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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_229</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_228.html">< previous page</a></td> <td id="ebook_previous" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_229</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_230.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 229</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">When the gyros selected for a given application do not allow heading initialization with the above methods, then alternative methods are required. Two common alternatives are magnetic-heading sensing with local magnetic-field compensation, or retrieval of the last stored heading for a system that is known to have been stationary between periods of operation. In either of these approaches, the accuracy will normally be low enough that on-line calibration of the azimuth error will be desirable by means of aiding sensors. A third option is the use of north-finding inertial instrumentation. Nonbenign environments (i.e., environments having disturbances or vibration) will result in inaccurate initialization of the attitude, which may necessitate on-line, aided error calibration.</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">6.8.1.2<br />Fine Initialization:<br />Physical Gyro Compassing</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 a mechanized system with accurately calibrated accelerometers and gyros, alignment can be achieved by a two-step process known as <i>gyro compassing</i> [18, 26, 69]. In the first step, referred to as <i>leveling</i>, the platform is torqued to null the outputs of the north and the east accelerometers. This step nominally aligns the <i>z</i> axis with the local gravity vector. The resulting leveling error is directly proportional to the uncompensated accelerometer bias. Since the east component of the earth rotation rate is known to be zero, the second step of the procedure rotates the platform about the <i>z</i> axis to as null the output of the east gyro. The second step is referred to <i>azimuth alignment</i>. At this point, at least theoretically, the north and the down gyros could be used to estimate and correct the platform latitude as </font><font face="Symbol" size="3">l</font><font face="Times New Roman, Times, Serif" size="3"> = arctan2(-</font><font face="Symbol" size="3">W</font><font face="Times New Roman, Times, Serif" size="1"><sub><i>D</i></sub></font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Symbol" size="3">W</font><font face="Times New Roman, Times, Serif" size="1"><sub><i>N</i></sub></font><font face="Times New Roman, Times, Serif" size="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">6.8.1.3<br />Fine Initialization:<br />Analytic Gyro Compassing</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">In strap-down system applications, the platform cannot be mechanically torqued to cause alignment with the gravity and earth rate vectors. Instead, the misalignment is estimated and used to correct the variables required for computing the direction cosine matrix.</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">From Sec. 6.4.1.3, the computed specific force vector in navigation frame 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="de83328a3a957d91ea5d4ecce3ae2442.gif" border="0" alt="0229-01.GIF" width="315" height="21" /></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="ca1c354f8f5bd0c5055c9476fe982a8e.gif" border="0" alt="0229-02.GIF" width="297" height="19" /></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 </font><font face="Symbol" size="3"><i>d</i></font><font face="Times New Roman, Times, Serif" size="3">f</font><font face="Times New Roman, Times, Serif" size="2"><sup><i>p</i></sup></font><font face="Times New Roman, Times, Serif" size="3"> represents the sum of the specific force measurement errors. If the navigation frame corresponds to the geographic frame, then</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="302d4aa54bd2f754876ff5bcb62be3ba.gif" border="0" alt="0229-03.GIF" width="139" height="19" /></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 f</font><font face="Times New Roman, Times, Serif" size="1"><sub><i>d</i></sub></font><font face="Times New Roman, Times, Serif" size="3"> represents the navigation-frame disturbances. These disturbances can be expected to be high frequency, but the amplitude and the spectral content are not accurately known and may be time varying. Disturbance motion can be decreased by mechanical isolation. The residual navigation-frame specific force</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_228.html">< previous page</a></td> <td id="ebook_next" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_229</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_230.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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