Sextant, Apollo Guidance and Navigation System
   An Apollo sextant and scanning telescope, from the collections of the National Air and Space Museum. The device penetrated the pressure hull of the Command Module.   Photo: National Air and Space Museum. 1.24078091106

Date manufactured: 1960's, Kollsman Instrument Company

Description: Between December 1968 and December 1972, a total of nine Apollo spacecraft carried human crews away from the Earth to another heavenly body. Primary navigation for these missions was done from the ground. As a backup, and for segments of the mission where ground tracking was not practical, an on-board inertial navigation system was used. Astronauts periodically used a sextant to sight on stars and the horizons of the Earth and Moon to align the inertial system, and to verify the accuracy of the Earth-based tracking data.

Item History: Project Apollo was inaugurated in the early 1960s, after President John F. Kennedy challenged the United States to put a man on the Moon and return him safely “before the decade is out.” At that time there were obviously a number of unknowns, especially whether it would be possible to navigate safely from the Earth to the Moon, a quarter of a million miles away. Early flights of unmanned Pioneer-series space probes revealed that small errors in the timing of the cut-off of a booster rocket would produce large errors in the final trajectory.

Thus it was not surprising that one of the first contracts NASA let for Apollo was for the guidance and navigation system for the journey. The MIT Instrumentation Laboratory in Cambridge, Massachusetts (later renamed the Charles Stark Draper Laboratory, after its Director) had experience in inertial systems for ballistic missiles, submarines, and aircraft. In 1957 The Lab made a proposal to the U.S. Air Force, for a 150 kg., unmanned probe, which would fly by Mars and take high-resolution photographs at close range. The system was primarily inertial, but to correct for drift, it would carry “…a space sextant to make periodic navigation angle measurements between pairs of celestial objects: the sun, the near planets, and selected stars..” Based on this experience, MIT Instrumentation Lab was chosen as the prime contractor for the Apollo’s guidance and navigation system. The challenges for a pure inertial navigation implementation were significantly different for a journey from the Earth to the Moon and return than for terrestrial applications. Of particular concern were the variable gravity field effects due to the changing proximities to the Earth and Moon and due to the anomalous nature of the Moon’s gravity.

The Apollo Guidance and Navigation System The sextant fulfilled the need for a device to aid the alignment and bound the drift of the inertial system. The instrument consisted of two telescopes. The first was a one-power, wide-field scanning telescope, which was used to locate a star or constellation in space. The second was a 28-power sextant, which took the actual reading. Although it did not look like a traditional sextant, it operated in a similar manner. The astronaut sighted on two heavenly bodies: two stars, or a star and the horizon of the Earth or Moon, adjusted the optics until they were aligned over one another, and then pressed a button marking the instrument’s reading and the time. One of the axes of the telescopes was fixed, so that the process of finding the Earth or Moon typically consisted of orienting the entire spacecraft around until that body came into the field of view. Once a reading was taken, the on-board Apollo Guidance Computer (AGC) computed the spacecraft’s position, based on those readings and data stored in its memory.

The Apollo 8 mission, in December, 1968, provided the first real test of this device, although it had been tested in Earth orbit for Apollo 7. Apollo 8 took three astronauts from the Earth to an orbit around the Moon and back again, safely. In the chronicles of Apollo, that mission marked a number of famous milestones: the first human crew lofted by the Saturn V booster, the Christmas Eve reading from the book of Genesis, and the famous “Earthrise” photograph showing a blue Earth rising above a barren and forbidding lunar horizon. It was also the first mission in which a human crew actively navigated across the depths of space from one heavenly body to another.

By 1968, however, the on-board system had evolved from the primary to the back-up to a ground-based navigation system. In the initial planning stages for Apollo, this extensive use of ground facilities was not foreseen. One concern was that the Soviet Union, whom the Americans were racing to the Moon, might jam the signals from Earth. This was a real possibility, but by 1968 NASA felt that it was less likely. Another advance was the completion by 1961 of the Deep Space Network: a set of three 85-foot parabolic dish antennas placed approximately 120 degrees apart on the Earth. These were able to track the Apollo spacecraft using several techniques, including precise timing of S-Band radio signals sent to the spacecraft and returned back to Earth. This technique depended on the dramatic improvement in atomic clocks during the early 1960s, also not foreseen by those involved in the early planning stages of Apollo. And finally, in 1960, Dr. Rudolf E. Kalman published a now-famous paper on linear filtering and prediction, which became the basis for the now-common Kalman Filter. In its initial description, the Kalman Filter was not easy to apply to aerospace navigation, but Stanley F. Schmidt, of the NASA-Ames Research Center, modified it and developed a version that proved to be applicable to Apollo navigation.

Nevertheless, the on-board system remained a critical part of Apollo, and the Apollo 8 mission tested it to its limits. James Lovell, appropriately a navy man, was the primary navigator for the mission, although the other astronauts, Frank Borman and William Anders, were trained to use the system. As the spacecraft left Earth orbit, Lovell found it difficult to get precise readings. It was hard to distinguish stars from pieces of ice and other small particles that surrounded the Command Module. As astronauts traveled toward the Moon, they never were in absolute darkness. Establishing the Earth’s limb was difficult due to the planet’s atmosphere, although the astronauts went through extensive training at the Instrumentation Lab to learn to recognize a point on the horizon that they could return to consistently. On the positive side, correcting for gyroscopic drift was relatively a straightforward process. The astronaut would key in the code for a star into the computer, and the computer in turn would rotate the spacecraft until that star appeared in the telescope’s cross-hairs. The extent to which a star was off-center indicated the amount of drift. The astronaut would manipulate the optics until the star was centered, then press a button, and the computer would realign the gyros.

By the second day, Lovell was making much more precise readings. Despite having an extremely limited memory compared to modern computers, the Apollo Guidance Computer was able to take Lovell’s readings and translate them into accurate position and velocity data.

With the position and velocity computed on-board agreeing closely with what was computed on the ground, the astronauts had confidence that they would safely pass by the Moon but not impact it. This was a genuine concern, as mission plans called for them to establish an orbit only 60 nautical miles above the Moon’s far side, where they would not be in radio contact with anyone on Earth. If their calculations were off by even a few percent, they would have been on a course to impact the far side of the Moon, with ground controllers unable to help them. The system performed equally well on the return journey as well, controlling the burn that took the astronauts out of lunar orbit on a trajectory back to Earth, aligning the Command Module to a precise angle to enter the Earth’s atmosphere, and bringing the crew down to a safe splashdown in the Pacific.

Apollo 8’s success set the stage for the successful landing of Apollo 11 in July 1969, and for the subsequent missions as well. The Apollo 8 mission played a central mission for the use of a sextant in space as astronauts sailed the ocean of space as their sea-faring counterparts did centuries before. A good book entitled “Digital Apollo” by David Mindell provides further detail on this topic.

Additional Photos:
An Apollo sextant and scanning telescope, from the collections of the National Air and Space Museum. The device penetrated the pressure hull of the Command Module. Photo: National Air and Space Museum.
An Apollo sextant and scanning telescope, from the collections of the National Air and Space Museum. The device penetrated the pressure hull of the Command Module. Photo: National Air and Space Museum.
Astronaut James Lovell sighting through the sextant, during the Apollo 8 mission, December 1968. His right hand is operating the controls that indicated to the computer that the bodies found in the telescope were aligned. Photo: NASA

For More Information, Contact:
Paul E. Ceruzzi, National Air and Space Museum

Submission authored by:
Paul E. Ceruzzi
National Air and Space Museum