matrix formalism. These cross-talk correction parameters (*G* and *H*) are stored in the SCC\_DB for each lidar channels (see section 3.1 in particular figure 3.2). Finally the *PLDR* is calculated using the backscatter coefficient profile and the molecular LDRP calculated by ELPP considering the center wavelength and bandwidth of the channels interference filter.
92
93 The *apparent calibration factor* :math:`\eta^*` is calculated by the **scc\_calibrator** module as the geometrical mean of the ratio of the +/-45 degrees reflected to the +/- 45 degrees transmitted signals within an altitude calibration range defined by the users in the raw data input files.
94
95 In case of +45 calibration method :math:`\eta^*` is calculated by:
96
97 .. math::
98 \eta^* = \frac{I_R}{I_T}(+45)
99
100 While in case of D90 calibration method:
101
102 .. math::
103 \eta^* = \sqrt{\frac{I_R}{I_T}(+45) \frac{I_R}{I_T}(-45)}
104
105 **ELDA** module calculates the “apparent” *VLDR*:
106
107 .. math::
108 \delta^* = \frac{K}{\eta^*} \cdot \frac{I_R}{I_T}
109
110 the *VLDR*
111
112 .. math::
113 \delta = \frac{\delta^*(G_T + H_T)-(G_R + H_R)}{(G_R - H_R) - \delta^*(G_T - H_T)}
114
115 and the *PLDR*
116
117 .. math::
118 \delta_{\alpha} = \frac{(1 + \delta_m)\delta R - (1 + \delta)\delta_m}{(1 + \delta_m)R - (1 + \delta)}
119
120 where:
121
122 - :math:`\eta^*` is the *apparent calibration factor* calculated by **scc\_calibrator**
123
124 - *K* is the *calibration factor correction* defined as polarization product option
125
126 - :math:`I_T` and :math:`I_R` are the transmitted and the reflected signals in the polarization detection set-up
127
128 - :math:`G_{T,R}` and :math:`H_{T,R}` are *polarization cross-talk correction parameters* for the transmitted and reflected signals used to correct for systematic errors. Both these factors are defined in the SCC\_DB for each lidar channel.
129
130 - :math:`\delta_m` is the molecular linear depolarization ratio calculated by ELPP
131
132 - *R* is the backscatter ratio
133
134 Please note once again that the polarization channels are described in terms of transmitted and reflected signals. This means that according to different lidar instrumental configurations, the transmitted or the reflected channel can contain total, perpendicular or parallel polarized signals.
135
136 In order to retrieve the backscatter profile the total signal must be obtained combining the transmitted and reflected polarized signals. The following formula is used:
137
138 .. math::
139 I_{total} \propto \frac{\eta^*}{K}H_R I_T - H_T I_R
140
141 The formulas above are general and can be adapted to all possible polarization lidar configurations selecting the right polarization cross-talk correction parameters (see Table 1.1).
142
143 Let's suppose, for example, we have the perpendicular polarized lidar signal on the transmitted channel and the parallel polarized on reflected channel. For an ideal system (no diattenuation and cross-talk) we have:
144
145 .. math::
146 G_T=1 , \qquad H_T=-1, \qquad G_R=1, \qquad H_R=1
147
148 If, on the other hands, we have the perpendicular polarized lidar signal on reflected channel and the total polarized on the transmitted for and ideal system we have:
149
150 .. math::
151 G_T=1 , \qquad H_T=0, \qquad G_R=1, \qquad H_R=-1
152
153
154 **Table 1.1:** Polarization cross-talk correction parameters for ideal systems
155
156 +----------------------+-----------------------------+-----------------+-----------------+-----------------+
157 | Laser polarization | Detected in lidar channel |
158 + +-----------------------------+-----------------+-----------------+-----------------+
159 | | Transmitted | Reflected |
160 + +-----------------------------+-----------------+-----------------+-----------------+
161 | | :math:`G_T` | :math:`H_T` | :math:`G_R` | :math:`H_R` |
162 +----------------------+-----------------------------+-----------------+-----------------+-----------------+
163 | total | 1 | 0 | 1 | 0 |
164 +----------------------+-----------------------------+-----------------+-----------------+-----------------+
165 | parallel | 1 | 1 | 1 | 1 |
166 +----------------------+-----------------------------+-----------------+-----------------+-----------------+
167 | cross | 1 | -1 | 1 | -1 |
168 +----------------------+-----------------------------+-----------------+-----------------+-----------------+
169
170 The *apparent calibration factor* (:math:`\eta^*`), *the calibration factor correction* (*K*) and the *polarization cross-talk correction parameters* are stored by **ELPP** module in the intermediate NetCDF files using the following variables:
171
172 - :code:`Polarization_Channel_Gain_Factor` (*apparent calibration factor* - :math:`\eta^*` )
173 - :code:`Polarization_Channel_Gain_Factor_Correction` (*calib. factor corr.* – *K*)
174 - :code:`G_T`
175 - :code:`H_T`
176 - :code:`G_R`
177 - :code:`H_R`
178
179 Finally new usecases have been defined to take into account all the possible lidar configurations. The details on that are provided as a separate file.
180
181 2. Changes of the SCC input format
182 ==================================
183
184 The following minor changes have been applied to raw SCC data format:
185
186 #. The optional variable *ID\_Range* has been **REMOVED**;
187 #. The **OPTIONAL** variable :code:`int Signal\_Type(channels)` has been added. The possible values are the same available in the SCC\_DB:
188
189 :code:`0` :math:`\rightarrow` :code:`elT`
190
191 :code:`1` :math:`\rightarrow` :code:`elTnr`
192
193 :code:`2` :math:`\rightarrow` :code:`elTfr`
194
195 :code:`3` :math:`\rightarrow` :code:`vrRN2`
196
197 :code:`4` :math:`\rightarrow` :code:`vrRN2nr`
198
199 :code:`5` :math:`\rightarrow` :code:`vrRN2fr`
200
201 :code:`6` :math:`\rightarrow` :code:`elPR`
202
203 :code:`7` :math:`\rightarrow` :code:`elPT`
204
205 :code:`8` :math:`\rightarrow` :code:`pRRlow`
206
207 :code:`9` :math:`\rightarrow` :code:`pRRhigh`
208
209 :code:`10` :math:`\rightarrow` :code:`elPRnr`
210
211 :code:`11` :math:`\rightarrow` :code:`elPRfr`
212
213 :code:`12` :math:`\rightarrow` :code:`elPTnr`
214
215 :code:`13` :math:`\rightarrow` :code:`elPTfr`
216
217 :code:`14` :math:`\rightarrow` :code:`vrRH2O`
218
219 :code:`15` :math:`\rightarrow` :code:`pRRhighnr`
220
221 :code:`16` :math:`\rightarrow` :code:`pRRhighfr`
222
223 :code:`17` :math:`\rightarrow` :code:`pRRlownr`
224
225 :code:`18` :math:`\rightarrow` :code:`pRRlowfr`
226
227 :code:`19` :math:`\rightarrow` :code:`vrRH2Onr`
228
229 :code:`20` :math:`\rightarrow` :code:`vrRH2Ofr`
230
231 :code:`21` :math:`\rightarrow` :code:`elTunr`
232
233 :code:`22` :math:`\rightarrow` :code:`+45elPT`
234
235 :code:`23` :math:`\rightarrow` :code:`+45elPR`
236
237 :code:`24` :math:`\rightarrow` :code:`-45elPT`
238
239 :code:`25` :math:`\rightarrow` :code:`-45elPR`
240
241 :code:`26` :math:`\rightarrow` :code:`+45elPTnr`
242
243 :code:`27` :math:`\rightarrow` :code:`+45elPTfr`
244
245 :code:`28` :math:`\rightarrow` :code:`+45elPRnr`
246
247 :code:`29` :math:`\rightarrow` :code:`+45elPRfr`
248
249 :code:`30` :math:`\rightarrow` :code:`-45elPTnr`
250
251 :code:`31` :math:`\rightarrow` :code:`-45elPTfr`
252
253 :code:`32` :math:`\rightarrow` :code:`-45elPRnr`
254
255 :code:`33` :math:`\rightarrow` :code:`-45elPRfr`
256
257 .. warning:: It this variable is found in the SCC input file the corresponding settings in the SCC database will be **OVERWRITTEN**. Unless you don't have any valid reason to overwrite the database value this variable should not be used.
258
259 3. The variables:
260
261 ::
262
263 double Pol\_Calib\_Range\_Min(channels)
264 double Pol\_Calib\_Range\_Max(channels)
265
266 have been added. Both these variable are **MANDATORY** for any calibration raw dataset. These variable should be included only the polarization calibration measurements and should specify the altitude range (meters) in which the polarization calibration should be made. For more details see section 3.3;
267
268 4. The variable :code:`Depolarization_Factor` has been **REMOVED**.
269
270 The SCC v3.11 used this variable to get polarization calibration factor for the calculation of the total signal out of cross and parallels ones. As the SCC v4.0 is able to calculate the same parameter by itself, the use of this variable is *NOT* possible anymore. The recommended way to get a valid and quality assured depolarization calibration factor is to submit to the SCC v4.0 a polarization calibration dataset and let the SCC to calculate such factor.
271
272 To make this change more smooth and to provide the users with the possibility to continue to analyze their data with the SCC v4.0 even if a calibration dataset has not been submitted yet, it will be possible for a **LIMITED** period of time to submit the calibration constant via the SCC web interface. The SCC will keep track of the used calibration method (automatic or manual).
273
274 .. warning:: After this transition period **ONLY** automatic calibration will be allowed!
275
276 5. The new **OPTIONAL** variable:
277
278 :code:`string channel\_string\_ID(channels)`
279
280 has been introduced.
281
282 Starting from SCC v4.0 the lidar channel can be identified not only by using integers (as it happened until SCC v3.11) but also by using strings.
283
284 The procedure implemented in the SCC v4.0 to recognize the lidar channel within the raw lidar data is fully backward compatible (old format files are accepted as they are by SCC v4.0).
285
286 .. warning:: Please note that the definition of the new string variable requires netCDF-4 format! The type *string* is not supported in netCDF-3 format!
287
288 3. Real Example
289 ===============
290
291 This section describes all the practical steps the users need to follow to switch from SCC v3.11 to new SCC v4.0.
292
293 :IMPORTANT:
294 If your lidar system is not equipped with any polarization channels **NO** changes are required. In this case, the SCC v4.0 should work using the same input files and the same database configurations you have used with the SCC v3.11. Anyway as in the SCC v4.0 several bugs have been fixed,it is recommended to re-run all the measurement IDs you have submitted. For doing that you just need to reprocess all your data without the need to submit raw data files already uploaded on the server.
295
296 The practical example reported below describes the modifications required to use the SCC v4.0 for lidar systems equipped with polarization channels.
297
298 3.1 Modification of polarization channel parameters
299 ---------------------------------------------------
300
301 In what it follows it is assumed you already have registered one or more lidar configurations in the SCC database and that such configurations have been already used to produce optical products (aerosol extinction and/or backscatter coefficients) by means of the SCC v3.11.
302
303 Let's assume your 3+2 system is registered in the SCC database and the settings used by the SCC v3.11 are the ones summarized in table 3.1.
304
305 :Table 3.1: Example of configuration in SCC v3.11
306
307 +----------------+--------------+----------------+-------------+-----------+
308 | Channel Name | Channel ID | Channel Type | nighttime | daytime |
309 +----------------+--------------+----------------+-------------+-----------+
310 | 355 | 1 | elT | x | x |
311 +----------------+--------------+----------------+-------------+-----------+
312 | 387 | 2 | vrRN2 | x | |
313 +----------------+--------------+----------------+-------------+-----------+
314 | 532 cross | 3 | elCP | x | x |
315 +----------------+--------------+----------------+-------------+-----------+
316 | 532 parallel | 4 | elPP | x | x |
317 +----------------+--------------+----------------+-------------+-----------+
318 | 607 | 5 | vrRN2 | x | |
319 +----------------+--------------+----------------+-------------+-----------+
320 | 1064 | 6 | elT | x | x |
321 +----------------+--------------+----------------+-------------+-----------+
322
323 We assume there are 2 system configurations called “nighttime” and “daytime”. The nighttime configuration contains all the available lidar channels (in order to calculate, for example, the aerosol extinction at 355 and 532nm and the aerosol backscatter at 355, 532 and 1064nm) while in daytime conditions only elastic channels are used (only elastic backscatter coefficients are generated).
324
325 To make these settings working with SCC v4.0 it is needed to modify :underline:ONLY` the products properties involving the polarization channels (532 cross and parallel). All the products not involving the polarization channels **DO NOT** need any modification and should work in the SCC v4.0 exactly as they did in SCC v3.11. In the example above the aerosol extinction and backscatter coefficient at 355nm, the extinction at 532nm as well as the backscatter coefficient at 1064nm do not required any
326 modification. Let's focus on the modifications needed for the calculation of backscatter at 532nm.
327
328 .. figure:: media/figure3.1.png
329 :height: 369
330 :width: 1037
331 :scale: 100 %
332 :align: center
333
334 **Figure 3.1**: How to select signal types
335
336 The first modification concerns the settings of the channel type for the 532 cross and 532 parallel polarization channels. Starting from SCC v4.0 polarization channels are identified as transmitted and reflected polarization channels and not on the base of their polarization state. So suppose if we suppose the cross polarized channel is transmitted by a polarizer beam splitter cube and the parallel is reflected the value reported in table 3.1 should be modified as they appear in table 3.2. So using the SCC web interface, the signal type of the 532 cross channel should be changed from :code:`elCP` to :code:`elPT` and in the same way the 532 parallel channel should be changed from :code:`elPP` to :code:`elPR` (see figure 3.1).
337
338 **Table 3.2:** The same of table 3.1 but with new channel types
339 introduced in SCC v4.0
340
341 +----------------+--------------+----------------+-------------+-----------+
342 | Channel Name | Channel ID | Channel Type | nighttime | daytime |
343 +----------------+--------------+----------------+-------------+-----------+
344 | 355 | 1 | elT | x | x |
345 +----------------+--------------+----------------+-------------+-----------+
346 | 387 | 2 | vrRN2 | x | |
347 +----------------+--------------+----------------+-------------+-----------+
348 | 532 cross | 3 | **elPT** | x | x |
349 +----------------+--------------+----------------+-------------+-----------+
350 | 532 parallel | 4 | **elPR** | x | x |
351 +----------------+--------------+----------------+-------------+-----------+
352 | 607 | 5 | vrRN2 | x | |
353 +----------------+--------------+----------------+-------------+-----------+
354 | 1064 | 6 | elT | x | x |
355 +----------------+--------------+----------------+-------------+-----------+
356
357 The other change about the polarization channels required to run the SCC v4.0 is the definition of the polarization crosstalk parameters for all the polarization channels available. Such parameters can be defined for each polarization channel using the SCC web interface (see figure 3.2). In particular among the channel parameters there is a new tab called *Polarization crosstalk parameters* where it is possible to insert the values from for the parameters *G* and *H* and the corresponding statistical and systematic errors if available. In case you have measured *G* and *H* for your polarization channels please insert the corresponding values there. Otherwise you can insert the ideal values as reported in table 1.1.
358
359 .. figure:: media/figure3.2.png
360 :height: 479
361 :width: 1890
362 :scale: 100 %
363 :align: center
364
365 **Figure 3.2:** Polarization crosstalk parameters tab in channel properties (SCC v4.0).
366
367 3.2 Definition of new calibration configuration and product
368 -----------------------------------------------------------
369
370 In this section we will see how to set the polarization calibration parameters: the calibration constant (called :math:`\eta^*` in section 1.3) and the correction to calibration constant (called K in section 1.3). In order to provide such parameters you need to define a new system configuration to be used **ONLY** for calibration purposes. Such new configuration should include the polarization channels in the measurement configuration used for the calibration. Let's suppose we want to use the :math:`\Delta90` calibration method.
371
372 In this case we need to define a new configuration (called for example “depol_calibration”) as reported in the table 3.3. As you can see the configuration “depol\_calibration” includes 4 “new” channels. Actually the channels “532 cross +45 degrees” (channel ID=10) and “532 cross -45 degrees” (channel ID=12) refer to the same physical channel “532 cross” reported with channel ID=3 in table 3.2. Anyway we need to define two new channel IDs to identify the “532 cross” channel in the two polarization rotated configurations (+45 and -45 degrees) needed to apply the D90 calibration method. The same is true for the “532 parallel” channel. The polarization rotated channels should be labeled with the corresponding signal type as reported in table 3.3 (see figure
373 3.1).
374
375 **Table 3.3:** Polarization calibration configurations assuming D90
376 calibration method
377
378 +----------------------------+--------------+----------------+----------------------+
379 | Channel Name | Channel ID | Channel Type | depol_calibration |
380 +----------------------------+--------------+----------------+----------------------+
381 | 532 cross +45 degrees | 10 | +45elPT | x |
382 +----------------------------+--------------+----------------+----------------------+
383 | 532 parallel +45 degrees | 11 | +45elPR | x |
384 +----------------------------+--------------+----------------+----------------------+
385 | 532 cross -45 degrees | 12 | -45elPT | x |
386 +----------------------------+--------------+----------------+----------------------+
387 | 532 parallel -45 degrees | 13 | -45elPR | x |
388 +----------------------------+--------------+----------------+----------------------+
389
390 Finally we should add to the configuration “depol_calibration” a product “*Linear polarization calibration”* to be used for the calibration. According to the example given above and to the usecase document attached we should use an usecase=4 for this example.
391
392 Other “*Linear polarization calibration”* options to be specified are reported in figure 3.3. The most important factor you should insert here is the *Pol calibration correction factor* (K). The ideal value for this parameter is 1. Anyway if you have measured the parameter K please fill in the measured value and the corresponding measurement errors.
393
394 .. figure:: media/figure3.3.png
395 :height: 495
396 :width: 1887
397 :scale: 100 %
398 :align: center
399
400 **Figure 3.3:** Options for *Linear polarization calibration product*.
401
402 As you can see it is possible to fill in only the K correction factor and not the calibration constant :math:`\eta^*`.
403
404 Actually for a **LIMITED** period of time it will be possible to fill in also the constant :math:`\eta^*` using a temporary tab called *Polarization calibration constant*. This has been done to provide the users with the possibility to continue to use the SCC even if an automatic calibration made by the SCC was not submitted yet. Anyway after a transition period it will be **NOT** possible to provide calibration constant using this procedure and the parameter :math:`\eta^*` can be calculated **ONLY** by the SCC as result of the submission of a proper calibration raw input dataset. The format of this input file is the same as the standard SCC input file. The only difference is that is should contain calibration measurements instead of standard measurements. Following our example, such file should contain the measurement performed at +45 and -45 degrees at 532nm. Also the channel IDs in the file should reflect the ones reported in table 3.3.
405
406 Moreover this raw input file has to contain the variables:
407 ::
408
409 double Pol_Calib_Range_Min(channels)
410 double Pol_Calib\_Range_Max(channels)
411
412 where to specify the altitude ranges in meters in which the polarization calibration should be done.
413
414 According to the table 3.3 this file should be something similar to:
415 ::
416
417 dimensions:
418 channels = 4 ;
419 nb\_of\_time\_scales = 1 ;
420 points = 16380 ;
421 scan\_angles = 1 ;
422 time = UNLIMITED ; // (3 currently)
423 variables:
424 int channel\_ID(channels) ;
425 double Background\_Low(channels) ;
426 double Background\_High(channels) ;
427 int id\_timescale(channels) ;
428 double Laser\_Pointing\_Angle(scan\_angles) ;
429 int Molecular\_Calc ;
430 int Laser\_Pointing\_Angle\_of\_Profiles(time, nb\_of\_time\_scales) ;
431 int Raw\_Data\_Start\_Time(time, nb\_of\_time\_scales) ;
432 int Raw\_Data\_Stop\_Time(time, nb\_of\_time\_scales) ;
433 int Laser\_Shots(time, channels) ;
434 double Raw\_Lidar\_Data(time, channels, points) ;
435 double Pressure\_at\_Lidar\_Station ;
436 double Temperature\_at\_Lidar\_Station ;
437 double Pol\_Calib\_Range\_Min(channels) ;
438 double Pol\_Calib\_Range\_Max(channels) ;
439
440 // global attributes:
441 :System = "mysystem" ;
442 :Longitude\_degrees\_east = 15.723771 ;
443 :RawData\_Start\_Time\_UT = "220000" ;
444 :RawData\_Start\_Date = "20130620" ;
445 :Measurement\_ID = "20130620po00" ;
446 :Altitude\_meter\_asl = 760. ;
447 :RawData\_Stop\_Time\_UT = "230333" ;
448 :Latitude\_degrees\_north = 40.601039 ;
449
450 data:
451 channel\_ID = 10, 11, 12, 13 ;
452
453 Background\_Low = 30000, 30000, 30000, 30000 ;
454
455 Background\_High = 50000, 50000, 50000, 50000 ;
456
457 id\_timescale = 0, 0, 0, 0 ;
458
459 Laser\_Pointing\_Angle = 0 ;
460
461 Molecular\_Calc = 0 ;
462
463 Laser\_Pointing\_Angle\_of\_Profiles =
464 0,
465 0,
466 0 ;
467
468 Raw\_Data\_Start\_Time =
469 0,
470 300,
471 600 ;
472
473 Raw\_Data\_Stop\_Time =
474 210,
475 510,
476 810 ;
477
478 Laser\_Shots =
479 1200, 1200, 1200, 1200,
480 1200, 1200, 1200, 1200,
481 1200, 1200, 1200, 1200 ;
482
483 Pressure\_at\_Lidar\_Station = 1010 ;
484
485 Temperature\_at\_Lidar\_Station = 14 ;
486
487 Pol\_Calib\_Range\_Min = 1000, 1000, 1000, 1000 ;
488
489 Pol\_Calib\_Range\_Min = 2000, 2000, 2000, 2000 ;
490
491 Raw\_Lidar\_Data = …...;
492
493 The file above assume the following calibration measurements have been done:
494
495 1. First +45 degrees acquisition followed by a corresponding -45 degrees acquisition
496
497 a. Measurement at +45 degrees
498
499 Start Time: 20130620 22:00:00
500
501 Stop Time: 20130620 22:01:00
502
503 Shots: 1200
504
505 b. Measurement at -45 degrees
506
507 Start Time: 20130620 22:02:30
508
509 Stop Time: 20130620 22:03:30
510
511 Shots: 1200
512
513 2. Second +45 degrees acquisition followed by a corresponding -45 degrees acquisition
514
515 a. Measurement at +45 degrees
516
517 Start Time: 20130620 22:05:00
518
519 Stop Time: 20130620 22:06:00
520
521 Shots: 1200
522
523 b. Measurement at -45 degrees
524
525 Start Time: 20130620 22:07:30
526
527 Stop Time: 20130620 22:08:30
528
529 Shots: 1200
530
531 3. Third +45 degrees acquisition followed by a corresponding -45 degrees acquisition
532
533 a. Measurement at +45 degrees
534
535 Start Time: 20130620 22:10:00
536
537 Stop Time: 20130620 22:11:00
538
539 Shots: 1200
540
541 b. Measurement at -45 degrees
542
543 Start Time: 20130620 22:12:30
544
545 Stop Time: 20130620 22:13:30
546
547 Shots: 1200
548
549 As you can see there are 3 cycles of consecutive measurements at +45 and -45 degrees. That way the dimension :code:`time` is set to 3.
550
551 The first +/-45 degrees measurement starts at “20130620 22:00:00” (start time of the first +45 measurement) and stops at “20130620 22:03:30” (stop time of the fist -45 measurement). As a consequence, according to the values of the global attributes :code:`RawData\_Start\_Date` and :code:`RawData_Start_Time_UT` we have to set:
552
553 :code:`Raw_Data_Start_Time[0]=0` (start of the first +45 measurement in
554 seconds since :code:`RawData_Start_Time\_UT`)
555
556 :code:`Raw_Data_Stop_Time[0]=210` (stop of the first -45 measurement in
557 seconds since :code:`RawData_Start_Time_UT`)
558
559 Following a similar procedure for the other 2 cycles we have:
560
561 :code:`Raw_Data_Start_Time[1]=300` (start of the second +45 measurement in seconds since :code:`RawData_Start_Time_UT`)
562
563 :code:`Raw_Data_Stop_Time[1]=510` (stop of the second -45 measurement in seconds since :code:`RawData_Start_Time\_UT`)
564
565 :code:`Raw_Data_Start_Time[2]=600` (start of the third +45 measurement in seconds since :code:`RawData_Start_Time_UT`)
566
567 :code:`Raw_Data_Stop_Time[2]=810` (stop of the third -45 measurement in seconds since :code:`RawData_Start_Time_UT`)
568
569 Moreover, according to the order of the channels in the :code:`channel_ID` variable, the :code:`Raw_Lidar_Data` array should be filled as it follows:
570
571 :code:`Raw_Lidar_Data[0][0][points]` :math:`\rightarrow` 1\ :sup:`st` measured transmitted signal at +45 degrees
572
573 :code:`Raw_Lidar_Data[0][1][points]` :math:`\rightarrow` 1\ :sup:`st` measured transmitted signal at +45 degrees
574
575 :code:`Raw_Lidar_Data[0][2][points]` :math:`\rightarrow` 1\ :sup:`st` measured transmitted signal at -45 degrees
576
577 :code:`Raw_Lidar_Data[0][3][points]` :math:`\rightarrow` 1\ :sup:`st` measured transmitted signal at -45 degrees
578
579 :code:`Raw_Lidar_Data[1][0][points]` :math:`\rightarrow` 2\ :sup:`nd` measured transmitted signal at +45 degrees
580
581 :code:`Raw_Lidar_Data[1][1][points]` :math:`\rightarrow` 2\ :sup:`nd` measured transmitted signal at +45 degrees
582
583 :code:`Raw_Lidar_Data[1][2][points]` :math:`\rightarrow` 2\ :sup:`nd` measured transmitted signal at -45 degrees
584
585 :code:`Raw_Lidar_Data[1][3][points]` :math:`\rightarrow` 2\ :sup:`nd` measured transmitted signal at -45 degrees
586
587 :code:`Raw_Lidar_Data[2][0][points]` :math:`\rightarrow` 3\ :sup:`rd` measured transmitted signal at +45 degrees
588
589 :code:`Raw_Lidar_Data[2][1][points]` :math:`\rightarrow` 3\ :sup:`rd` measured transmitted signal at +45 degrees
590
591 :code:`Raw_Lidar_Data[2][2][points]` :math:`\rightarrow` 3\ :sup:`rd` measured transmitted signal at -45 degrees
592
593 :code:`Raw_Lidar_Data[2][3][points]` :math:`\rightarrow` 3\ :sup:`rd` measured transmitted signal at -45 degrees
594
595 Once this file has been created it needs to be submitted to the SCC and linked to the configuration “depol\_calibration”. The result of the SCC analysis on this file will be the calculation of the calibration constant h\ :sup:`\*` that will be logged into the SCC database and can be used to calibrate Raman/Elastic backscat ter products (see section 3.3).
596
597 3.3 Definition of “Raman/Elastic backscatter and linear depolarization ratio”
598 -----------------------------------------------------------------------------
599
600 In order to calculate the *PLDR* we need to modify the polarization related products linked to the “standard” measurement configurations (the configuration called “nighttime” and/or “daytime” in table 3.2).
601
602 Let's suppose we have defined the following products (defined already in SCC v3.11):
603
604 **Table 3.4:** Example of products configuration in SCC v3.11
605
606 +-----------------------+--------------+-----------------------+-------------+-----------+
607 | Product Name | Product ID | Product Type | nighttime | daytime |
608 +-----------------------+--------------+-----------------------+-------------+-----------+
609 | Raman backscatter | 1 | Raman backscatter | x | |
610 | | | | | |
611 | 355nm | | | | |
612 +-----------------------+--------------+-----------------------+-------------+-----------+
613 | Extinction | 2 | Extinction | x | |
614 | | | | | |
615 | 387nm | | | | |
616 +-----------------------+--------------+-----------------------+-------------+-----------+
617 | Raman backscatter | 3 | Raman backscatter | x | |
618 | | | | | |
619 | 532nm | | | | |
620 +-----------------------+--------------+-----------------------+-------------+-----------+
621 | Extinction | 4 | Extinction | x | |
622 | | | | | |
623 | 532nm | | | | |
624 +-----------------------+--------------+-----------------------+-------------+-----------+
625 | Elastic backscatter | 5 | Elastic backscatter | | x |
626 | | | | | |
627 | 355nm | | | | |
628 +-----------------------+--------------+-----------------------+-------------+-----------+
629 | Elastic backscatter | 6 | Elastic backscatter | | x |
630 | | | | | |
631 | 532nm | | | | |
632 +-----------------------+--------------+-----------------------+-------------+-----------+
633 | Elastic backscatter | 7 | Elastic backscatter | x | x |
634 | | | | | |
635 | 1064nm | | | | |
636 +-----------------------+--------------+-----------------------+-------------+-----------+
637
638 Product ID=1, 2, 4, 5, 7 do not need any modification as they do not involve polarization channels. The only product that need to be modified are the Product ID=3 and 6. To produce b532 files containing also *PLDR* we need to modify the “nighttime” and “daytime” configurations to include a product of type “Raman bakscatter and linear depolarization ratio” or “Elastic bakscatter and linear depolarization ratio” respectively. So the configuration reported in table 3.4 should be
639 changed to match what is included in table 3.5.
640
641 **Table 3.5:** The same of table 3.4 but with new product types introduced in SCC v4.0
642
643 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
644 | Product Name | Product ID | Product Type | nighttime | daytime |
645 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
646 | Raman backscatter | 1 | Raman backscatter | x | |
647 | | | | | |
648 | 355nm | | | | |
649 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
650 | Extinction | 2 | Extinction | x | |
651 | | | | | |
652 | 387nm | | | | |
653 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
654 | Raman backscatter | 10 | **Raman backscatter and linear depolarization ratio** | x | |
655 | | | | | |
656 | 532nm | | | | |
657 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
658 | Extinction | 4 | Extinction | x | |
659 | | | | | |
660 | 532nm | | | | |
661 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
662 | Elastic backscatter | 5 | Elastic backscatter | | x |
663 | | | | | |
664 | 355nm | | | | |
665 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
666 | Elastic backscatter | 11 | **Elastic backscatter and linear depolarization ratio** | | x |
667 | | | | | |
668 | 532nm | | | | |
669 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
670 | Elastic backscatter | 7 | Elastic backscatter | x | x |
671 | | | | | |
672 | 1064nm | | | | |
673 +-----------------------+--------------+-----------------------------------------------------------+-------------+-----------+
674
675 As you can see in table 3.5, the old product IDs=3 and 6 (present in table 3.4) have been replaced with the new product ID=10 and 11 to guarantee the calculation of *PLDR*.
676
677 It is important to set among the product options of the product ID=10 and 11 which calibration product we want to use for calibration (see section 3.2). This can be done using the SCC web interface setting the appropriate setting in the tab *Polarization calibration products* (see figure 3.4). According to the current example you should set here the calibration product defined in section 3.2.
678
679 .. figure:: media/figure3.4.png
680 :height: 102
681 :width: 1895
682 :scale: 100 %
683 :align: center
684
685 **Figure 3.4:** How to link a product to calibrate with a calibration product.
686
687 .. warning:: Please not that also *Raman/Elastic backscatter products* need to be linked to a calibration product because the calibration constant and the corresponding correction factor is needed to calculate the total signal out of the two polarization components even if the *PLDR* is not involved in the product calculation.

EARLINET Repository: SCC Documentation

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