Description of SPD_ASCII format.

   A file in the SPD_ASCII format contains results of computing slant path 
delay of electromagnetic wave propagation through the 3D, continuous, 
heterogeneous atmosphere from the outer space to a list of stations. 
The results include 1) slant path delay at an elevation-azimuth grid; 
b) surface atmospheric pressure; c) surface air temperature; d) (optionally) 
optical thickness of the atmosphere at a station at the specified 
frequencies; e) (optionally) atmosphere brightness temperature at a station 
at specified frequencies.

   The path delay is defined as a difference in the propagation time between 
the emitter and the receiver through the atmosphere and the propagation 
between these two points along the geodetic line in vacuum. The path delay 
is computed for a set of stations for a certain epoch at a grid 
of azimuths and elevations. The grid is equidistant at azimuths and 
non-equidistant at elevation angles. The values of path delays computed 
on a grid can be used for computing coefficients of the interpolating 
functions that enable us to compute the path delay in an arbitrary direction.

   A file in the SPD_ASCII format consists of records of variable
length in ASCII coding. The records are separated by a character with
decimal code 13. The records are grouped into sections. Sections follow
in this order:

 1) Header record -- the first record should have a signature:
                     SPD_ASCII  Format version of 2008.11.30

                     The header record allows to distinguish a valid
                     file in the SPD_ASCII  format from files in other
                     formats.

 2) N-record -- defines the number of records in further other
                sections.

 3) M-record -- provides text with a very brief algorithm description and
                options used during computations.

 4) I-record -- provides text with a brief description of the numeric
                model of the atmosphere used during computation.

 5) U-record -- defines which components of the path delay have been computed.

 6) T-record -- defines the time epoch of the path delay.

 7) F-record -- defines frequencies for atmosphere optical thickness
                and brightness temperature computation.
                NB: F-record may be missing.

 8) S-record -- defines name and coordinates of stations for which path
                delay have been computed.

 9) E-record -- defines elevations for which path delay is computed.

10) A-record -- defines azimuths for with the path delay is computed.

11) P-record -- defines the atmospheric pressure and air temperature
                for each stations.

12) D-record -- defines the path delay for each component and each station.

13) O-record -- defines the optical thickness of the atmosphere and 
                atmosphere brightness as seen at each station.
                NB: O-record may be missing.

14) Trailer record -- the last record is the same as the header record.


Format of an N-record:
----------------------

Field  1:1   A1    -- Record ID: letter "N". 
field  2:3   a2       delimiter: blank.
Field  4:7   I4    -- Number of M-records in the file. i.e. brief algorithm
                      description and options used during computations.
field  8:9   a2       delimiter: blank.
field 10:13  I4    -- Number of I-records in the file, i.e. the number of
                      lines with a brief description of the numeric
                      model of the atmosphere used during computation.
field 14:15  a1       delimiter: blank.
field 16:21  I7    -- Number of S-records in the file, i.e. the number
                      of stations.
field 22:23  a2       delimiter: blank.
field 24:27  I4    -- Number of E-records in the file, i.e. the number
                      elevations in the grid.
field 28:29  a2       delimiter: blank.
field 30:33  I4    -- Number of A-records in the file, i.e. the number
                      azimuths in the grid.
field 34:35  a2       delimiter: blank.
field 36:39  I4    -- Number of F-records in the file, i.e. the number
                      frequencies. It may be zero.


Format of an M-record:
----------------------

Field  1:1   A1    -- Record ID: letter "M".
field  2:3   a2       delimiter: blank.
Field  4:7   I4    -- Index of the current M-records counted from 1.
field  8:9   a2       delimiter: blank.
Field 10:73  A64   -- arbitrary characters in the range 32-255 (decimal).


Format of an I-record:
----------------------

Field  1:1   A1    -- Record ID: letter "I".
field  2:3   a2       delimiter: blank.
Field  4:7   I4    -- Index of the current M-records counted from 1.
field  8:9   a2       delimiter: blank.
Field 10:73  A64   -- arbitrary characters in the range 32-255 (decimal).


Format of a U-record:
---------------------

Field  1:1   A1    -- Record ID: letter "U".
field  2:3   a2       delimiter: blank.
field  4:6   A3    -- Code of the first component of delay.
                      One of TOT, WAT
field  7:8   a2       delimiter: blank.
field  9:11  A3    -- Code of the second component of delay.
                      One of TOT, WAT
field 12:13  a2       delimiter: blank.
field 14:16  A3    -- Code of the third component of delay.
                      One of TOT, WAT or blank.

Meaning of components codes:

        TOT -- total path delay.

        WAT -- Path delay due to contribution of partial pressure of
               water vapor.

Format of a T-record:
---------------------

Field  1:1   A1    -- Record ID: letter "T".
field  2:3   a2       delimiter: blank.
field  3:27  A27   -- Date and time for path delay in TAI.
                      Format of the date : YYYY.MM.DD-hh:mm:ss.ffff
                      where YYYY -- integer year
                              MM -- integer month;
                              DD -- integer day of the month;
                              hh -- integer hour;
                              mm -- integer minute after the hour;
                              ss -- integer second after the minute;
                            ffff -- integer second fraction.


Format of an F-record:
----------------------

Field  1:1   A1    -- Record ID: letter "F".
field  2:3   a2       delimiter: blanks.
field  4:7   I4    -- frequency index.
field  8:9   a2       delimiter: blanks.
field 10:24  F15.2 -- Frequency of electromagnetic wave.


Format of an S-record:
----------------------

Field  1:1   A1    -- Record ID: letter "S".
field  2:3   a1       delimiter: blanks.
field  4:9   I7    -- station index
field 10:11  a2       delimiter: blanks.
Field 12:19  A8    -- 8-letter long site identifier. May contain
                      any characters with decimal codes 32-255, but
                      blanks are allowed only at the end of the site
                      identifier. This site identifier should be unique
                      among S-records. This field should not
                      necessarily have a special meaning. Parsing
                      software SHOULD NOT TRY TO INTERPRET it.
field 20:21  a2       delimiter: blanks.
Field 22:33  F12.3 -- X site coordinate in a crust fixed reference
                      frame. Units: meters.
field 34:34  a1       delimiter: blank.
Field 35:46  F13.4 -- Y site coordinate in a crust fixed reference
                      frame. Units: meters.
field 47:47  a1       delimiter: blank.
Field 48:59  F13.4 -- Z site coordinate in a crust fixed reference
                      frame. Units: meters.
field 60:61  a2       delimiter: blanks.
Field 62:69  F8.4  -- Site geocentric latitude, positive to north,
                      in degrees. This field is for information only.
                      Parsing software MUST ignore it.
field 70:70  a1       delimiter: blank.
Field 71:78  F8.4  -- Site longitude, positive towards east,
                      in degrees. This field is for information only.
                      Parsing software MUST ignore it.
field 79:80  a2       delimiter: blank.
Field 81:86  F6.1  -- Site height above the reference ellipsoid in
                      meters. This field is for information only.
                      Parsing software MUST ignore it.
field 87:87  a1       delimiter: blank.
Field 88:93  F6.1  -- Site height above the geoid in meters. This field
                      is for information only.
                      Parsing software MUST ignore it.


Format of an E-record:
----------------------

Field  1:1   A1    -- Record ID: letter "E".
field  2:3   a2       delimiter: blanks.
field  4:7   I4    -- elevation record index.
field  8:9   a2       delimiter: blanks.
field 10:19  F10.6 -- elevation angle above the horizon in degrees:
                      the angle between the direction to the emitter,
                      undisturbed by refraction and the plane perpendicular
                      to the normal to the reference ellipsoid.


Format of an A-record:
----------------------

Field  1:1   A1    -- Record ID: letter "A".
field  2:3   a2       delimiter: blanks.
field  4:7   I4    -- azimuth record index.
field  8:9   a2       delimiter: blanks.
field 10:19  F10.6 -- azimuth of the emitter in degrees, undisturbed by
                      refraction, counted from North to East.


Format of an P-record:
----------------------


Field  1:1   A1    -- Record ID: letter "P".
field  2:3   a1       delimiter: blanks.
field  4:9   I7    -- station index
field 10:11  a2       delimiter: blanks.
field 12:19  F8.1  -- atmospheric pressure at the station in Pa.
field 20:21  a2       delimiter: blanks.
field 22:29  F8.2  -- partial pressure of water vapor at the station in Pa.
field 30:31  a2       delimiter: blanks.
field 32:36  F5.1  -- air temperature at the station in Kelvin.


Format of a D-record:
---------------------

Field  1:1   A1    -  - Record ID: letter "F".
field  2:3   a1         delimiter: blanks.
field  4:9   I7      -- station index
field 10:11  a2         delimiter: blanks.
field 12:15  I4      -- elevation index
field 16:17  a2         delimiter: blanks.
field 18:21  I4      -- azimuth index
field 22:23  a2         delimiter: blanks.
field 24:35  1PD12.6 -- path delay, first component, in sec.
field 36:37  a2         delimiter: blanks.
field 38:49  1PD12.6 -- path delay, second component, in sec.

NB:  fields 38:49 can be omitted


Format of a O-record:
---------------------

Field  1:1   A1    -  - Record ID: letter "F".
field  2:3   a1         delimiter: blanks.
field  4:9   I7      -- station index
field 10:11  a2         delimiter: blanks.
field 12:15  I4      -- elevation index
field 16:17  a2         delimiter: blanks.
field 18:21  I4      -- azimuth index
field 22:23  a2         delimiter: blanks.
field 24:27  I4      -- frequency index
field 28:29  a2         delimiter: blanks.
field 30:35  F6.4    -- Atmosphere optical thickness, i.e. natural
                        logarithm of the ratio of the brightness temperature
                        of a source at the top of the atmosphere to the
                        brightness temperature at the station.
field 36:37  a2         delimiter: blanks.
field 38:43  F6.2    -- Brightness of the atmosphere at the station in K.

NB: record O may be missing.
