GPSbabel: how do I define the layout of my input data

I have collected GPS data on a journey by extracting it every second from the $GPRMC NMEA sentence, with each reading saved in the form:

time,latitude,longitude,speed,direction

a typical couple of lines are.

163044,5050.8427,-00049.8211,047.4,107.6
163045,5050.8387,-00049.8012,047.2,106.5

Note: I am using Linux, and a Processing sketch to extract the data and save it in a file made unique by including the date and time of its creation in its name, e.g. posns20150321131751.txt

I intend to plot this as a trace on an OpenStreetMap which requires data in osm format.

This is my Linux command line and error message:

[Harry@localhost]~/Gpsbabel% gpsbabel -i xcsv,style=/home/Harry/Gpsbabel/myidata.style -f /GPStalk/posns20150321131751.txt -o osm ~/GPStalk/posn01.osm 
CSV_UTIL : xcsv style "%H%M%S" is missing default. 

My style file myidata.style is:

FIELD_DELIMITER    COMMA
IFIELD GMT_TIME,"","%H%M%S"
IFIELD LAT_NMEA
IFIELD LON_NMEA
IFIELD IGNORE
IFIELD IGNORE

I find the documentation in the GPSBabel documentation difficult to follow, so my question is, please, where have I gone wrong? In particular, what is wrong with my style file myidata.style?