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 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