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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_17</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_16.html">< previous page</a></td> <td id="ebook_previous" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_17</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_18.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 17</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">First, consider the column titled Implementation. The design of ground-based systems requires that the center frequency be selected as a tradeoff between accuracy and range. Low-frequency signals have longer range, since they are reflected by the ionosphere, and can therefore be received over the horizon. However, lower-frequency (longer-wavelength, </font><font face="Symbol" size="3">l</font><font face="Times New Roman, Times, Serif" size="3">=<i>c/f</i>) signals result in less position-determination accuracy. Since the signals from space-based systems must penetrate the ionosphere to reach earth-bound users, space-based systems all have high center frequencies. In fact, the prime motivation for space-based radio-navigation systems is to achieve the high accuracies possible from high-frequency systems without sacrificing range. A single space-based transmitter can broadcast to slightly less than half the globe. A constellation of space-based transmitters can provide multisatellite coverage for the entire globe.</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">Accuracy, update rate, reliability, and hardware availability are four factors on which the space-based systems can be compared. The Transit system is the United States' oldest family of radio-navigation satellites and is in the process of being phased out. One of its major drawbacks was its low sampling rate (</font><font face="Symbol" size="3">禄</font><font face="Times New Roman, Times, Serif" size="3">1 sample per hour). Global navigation satellite system (Glonass) and GPS are comparable in various aspects of system design. Both systems offer comparable accuracies and update rates (1.0 sample per second is typical). This text is focusing on GPS rather than Glonass for the following reasons: (1) GPS is currently much more widely used, (2) GPS receivers are widely available from numerous manufacturers at lower cost, (3) in past experience the GPS system with the backing of the U.S. government has been very reliable, and (4) GPS is currently being used or considered in various large-scale air and land navigation applications [101]. Although the receiver output and the pseudorange processing would change for a navigation system for which Glonass is used, the system theory and INS aiding concepts are still directly applicable.</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 major benefits of a combined GPS/Glonass implementation would be in the areas of availability and integrity monitoring. Availability refers to the percentage of time that a user at a given location (or region) can be guaranteed a stated level of position-estimation accuracy. Availability would increase since a two-system approach increases the number of available satellites. In addition, most error sources (other than atmospheric) would be uncorrelated. Integrity monitoring refers to the ability of the receiver to detect the loss of system integrity. A two-system approach would improve system integrity, since two systems with completely independent space and control segments are extremely unlikely to fail at the same time in the same manner. If the observables of both systems were observed, it would be possible to determine when one system was misbehaving. It might be possible to determine which system was at fault also by use of the INS information.</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="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">1.6<br />Complementary Filters</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">As stated above, GPS and INS have complementary characteristics. GPS pseudorange measurements are available at relatively low sample rates and provide</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_16.html">< previous page</a></td> <td id="ebook_next" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_17</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_18.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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