船员、飞行员使用的无线电信标（EPIRB/ELT）已经有很多年头了，从技术上看的确谈不上新鲜。随着微电子技术、卫星技术的发展，个人版的PLBs是在最近几年才开始慢慢实用化的，即使在美国，对许多人来说也是新鲜玩意。
去年12月一个家伙在美国德克萨斯州Big Bend国家公园爬Elephant Tusk峰，爬到一半上不去下不来了，后来靠ARC公司的PLBs和一张space救生毯获救。
PLBs提供两种定位方法，今年新出的PLBs都集成了GPS芯片(如ACR的MicrOFix / ResQFix)：
1) The basic process involves Doppler shift measurement techniques that allow the low-earth orbiting (LEO) satellites passing overhead to determine a location. The location is determined from the Doppler shift in the carrier frequency.
The Doppler effect is the perceived change in frequency resulting from the relative movement of the source, the PLB, and the receiver, the LEO satellite. As the satellite approaches the PLB, the frequency received by the instruments on board the satellite will be higher than the actual transmitted frequency; as the satellite passes by the source of the signal the transmitted frequency will become lower than the transmitted frequency. At the point on the Doppler curve that the received and transmitted frequencies are equal, the position of the transmitter will be perpendicular to the satellite ground track.
The strong 5 watt 406 MHz signal makes this process easier and more accurate than the older, low-powered 121.5 MHz signal; sufficiently better that a reliable and accurate location can usually be determined on the first pass, not the case with the weaker 121.5 MHz signal which require multiple passes.
Explanation of Doppler effect for location determination
2) PLBs equipped to transmit GPS derived location information can provide location as quick as a few minutes when in view of a geostationary (GEO) satellite, essentially between 70 degrees North and South latitudes, assuming a clear view of the horizon at higher latitudes. This also assumes that the GPS is able to get a location, which is another issue entirely. See: "GPS and GPS Limitations."
[ 本帖最后由 cachet 于 2007-7-15 23:15 编辑 ] |
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发表于 2007-7-13 22:25
