Java tutorial
/** predict4java: An SDP4 / SGP4 library for satellite orbit predictions Copyright (C) 2004-2010 David A. B. Johnson, G4DPZ. This class is a Java port of one of the core elements of the Predict program, Copyright John A. Magliacane, KD2BD 1991-2003: http://www.qsl.net/kd2bd/predict.html Dr. T.S. Kelso is the author of the SGP4/SDP4 orbital models, originally written in Fortran and Pascal, and released into the public domain through his website (http://www.celestrak.com/). Neoklis Kyriazis, 5B4AZ, later re-wrote Dr. Kelso's code in C, and released it under the GNU GPL in 2002. PREDICT's core is based on 5B4AZ's code translation efforts. Author: David A. B. Johnson, G4DPZ <dave@g4dpz.me.uk> Comments, questions and bugreports should be submitted via http://sourceforge.net/projects/websat/ More details can be found at the project home page: http://websat.sourceforge.net This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, visit http://www.fsf.org/ */ package com.github.amsacode.predict4java; import java.util.ArrayList; import java.util.Calendar; import java.util.Date; import java.util.List; import java.util.TimeZone; import org.apache.commons.logging.Log; import org.apache.commons.logging.LogFactory; /** * Class which provides Pass Prediction. * * @author David A. B. Johnson, g4dpz * */ public class PassPredictor { private static final String UTC = "UTC"; private static final String SOUTH = "south"; private static final String NORTH = "north"; private static final double SPEED_OF_LIGHT = 2.99792458E8; private static final double TWOPI = Math.PI * 2.0; private static final String DEADSPOT_NONE = "none"; /** The time at which we do all the calculations. */ static final TimeZone TZ = TimeZone.getTimeZone(UTC); private static Log log = LogFactory.getLog(PassPredictor.class); private final TLE tle; private final GroundStationPosition qth; private final Satellite sat; private int iterationCount; /** * Constructor. * * @param tle * the Three Line Elements * @param qth * the ground station position * @throws IllegalArgumentException * bad argument passed in * @throws SatNotFoundException */ public PassPredictor(final TLE theTLE, final GroundStationPosition theQTH) throws IllegalArgumentException, SatNotFoundException { if (null == theTLE) { throw new IllegalArgumentException("TLE has not been set"); } if (null == theQTH) { throw new IllegalArgumentException("QTH has not been set"); } this.tle = theTLE; this.qth = theQTH; sat = SatelliteFactory.createSatellite(tle); if (null == sat) { throw new SatNotFoundException("Satellite has not been created"); } else if (!sat.willBeSeen(qth)) { throw new SatNotFoundException("Satellite will never appear above the horizon"); } } /** * Gets the downlink frequency corrected for doppler. * * @param freq * the original frequency in Hz * @return the doppler corrected frequency in Hz * @throws InvalidTleException * bad TLE passed in * @throws SatNotFoundException */ public Long getDownlinkFreq(final Long freq, final Date date) throws SatNotFoundException { // get the current position final Calendar cal = Calendar.getInstance(TZ); cal.clear(); cal.setTimeInMillis(date.getTime()); final SatPos satPos = getSatPos(cal.getTime()); final double rangeRate = satPos.getRangeRate(); return (long) ((double) freq * (SPEED_OF_LIGHT - rangeRate * 1000.0) / SPEED_OF_LIGHT); } public SatPos getSatPos(final Date time) throws SatNotFoundException { this.iterationCount++; return sat.getPosition(qth, time); } public Long getUplinkFreq(final Long freq, final Date date) throws SatNotFoundException { final Calendar cal = Calendar.getInstance(TZ); cal.clear(); cal.setTimeInMillis(date.getTime()); final SatPos satPos = getSatPos(cal.getTime()); final double rangeRate = satPos.getRangeRate(); return (long) ((double) freq * (SPEED_OF_LIGHT + rangeRate * 1000.0) / SPEED_OF_LIGHT); } public SatPassTime nextSatPass(final Date date) throws SatNotFoundException { return nextSatPass(date, false); } public SatPassTime nextSatPass(final Date date, final boolean windBack) throws SatNotFoundException { int aosAzimuth = 0; int losAzimuth = 0; double maxElevation = 0; double elevation = 0; String polePassed = DEADSPOT_NONE; // get the current position final Calendar cal = Calendar.getInstance(TZ); cal.clear(); cal.setTimeInMillis(date.getTime()); // wind back time 1/4 of an orbit if (windBack) { double meanMotion = tle.getMeanmo(); cal.add(Calendar.MINUTE, (int) (-24.0 * 60.0 / meanMotion / 4.0)); } SatPos satPos = getSatPos(cal.getTime()); SatPos prevPos = satPos; // test for the elevation being above the horizon if (satPos.getElevation() > 0.0) { // move time forward in 30 second intervals until the sat goes below // the horizon do { satPos = getPosition(cal, 60); } while (satPos.getElevation() > 0.0); // move time forward 3/4 orbit cal.add(Calendar.MINUTE, threeQuarterOrbitMinutes()); } Date tca = null; // now find the next time it comes above the horizon do { satPos = getPosition(cal, 60); final Date now = cal.getTime(); elevation = satPos.getElevation(); if (elevation > maxElevation) { maxElevation = elevation; tca = now; } } while (satPos.getElevation() < 0.0); // refine it to 5 seconds cal.add(Calendar.SECOND, -60); do { satPos = getPosition(cal, 5); final Date now = cal.getTime(); elevation = satPos.getElevation(); if (elevation > maxElevation) { maxElevation = elevation; tca = now; } prevPos = satPos; } while (satPos.getElevation() < 0.0); final Date startDate = satPos.getTime(); aosAzimuth = (int) ((satPos.getAzimuth() / (2.0 * Math.PI)) * 360.0); // now find when it goes below do { satPos = getPosition(cal, 30); final Date now = cal.getTime(); final String currPolePassed = getPolePassed(prevPos, satPos); if (!currPolePassed.equals(DEADSPOT_NONE)) { polePassed = currPolePassed; } log.debug("Current pole passed: " + polePassed); elevation = satPos.getElevation(); if (elevation > maxElevation) { maxElevation = elevation; tca = now; } prevPos = satPos; } while (satPos.getElevation() > 0.0); // refine it to 5 seconds cal.add(Calendar.SECOND, -30); do { satPos = getPosition(cal, 5); final Date now = cal.getTime(); elevation = satPos.getElevation(); if (elevation > maxElevation) { maxElevation = elevation; tca = now; } } while (satPos.getElevation() > 0.0); final Date endDate = satPos.getTime(); losAzimuth = (int) ((satPos.getAzimuth() / (2.0 * Math.PI)) * 360.0); return new SatPassTime(startDate, endDate, tca, polePassed, aosAzimuth, losAzimuth, (maxElevation / (2.0 * Math.PI)) * 360.0); } /** * @param cal * @param offSet * @return * @throws InvalidTleException * @throws SatNotFoundException */ private SatPos getPosition(final Calendar cal, final int offSet) throws SatNotFoundException { SatPos satPos; cal.add(Calendar.SECOND, offSet); satPos = getSatPos(cal.getTime()); return satPos; } /** * Gets a list of SatPassTime * * @param start * Date * * newTLE = true; validateData(); * @param end * Date * @param firstAosLimit * in hours * @return List<SatPassTime> * @throws SatNotFoundException * @throws InvalidTleException */ public List<SatPassTime> getPasses(final Date start, final int hoursAhead, final boolean windBack) throws SatNotFoundException { this.iterationCount = 0; boolean windBackTime = windBack; final List<SatPassTime> passes = new ArrayList<SatPassTime>(); Date trackStartDate = start; final Date trackEndDate = new Date(start.getTime() + (hoursAhead * 60L * 60L * 1000L)); Date lastAOS; int count = 0; do { if (count > 0) { windBackTime = false; } final SatPassTime pass = nextSatPass(trackStartDate, windBackTime); lastAOS = pass.getStartTime(); passes.add(pass); trackStartDate = new Date(pass.getEndTime().getTime() + (threeQuarterOrbitMinutes() * 60L * 1000L)); count++; } while (lastAOS.compareTo(trackEndDate) < 0); return passes; } /** * Returns the iterationCount. @VisibleForTesting * * @return the iterationCount */ final int getIterationCount() { return iterationCount; } /** * @return time in mS for 3/4 of an orbit */ private int threeQuarterOrbitMinutes() { return (int) (24.0 * 60.0 / tle.getMeanmo() * 0.75); } private String getPolePassed(final SatPos prevPos, final SatPos satPos) { String polePassed = DEADSPOT_NONE; final double az1 = prevPos.getAzimuth() / TWOPI * 360.0; final double az2 = satPos.getAzimuth() / TWOPI * 360.0; if (az1 > az2) { // we may be moving from 350 or greateer thru north if (az1 > 350 && az2 < 10) { polePassed = NORTH; } else { // we may be moving from 190 or greateer thru south if (az1 > 180 && az2 < 180) { polePassed = SOUTH; } } } else { // we may be moving from 10 or less through north if (az1 < 10 && az2 > 350) { polePassed = NORTH; } else { // we may be moving from 170 or more through south if (az1 < 180 && az2 > 180) { polePassed = SOUTH; } } } return polePassed; } /** * Calculates positions of satellite for a given point in time, time range * and step increment. * * @param referenceDate * @param incrementSeconds * @param minutesBefore * @param minutesAfter * @return list of SatPos * @throws SatNotFoundException * @throws InvalidTleException */ public List<SatPos> getPositions(final Date referenceDate, final int incrementSeconds, final int minutesBefore, final int minutesAfter) throws SatNotFoundException { Date trackDate = new Date(referenceDate.getTime() - (minutesBefore * 60L * 1000L)); final Date endDateDate = new Date(referenceDate.getTime() + (minutesAfter * 60L * 1000L)); final List<SatPos> positions = new ArrayList<SatPos>(); while (trackDate.before(endDateDate)) { positions.add(getSatPos(trackDate)); trackDate = new Date(trackDate.getTime() + (incrementSeconds * 1000)); } return positions; } }