longrangenavigation-loran,(classiinavigation)远程导航-罗兰，（ii级导航）.ppt

LOng RAnge Navigation- LORAN, (Class II navigation),AST 241 Dr. Barnhart,LORAN Overview,1st Modern practical area navigation system for use in general aviation. Developed for maritime navigation. Early LORAN A’s were exclusively for marine use Still in use today although its use beyond 6 – 8 years from now is uncertain,LORAN Overview,Certain LORAN-C receivers are approved for IFR Enroute and Terminal navigation but not for approaches. Most LORAN-C recievers are used for VFR navigation For all practical purposes (for the pilot) LORAN-C provides the same type of nav. Data to the pilot ( ground track, desired track, groundspeed, ETE),LORAN-C Theory,Operates in the LOW frequency electromagnetic energy radio bandwidth which follows the curvature of the earth therefore making it suitable for longer ranges,LORAN Theory,The LORAN-C concept is based on groups or “chains” of stations around the world. Worldwide there are 28 chains. Each chain consists of one master station and 4 – 6 secondary stations in a geographic area (ie. Northeast U.S.chain, Great Lakes U.S. chain, etc.),LORAN Theory,The master station of each chain broadcasts a continuous string of Low Frequency pulses (measures in microseconds). The unique time between the start and stop of each pulse identifies the particular chain.,LORAN Theory,This time is known as the Group Repetition Interval or GRI. For instance the time between the start and stop of each pulse in the northeast U.S. chain is 99,600 microseconds or 9960 for short. 9960 is identified as the great lakes chain,LORAN Theory,The GRI is entered into the LORAN-C unit in the aircraft upon startup along with the approximate LAT./Long. Position during the initialization process. This helps the unit find its position more quickly.,LORAN Theory,The LORAN-C unit then searches for that master station and the two strongest secondary signals. The master station pulse triggers a response from the secondary stations. The receiver corrects for any time lag in pulse reply.,LORAN Theory,The receiver then calculates the time it takes these signals to reach the aircraft and essentially generates hyperbolic Lines of Position (LOP) from each station- known as a hyperbolic lines of position since the lines look like a parabola.,LORAN Theory,Two intersecting hyperbolas produce two possible points for the aircraft position. Using a second slave (or secondary) station the receiver then pinpoints its position. Entering the Lat./long. During initialization also aids in this process.,LORAN Limitations,Area of coverage limited to chains which don’t give global coverage The “sky wave” component of the signal bounces off the ionosphere causing navigational errors when traveling greater than 1,000 NM from the farthest master or secondary station- causes errors in position of up to three miles (greater in some cases).,LORAN Limitations- sky waves,1,000 miles is significant as at ranges less than this, the ground wave is significantly greater in strength than the sky waves therefore the receiver is programmed to reject the weaker sky waves.,LORAN Limitations- sky waves,At distances of between 1,000 and 1,400 NM the ground and sky waves are approximately the same strength making the signals seem the same to the receiver. Since the sky waves take longer to reach the receiver than the ground waves, this leads to position errors.,LORAN Limitations- sky waves,Manufacturers deal with in one of two ways Programming the receivers to reject all sky waves thereby reducing the effective range of the unit (but maximizing the accuracy) Informing the user that nav. Data may be inaccurate between 1,000 and 1,400 NM At ranges greater than 1,400 NM, the receivers can be programmed to use the sky wave,LORAN Limitations,As the aircraft moves through precipitation, it generates static electricity which generates electromagnetic “noise” around the aircraft thereby interfering with the LORAN signal- causing the Signal Strength to Noise ratio (SNR ratio) to go to unacceptable limits. SNR ratio is a function of precip. Intensity and distance from the stations.,LORAN- Accuracy,LORAN accuracy is also best over water during the day. Conversely it is least accurate over land masses at night (sky waves more intense at night).,LORAN Accuracy,Repeatable Accuracy- ability to return to a pre-set position for LORAN-C is .01 NM or 60ft. Absolute Accuracy- ability of the receiver to determine its position independently is generally within at least 3 NM for distances up to 2,800 NM.,。