Autonomous planetary landing guidance by optical correlation

We describe our planned use of optical correlation in landmark navigation associated with planetary landing. Standard correlation provides 'pointing-to' information, giving the vector to the landmark from the spacecraft in the spacecraft frame. The synthetic estimation filter (SEF) provides 'pointing-from' information, estimating the vector from the landmark to the spacecraft. Digital and optical SEFs were constructed and compared using a Martian-like 3D modelboard to provide test images. The digital SEF worked reasonably well, but the optical SEF did not perform as expected. The optical SEF was implemented with a liquid crystal television (LCTV) correlator that had been used successfully in previous SEF experiments using a spacecraft model. The results suggest the SLM model for the LCTV needs further refinements. Both the digital and optical filter provided good pointing-to results. We did not plot the correlation surface for the digital SEF response, and though it is less sharp than the POF, it had only a one-pixel variation in the peak location. Surface plots for the conventional optical phase-only filter produced a correlation peak that was sharp enough to be located within two pixels.