Processing of dual-band data with Calc/Solve.
---------------------------------------------

  It should be noted that Calc/Solve treatment of dual-band is a little 
bit tricky. Due to historical reasons, Calc/Solve considers observations 
at two bands as different experiments. This approach was justified for 
processing early data at site ONSALSA. That site has two antennas: ONASLA85
which had S-band only receiver and ONSALA60 which before 1988 did not 
had S-band received, but had only X-band receiver. This was an exceptional
transitional situation, but, unfortunately, for a long it was treated as 
a rule. Program DBEDIT creates two separate files for a VLBI experiment:
one for low band (S), another for the high band (X). Calc/Solve does not
have an ability to process gently data with three or mode bands.

  Initially, the database version 1 that has letter X in the suffix of its 
name has only results of post-correlator processing of X-band data, and the 
database with letter S in the suffix has results of processing of S-band 
data. Such databases are called incomplete. In  order to make the database 
complete a user should 1) first load the database with letter X in the suffix;
2) load the database with letter S in the the suffix; 3) run either program
GAMB, which called by striking letter A in OPTIN menu, or program IONO 
which called by striking letter [ in OPTIN menu. These programs are for 
resolving group delay ambiguities and computing ionospheric contribution to 
delay (GAMB) or only for computing ionospheric contribution to the time 
delay (IONO). In addition to their main business, they performs the second 
very important function: they copy 27 lcodes with S-band related information 
from the database file with suffix S in its name to the database with 
suffix X in its name. After that the database with suffix S will not be needed 
for Calc/Solve, since Solve can find everything that it is able to digest in 
the database with suffix X in its name. GAMB and IONO must be the first 
programs which are executed when an incomplete database is processed. 
The database becomes complete.

  A complete database contains 8 observables per observation:

    X-band group delay
    X-band phase delay
    X-band single band delay (or narrow-band delay)
    X-band phase delay rate
    S-band group delay
    S-band phase delay
    S-band single band delay (or narrow-band delay)
    S-band phase delay rate

  When Solve builds the equations of conditions which arise in the LSQ problem
it puts a linear combination of these eight observables in the right-hand 
side. The specific choice of the coefficients of of this linear combination is 
called "solution type". Calc/Solve supports 21 solution types for complete
database and 8 solutions types for an incomplete database.

  Solution type is selected by hitting + in the last page of OPTIN menu
in interactive solutions, or by specifying  keyword TYPE in the $DATA section
of a batch control file. Solution types, which are unavailable for an 
incomplete database, are marked with asterisk in the menu.

  Among other solutions types, Solve supports "Group delay X-band" and
"Group delay S-band" solution type. So one can make S-band solution using
the database with suffix with letter "X". In the past database with letter
X in the suffix were called "X-band database", and database with letter S
in the suffix were called "S-band databases". This is misleading. A complete
database with letter X in the suffix should be considered as an XS database.
S-band in the XS databases were temporarily separated from the X-band database 
by DBEDIT and happily re-united together with Solve a little bit later. 

  So, a compete database has both X and S band data. An interactive Solve 
allows to switch solution type any time using menu. In order to make an S-band
solution, one should not look for another database file, but it is sufficient
to select "group delay S-band" in the many at the last page of OPTIN, run
solution and get residuals. Results will be the same as if the database file
with letter S in the suffix were loaded.

  It is legitimate to make the following trick: to run interactive solution
with one solution type, for example, "group delay S-band", then switch solution 
type, for instance to "group delay X-band" and then compute residuals 
once again without running new solutions, by hitting key @ in the OPTIN
menu. This trick will produce X-band residuals with respect to the S-band group 
delay solutions. The technique like that may be useful for resolving group 
or phase ambiguities in difficult cases.

  If the database is incomplete, then the only way to use dual-band 
ionosphere-free linear combination of observables is to select solution
type "Group delay only" and apply so-called calibration for ionosphere.
This means that a quantity <ionosphere free linear combination of X/S group 
delay observables minus X-band group delay> previously computed and stored
in the databases will be added to group delay observables. This is equivalent
to using "G-GXS combination" soltution type for a complete database. 
Sometimes we have to use incomplete databses because S-band data were either
unavailable or lost, but ionosphere contribution can be obtained from
external datasets.

  A caveat. Although Solve supports 21 solutions types, it has only
two slots for suppression flags: one for all group delay type of observation
types and another for phase delay observation types. This means that a user
switches from one group delay solution type to another, for example from
"G-Gxs combination" to "Group delay S-band", the same suppression status
will be applied which may not always be desirable.

  Read also $MK5_ROOT/help/elim_05.hlp
