As far as I can tell this file type is proprietary for Trimble hardware and will need to be converted with Trimble software....
In the field, one of the first actions performed is to open a file into
which your data will be stored. Some Trimble mapping receivers can
record two types of data files: rover and base. Data is also stored as
waypoints and in an almanac file. Each of these file types is explained
in the following sections.
These files are stored in a Trimble proprietary file format known as
Standard Storage Format (SSF). After being transferred to a PC these
files usually have the extension .ssf. In postprocessing, new files are
created. Although these files are named with a variety of other
extensions (such as .cor, .adj, .ssx) they are also stored in the SSF
format. Files created by the export modules of the postprocessing
software are an exception. These files are named with extensions
unique to your GIS, CAD ASCII, or database format."
From this link