Идиот-клуб нумер [7]

Not all data in the AGC originates from internal calculations or crew inputs. Several
times during a mission, flight controllers enter data directly into the computer using the special uplink frequency. Adjustments to the mission clock, landmark locations and system variables occur regularly, but the most frequent and important update is to the state vector, the variable that defines the position and velocity of the vehicle in space. Three primary tracking stations, equally spaced around the world, monitor the position, distance and velocity of the vehicle, and relay the data to Houston where controllers perform the state vector calculations. At the same time, the crew takes sextant navigation fixes to calculate their own version of the state vector. Although the onboard solution is often as accurate as the ground-based calculations, conservative mission rules dictate that the ground controllers uplink their solution to the computer.

 

The state vector is not a trivial piece of data, and consists of six double-precision values. As an argument for the need to automate computer inputs, consider that updating this data by hand requires nearly 100 flawless keystrokes on the DSKY. As the numbers are encoded in fractional notation, it is impractical for the crew to decide whether the data is reasonable by simply looking at it. For critical information like the state vector, speed and accuracy requirements demand removing the human from the update process. Sending the update directly from ground computers neatly bypasses the most error-prone element in the process,
and even lengthy updates require only seconds to perform. This process does not occur automatically, and two safeguards exist to prevent undesired or spurious updates. First, the AGC must be in an idle state, running Program 00, before an update may begin. Once an update begins, the Major Mode automatically switches to Program 27 which automatically terminates any previously running program. Next, the astronauts must flip the telemetry uplink switch from Block to Accept to physically allow the updates to enter the computer. Without these two
conditions in place, no AGC updates are possible. In a stroke of brilliant simplicity, the format of the uplink data uses DSKY key sequences to enter and manipulate data. An uplink consists of the same key codes used by the physical DSKY keyboards, and uses the conventional Verb-Noun format for updating memory.
During the mission, tracking stations maintain nearly continuous transmissions to
both the Command Module and Lunar Module. Both communication links transmit
in the S-Band radio frequencies, with 2106.40625 MHz used for the CM and 2101.8
MHz for the LM.

 

Update from MSFN: time to landing: 4:20

In anticipation of the LM's flight over the landing site, the crew receives the Time of Closest Approach to the landing site. At that time, the crew will get an excellent look at the site from an altitude of about 50,000 feet (15 km). The two spacecraft are now moving apart, with the CSM actively thrusting ``backwards'' to ensure separation (initially, it is moving radially downward towards the Moon). This maneuver alters the CSM's orbit slightly, and the LM computer needs to be aware of this change. Radar tracking by the MSFN recomputes the CSM's orbit, and flight controllers uplink this data directly into the LM's computer.

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Update from MSFN: time to landing: 2:25

In tracking the CSM and LM over the last ten to fifteen minutes, ground-based radar and tracking systems derive accurate position and velocity information. This is uplinked to the AGC in the form of the state vector, and will eventually be used to update the AGS.

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Update from MSFN: time to landing: 0:30

With the powered descent imminent, the crew receives the final updates from ground controllers. An improved state vector and the latest update on the landing site coordinates are uplinked to the computer. The final guidance switch settings are made, giving the AGC primary computer control over the spacecraft, with the AGS ready to take over if needed.

 

For the next several minutes, the descent engine is maintained at full thrust, to gradually slow the LM from orbital velocity. For the crew, a large part of the time is spent monitoring the progress of the descent. As they pass about 40,000 feet, approximately three minutes from the time of ignition, the landing radar locks onto the surface, providing accurate altitude and altitude-rate data to the computer. In all missions except Apollo 11, tracking from the ground provided data to refine the targeting to the landing site. Using doppler radar data from Earth, combined with navigation data from the LM's guidance system, ground controllers were able to identify targeting errors and calculate updates for the computer to insure an accurate landing. These updates were relayed verbally, and the crew entered the data manually into the computer. In what may seem to be reverse logic, the computer is not told that it is off course, but rather that the landing site has moved! While the targeting could be altered in all three axes (up- or down-range, or north or south of track, and accommodate differences in altitude), it was only used for up- and down-range corrections.