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<html><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><!--Converted with jLaTeX2HTML 2002 (1.62) JA patch-1.4patched version by: Kenshi Muto, Debian Project.LaTeX2HTML 2002 (1.62),original version by: Nikos Drakos, CBLU, University of Leeds* revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan* with significant contributions from: Jens Lippmann, Marek Rouchal, Martin Wilck and others --><HTML><HEAD><TITLE>17.2.1 Nodes and node positions</TITLE><META NAME="description" CONTENT="17.2.1 Nodes and node positions"><META NAME="keywords" CONTENT="everything"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"><META NAME="Generator" CONTENT="jLaTeX2HTML v2002 JA patch-1.4"><META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css"><LINK REL="STYLESHEET" HREF="everything.css" tppabs="http://www.isi.edu/nsnam/ns/doc/everything.css"><LINK REL="next" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html"><LINK REL="previous" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><LINK REL="up" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><LINK REL="next" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html"></HEAD><BODY ><!--Navigation Panel--><A NAME="tex2html4362" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html4356" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html4350" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html4358" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html"><IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="file:/usr/share/latex2html/icons/contents.png"></A> <A NAME="tex2html4360" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html"><IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="file:/usr/share/latex2html/icons/index.png"></A> <BR><B> Next:</B> <A NAME="tex2html4363" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html">17.2.2 Satellite links</A><B> Up:</B> <A NAME="tex2html4357" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html">17.2 Using the satellite</A><B> Previous:</B> <A NAME="tex2html4351" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html">17.2 Using the satellite</A>   <B> <A NAME="tex2html4359" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html4361" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html">Index</A></B> <BR><BR><!--End of Navigation Panel--><H2><A NAME="SECTION031421000000000000000"></A><A NAME="sec:satellite_usage_nodes"></A><BR>17.2.1 Nodes and node positions</H2><P>There are two basic kinds of satellite nodes: <EM>geostationary</EM> and <EM>non-geostationary</EM> satellite nodes. In addition, <EM>terminal</EM> nodescan be placed on the Earth's surface. As is explained later in Section <A HREF="node210.html#sec:satellite_implementation" tppabs="http://www.isi.edu/nsnam/ns/doc/node210.html#sec:satellite_implementation">17.3</A>,each of these three different types of nodes is actually implemented with the same <TT>class SatNode</TT> object, but with different position,handoff manager, and link objects attached. The position object keeps track of the satellite node's location in the coordinate system as a function of the elapsed simulation time.This position information is used to determine link propagation delays andappropriate times for link handoffs. Section <A HREF="node46.html#sec:node:nodeconfig" tppabs="http://www.isi.edu/nsnam/ns/doc/node46.html#sec:node:nodeconfig">5.3</A> introduced the "node-config" utility used to prime the node generator for differenttypes of satellite nodes. <P>Figure <A HREF="node199.html#fig:spherical" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html#fig:spherical">17.2</A> illustrates the spherical coordinate system,and the corresponding Cartesian coordinate system.The coordinate system is centered at the Earth's center, and the axis coincides with the Earth's axis of rotation. <!-- MATH $(R,\theta,\phi) = (6378 km, 90^o, 0^o)$ --> corresponds to longitude (prime meridian) on the equator.<P>Specifically, there is one class of satellite node <TT>Class Node/SatNode</TT>,to which one of three types of <TT>Position</TT> objects may be attached. Each <TT>SatNode</TT> and <TT>Position</TT> object is a split OTcl/C++ object,but most of the code resides in C++. The following types of position objects exist: <UL><LI><TT>Position/Sat/Term</TT> A terminal is specified by its latitude andlongitude. Latitude ranges from and longitude ranges from, with negative values corresponding to south and west, respectively. As simulation time evolves, the terminals move alongwith the Earth's surface. The node generator can be used to create a terminal with an attached position object as follows:<PRE>$ns node-config -satNodeType terminal \bs (other node config commands go here...)set n1 [$ns node]$n1 set-position $lat $lon; # in decimal degrees</PRE></LI><LI><TT>Position/Sat/Geo</TT> A geostationary satellite is specified by its longitude above the equator. As simulation time evolves, the geostationarysatellite moves through the coordinate system with the same orbital periodas that of the Earth's rotation. The longitude ranges from degrees. As we describe further below, two flavors of geostationary nodesexist: ``geo'' (for processing satellites) and ``geo-repeater'' (for bent-pipesatellites). The node generator can beused to create a geostationary satellite with an attached position object as follows:<PRE>$ns node-config -satNodeType geo (or ``geo-repeater'') \bs (other node config commands go here...)set n1 [$ns node]$n1 set-position $lon; # in decimal degrees</PRE></LI><LI><TT>Position/Sat/Polar</TT> A polar orbiting satellite has a purelycircular orbit along a fixed plane in the coordinate system; the Earthrotates underneath this orbital plane, so there is both an east-west anda north-south component to the track of a polar satellite's footprint onthe Earth's surface. Strictly speaking, the polar position object canbe used to model the movement of any circular orbit in a fixed plane; we use the term ``polar'' here because we later use such satellites to model polar-orbiting constellations.<P>Satellite orbits are usually specified by six parameters: <EM>altitude</EM>,<EM>semi-major axis</EM>, <EM>eccentricity</EM>, <EM>right ascension of ascending node</EM>, <EM>inclination</EM>, and<EM>time of perigee passage</EM>. The polar orbiting satellites in havepurely circular orbits, so we simplify the specification of the orbits toinclude only three parameters: <EM>altitude</EM>, <EM>inclination</EM>, and<EM>longitude</EM>, with a fourth parameter <EM>alpha</EM> specifying initial position of the satellite in the orbit, as described below.<B>Altitude</B> is specified in kilometers above the Earth's surface, and <B>inclination</B> can range from degrees, with correspondingto pure polar orbits and angles greater than degrees correspondingto ``retrograde'' orbits. The <EM>ascending node</EM> refers to the pointwhere the footprint of the satellite orbital track crosses the equator moving from south to north. In this simulation model, the parameter <B>longitude of ascending node</B> specifies the earth-centric longitude at which the satellite's nadir point crosses the equator moving southto north.<A NAME="tex2html36" HREF="footnode.html#foot8044" tppabs="http://www.isi.edu/nsnam/ns/doc/footnode.html#foot8044"><SUP>17.3</SUP></A> <EM>Longitude of ascending node</EM> can range from degrees. The fourth parameter,<B>alpha</B>, specifies the initial position of the satellite along thisorbit, starting from the ascending node. For example, an <EM>alpha</EM> of degrees indicates that thesatellite is initially above the equator moving from north to south.<EM>Alpha</EM> can range from degrees.Finally, a fifth parameter, <B>plane</B>, is specified when creatingpolar satellite nodes- all satellites in the same plane are given thesame plane index.The node generator used to create a polar satellite with an attached position object as follows:<PRE>$ns node-config -satNodeType polar \bs (other node config commands go here...)set n1 [$ns node]$n1 set-position $alt $inc $lon $alpha $plane</PRE><P></LI></UL><P><HR><!--Navigation Panel--><A NAME="tex2html4362" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html4356" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html4350" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html4358" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html"><IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="file:/usr/share/latex2html/icons/contents.png"></A> <A NAME="tex2html4360" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html"><IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="file:/usr/share/latex2html/icons/index.png"></A> <BR><B> Next:</B> <A NAME="tex2html4363" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html">17.2.2 Satellite links</A><B> Up:</B> <A NAME="tex2html4357" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html">17.2 Using the satellite</A><B> Previous:</B> <A NAME="tex2html4351" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html">17.2 Using the satellite</A>   <B> <A NAME="tex2html4359" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html4361" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html">Index</A></B> <!--End of Navigation Panel--><ADDRESS>2003-09-23</ADDRESS></BODY></HTML>⌨️ 快捷键说明
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