No, the NMEA 0183 is a very loosely defined protocol. If you want to make a system that is compatable with more than one perticular brand of GPS units, then you have to count the commas. The size of the fields can and does change. I encountered a similar problem/situation during my final year project. some fields had zero (0) size. that is, the start of field comma was immediately followed by another comma for the next field.
My design used four septate controllers 3 x AT89C2051 and one AT89C52. Two of the 2051s (I call them port controllers) were used to gather data from electronic compass and the GPS unit. The port controllers acted as buffers, getting the required data from monitering the NMEA output. The system would wait for serial NMEA data to arive and check if the recieved character was a "$". $ is the string start character. after this character, a two character device ID and a three character string ID is sent. if the string ID is the desired one, continue to count commas till the required field is reached. get the data and store it at some location in the RAM.if you want to get multiple fields, continue with counting the field limit commas. else return to checking the "$" character.
Hope this helps. If you want, I can send a portion of my code to you.(Sorry cant send the entire file.... corporate restrictions).

take care.
Musharraf Hanif

ps. this is my first post to this forum. if there are some shortcomings(like detail, clarity,etc) plz infom me