Since the beginning of warp travel there have been those psykers that are able to guide the spacecraft across the vast expanses of space using the beacons of light that are populated planets. Since the rise of the Astronomicon and the Emperor's light, travel has become easier and faster for these individuals.
Over the millenia these Navgators have intermarried and there are now vast families located amongst the stars, rarely setting foot on a planet for very long before entering space again. Life amongst the start has it's drawbacks, the low gravity had made them tall and spindly to look at with fairly weak bones, unused to the pressures of planetary gravity.
Psi-track, telepathy and sanctuary are all 15 difficulty points less (down to 0) for navigators to cast as they are constantly using these kind of skills whilst plotting courses through the warp, using the Astronomicon as their psycic beacon. All others are at +10 difficulty points as they really aren't combat orientated at all.
Navigators will have access to all forms of equipment as they are constantly travelling amongst the stars.