Primary Guidance, Navigation and Control System

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The Primary Guidance, Navigation and Control System ( PGNCS (pronounced: pings )) was the independent inertial navigation system of the Apollo spacecraft . It enabled the Apollo spacecraft to perform their operations when communications with Earth were interrupted, for example when the spacecraft was behind the moon or when there was a communication failure.

The user interface of the Apollo Guidance Computer in the command module

Both the command module (CM) of the Apollo spacecraft and the lunar module ( LM) were each equipped with a version of the PGNCS. It was developed under the direction of Charles Stark Draper at the MIT Instrumentation Laboratory . The main contractor for the construction of the PGNCS and manufacturer of the inertial measurement unit was the Delco Division of General Motors .

description

The Primary Guidance, Navigation and Control System (PGNCS) comprised the following components:

  • the Inertial Measurement Unit ( IMU ),
  • the Apollo Guidance Computer (AGC) ,
  • the coordinate converter for converting the coordinates of the inertial measuring unit into signals for the servo control,
  • the optical unit ( Optical Unit ),
  • a mechanical frame, called Navigation Base (or Navbase ), to connect the optical unit, and in the case of the lunar module , the rendezvous radar , with the inertial measuring unit and
  • the AGC software .

Versions

The command module and the lunar module used the same AGC, the same inertial measuring unit and the same coordinate converters. One difference was the optical unit. Due to the different design of the assembly, the navbase of the command module and the lunar module was also different. The lunar module's rendezvous radar was also connected to the navbase.

There were two versions of the PGNCS, Block I and Block II, corresponding to the two generations of command modules. After the Apollo 1 fire, which occurred in a Block I command module, NASA decided that the Block I command module would not be used on any manned Apollo flight. (However, these continued to be used for unmanned flights). The main modifications to the block II PGNCS consisted of the replacement of the electro-mechanical coordinate converter against fully electronic equipment and exchange the group consisting of beryllium fabricated Navbase against a Navbase whose frame with polyurethane foam -filled aluminum tubes consisted. This Block II Navbase was lighter, cheaper and still more rigid than the Block I Navbase. The Apollo Guidance Computer was also improved in Block II.

Some components of the PGNCS were later used by Charles Stark Draper for the Deep Submergence Rescue Vehicle ( DSRV ) of the United States Navy .

Inertial measurement unit

The inertial measurement unit ( IMU ) was gimbaled in three axes and its gyro platform consisted of a beryllium cube with an edge length of 15 cm (6 inches). It was equipped with three gyroscopes and three accelerometers . Feedback, including the coordinate converters, used the signals from the gyroscopes to control the motors for each axis. The gyro platform was kept stable by this servo system. The inertial measuring unit of the PGNCS was derived from the control system that Charles Stark Draper had developed for the Polaris rocket . The IMU had been simplified compared to that of the Gemini missions, which had four gimbal axes, and had less drift, on the other hand there was a need to avoid a gimbal lock , as this would have led to time-consuming initialization and realignment.

The measurements of the PGNCS had an angular deviation (drift) of approx. 1 mrad / h. That is why the gyro platform of the inertial measuring unit was regularly aligned using the fixed stars.

Optical unit

The command module had a fixed sextant for measuring the angles between fixed stars , orientation aids on earth or moon and planetary horizons. This unit consisted of a scanning telescope for aiming at the stars and was used to determine the position and orientation of the spacecraft in space.

In contrast, the lunar module had the so-called Alignment Optical Telescope ( AOT ). This could only determine the position of the lunar module in space. The AOT essentially consisted of a sun-protected prism that could be rotated around three axes relative to the lunar module so that a large part of the lunar sky could be covered. The position of the AOT was read by the AGC. After targeting several fixed stars by the AOT, the AGC was able to determine the position of the lunar module.

software

The control software running on the AGC used Kalman filters in order to carry out an optimal position determination based on the data from several position measurements . A coordinate transformation between the gyro platform of the IMU and two reference coordinate systems , one with the earth as the center and one with the moon as the center, served as the basis. The resulting matrix was called REFSMMAT (for: Reference to a Stable Member Matrix ).

Use of the PGNCS

Contrary to what the term " primary " (German: basic , mainly ) suggests, the data generated by the PGNCS were not the main source of information on the navigation of the Apollo spacecraft. Orbit tracking data from NASA's Deep Space Network was processed by computers at the Mission Control Center in Houston using the least squares method . The position and speed of the Apollo spacecraft determined in this way were more accurate than the data generated by the PGNCS. The astronauts regularly updated the REFSMMAT with this data transmitted from Earth. The main purpose of the PGNCS was to maintain the spacecraft's attitude in space and to control the rockets during flight maneuvers, including landing on the moon and taking off from the moon. During planned and unplanned disruptions in communications with Earth, the PGNCS was the primary source of navigational data. In addition, the navigation data transmitted from Earth was checked by the PGNCS.

The lunar module had a third navigation option, the Abort Guidance System ( AGS ) built by TRW . This system, which should be used in the event of a PGNCS failure, could be used to take off the Lunar Module from the moon and to rendezvous with the command module, but not to land on the moon.

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