> What if we can only carry one sensor? > That is what happened exactly in the Apollo rocket, back in the 60s the IMUs were veeery heavy and they could only ca

> What if we can only carry one sensor? > That is what happened exactly in the Apollo rocket, back in the 60s the IMUs were veeery heavy and they could only carry one. The first and most famous application of a Kalman filter in the Apollo program was for the problem of midcourse navigation. In this application, the Apollo PGNCS used the Kalman filter to combine the calculated trajectory based on vehicle dynamics with the sextant measurements (optical starsighting). The IMU was not used for midcourse navigation. See e.g. Figures 1-6 and 5-5 in http://klabs.org/history/history_docs/mit_docs/1697.pdf http://klabs.org/history/history_docs/mit_docs/1697.pdf.