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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>page_222</title> <link rel="stylesheet" href="reset.css" type="text/css" media="all"> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> </head> <body> <table summary="top nav" border="0" width="100%"> <tr> <td align="left" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_221.html">< previous page</a></td> <td id="ebook_previous" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_222</strong></td> <td align="right" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_223.html">next page ></a></td> </tr> <tr> <td id="ebook_page" align="left" colspan="3" style="background: #ffffff; padding: 20px;"> <table border="0" width="100%" cellpadding="0"><tr><td align="center"> <table border="0" cellpadding="2" cellspacing="0" width="100%"><tr><td align="left"></td> <td align="right"></td> </tr></table></td></tr><tr><td align="left"><p></p><table border="0" cellspacing="0" cellpadding="0" width="100%"><tr><td align="right"><font face="Times New Roman, Times, Serif" size="2" color="#FF0000">Page 222</font></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="17"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">6.6<br />The Earth Geoid and Gravity Model</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">Earth-relative INSs require an accurate model of the local gravity vector to compensate for the specific force measurement to produce an accurate measure of acceleration. In the following sections the need for and the definition of the earth geoid and the gravity vector relative to this geoid are briefly discussed.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3"><i>6.6.1<br />Earth Geoid</i></font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The gravity vector is the vector sum of the gravitational force of the earth mass and the centrifugal force due to earth rotation. The relative size and orientation of these two forces are dependent on the point of evaluation. For example, the centrifugal force is maximum at the equator and zero along the earth's axis of rotation. The geodetic surface of the earth is defined to be everywhere normal to the gravity vector (i.e., an equipotential surface relative to the force of gravity). The geodetic surface is distinct from the actual irregular shape of the surface, but may be imagined as the mean shape that the earth would take if the solid surface of the earth were completely covered with sea water.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The geodetic surface of the earth is closely approximated by an ellipsoid of revolution about its minor axis. Such an ellipsoid can be defined by the length of the semimajor axis a and the flattening </font><font face="Symbol" size="3">娄</font><font face="Times New Roman, Times, Serif" size="3">. The gravity vector is specified by the angular rate of earth rotation </font><font face="Symbol" size="3">w</font><font face="Times New Roman, Times, Serif" size="1"><sub><i>ie</i></sub></font><font face="Times New Roman, Times, Serif" size="3"> and the equatorial value of gravity </font><font face="Symbol" size="3">g</font><font face="Times New Roman, Times, Serif" size="1"><sub><i>a</i></sub></font><font face="Times New Roman, Times, Serif" size="3">. These four values specifying the geodetic surface and gravity vector can be determine based on experimental observations. Because of the change in measurement accuracy over the decades, various sets of geodetic and gravity parameters have been determined and accepted by international bodies. Two sets of accepted values are presented in the first four rows of Table 6.3.</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The first column of values was determined based on experimental data by Hayford in 1909 to determine the geodetic parameters and data by Heiskanen in 1928 to determine </font><font face="Symbol" size="3">g</font><font face="Times New Roman, Times, Serif" size="1"><sub><i>a</i></sub></font><font face="Times New Roman, Times, Serif" size="3">. The first column of data was accepted as the international standard in 1930. As experimental techniques have improved, the geodetic and gravity model parameters have been refined. In particular, improved accuracy was made possible by the analysis of the trajectories of artificial satellites. The</font><font face="Times New Roman, Times, Serif" size="3" color="#FFFF00"></font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table cellpadding="0" cellspacing="0" border="0" width="100%"><tr><td height="12"></td></tr><tr><td><table cellspacing="0" width="421" cellpadding="4"><tr><td colspan="4" valign="top"><font face="Times New Roman, Times, Serif" size="1">TABLE</font><font face="Times New Roman, Times, Serif" size="2"> 6.3 Geodetic and Gravitational Parameters</font></td></tr><tr><td valign="top"><font face="Times New Roman, Times, Serif" size="2">Constant</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">International</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">WGS-84</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">Units</font></td></tr><tr><td valign="top"><font face="Times New Roman, Times, Serif" size="2"><i>r</i></font><i><font face="Times New Roman, Times, Serif" size="1"><sub>e</sub></font><font face="Times New Roman, Times, Serif" size="2"></font></i><font face="Times New Roman, Times, Serif" size="2">= <i>a</i></font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">6378338.0</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">6378137.0</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">m</font></td></tr><tr><td valign="top"><font face="Symbol" size="2">娄</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">1/297.000</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">1/298.257</font></td><td valign="top"></td></tr><tr><td valign="top"><font face="Symbol" size="2"><i>g</i></font><i><font face="Times New Roman, Times, Serif" size="1"><sub>a</sub></font></i></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">9.78049000</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">9.780327</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">m/s<sup>2</sup></font></td></tr><tr><td valign="top"><font face="Symbol" size="2"><i>w</i></font><i><font face="Times New Roman, Times, Serif" size="1"><sub>ie</sub></font></i></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">7.292115110</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">7.292115</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">脳10<sup>-5</sup> rad/s</font></td></tr><tr><td valign="top"><font face="Times New Roman, Times, Serif" size="2"><i>b</i></font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">6356911.9</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">6356752.3</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">m</font></td></tr><tr><td valign="top"><font face="Symbol" size="2"><i>m</i></font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">3986329</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">3986005</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">脳10<sup>8</sup> m<sup>3</sup>/s<sup>2</sup></font></td></tr><tr><td valign="top"><font face="Times New Roman, Times, Serif" size="2"><i>J</i></font><font face="Times New Roman, Times, Serif" size="1"><sub>2</sub></font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">0.0010920</font></td><td valign="top"><font face="Times New Roman, Times, Serif" size="2">0.00108263</font></td><td valign="top"></td></tr></table></td></tr></table><br /></td></tr></table><p><font size="0"></font></p>聽 </td> </tr> <tr> <td align="left" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_221.html">< previous page</a></td> <td id="ebook_next" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_222</strong></td> <td align="right" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_223.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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