Underwater Navigation
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There are a number of methods that are used for underwater navigation but by far the most practical for gliders is a simple dead reckoning.

Dead reckoning
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This is the basic way all gliders currently navigate when underwater. The only inputs available to the glider at this point are its heading, attitude and depth. Therefore the glider inputs these parameters in its flight model and calculates where it thinks it should be at any moment. Once it completes a yo and surfaces, it can then measure the difference between where it thought it surface with where it actually  ended up by getting a new GPS fix, therefore calculating the average current faced by the glider during that yo which is called the depth-averaged current. 

Under-Ice Navigation
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This is of great interest to scientists due to the Arctic's critical role in regulating the global climate, coupled with the fact that it remains one of the least studied regions on Earth. Due to their endurance, gliders are an ideal platform for this operation; however, developing a power-efficient navigation method that eliminates the need for deploying massive infrastructure remains a significant challenge.