curious question: can navigational computers (on planes, though maybe this can be asked about other systems as well) not take an initial position-reading from g

curious question: can navigational computers (on planes, though maybe this can be asked about other systems as well) not take an initial position-reading from gps or radio and then compute their location then-on based on internal gyros, compasses, accelerometers, etc.? if outside navigation aids fail, are these systems not capable of internally simulating and determining their own position, or am i underestimating how much compute power this would take?