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Re: MHR1 replica and is my GPS wrong?
From: Paul Hirose
Date: 2012 Feb 28, 21:08 -0800
From: Paul Hirose
Date: 2012 Feb 28, 21:08 -0800
Örjan Sandström wrote: > Any hints as to as to why my terrestrial cel-nav intercepts if I use the > symedian point more or less constantly show me to be 0.5-0.7' east of > GPS even with over 20 ? If your fixes were significantly *west* of the true location, I would suspect a systematic late time, e.g., from not reading the chronometer second hand first. In that case, if you observe a body to the east (west), the computed altitude is too high (low), and so a perfect altitude observation is lower (higher) than the computed altitude. This shifts the fix to the west. But your error is to the east, so that explanation doesn't work. I don't believe someone would consistently read the time early by 2 or 3 seconds. And I don't think your chromometer checks by radio would have a systematic error that large. You said, "whatever azimuth body has seems to make little difference," but it's not clear if you have observed bodies to the east and the west. If observations in both directions agree, that would exclude sextant error and personal error. The GPS might be wrong – if the wrong datum is selected. WGS84 is a good worldwide datum. But some of the local datums can give large errors if you apply them in the wrong part of the world, e.g., if you use the old Tokyo datum in the U.S. Unlikely but possible – deflection of the vertical. The coordinates from a map or GPS are with respect to a mathematically perfect surface, an ellipsoid. Due to Earth's irregular density and shape, a gravity vertical is not exactly perpendicular to the ellipsoid. In most places the error is only a few seconds of arc and is not significant in practical navigation. However, deflection of the vertical may be apparent to a careful observer in excellent conditions. There are areas in the U.S. where the deflection of the vertical is as large as the anomaly you report. http://www.ngs.noaa.gov/GEOID/DEFLEC09/ Unfortunately, the online calculator on that page is good only in U.S. territory. The NGA has a worldwide calculator, but it gave an error message when I tried it today. http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/intpt.html --