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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_268</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_267.html">< previous page</a></td> <td id="ebook_previous" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_268</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_269.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 268</font></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">where <i>c</i> </font><font face="Symbol" size="3">d</font><font face="Times New Roman, Times, Serif" size="3"><i>t</i></font><i><font face="Times New Roman, Times, Serif" size="1"><sub>o</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>c</i> </font><font face="Symbol" size="3">d</font><font face="Times New Roman, Times, Serif" size="3"><i>t</i></font><font face="Times New Roman, Times, Serif" size="1"><sub>sv</sub></font><font face="Times New Roman, Times, Serif" size="3"> represent the residual base and satellite clock errors. The compensation and the effect of residual base clock error are discussed in the next section. The residual satellite clock error is small and common to all receivers. Therefore it will be removed through differential operation. Removal of base and satellite clock errors before broadcasting the corrections greatly decreases the magnitude of the broadcast corrections without sacrificing any accuracy.</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>7.5.2<br />Clock Bias Compensation</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 effects of the base receiver clock error should be removed from the corrections [102, 127]. This is done for two reasons. The first reason is to decrease the dynamic range of the corrections. The clock bias can result in errors as large as 0.001 * <i>c</i> </font><font face="Symbol" size="3">禄</font><font face="Times New Roman, Times, Serif" size="3"> 300 km. </font><font face="Symbol" size="3">D</font><font face="Times New Roman, Times, Serif" size="1"><sub>DGPS</sub></font><font face="Times New Roman, Times, Serif" size="3">(<i>t</i>) of Eq. (7.54) is normally less than 300 m. Second, the RTCM standard recommends that the </font><font face="Symbol" size="3">D</font><font face="Times New Roman, Times, Serif" size="1"><sub>DGPS</sub></font><font face="Times New Roman, Times, Serif" size="3">(<i>t</i>) corrections be slowly varying continuous functions (so that they can be accurately fit by a first-order polynomial); since the receiver clock bias can change discontinuously, they should not be included in the corrections. Since the C/A code has a 1-ms period, a receiver clock bias greater than this amount results in overflow (i.e., a bias of 1.1 ms appears as 0.1 ms.). This clock bias overflow corresponds to a 0.001 * <i>c</i> </font><font face="Symbol" size="3">禄</font><font face="Times New Roman, Times, Serif" size="3"> 300 km discontinuous change in the measured pseudorange, as shown in Fig. 5.2. The otherwise smooth corrections should not reflect these discontinuities.</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">One method of removing clock bias at the base is to estimate it as the least-squares estimate over all satellites available to the base. Since</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"><img src="99dd6c976ede8f41d25be1fd9a147c10.gif" border="0" alt="0268-01.GIF" width="484" height="43" /></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">a least-squares estimate of the clock bias is</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"><img src="ddf4806b016f8aae1b95c1f51e0dbeb0.gif" border="0" alt="0268-02.GIF" width="372" height="47" /></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">where <i>M</i> is the number of satellites available and <i>i</i> is an index counting over these satellites. An alternative approach is to select <img src="0c533d67ebe8f9017da429e439531296.gif" border="0" alt="c0268-01.GIF" width="67" height="18" /> to minimize the magnitude of the maximum correction. For a fixed set of satellites, the clock bias measurement of Eq. (7.57) includes a slowly varying additive bias error equivalent to the average of the raw computed range corrections from Eq. (7.55). The clock bias measurement error standard deviation (at a given time instant) is then <img src="029385f8a9f2597c0016f48f69c53676.gif" border="0" alt="C0268-02.GIF" width="67" height="17" /> m and has a significant correlation time. However, when a new satellite becomes available or an old satellite drops out, the measurement bias error will change discontinuously by the satellite differential range correction divided by <i>M</i>. Two questions are of interest: (1) does the slowly varying bias on the clock measurement matter, and (2) do the discontinuities in the clock measurement bias matter?</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">For applications not requiring accurate time transfer, the answer to the first question is a qualified no. If the clock estimate removed from the broadcast corrections is in error by a constant </font><font face="Symbol" size="3">t</font><font face="Times New Roman, Times, Serif" size="3">, then after differential correction, the user ranges would also all be identically biased by the constant </font><font face="Symbol" size="3">t</font><font face="Times New Roman, Times, Serif" size="3">. Since, as</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_267.html">< previous page</a></td> <td id="ebook_next" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_268</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_269.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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