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Garmin Gps/sonar combo internal or external antenna, dual or single frequency?

• I want to buy a new GPS/SONAR for my new boat. Anyone have one with an internal antenna? I would like to know if you ever lose signal. Whats the difference between the single and double frequency? This is the model Im looking at buying, Eagle SeaCharter 642C DF iGPS but Im interested in the new lowrance hds models also. Any info would be appreciated.

• Answer:

  Internal will be fine if the unit is mounted outside the cabin (I'm not a boater so I don't know if that is the proper term). If its mounted "indoors" then you should go with the external antenna. Single vs dual frequency in GPS terms means whether or not you can track L1 only (i.e. single freq) or L1 and L2 (i.e. dual freq). Generally, dual frequency receivers in GPS have a specific [high-end] use (engineering or military). Its not likely to do the consumer any good. However, looking at the spec sheet for this model (http://www.eaglenav.com/en/Products/Fishfinder-Chartplotters/SeaCharter-642C-DF-iGPS/), I believe the dual freq refers to the sonar, not GPS. (I have no knowledge of sonars, but I suspect this means the sonar operates at 2 frequencies for better precision)