World's Toughest R/C Job - Guidance in Outer Space
May 1967 Radio-Electronics

May 1967 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

SpaceX and Blue Origin have been in the news for the last decade for their efforts (some successful, some not) to autonomously land a spacecraft vertically under its own power. Love (which I still do, in spite of its crippling "wokeness") it or hate it, NASA has been doing that for nearly six decades. Granted, it was on celestial bodies with lower gravitational acceleration than on Earth, but the earliest craft (Surveyor 1, 1966) had relatively crude electronics aboard, including a Doppler radar, flight computer, and video camera. The now legendary Apollo Guidance Computer has been written about extensively, and is a testimony to the brilliance of the scientists, engineers, managers, operators, and technicians who built and flew it. Articles like this one in the May 1967 issue of Radio-Electronics magazine are a very valuable part of the historical record for America's manned space flight programs. It reports on the remote control portion of the flights rather than the autonomous parts. Here is the Surveyor 1 Flight Performance Report.

World's Toughest R/C Job - Guidance in Outer Space

By Allen B. Smith

NASA's spectacular success in soft-landing Surveyor I on the moon, and in obtaining high-resolution photographs of the lunar surface from Lunar Orbiter, has generated a tremendous surge of interest in America's plans to explore outer space. Project Apollo, though struck by the tragic fire that took the lives of astronauts Grissom, White and Chaffee, continues to captivate the interest of the world public. By the end of 1968 or early in 1969, three American astronauts will embark on an historic 8-day trip to the moon and return to earth. Two of the three-man Apollo crew will explore the moon's surface during a projected 35-hour encampment.

Virtually every system in NASA's diversified collection of spacecraft de-pends heavily on electronic instrumentation. Of all the electronics, the guidance and navigation package in all missiles and spacecraft is particularly interesting.

In NASA's space-exploration program there is a bewildering array of spacecraft. They fall roughly into three major categories: missiles, unmanned probes, and manned spaceships and capsules. Each is designed for a specific purpose, and each has a unique physical configuration, Yet in the center of all those vehicles nestles a group of gyroscopes and accelerometers, a computer and other precision components. These comprise the nucleus of the remarkable inertial guidance and navigation unit.

Ballistics missiles like Redstone, Jupiter, Atlas and Titan, originally conceived as force-deterrent weapons, have been developed into highly dependable vehicles for thrusting American satellites and spacecraft into space. This family of rocket-engined craft uses the most direct form of inertial guidance. The system's computer is preprogramed to control the vehicle during its flight without reference to external signals.

Unmanned, instrumented space-craft are far more varied than missiles. Explorer, Pioneer, Nimbus, Tiros, Early Bird and other orbital craft employ an inertial-guidance system, but only to maintain or correct the attitude of the satellite relative to the earth. Lunar probes of the Ranger and Lunar Orbiter series are equipped with restartable rocket engines and may be required to make midcourse corrections. They must, therefore, be able to receive commands from ground-based stations so that their computers can develop course-change commands to the attitude-control system and the main propulsion engine. Surveyor-series craft can do that also.

Inertial guidance and navigation equipment in manned spacecraft is much more complex. It has a variety of subsystems that augment the basic inertial system and insure absolute control and pinpoint accuracy.

In simplest terms, an inertial-guidance system is a self-contained instrument package requiring no external reference signals or commands once it has been programed to guide a vehicle to its destination. It must have three basic elements: (1) a directional reference provided by gyroscopes, (2) a memory device - usually a compact computer, and (3) velocity-sensitive accelerometers to detect external forces that alter the vehicle's course in any direction. The computer integrates information obtained from the gyros and accelerometers to produce electrical error signals that command the attitude-control system and main rocket engine.

The heart of the inertial system consists of the stable platform, also known as the inertial reference unit (IRU), or the inertial measuring unit (IMU). The platform is a three-axis gyro system. It establishes a stable element to which the accelerometers are mounted. Fig. 1 shows how the three gyros (G) and three accelerometers (A) are suspended by the innermost member of a gimballed support framework. Gimbals permit the gyro assembly to tilt freely in any direction. Axes of the gyros and accelerometers coincide to make a kind of artificial horizontal/vertical frame of reference against which spacecraft motion is measured and computed.

The gyro platform "wants" to remain fixed as the vehicle tips and pitches around it. When the gyros are disturbed in any axis or combination of axes, output signals are obtained. These signals are applied to small torque motors which reposition the stable platform. This action compensates for the axial disturbance and restores the correct frame of reference. Gyro output information also is fed into the computer to record the history of the flight for reference in future computations. As long as the computer knows where it is going and where it has been (in the form of integrated data from the stable platform), it can compute flight-path corrections to arrive exactly at the destination.

A gyro used in inertial systems is so sensitive that the weight of an oily fingerprint on its balance wheel would cause the entire gyro to drift enough to cause a serious aiming error and throw the spacecraft completely off course. The accelerometers, too, have startling capabilities. A typical unit is capable of measuring velocity changes as great as 10 G's (equivalent to accelerating a body from 0 to 60 mph in 1/4 sec) or as slight as 0.0008 G (from 0 to 6 mph in 3 hours).

These elements are necessary to any inertial system. While such basic packages have been used in orbiting spacecraft and in missile and booster rockets, most of the state-of-the-art inertial systems (Lunar Orbiter and Apollo particularly) have complementary subsystems to increase the effectiveness of the guidance and navigation equipment. The Apollo, for example. uses a scheme like the one diagramed in Fig. 2.

The most spectacular performances of inertial guidance and navigation equipment to date have occurred in the Mariner, Lunar Orbiter and Surveyor spacecraft.

Launched on November 28, 1964, Mariner IV was lofted into an initial parking orbit around the earth by an Atlas booster. Reaching the optimum point on this orbital path from which it would begin its trajectory to Mars, the restartable Agena second stage thrust the craft into its interplanetary path. Separated from the booster, Mariner then was free to follow the commands programed into its computer sequencer or to receive commands from the control center on earth.

In flight for more than 7,375 hours before passing beyond the range of the Deep Space Network, Mariner performed literally thousands of attitude corrections and major orientation changes. Data, both from the scientific instruments carried on board and from the spacecraft support telemetry system, were fed back to earth continuously. The final signals, received far beyond the orbit of Mars, were at a level of 10¹⁸ (a billion-billionth) watt!

The first operation required of Mariner after separating itself from the Agena booster was to position itself to receive light from the sun. The craft's sun sensor locked on the sun, positioning the solar-cell power panels that generated electrical power for all systems. This maneuver assured a continuing supply of power for the duration of the flight.

With Mariner locked onto the sun along its roll axis, the CC&S (central computer and sequencer) activated a second optical unit, this time to identify the bright star Canopus. Lying about 15° from the spacecraft's south rotational pole, perpendicular to the sun-lock roll axis, Canopus was used to position the craft's antenna systems. This maneuver enabled Mariner to receive ground-control commands required for midcourse correction. It also aimed the two 10-watt transmitters used to relay scientific data back to the earth-based stations.

Midcourse commands were the only signals Mariner had to receive from ground control; the CC&S was capable of handling all other en-route computations and commands. Still, Mariner's computer could respond to 28 additional ground-based commands, if an unplanned series of operations had to be performed during the voyage.

Midcourse trajectory was corrected December 5, 1964. Commands received by Mariner resulted in a change in attitude of 39° in pitch, 156° in roll.

A burn of 20 seconds by the rocket propulsion system changed the craft's course 0.25° and increased its speed 37 mph. Mariner IV was now on a trajectory that intercepted Mars on the 228th day of its voyage. At the time of the Mars encounter, Mariner was 134 million miles from earth. The CC&S then triggered the television tape-recording system which transmitted pictures back to the ground-control stations.

Twenty-two complete electronic photographs were recorded by Mariner's camera system, and data on tape were transmitted to earth at 8 1/3 bits per sec, requiring 8 2/3 hr for each of the 22 pictures. The tape was played back twice to make sure a complete set of video data had been sent down. Signals required slightly more than 12 minutes to make the trip from Mars.

The inertial guidance system had to do two things during this epic solar- system exploration: It had first to position the craft by the sun and Canopus sensors. Second, it had to function in  harmony with the ground-control commands to execute the midcourse maneuver and assure an accurate trajectory. Because gyros do drift and because magnetic, gravitational and light-pressure forces tend to change the attitude of spacecraft, the inertial system had constantly to detect minute variations in attitude and correct them to maintain the craft's position in space. No other kind of system could do the job.

Lunar Orbiter

A more recent unmanned flight, the highly productive photoreconnaissance mission of Lunar Orbiter, also had an eventful space history. As you'll note in Fig. 3, several complex maneuvers were executed successfully during the 90-hr trip to the moon. Launched from Cape Kennedy at 19 hr 26 min GMT on August 10, 1966, the 850-lb photographic spacecraft was boosted into its initial parking orbit by the reliable Atlas/Agena rocket launch system. After 38 min and 3 sec in this preliminary orbit, during which the inertial system positioned the craft correctly for injection into the translunar trajectory, the Agena was restarted. The vehicle reached its escape velocity of approximately 25,000 mph, and the Agena parted from the spacecraft. Within 3 minutes, the high-gain and omnidirectional antennas and the four solar-power panels were deployed. The Woomera, Australia, station of the Deep Space Net picked up the craft at 20 hr 13 min 38 sec GMT. From that moment on, the spacecraft was under the control of NASA and Boeing Co. scientists and engineers from the Langley Research Center.

The success of these early operations was due to the 13.2-lb inertial reference unit (IRU) shown in the photo. Designed and built by Sperry Gyroscope, the IRU supplied continuous attitude- and rate-error signals for the computer sequencer to use in executing commands from its programmer and from ground-based stations.

Like Mariner IV, Lunar Orbiter carried sun and Canopus sensors for positional references during the lunar voyage. Sun-lock was achieved without trouble at 20 hr 32 min; but when the Canopus sensor was activated, it saw a visual light level of nearly 3 volts rather than the expected dark level of 0.56 Volt, as shown by an examination of the telemetric data tapes. The 3-volt level was more than enough to blind the Canopus sensor, preventing it from locking on the star to position the craft before midcourse maneuver. Scientists eventually determined that the sensor was seeing light reflected from some part of the spacecraft itself, probably from the omnidirectional antenna.

For several hours it seemed that the mission might be doomed by this unforeseen development. By analyzing the telemetric data received from Orbiter, however, and by rolling the craft around its sun-locked axis, scientists in the control center discovered that the sensor could be locked onto the moon itself. Appearing to the sensor as a thin crescent, the angular dimension of the moon was small enough to provide an accurate position referenced to the IRU. This was possible only because a 1-day delay in launching put Orbiter in the only translunar flight path on which it would be able to see the moon within the narrow angle of recognition of the Canopus sensor! So coincidence, telemetric data analysis and remote-control capability all contributed together to the mission's success.

Once locked to the moon and the sun, the spacecraft was commanded to realign itself at midnight GMT on August 11. The needed thrust computed from data received from the IRU and from the ground-control center was supplied by the craft's 100-lb-thrust reaction engine, which burned for 32 sec. This maneuver changed the craft's course from a lunar-miss distance of 5,200 miles to an aiming point 3,950 miles behind the moon's trailing edge. Attaining the planned trajectory got the mission over one major obstacle, but for the critical retro-firing that would place the craft into an orbit around the moon, the Canopus sensor had somehow to be locked onto its star. Locked onto the moon, positional accuracy was rather crude - about 2°; an error signal accurate to 0.1° was required for the de-boost engine to start.

By rotating the spacecraft so the Canopus sensor's viewing angle began to include the sun. a protective bright-object sensor was triggered into closing a shutter over the Canopus sensor's eye, and a normal background level of 0.56 volt was obtained. This confirmed that the electronics in the star tracker was operating as designed.

A specialist on the system was flown to the control center from Boeing's Seattle plant, and he suggested that the sensor might lock on Canopus despite the high background that reappeared when the craft was once again stabilized in its sun-lock axis, if the spacecraft could be pointed directly at the star before the lock circuits were energized. Orbiter was then stepped in small rotational increments around its sun-lock axis across the area in which Canopus was expected to be found. After several attempts, the star was located, and the system locked on at 13 hr 49 min GMT on August 13.

Rather than place the sensor in its automatic-track mode, however, the loop was kept open to include corrections fed from ground-control equipment. This removed the possibility that the sensor might lose the star momentarily and begin a searching-mode roll that could not be stopped by the attitude-control system.

The retro-fire maneuver was executed at 15 hr 34 min GMT August 14, and the spacecraft was injected into its intended lunar orbit. The craft's rocket engine fired for 9 min 49 sec, the IRU accelerometer cutting off the engine when the velocity reached 790 meters/sec. The planned orbit was inclined at an angle of 12.04° to the lunar equator and had an apolune (high point) of 1,150 miles and a perilune (low point) of 124 miles. So great was the accuracy of the inertial system that the actual orbit was inclined at 12.18° with an apolune of 1,158.7 miles and a perilune of 117.4 miles. The orbital period was 3 hr 37 min 45 sec.

On August 18, Orbiter began taking the first pictures of the moon from a lunar orbit, and the IRU once again correctly positioned the spacecraft to point the cameras to the desired sectors of the lunar surface. On August 21, a second retro-fire burn of 24 sec was calculated by the inertial system, lowering the craft to within 30 miles of the surface. Velocity data supplied by the PIP accelerometers provided the cutoff signal at the calculated de-boost speed. Picture-taking continued through August 29, at which time the IRU locked onto earth to permit transmission of the video information. Everyone is aware by now of the spectacular quality of Orbiter's photography.

Thanks to the remarkable capability of the Deep Space Network, and to the analytical ability of the scientists at the control center, Lunar Orbiter was an outstanding success. It furnished us new scientific data about the moon.

We're on Our Way

Inertial systems like those in Mariner and Lunar Orbiter have enabled NASA's space scientists to expand tremendously the range of man's exploration in our solar system and of what lies far beyond. Within 2 or 3 years, the mighty Apollo moonship - 364 feet high on its launching pad - will soar to the moon carrying three astronauts, two of whom will step out of their lunar module onto the moon's surface. Guided by inertial systems and a full range of complementary systems, men will take their first steps toward the stars.

Posted August 22, 2024