Towers are only necessary for the maps, unless the app has the capability of predownloading the necessary mapset before loss of cell signal. Backcountry navigator has this capability for example. This allows the user to take advantage of the gps antenna when no cell signal is avail. In the earlier days I wo,dered why one couldnt just get the coordinates from the gps antenna so one didnt need a digital map, ala the old gps devices. But then bcnav came around with the downloading maps options.
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