As seen in Fig. 1, several alternatives may be included for providing the integrated inertial navigation systems with position updates. Obtaining GPS surface fixes is the most intuitive method and should be used whenever practical. The time between surfacing depends on the required navigation accuracy and additional aiding sensors. The following GPS positioning services can be used:
• GPS Standard Positioning Service (SPS)
• GPS Precise Positioning Service (PPS)
• Differential GPS (DGPS)
• Real-Time Kinematic GPS (RTKGPS)
GPS SPS is available to all users worldwide. GPS PPS is available only to authorised users and primarily intended for military purposes. GPS PPS receivers should be the choice for military AUVs, at least for operations in denied areas. Compared to SPS, PPS is more resistant to jamming and deception. SPS and PPS have comparable accuracy. AUVs for detailed seabed mapping will typically be equipped with DGPS, or in some cases even RTKGPS. An example of a mission where GPS was applied is shown in Fig. 3.
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