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Access control is often a system enabling authorities to control use of areas and resources inside a given physical facility or computer-based information system. Access control is, actually, a typical, everyday phenomenon. For example, a lock on the car door is essentially a kind of access control. The PIN over a bank's ATM method is another way of access control. Even nightclub bouncers, standing in front of your door, are another illustration of (perhaps) a much more primitive mode of access control. Using an access control mechanism is essential when persons seek to secure confidential, important, or sensitive information and equipment.
When the dog chewed the leash the lead went in the case and unraveled the coil spring. With the case facing you possess it steady and grab the loose end of the lead and pull on the cause add tension. Now that you just have about two feet of lead out, support the mechanism while preventing the lead wheel assembly from unwinding rewind the extra lead that you just got out, without releasing the tension.
Spacecraft flew hundreds of thousands to hundreds of millions of kilometers from the earth, injecting speed when the size and direction of a slight error, arrived at target planets will appear large deviation error, for example, when Mars probe injecting speed 1 m/s speed (about one over ten thousand), arrived at Mars, about 100000 km distance deviation. Therefore, precise control and navigation must be carried out during the long flight. Flying to the moon is usually controlled by the orbital control system of the ground and space probe (see orbit control). Interplanetary flight distances, radio signal transmission time is long, the real-time remote control, can't so planets and interplanetary spacecraft orbit control system shall have the ability of autonomous navigation (see interstellar travel navigation and control). The United States, for example, "pirates" spacecraft in space flight of more than eight hundred million km, for 11 months of 2000 times of autonomous orbit adjustment, the final surface to achieve a soft landing on Mars, placement accuracy up to 50 km. In addition, in order to ensure the accuracy of the orbit control engine, the communication antenna is always aimed at the earth, and other systems work normally, and the detector has the autonomous attitude control ability
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