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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_144</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_143.html">< previous page</a></td> <td id="ebook_previous" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_144</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_145.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 144</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">techniques, described in Sec. 5.8, can significantly improve these accuracies. Full SPS operational capability was achieved in late 1993.</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">PPS is a more accurate positioning, velocity, and timing service that is available only to users authorized by the U.S. government. Access to this service is controlled by two techniques known as AS and <i>selective availability</i> (SA). AS is implemented by replacing the P code with the classified Y code. SA is implemented by purposefully degrading the satellite clock and ephemeris data available to nonauthorized users. Authorized user receivers have the ability to access the Y code and remove the effects of SA. Predictable accuracy of PPS is 22 m (2 drms) horizontal, 27.7 m (95%) vertical, and 200 ns (95%) time [35]. Full PPS operational capability was achieved in Spring 1995.</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">5.2<br />The Mathematics of the GPS</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 GPS is based on time-of-arrival ranging [93]. The GPS receiver contains an internal clock that is used to note the time of arrival of the satellite ranging code (C/A, P, or Y depending on mode of operation). From this information, the GPS receiver determines the time required for the signal to propagate from the satellite to the receiver. Since the signal travels at the speed of light, this time interval can be converted to a distance when it is multiplied by the speed of light, denoted by <i>c</i>. These range measurements are biased by the receiver and satellite clock errors and therefore are referred to as <i>pseudoranges</i>. A timing error of 1 ms results in a range error of ~300 km; therefore time bias will need to be accurately compensated.</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">If the satellite location is known, then, neglecting clock errors, the measured satellite-to-receiver range defines a sphere of possible receiver locations. The intersection of two such spheres defines a circle of possible receiver locations. The intersection of three spheres results in two possible receiver locations. With either a prior estimate of the receiver location or another range estimate, a single receiver position estimate would result. This geometrical example is equivalent to the idea that three independent equations are necessary to determine three unknowns. The three unknowns are the coordinates of the receiver antenna in the three-dimensional world. When the receiver clock error is accounted for, all simultaneous range measurements are biased by the same receiver clock error. Therefore, in any set of simultaneous range measurements there are four unknowns and at least four range measurements will be required for a unique solution to exist.</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 control segment monitors each satellite clock and determines the offset, drift rate, and drift acceleration with respect to GPS time. These parameters are subsequently uploaded to the satellites and transmitted as part of the navigation message broadcast by the satellites. GPS receivers use the satellite clock offsets to correct the measured pseudoranges based on the formulas described in App. E.</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">Let the receiver clock offset from the satellite system time be denoted by </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>r</sub></font></i><font face="Times New Roman, Times, Serif" size="1"><sub></sub></font><font face="Times New Roman, Times, Serif" size="3">. Then the standard GPS positioning problem involves four unknown quantities</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_143.html">< previous page</a></td> <td id="ebook_next" align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_144</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_145.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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