Bah! Be a man and use dead rekoning

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If you don't mind the fairly high workload, there's always "pure" radio navigation. Couple RN with the FS GPS, or the RealityXP add-on GPS units (all three of them... the Apollo, and the Garmin 430 and 530 units...) and you have more than enough for online flying (which of course you are already doing as a Sim-Pilot

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FS-Nav and/or FS-Build 2 are the logical step up from RN and GPS.
Then top of the pile, and as already mentioned, the bigger birds often have an FMC (PMDG, LevelD, PSS and a number of others have really accurate FMC simulations): Project Magenta do both Boeing-type and Airbus-type separate FMS's when you are ready to take your simulation to the next level, and of course, there are a few companies that make plug'n'go USB/serial/keyboard CDU units to control the PM CDU (just need to do the final building of my FDS Elite CDU and couple it to my PM GC/CDU comp... so much unpacking, so little time...)