很可惜台灣在差分GPS訊號僅能存在於封閉系統eGPS中, 常人根本無法無償取得這樣的高精度定位服務! 海洋大學的海事頻段DGPS似乎是唯一裝置, 中華電信則在籌建全台超長波50kHz附近的授時台, 也許會加入DGPS訊號吧!
http://www.ja-gps.com.au/dgpsprod_garmin.html 有列出Garmin GBR21, GBR23(可惜停產)就是這樣的接收裝備(天線+decoder), 讓 Garmin gps也可以有1m定位水準以上精度.
Differential Global Positioning System (DGPS)
The marine nondirectional beacons in Band B (285 to 325 kHz) have been phased out and replaced by marine DGPS beacons. In the US, these are also known as National DGPS or NDGPS. They use 200 bps MSK to relay information from a GPS monitoring station to allow significant improvement in the accuracy of a GPS position fix of a nearby receiver. The format has been standardized by RTCM, and DGPS beacons are also found in this band in Regions 1 and 3.
The US Coast Guard (USCG), in coopera-tion with six other federal agencies such as highways (FHWA) and railroads (FRA), is undertaking a significant expansion of the differential GPS (DGPS) system.3 This includes a faster data rate (500 or 1000 bps, versus the 200 bps) for the DGPS beacons at 300 kHz. The plan is to have coverage from at least two beacons everywhere in the continental US. The higher data rate allows the use of more monitors and the use of carrier phase. This gives them centimeter accuracy and accurate velocity for 「real-time kinematics」 (RTK). There is a wide variety of applications ranging from tracking vehicles to guiding crop dusters.
Many of the applications are for terrestrial navigation such as tracking cars and trains and knowing which lane or track they are on. The MF transmissions are well suited for this because they can be received at all altitudes beyond line of sight. Two such transmitters (Pennsylvania and Virginia, 454 and 456 kHz) have recently been put on the air.
The USCG plans to use the entire Band D from 435 to 495 kHz for these new high-accuracy
DGPS beacons. This frequency allocation has been cleared through the NTIA. 此一系統稱之為Nationwide DGPS, 詳見: http://www.tfhrc.gov/its/ndgps/ 及 http://www.navcen.uscg.gov/ndgps/default.htm (內有評估報告194頁)
PS: 定位精度---Currently, the GPS service offers a 4- to 20-meter (m) navigational accuracy. NDGPS provides accurate dynamic navigation information to travelers with 1- to 2- meter accuracy (and possibly better in the future). When used in a static mode, NDGPS offers a .1 to 1 meter service. HA(High Accuracy)-NDGPS to provide 10 cm horizontal and 20 cm vertical (95 percent) corrections to users.
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