Spacecraft-Based Navigation Spacecraft-borne navigation measurement tends more to optical frequency direction measurement rather than the radio frequency direct ranging that is so accurate for ground tracking. For relatively close work, however, direct ranging using radio frequencies with rendezvous or landing radars becomes possible, albeit necessary. But further from the planets and other targets, direct measurement of range or range rate, or the use of radio frequencies has not appeared attractive to the designers due to the weight and power penalties. Spacecraft onboard directional measurements are those made to the near bodies... the sun, moon, earth, and other planets. The stars provide no position data because of their extreme distances. But because of this distance they are most excellent references against which to measure directions to the nearer bodies.
In a sense, then, onboard navigation is performed by observing the near bodies relative to the background stars. This can be done indirectly by measuring the angles sequentially from a gyro stabilized base to the stars and the near body. Alternately a direct and simultaneous measurement of the angle between a reference star and the near body with a suitable sextant-like instrument avoids an accumulation of errors with which the former sequential technique must cope. The ancient sextant, updated and refined with a suitable telescope for image resolution and with a precision angle readout of the deflecting mirror, can provide in a reasonable size an accurate measure of the angle between a feature of a near body and a star superimposed upon that feature in the field of view.
The "feature" alluded to above is some distinct point of known coordinates on the planet to which the direction is being measured. The center of the planetary disk naturally comes to mind, but identifiable surface landmark features and horizons which can be related to planet coordinates are easier and more accurate for visual use, particularly under crescent illumination.
Post 202 was supposed to be directed to you. You would enjoy learning the mechanics of Apollo navigation.