http://www.gpsinformation.org/dale/dgps.htm 第五個FAQ提到GPS定位誤差的幾個原因;
4.0m Ionosphere(電離層誤差)
2.1m Clock(時錶誤差)
2.1m Ephemeris(星曆誤差)
0.7m Troposphere(對流層誤差)
0.5m Receiver(接收機誤差)
1.0m Multipath(多重路徑誤差)
===================
10.4m Total(總誤差)
http://www.gpsinformation.net/waas/vista-waas.html Observations:GPS accuracy is generally better at night when the ionospheric errors are less. WAAS ionospheric corrections are therefore less apparent at night. The reference-point accuracy is confirmed with the average night error of 0.8m. 剛好發現美國實驗夜間進行GPS定位精度實驗, 發現夜間Ionosphere誤差降到最低, 這也符合我對無線電研究的心得與理論, 不妨多利用精確基點核對看看是否符合這個觀察心得.
當然上述數據也反駁了網路上常懷疑天氣(Troposphere)影響GPS精度的傳言, 因為前三項誤差(8.2m)因素遠高於對流層(0.7m)的影響.
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有關于"GPS Error Budget"文章，網絡上不勝枚舉，而且估算方式和數字各有不同。
最常被引用到的文獻包括
- Langley, R. B. (1997), The GPS error budget. GPS World , Vol. 8, No. 3, pp. 51-56.
- Hoffmann-Wellenhof et al. (1997), GPS: Theory and Practice, 4th Ed., Springer.
- Bradford W. Parkinson et al. (1996), Global Positioning System: Theory and Applications, pp. 478-483.
不過前述所引用網頁的 "將所有 error source 加總在一起"，好像不是常見的估算方式。可以參考以下的簡單說明：
(from p. 44, Ahmed El-Rabbany (2002), Introduction to GPS: the Global Positioning System, Artech House)
A more simplified way of examining the GPS positioning accuracy may be achieved through the introduction of the ). Assuming that the measurement errors for all the satellites are identical and independent, then a quantity known as the UERE may be defined as the root-sum-square of the various errors and biases discussed earlier [3]. Multiplying the UERE by the appropriate DOP value produces the expected precision of the GPS positioning at the one-sigma (1-s) level.
To obtain the precision at the 2-s level, sometimes referred to as approximately 95% of the time, we multiply the results by a factor of two. For example, assuming that the UERE is 8m for the standalone GPS receiver, and taking a typical value of HDOP as 1.5, then the 95% positional accuracy will be 8 × 1.5 × 2 = 24m.
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i_am_bear wrote:
謝謝大大的文章。看了...
To obtain the precision at the 2-s level, sometimes referred to as approximately 95% of the time, we multiply the results by a factor of two. For example, assuming that the UERE is 8m for the standalone GPS receiver, and taking a typical value of HDOP as 1.5, then the 95% positional accuracy will be 8 × 1.5 × 2 = 24m.
(恕刪)
真幸運又遇到內行人, http://edu-observatory.org/gps/gps_accuracy.html是我近日找到的資料, 您的說明讓我終於看懂下面這段文字:
Estimated Position Error (EPE) and Error Sources
EPE (1-sigma) = HDOP * UERE (1-sigma) (1)
Multiplying the HDOP * UERE * 2 gives EPE (2drms) and is commonly taken as the 95% limit for the magnitude of the horizontal error. The probability of horizontal error is within an ellipse of radius 2drms ranges between 0.95 and 0.98 depending on the ratio of the ellipse semi-axes.
這樣就與我先前用Garmin Vista HCx的估計誤差(EPE)來推算實際誤差是一致的, 同時也與我的實際測量經驗相符.
至於第一篇的誤差數字應該是1 sigma的結果, 而且極可能是舊式傳統接收晶片的表現, 以前用Garmin eTrex Summit的最小估計誤差還是7m, 而現在Vista HCx(高感度晶片)已經可以縮小到3m, 但是網路上看到的似乎沒有以高感度晶片的測試數據. 上面連結中的 Table 2 : Standard error model - L1 C/A (no SA) 也是相同條件下的誤差(1-sigma).
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flycode wrote:
電離層高度約 50 – 2千 公里間
雲的高度大約是 2~12公里,雨層雲的高度約2公里以內
那看起來,下雨確實影響GPS準確度很少很少。...(恕刪)
http://en.wikipedia.org/wiki/Ionosphere電離層沒有這麼厚啦, 範圍僅有在50--400km間而已. 你用厚度來比較似乎還言之成理: 10km vs 350km, 一般人應該很容易理解吧!!
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millerliu wrote:
http://en....(恕刪)
電離層的所在高度,我也是抄來的啦!有的是寫50~2000公里,有的是寫50~1500公里
http://www.geodetic.gov.hk/smo/gsi/programs/tc/satref2.htm
電離層是大氣層較高部分的由負電荷的自由電子及正離子組成。它位於距離地面50至1500公里，其中電子密度最高的部分位於200至300公里處(Torge, 2001)。
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而電離層厚度多少我就不清楚了!? 根據直覺推測,電離層的電漿密度的影響應該較大,
但把電離層的高度抓出來,主要是排除在2~12公里間雲層/下雨對GPS精度的影響,
要不然,天黑黑要下雨,佷多人就會"感覺到"GPS訊號也受到影響。
業代殺手,斷人財路!
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rediku wrote:
天氣差應該是信號強度會變差 ....
但只要強度夠大 ...
就可以做定位 .......(恕刪)
第一篇的內容可是多數論文的結論, http://edu-observatory.org/gps/gps_accuracy.html是另一位教授的文章, 您的想法不知道是否有根據, 不然我們的討論也是不會有焦點與討論基礎. 下面這段已經推翻了你的想法:
E. Troposphere Errors
Another deviation from the vacuum speed of light is caused by the troposphere. Variations in temperature, pressure, and humidity all contribute to variations in the speed of light of radio waves. Both the code and carrier will have the same delays. This is described further in the chapter devoted to these effects, Chapter 13 of this volume. For most users and circumstances, a simple model should be effectively accurate to about 1 m or better.
