The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. In this example we implement algorithm to plot GPS ground track. Same algorithm could be used for any satellite.
clear all; clc; % Earth topographic map figure(1); xwidth = 820; ywidth = 420; hFig = figure(1); set(gcf,'PaperPositionMode','auto') set(hFig, 'Position', [100 100 xwidth ywidth]) hold on; grid on; axis([0 360 -90 90]); load('topo.mat','topo','topomap1'); contour(0:359,-89:90,topo,[0 0],'b') axis equal box on set(gca,'XLim',[0 360],'YLim',[-90 90], ... 'XTick',[0 60 120 180 240 300 360], ... 'Ytick',[-90 -60 -30 0 30 60 90]); image([0 360],[-90 90],topo,'CDataMapping', 'scaled'); colormap(topomap1); ylabel('Latitude [deg]'); xlabel('Longitude [deg]'); title('GPS BII-10 ground track'); R_e = 6378; % Earth's radius mu = 398600; % Earth’s gravitational parameter [km^3/s^2] J2 = 0.0010836; we = 360*(1 + 1/365.25)/(3600*24); % Earth's rotation [deg/s] % GPS BII-10 Orbital Parametres rp = 19781 + R_e; % [km] Perigee Radius ra = 20582 + R_e; % [km] Apogee Radius theta = 25; % [deg] True anomaly RAAN = 229.9128 ; % [deg] Right ascension of the ascending node i = 54.4303 ; % [deg] Inclination omega = 335.2539 ; % [deg] Argument of perigee a = (ra+rp)/2; % Semimajor axis e = (ra -rp)/(ra+rp) ; % Eccentricity h = (mu*rp*(1 + e))^0.5; % Angular momentum T = 2*pi*a^1.5/mu^0.5; % Period dRAAN = -(1.5*mu^0.5*J2*R_e^2/((1-e^2)*a^3.5))*cosd(i)*180/pi; domega = dRAAN*(2.5*sind(i)^2 - 2)/cosd(i); % Initial state [R0 V0] = Orbital2State( h, i, RAAN, e,omega,theta); [ alfa0 ,delta0 ] = R2RA_Dec( R0 ); scatter(alfa0,delta0,'*k'); ind = 1; eps = 1E-9; dt = 20; % time step [sec] ti = 0; while(ti <= 3*T); E = 2*atan(tand(theta/2)*((1-e)/(1+e))^0.5); M = E - e*sin(E); t0 = M/(2*pi)*T; t = t0 + dt; M = 2*pi*t/T; E = keplerEq(M,e,eps); theta = 2*atan(tan(E/2)*((1+e)/(1-e))^0.5)*180/pi; RAAN = RAAN + dRAAN*dt ; omega = omega + domega*dt; [R V] = Orbital2State( h, i, RAAN, e,omega,theta); % Considering Earth's rotation fi_earth = we*ti; Rot = [cosd(fi_earth), sind(fi_earth),0;... -sind(fi_earth),cosd(fi_earth),0;0,0,1]; R = Rot*R; [ alfa(ind) ,delta(ind) ] = R2RA_Dec( R ); ti = ti+dt; ind = ind + 1; end scatter(alfa,delta,'.r'); text(280,-80,'smallsats.org','Color',[1 1 1], 'VerticalAlignment','middle',... 'HorizontalAlignment','left','FontSize',14 );
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