Skylab continued orbiting the Earth once each 93 minutes.
Without a crew aboard, its requirements for system operation were reduced. But it still needed electrical power for the transmission of technical data to ground controllers, and for retaining a spacecraft attitude and environment suitable for systems operations.
After the first crew departed from the space station, ground controllers lowered workshop internal pressures from 5 to 2 pounds per square inch. This reduced the dewpoint to about 35°F and prevented condensation in the workshop, as temperatures in the unoccupied workshop decreased.
Communicating With Skylab
The ability of ground crews to determine the exact condition of components of operating systems aboard Skylab was vital to mission success. Even with highly skilled crews aboard, it would have been impossible for crewmen to record the vast amount of data, to analyze it, and to take corrective action in the limited time available. With this burden, they would not have had time to carry out their assigned activities.
A dependable system for acquiring and communicating information and for providing radio contact for the control of Skylab was extremely important.
Throughout the Skylab mission, this instrumentation and communication system provided a continuous monitoring of the space station during both manned and unmanned portions of the mission. It provided two-way voice communication between flight crews and ground personnel, and it included a means for transmitting printed instructions to the crew each day. Data obtained during experiments and science demonstrations were transmitted over this system. And crew activities and some scientific observations were transmitted to ground observers by means of television.
More than 2000 separate measurements were made of temperatures, pressure, displacement, flow, voltage, current, vibration, the position of mechanical devices, and other necessary information. Such measurements, made by carefully placed sensors, were converted into electrical signals, which were then transmitted to ground receiving stations.
Thirteen ground stations, located throughout the world, made up the Skylab tracking and data network. They received data directly from Skylab about 32 percent of the time; the remainder of the time, data were recorded on tape and subsequently transmitted to a ground station when Skylab passed over it.
The frequency of measurements varied widely. Since most temperature changes occurred relatively slowly, temperature measurements were made at a rate of from 0.42 to 1.25 samples each second. Some of the solar observatory computer output measurements were made at a rate of 120 samples...
....per second, and many of the biomedical measurements were made at the maximum rate possible.
Two separate command systems made it possible to control Skylab from the laboratory or from the ground. The solar observatory system allowed the crew full control while the laboratory was manned and allowed limited control of the solar observatory from the ground. The second system provided both crewmen and ground controllers a means of controlling systems operation in the airlock, the docking adapter, and the workshop.
Command signals to the solar observatory, transmitted from ground stations, were coded so that specific functions would be performed.
A digital command subsystem made possible some 540 distinct commands, which permitted positive control over the space station at all times. Such commands were received, transmitted through command receivers and decoders, then translated into commands.
The workshop decoder also provided a digital output to the teleprinter, the first such device ever used in a spacecraft. The teleprinter received coded data, converted the data into dots on a matrix, and printed patterns of dots to form messages on thermally sensitive paper. This teleprinter played an important role in the daily lives of the crews, as almost all instructions to them were relayed by this means while they slept. Without it, much more extensive and time-consuming voice communica....
....tion would have been required, with considerably greater possibilities for error.
The capability to observe operation of the space station systems and to control them through selected commands made it possible to continue a program of scientific observation even with the space station unmanned.
Solar Observations Continued
Solar observations were continued in an effort to learn more about the star whose existence makes possible life on Earth. The solar observatory's telescopes had given scientists an intimate look at a Sun radically different from that which they had known before. Unhindered by Earth's atmosphere, its eight telescopes had observed a surprisingly violent surface with mysterious bright points enveloped in a vast and turbulent corona. Ground controllers operated the fine pointing control system to observe selected targets on the Sun.
Use of this control system was limited to the daylight portions of each orbit. At true sunrise, the computer opened the door protecting the precise Sun sensor. At effective sunrise (when the Sun was above Earth's atmosphere) the computer released the Sun sensor and commanded the controller. The controller then uncaged the canister housing the telescopes by disengaging the orbital locks and began commanding the up-down and left-right actuators which accomplish precise pointing.
On the 64th day of the mission, while Skylab was still unmanned, the primary up-down rate gyroscope processor became very noisy and eventually failed. Since Skylab was out of contact with the ground tracking network when the failure occurred, an entire orbital day phase passed before the problem became evident. The canister began to oscillate about the up-down axis until the actuators overheated and seized, and the system was turned off. Three days later, after extensive analysis by ground crews, the secondary up-down rate gyroscope was selected, and the system was turned on again. No further problems were encountered with the actuator.
Electrical Energy Vital to Station Operations
Skylab was exposed to sunlight for varying periods of time, because of the orbital inclination of the Skylab mission. The maximum period of...
....darkness was 26 minutes. There were orbits in which Skylab was in continuous sunlight, during which orbital night did not occur. The continuous sunlight periods occurred only three times during the 273-day mission, for a total duration of 10 days. While Skylab was in orbital daylight, its solar cells drew energy from the Sun for conversion into electrical power. But when it passed through periods of orbital night, power had to come from the batteries. This required that sufficient power be generated during orbital daylight to operate the
Skylab systems and also to charge the batteries. Calculations resulted in the decision that the solar cell array be sized electrically to be 2 1/2 times the space station load.
Power sharing between the solar observatory and the workshop electrical power systems was controlled by adjusting the voltage of the laboratory power system. By increasing the workshop output voltage, its system supplied a larger percentage of the load. Conversely, by decreasing the workshop output voltage, the solar observatory supplied an  increasing percentage of the load. If equal load sharing was desirable, the workshop voltage could be adjusted so that both systems supplied one-half of the total load.
To transfer power from Skylab to the command and service module, the astronauts connected an umbilical cable in the workshop to the spacecraft, and circuit breakers and switches were operated for power transfer. The cable was installed at the beginning of each manned mission.
The flexibility of this system was demonstrated vividly during the early days of the Skylab mission.
For the first 11 days, the solar observatory electrical power system was the only source of power. On the 1 2th day, the Apollo command and service module docked with Skylab. Its fuel cells continued to supply the command and service module load, while the solar observatory system provided all the power for Skylab activities until the workshop solar wing was deployed on the 14th day of the first manned period.
Power Management Needed for Mission Success
Skylab designers had foreseen the possibility of problems arising and had made plans for management of electrical power under a wide range of conditions. During the initial period of dependence upon the solar observatory power generation system, prior to freeing and deployment of the....
....workshop solar wing, it became necessary to implement effective management of the available power. This required constant monitoring of all electrical power system parameters, knowledge of the exact vehicle electrical load at all times, and precise knowledge of the requirements of each subsystem for all mission phases. With this information, engineers reviewed all planned Skylab activities and evaluated the capability of the electrical power system to support them. Next, they predicted total electrical load requirements and system performance. And finally, power management techniques were developed to satisfy the requirements.
For the first 11 days of the mission, the principal technique was simply to eliminate or reduce power requirements as much as possible. Power to nonessential, manageable loads, such as internal heaters, coolant pumps, telemetry transmitters, and redundant gyroscopes, was either turned off or reduced.
A second means of power management was reduction of the time Skylab was not pointing its arrays at the Sun. But this could be done only at the expense of other vital considerations, such as holding temperatures to a tolerable level within the workshop.
The first crew was very conscious of the need for careful power management. They were careful to turn off lights, fans, food trays, cameras, experiments, and other power-consuming pieces of equipment when they were not needed.
Preparation for the Second Manned Period
While Skylab, unmanned, continued its orbit, ground crews were busily preparing for the second manned period.
Alan Bean, the commander of the second crew, had explored the Moon. Owen Garriott, the scientist pilot, had taught electrical engineering at Stanford University. Jack Lousma, the pilot, had served as the communicator in Mission Control during the Apollo 13 flight. All had gone through extensive training in preparation for the Skylab mission.
The crew had spent many hours training in the deployment of a second parasol, which had been developed and fabricated between the first and second manned periods and in the deployment of the twin-pole shield stowed in the workshop by the first crew.
As they roared into space just after 7 o'clock on the morning of July 28, 1973, they carried with them a "six-pack" of rate gyroscopes, cables, the improved parasol, experiment film, food for a 3-day mission extension, various assemblies needed to replace failed experiment components, and two laboratory data tape recorders. Also on board were 2 Mummichog minnows and 50 minnow eggs, 6 pocket mice, 720 fruitfly pupae, and 2 common Cross spiders named Arabella and Anita.
The launch had originally been scheduled for 3 weeks later, but the rate gyroscopes aboard Skylab were degrading rapidly, and it was possible that the thermal shield was deteriorating. This made it desirable to have repairs made as soon as possible. Also, the second manned period was extended 3 days beyond the planned 56 days to allow more worktime and to permit a more favorable splashdown area.
Eight hours after launch, Lousma called out, "Here's our home in the sky." Skylab had been sighted.
Two methods were used for rendezvous and docking of the command module with Skylab.  Electronic equipment provided radio contact for ranging information and visual aids were used for the final phase of rendezvous maneuvering and docking.
The very-high-frequency rendezvous and ranging equipment determined the closing rate and distance between Apollo and Skylab. During rendezvous the Apollo command and service module transmitted a tone-modulated signal to the space station where a transponder received and retransmitted this signal. Command and service module electronics demodulated the retransmitted signal and measured the phase differences to compute the distance between the two vehicles.
Skylab was visually sighted from as far away as 390 miles by its crew. Four flashing lights, mounted on the solar observatory, made Skylab clearly visible. Eight docking lights were mounted on the workshop structure and color coded to indicate its orientation. Four smaller bulbs illuminated the tips of the discone antennas. The astronauts used these lights to orient the Apollo module for safe docking, after which they turned them off until their departure at the end of their mission.
Exterior lights illuminated the workshop, airlock, and the solar observatory work station during extravehicular activities.
Once docked, the crew opened the hatch leading to Skylab and began the job of transferring supplies and preparing Skylab for the second manned period.