The Cobra-Tac (Teledyne RD Instruments) is a self-contained diver navigation system based on acoustic Doppler velocity log (DVL) technology that uses dead reckoning to compute displacements from a known starting point The navigational accuracy of the system was evaluated using a series of field tests with known solutions Initial test data had an obvious directional bias in the navigation measurements. with positional errors as large as 5% of the track length An analysis of this error showed that the DVL speed measurements were extremely accurate and that most of the position error could be explained by heading-dependent compass error Compass error versus heading curves were different depending on whether a given route was mapped in a clockwise or counterclockwise direction. and further testing indicated that this was clue to a combination of local magnetic field anomalies at the test site and instrument tilt A post processing correction procedure. based on a one-cycle compass error model, was derived that significantly improves Cobra-Tac position estimates After correction. position errors were well within manufacturer specifications Further tests using new firmware that permitted in situ underwater compass calibration gave position errors of less than 1% of the track length This level of accuracy should be sufficient for a wide range of scientific applications. Collectively. the test results and analyses indicate that Cobra-Tac can give very accurate navigation results, but the accuracy depends strongly on compass calibration and diver skill with the system