Whatever else may be said about man's first landing on the Moon, it was surely the most spectacular navigational achievement of all time.

In the following paper Dr. Mayer, Chief of the Mission Planning and Analysis Division at the Manned Space Flight Center, Houston, and Mr Osburn give a general outline of the operational use of the space navigation systems which contributed to the success of the first manned lunar exploration mission. The ground and onboard navigation systems, their interfaces with the Apollo guidance system, and their uses in various phases of lunar landing are reviewed. The navigation procedures are then discussed, with particular emphasis on the Apollo 11 mission, the first manned Moon landing. Post-flight analyses and actual mission results from Apollo 11 are examined to give an accurate assessment of the overall performance of the navigation system.

The paper was presented by Dr. Mayer at a joint meeting of the Institute and the Royal Astronomical Society held in London at the Royal Geographical Society on 1 October 1969 with Professor Sir Bernard Lovell, F.R.S., in the Chair.

A list of the terms and abbreviations used in the paper is given on page 148.

The Apollo navigation problem may be divided into two general areas. The position and velocity of the vehicle must be determined at some specified time, present or immediate past. (This will be referred to as orbit determination.) This information must then be utilized to determine the future course of the spacecraft. (This is referred to as trajectory prediction.) Ground orbit determination may be performed in either a low-speed or a high-speed mode. Low-speed computations require 5 to 10 minutes for processing, while high-speed solutions are available virtually instantaneously. The ground system utilizes both types of processors. Low-speed solutions are used for trajectory prediction and, in a few cases, for processing astronaut observations. The high-speed processor is used to monitor manœuvres such as the descent to the lunar surface and for special situations where near real-time orbit solutions are necessary. The spacecraft system uses only a high-speed processor, modified slightly to facilitate manual use by the astronaut in some cases.