1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-18a71 Center Point of Contact: KSCa71 Submitted by: Wil HarkinsSubject: Global Positioning System (GPS) Timing System Practice: Use of the Global Positioning System (GPS) to provide a timing system with improved reliabilit
2、y and accuracy over the previous system.Programs that Certify Usage: This practice has been used on Space Shuttle Program Kennedy Space Center (KSC) Launch Control Center (LCC) Timing SystemCenter to Contact for Information: KSCImplementation Method: This Lesson Learned is based on Reliability Pract
3、ice No. PD-ED-1234; from NASA Technical Memorandum 4322A, NASA Reliability Practices.Benefit:In addition to improving the timing systems overall reliability by utilizing multiple timing sources, the upgrade from the previous Apollo-era designed system (using LORAN and WWV) provides improvements in t
4、he accuracy, monitoring and feedback capabilities. The timing system is used to provide timing commonality between instrumentation systems so data can be referenced with respect to time. Improving the reliability and accuracy of this system improves the time reference capabilities.Provided by IHSNot
5、 for ResaleNo reproduction or networking permitted without license from IHS-,-,-Implementation Method:The primary time sources of the GPS Timing System are two GPS receivers and an Eastern Test Range UHF timing receiver. Each receivers output feeds a Time Code Generator whose output feeds an Error D
6、etection and Switching System. The inputs from different receivers are compared for bit error and phase error. Switching to a secondary input is performed if a fault is detected. The output is fed to Distribution Amplifiers for distribution throughout the KSC launch area (see Figure 1).refer to D de
7、scriptionProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-D Figure 1. GPS Timing Block Diagram The GPS Timing System is equipped with a computer system that allows the operator to monitor all timing outputs including the signal source, signal level, d
8、istribution communication circuit number and end user. The timing signal is also displayed on an oscilloscope. This allows operators to quickly identify any distribution problems.On a daily basis, operators can page through every timing signal distributed from the LCC, checking the code and level of
9、 each signal. After checking all signals, the operator places the computer in auto mode, and the computer automatically checks the time difference between a cesium clock and all other timing units (two GPS receivers, the Eastern Test Range UHF timing receiver, two LORAN receivers, the Communication
10、Instrumentation Facility time, ETR time and a portable clock time).The time differences are logged on a printer along with the following information from each GPS receiver:a71 Greenwich Mean Timea71 Satellites being trackeda71 Oscillator control voltagea71 Satellites selecteda71 Satellites deselecte
11、da71 Antenna statusThe printout can then be reviewed by the operator and filed as part of the station log for historical purposes.The computer system is also equipped with a program that monitors the Timing Station at night and during other unmanned periods. The program pages through each time unit
12、every 10 minutes, checking the time difference between the units and the Cesium clock. The time difference is logged on disk for use during the next work period when it is imported to a software application that can generate graphs for system analysis.Technical Rationale:Engineers at Kennedy Space C
13、enter require timing commonality between instrumentation systems so data can be referenced with respect to time. The development of the GPS allowed for the development of a timing system that is more accurate, more reliable and provides greater monitoring and feedback capabilities than the Apollo-er
14、a design it replaces. That system, using LORAN and WWV, resolved time to an accuracy of +- 1 microsecond. The GPS system is accurate to within +- 200 nanoseconds. Another advantage is that unlike LORAN, GPS satellites are capable of supplying Provided by IHSNot for ResaleNo reproduction or networkin
15、g permitted without license from IHS-,-,-the time and need not be used in conjunction with any other system.References:1. KSC Drawing - G79K20503, “KSC Timing System Diagram“2. KSC Drawing - 80K54337, “LCC, Rm 2P22, Timing System“Impact of Non-Practice: The use of an older, less accurate timing syst
16、em would not provide the reliability, accuracy and the monitor and feedback capabilities that may be required or desired for timing of some present-day systems.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-18a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
