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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_157</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_156.html">< previous page</a></td> <td id="ebook_previous" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_157</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_158.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 157</font></td></tr></table><table border="0" cellspacing="0" cellpadding="0" width="100%"><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 align="center"><font face="Times New Roman, Times, Serif" size="3"><img src="a8904e332a9a3890ae1c5e0eab9d6df9.gif" border="0" alt="0157-01.GIF" width="474" height="290" /></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" width="100%"><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 align="center"><font face="Times New Roman, Times, Serif" size="2">Figure聽5.4<br />Tropospheric聽delay聽compensation聽models.聽Heights聽as聽indicated聽in聽the聽legend<br />correspond聽to聽the聽plots聽from聽top聽to聽bottom.</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">With this definition, the L1 and the L2 ionospheric delays are (<i>f</i></font><font face="Times New Roman, Times, Serif" size="1"><sub>2</sub></font><font face="Times New Roman, Times, Serif" size="3">/<i>f</i></font><font face="Times New Roman, Times, Serif" size="1"><sub>1</sub></font><font face="Times New Roman, Times, Serif" size="3">)<i>I</i></font><i><font face="Times New Roman, Times, Serif" size="1"><sub>a</sub></font></i><font face="Times New Roman, Times, Serif" size="1"><sub></sub></font><font face="Times New Roman, Times, Serif" size="3"> and (<i>f</i></font><font face="Times New Roman, Times, Serif" size="1"><sub>1</sub></font><font face="Times New Roman, Times, Serif" size="3">/<i>f</i></font><font face="Times New Roman, Times, Serif" size="1"><sub>2</sub></font><font face="Times New Roman, Times, Serif" size="3">)<i>I</i></font><i><font face="Times New Roman, Times, Serif" size="1"><sub>a</sub></font></i><font face="Times New Roman, Times, Serif" size="1"><sub></sub></font><font face="Times New Roman, Times, Serif" size="3">, respectively. Two-frequency receivers can take advantage of the frequency dependence of this delay, as described in Sec. 5.6.</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">Single-frequency receivers must rely on either differential operation or an ionospheric delay model. One such approach based on parameters broadcast by the satellite is described in Sec. E.3 of App. E. This Klobuchar model is expected to compensate for approximately 50% of the ionospheric delay [73]. Since TEC is path dependent, the ability of DGPS techniques to compensate for ionospheric errors will depend on the user-to-reference station baseline vector.</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>5.4.4<br />Selective Availability</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">SA errors are artificially added to the satellite signals by the U.S. government to degrade the position and the velocity accuracy that can be attained with standard GPS receiver technology. SA error is induced in two forms [119, 127]. First, the broadcast ephemeris data may be purposely corrupted. This is referred to as the </font><font face="Symbol" size="3">e</font><font face="Times New Roman, Times, Serif" size="3"> form of selective availability. Second, the satellite oscillator frequency may be intentionally dithered. This is referred to as the </font><font face="Symbol" size="3">d</font><font face="Times New Roman, Times, Serif" size="3"> form of selective availability. The </font><font face="Symbol" size="3">e</font><font face="Times New Roman, Times, Serif" size="3"> form of SA would result in a very slowly changing error in the calculated satellite position. Note that the actual position of the satellite is unaffected. The </font><font face="Symbol" size="3">e</font><font face="Times New Roman, Times, Serif" size="3"> form has limited utility because of its long correlation time (i.e., hours). The </font><font face="Symbol" size="3">d</font><font face="Times New Roman, Times, Serif" size="3"> form of SA does affect the actual satellite clock. The clock dithering is designed to produce range errors with a correlation time of only a few minutes. 1-</font><font face="Symbol" size="3">s</font><font face="Times New Roman, Times, Serif" size="3"> values for the range error are typically 24.0 m;</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></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_156.html">< previous page</a></td> <td id="ebook_next" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_157</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_158.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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