Note that a skeleton file contains RINEX header information such as receiver and antenna types. In case of stream conversion to RINEX Version 3, a skeleton file should also contain information on potentially available observation types. A missing skeleton file will force BNC to only save a default set of RINEX 3 observation types.
++A skeleton file carrying only RINEX Version 2 style observation types (2-character codes, e.g. 'C1', 'L1') cannot supply the tracking-mode +attribute a RINEX Version 3/4 header needs, so BNC will not use such a skeleton when producing Version 3/4 files. It is treated the same +way as a missing skeleton: with 'Skeleton mandatory' ticked, no RINEX file is produced for the affected stream; otherwise BNC falls back +to a default set of RINEX 3/4 observation types. Conversely, a skeleton carrying RINEX Version 3/4 style observation types +(3-character codes) is always usable when producing RINEX Version 2 files, because the Version 3/4 codes can be unambiguously mapped +down to Version 2 codes (see section 'Version 2'). +
+For the same reason, a RINEX header skeleton file that only carries RINEX Version 2 style observation types +(2-character codes) cannot be used to build a RINEX Version 3/4 header. BNC ignores such a skeleton for that +purpose and falls back to a default set of RINEX 3/4 observation types instead, see section 'Skeleton Mandatory'.