PredictGlossary

Using Predict NA S D FIRST PROGRAM PREDICTING SINGLE SOLAR SOLAR OPERATION COMMAND QUIET AUTOMATIC ADDITIONAL SERVER ADDING NEAT GLOSSARY Anomalistic Ascending Descending Doppler Orbital Path Nodal Slant Sub-Satellite ADDITIONAL KD2BD class="anchor" id="line-2">
PREDICT(1)                KD2BD Software               PREDICT(1) ME predict  - Track and predict passes of satellites in Earth orbit YNOPSIS predict [-u tle_update_source] [-t tlefile]  [-q  qthfile] [-a  serial_port]  [-a1 serial_port] [-n network_port] [-f sat_name starting_date/time ending_date/time] [-p sat_name starting_date/time]  [-o output_file] [-s] [-east] [-west] [-north] [-south] ESCRIPTION PREDICT is a multi-user  satellite  tracking  and  orbital prediction  program written under the Linux operating sys- tem by John A. Magliacane, KD2BD. PREDICT  is  free  soft- ware.  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 any later version. PREDICT is distributed in the hope that it will be useful, but  WITHOUT  ANY  WARRANTY, without even the implied war- ranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR  PUR- POSE. See the GNU General Public License for more details. TIME USE PREDICT tracks and predicts passes of satellites based  on the  geographical location of the ground station, the cur- rent date and time as provided by  the  computer  system's clock,  and  Keplerian  orbital data for the satellites of interest to the ground station. First time users  of  PRE- DICT  are  provided  default  ground  station location and orbital data information files. These files are managed by the  program,  and  are  normally located in a user's home directory under  a  hidden  subdirectory  named  .predict. First  time  users will be prompted to supply PREDICT with their geographical location (the same as selecting  option [G]  from the program's main menu) the first time the pro- gram is run. Latitude is  normally  expressed  in  degrees north  with  latitudes  south  of the equator expressed in negative  degrees.  Longitude  is  normally  expressed  in degrees west with eastern longitudes expressed in negative degrees. This behavior can  be  modified  by  passing  the -east or -south commmand line switches to PREDICT. Latitudes  and longitudes may be either entered in decimal degrees, or in degrees,  minutes,  seconds  (DMS)  format. Station  altitude  is  entered as the number of meters the ground station is located above sea level.  This parameter is  not  very critical.  If unsure, make a realistic guess or simply enter 0. Users of PREDICT  need  Keplerian  orbital  data  for  the satellites  they wish to track that is preferably no older than one month. The default orbital data supplied with the program  is liable to be quite old, and so must be brought up to date if accurate results are to  be  expected.  This may be accomplished by selecting option [E] from PREDICT's main menu and manually entering Keplerian  data  for  each satellite  in  the  program's  database,  or  by selecting option [U] and specifying a file containing recent  2-line Keplerian  element data sets that correspond to the satel- lites in the program's database.  Keplerian  orbital  data is   available   from  a  variety  of  sources,  including http://www.celestrak.com/,    http://www.space-track.org/, and http://www.amsat.org/. OPERATION The  start-up  screen  of PREDICT lists the program's main functions.  Several tracking and orbital prediction  modes are  available, as well as several utilities to manage the program's orbital database. SATELLITE PASSES Orbital predictions are useful for determining in  advance when  a  satellite  is  expected to come within range of a ground station. They can also be used to look back to pre- vious  passes to help to confirm or identify past observa- tions. PREDICT includes two orbital prediction modes  to  predict any pass above a ground station (main menu option [P]), or list only those passes that might be visible to  a  ground station  through  optical means (main menu option [V]). In either mode, the user is asked to select  a  satellite  of interest from a menu, and then asked to enter the date and time (in UTC)  at  which  prediction  calculations  should start. The  current  date  and time may be selected by default by entering nothing and hitting simply  the  ENTER  key  when prompted to enter the starting date and time. Otherwise, the starting date and time should be entered in the form: DDMonYY HH:MM:SS Entering the time is optional.  If it is omitted, midnight (00:00:00)  is  assumed.   Once complete, orbital calcula- tions are started and prediction information is  displayed on the screen. The  date and time in UTC, along with the satellite's ele- vation above ground, azimuth heading, modulo  256  orbital phase,  sub-satellite  point latitude and longitude, slant range between the ground station and  the  satellite,  and the satellite's orbit number are all displayed.  If space- craft attitude parameters (ALAT,  ALON)  are  included  in PREDICT's   transponder  database  file,  then  spacecraft antenna squint angles are displayed instead of orbit  num- bers in the orbital prediction output. An asterisk (*) displayed to the right of the orbit number or squint angle means the satellite is in sunlight at  the date  and time listed on the line. A plus symbol (+) means the satellite is in sunlight while the ground  station  is under  the  cover of darkness at the time and date listed. Under good viewing conditions, large  satellites  such  as the  International Space Station (ISS), the US Space Shut- tles, and Hubble Space Telescope, and the Upper Atmosphere Research Satellite (UARS) are visible to the naked eye. If no symbol appears to the right  of  each  line,  then  the satellite  is  in  the Earth's shadow at the time and date listed, and is not receiving  any  illumination  from  the sun. Pressing  the  ENTER  key,  the  'Y' key, or the space bar advances the orbital predictions to a screen  listing  the next  available  passes.   Pressing the 'L' key allows the currently displayed screen plus any subsequent screens  to be  logged  to  a text file in your current working direc- tory. The name given to this  file  is  the  name  of  the satellite  plus a ".txt" extension.  Any slashes or spaces appearing in the satellite name are replaced by the under- score  (_)  symbol.  The logging feature may be toggled on and off at any time by pressing the 'L' key.  Exiting  the orbital  prediction  mode  by  pressing 'N' or hitting the ESCape key will also close the log file. The log file will be appended with additional information if additional pre- dictions are conducted for the  same  satellite  with  the logging feature turned on. Selecting  [V]  from  PREDICT's  main  menu  will permit a ground station to only predict passes for satellites  that are  potentially  visible through optical means. Since all other passes are filtered out in this mode, and since some satellites  may  never  arrive  over a ground station when optical viewing conditions are possible, the program  pro- vides the option of breaking out of visual orbital predic- tion mode by pressing the [ESC]ape key as calculations are made. A prompt is displayed at the bottom of the screen to alert the user of this option. In either orbital prediction mode, predictions will not be attempted  for  satellites  that  can never rise above the ground station's horizon, or for satellites in geostation- ary  orbits.  If  a  satellite is in range at the starting date and time specified, PREDICT will adjust the  starting date  back in time until the point of AOS so that the pre- diction screen displays the first  pass  in  its  entirety from start to finish. SATELLITE TRACKING MODE In addition to predicting satellite passes, PREDICT allows satellites to be tracked in real-time using PREDICT's Sin- gle  Satellite  Tracking  Mode  (main menu option [T]), or simultaneously as a group of 24 using the program's Multi- Satellite Tracking Mode (main menu option [M]).  The posi- tions of the Sun and Moon are also displayed when tracking satellites in real-time. Selecting  option  [T] from PREDICT's main menu places the program in Single Satellite Tracking Mode. The  user  will be  prompted  to  select  the satellite of interest, after which a screen will appear and display tracking  positions for the satellite selected. In Single Satellite Tracking Mode, a wealth of information related to tracking a spacecraft and communicating through its  transponder  is displayed.  The current date and time is displayed  along  with  the  satellite's  sub-satellite point, its orbital altitude in both kilometers and statute miles, the slant range distance between the ground station and  the  satellite  in both kilometers and statute miles, the current azimuth  and  elevation  headings  toward  the satellite,  the  orbital velocity of the satellite in both kilometers per  hour  and  statute  miles  per  hour,  the footprint  of the satellite in both kilometers and statute miles, the modulo 256 orbital phase of the satellite,  the eclipse  depth,  the  spacecraft antenna squint angle, and orbital model in use, as well as the current orbit  number are also displayed.  The date and time for the next AOS is also provided. Additionally, if the satellite is currently  in  range  of the  ground  station,  the amount of Doppler shift experi- enced on uplink and downlink frequencies, path loss, prop- agation  delay,  and  echo  times are also displayed.  The expected time of LOS is also provided. Uplink and downlink  frequencies  are  held  in  PREDICT's transponder   database   file   predict.db  located  under $HOME/.predict.  A default file is provided with  PREDICT. Transponders  may  be  selected by pressing the SPACE BAR. The passband of the transponder may  be  tuned  in  1  kHz increments by pressing the < and > keys.  100 Hz tuning is possible using the , and . keys.  (These are simply the  < and > keys without the SHIFT key.) If  no transponder information is available, the data dis- played on the tracking screen is abbreviated. The features available in the  Single  Satellite  Tracking Mode  make  it possible to accurately determine the proper uplink frequency to yield a given downlink  frequency,  or vice  versa.   For  example,  if one wishes to communicate with a station heard on  435.85200  MHz  via  FO-29,  then 435.85200  MHz  can  be selected via the keyboard as an RX frequency using the tuning keys while tracking FO-29,  and the corresponding ground station TX frequency will be dis- played by PREDICT. Obviously, an accurate system clock and up-to-date orbital data are required for the best tuning accuracy. If  a sound card is present on your machine and the Single Satellite Tracking Mode is invoked with an  uppercase  'T' rather  than  a  lowercase 't', PREDICT will make periodic voice announcements stating the satellite's tracking coor- dinates in real-time. Announcements such as: "This  is  PREDICT.   Satellite  is  at  fifty six degrees azimuth and forty five degrees elevation, and is approach- ing.  Satellite is currently visible." are  made  at intervals that are a function of how quickly the satellite is moving across the sky. Announcements  can occur  as frequently as every 50 seconds for satellites in low earth orbits such as the International  Space  Station (370 km), or as infrequently as every 8 minutes for satel- lites in very high orbits, such as the AMC-6 geostationary satellite (35780 km). Voice announcements are performed as background processes so as not to interfere with  tracking calculations  as  the  announcements  are made. Alarms and special announcements are made when  the  satellite  being tracked  enters  into or out of eclipse. Regular announce- ments can be forced by pressing  the  'T'  key  in  Single Satellite Tracking Mode. MULTI-SATELLITE TRACKING MODE Selecting  [M] from PREDICT's main menu places the program in a real-time  multi-satellite  tracking  mode.  In  this mode,  all  24  satellites  in  the program's database are tracked simultaneously along with the positions of the Sun and Moon. Tracking data for the satellites is displayed in two columns of 12 satellites each. The name, azimuth head- ing,  elevation,  sub-satellite point latitude (in degrees North) and longitude (in degrees West) positions are  pro- vided,  along  with  the  slant range distance between the satellite and the ground station (in kilometers). A letter displayed to the right of the slant  range  indi- cates  the satellite's sunlight and eclipse conditions. If the satellite is experiencing an eclipse period, an  N  is displayed.  If the satellite is in sunlight and the ground station is under the cover of darkness, a V  is  displayed to  indicate the possibility that the satellite is visible under the current conditions. If the satellite is in  sun- light  while conditions at the ground station do not allow the satellite to be seen, a D is displayed.  Satellites in range  of the ground station are displayed in BOLD letter- ing. The AOS dates and times for the next three satellites predicted  to  come into range are displayed on the bottom of the screen between the tracking coordinates of the  Sun and Moon.  Predictions are not made for satellites in geo- stationary orbits or for satellites so low in  inclination and/or altitude that they can never rise above the horizon of the ground station. ILLUMINATION PREDICTIONS Selecting [S] from PREDICT's main menu  will  allow  solar illumination  predictions  to  be made.  These predictions indicate how much sunlight  a  particular  satellite  will receive  in  a  24 hour period.  This information is espe- cially valuable  to  spacecraft  designers  and  satellite ground  station  controllers  who  must monitor spacecraft power budgets or thermal conditions on-board their  space- craft due to sunlight and eclipse periods.  It can even be used to predict the optimum times for astronauts  to  per- form  extra-vehicular activities in space. Solar illumina- tion predictions may be logged to a file in the same  man- ner  that  orbital  predictions may be logged (by pressing L). AND LUNAR ORBITAL PREDICTIONS In addition to making orbital predictions  of  spacecraft, PREDICT  can  also  predict  transits  of  the Sun and the Moon.  Lunar predictions are initiated  by  selecting  [L] from  PREDICT's Main Menu.  Solar predictions are selected through Main Menu option [O]. When making solar and lunar orbital  predictions,  PREDICT provides  azimuth and elevation headings, the right ascen- sion, declination,  Greenwich  Hour  Angle  (GHA),  radial velocity,  and  normalized  distance (range) to the Sun or Moon.  Declination and Greenwich Hour Angle correspond  to the  latitude  and longitude of the object's sub-satellite point above the Earth's surface.  The radial velocity cor- responds  to the speed and direction the object is travel- ing toward (+) or away (-) from the ground station, and is expressed  in meters per second.  When the radial distance of the Moon is close to zero, the amount of Doppler  shift experienced  in Moonbounce communications is minimal.  The normalized distance corresponds  to  the  object's  actual distance   to  the  ground  station  divided  its  average distance.  In practice, the normalized distance can  range from about 0.945 to 1.055 for the Moon, and about 0.983 to 1.017 for the Sun. Note that the  effects  of  atmospherics  are  ignored  in determining  the  elevation  angles  for the Sun and Moon. Furthermore, the data provided by PREDICT  corresponds  to the  object's  center, and not the upper or lower limb, as is sometimes done when predicting the rising  and  setting times of these celestial objects. UNDER THE X-WINDOW SYSTEM PREDICT  may  be run under the X-Window System by invoking it through the xpredict script contained with  this  soft- ware. xpredict can invoke rxvt, xterm, Eterm, gnome-termi- nal, or kvt, and display PREDICT  in  a  virtual  terminal window.   xpredict  should be edited for best results.  In many cases, holding down the SHIFT key while pressing  the plus  (+) and minus (-) keys allows PREDICT's window to be re-sized when started under xpredict. LINE ARGUMENTS By default, PREDICT  reads  ground  station  location  and orbital  data  information from a pair of files located in the user's home  directory  under  a  hidden  subdirectory named  .predict.  Ground  station  location information is held in a  file  named  predict.qth,  while  orbital  data information for 24 satellites is held in a file named pre- dict.tle. If we wish  to  run  PREDICT  using  data  from  alternate sources  instead of these default files, the names of such files may be passed to PREDICT on the  command  line  when the  program  is  started. For example, if we wish to read the TLE file visual.tle and the QTH  file  beach_house.qth rather  than the default files, we could start PREDICT and pass the names of these alternate files to the program  in the following manner: predict -t visual.tle -q beach_house.qth or predict -q beach_house.qth -t visual.tle If  the  files  specified  are  not located in the current working directory, then their relative or  absolute  paths should  also  be specified along with their names (predict -t /home/kd2bd/orbs/visual.tle). It is also possible to specify  only  one  alternate  file while using the default for the other. For example, predict -t visual.tle reads  QTH  information  from  the default ~/.predict/pre- dict.qth location, and TLE  information  from  visual.tle, while predict -q bobs.qth reads  QTH  information  from bobs.qth and TLE information from the default ~/.predict/predict.tle location. ORBITAL DATABASE UPDATES It is also possible to update PREDICT's satellite  orbital database  using  another  command line option that updates the database from a NASA two-line element data  set.  PRE- DICT then quietly exits without displaying anything to the screen, thereby eliminating the need for entering the pro- gram  and  selecting  the  appropriate  menu options. This option is invoked using the -u command line switch as fol- lows: predict -u orbs248.tle This  example  updates  PREDICT's default orbital database with the Keplerian elements found in the file orbs248.tle. PREDICT may be updated from a list of files as well: predict -u amateur.tle visual.tle weather.tle If an alternate datafile requires updating, it may also be specified on the command line using the -t switch as  fol- lows: predict -t oscar.tle -u amateur.tle This  example  updates the oscar.tle orbital database with the two-line element data contained in amateur.tle. These options permit the  automatic  update  of  PREDICT's orbital  data  files using Keplerian orbital data obtained through automatic means  such  as  FTP,  HTTP,  or  pacsat satellite download. For  example,  the  following script can be used to update PREDICT's orbital database via the Internet: #!/bin/sh wget  -qr  www.celestrak.com/NORAD/elements/amateur.txt -O amateur.txt wget -qr www.celestrak.com/NORAD/elements/visual.txt -O visual.txt wget  -qr  www.celestrak.com/NORAD/elements/weather.txt -O weather.txt /usr/local/bin/predict    -u   amateur.txt   visual.txt weather.txt To truly automate the process  of  updating  your  orbital database,  save  the above commands to a file in your home directory (such as kepupdate), and add the following  line to your crontab (type crontab -e to edit your crontab): 0 2 * * * kepupdate and  PREDICT  will automatically update its database every day at 2:00 AM. ANTENNA TRACKING PREDICT is compatible with  serial  port  antenna  rotator interfaces conforming to the EasyComm 2 protocol standard. This includes the PIC/TRACK interface developed by Vicenzo Mezzalira,   IW3FOL  <http://digilander.iol.it/iw3fol/pic- track.html>, TAPR's EasyTrak Jr.  (currently under  devel- opment),  and  Suding  Associates Incorporated's Dish Con- trollers         <http://www.ultimatecharger.com/Dish_Con- trollers.html>.    The   FODTRACK   rotator  interface  is supported through the use of Luc Langehegermann's  (LX1GT) fodtrack utility written for and included with PREDICT. Using  any of these hardware interfaces, PREDICT can auto- matically control the position of AZ/EL antenna  rotators, and  keep  antennas  accurately pointed toward a satellite being tracked by PREDICT.   In  operation,  tracking  data from  PREDICT  is  directed  to  the specified serial port using the -a command line option.  For example: predict -a /dev/ttyS0 will send AZ/EL tracking data to  the  first  serial  port when  the  program  is  tracking a satellite in the Single Satellite Tracking Mode.  The data sent to the serial port is  of  the  form:  AZ241.0 EL26.0 using 9600 baud, 8-data bits, 1-stop bit, no parity, and no handshaking.  Data  is sent to the interface if the azimuth or elevation headings change by one degree or more.   For  interfaces  requiring keepalive  updates  at  least  once per second whether the AZ/EL headings have changed or not (such as  the  ones  by SAI), the -a1 option may be used: predict -a1 /dev/ttyS0 OPTIONS The  -f  command-line option, when followed by a satellite name or object number and starting date/time, allows  PRE- DICT  to  respond  with  satellite positional information. This feature allows PREDICT to  be  invoked  within  other applications  that  need  to  determine  the location of a satellite at a particular point in time, such as the loca- tion  of  where  a  CCD camera image was taken by a Pacsat satellite based on its timestamp. The information produced includes the  date/time  in  Unix format  (the  number of seconds since midnight UTC on Jan- uary 1, 1970), the date/time in ASCII (UTC), the elevation of  the  satellite  in degrees, the azimuth heading of the satellite, the orbital phase (modulo  256),  the  latitude and  longitude  of  the satellite's sub-satellite point at the time specified, the slant range to  the  satellite  in kilometers  with respect to the ground station's location, the orbit number, and the spacecraft's sunlight visibility information. The  date/time must be specified in Unix format (number of seconds since midnight UTC on January  1,  1970).   If  no starting   or   ending  time  is  specified,  the  current date/time is assumed and a single line of output  is  pro- duced.   If  a  starting  and ending time are specified, a list of coordinates beginning at  the  starting  time/date and  ending  with the ending time/date will be returned by the program with a one second resolution.  If the letter m is  appended  to  the  ending  time/date,  then  the  data returned by the program will have a one minute resolution. The  -o  option allows the program to write the calculated data to an output file rather than  directing  it  to  the standard output device if desired. The proper syntax for this option is as follows: predict -f ISS 977446390 977446400 -o datafile A  list  of  coordinates starting at the current date/time and ending 10 seconds later may be produced by the follow- ing command: predict -f ISS +10 If  a  list  of coordinates specifying the position of the satellite every minute for the next 10 minutes is desired, the following command may be used: predict -f ISS +10m If  a satellite name contains spaces, then the entire name must be enclosed by "quotes". The -p option allows orbital predictions for a single pass to  be  generated  by  PREDICT  via the command-line.  For example: predict -p OSCAR-11 1003536767 starts predictions for the OSCAR-11 satellite  at  a  Unix time  of  1003536767  (Sat  20Oct01 00:12:47 UTC).  If the starting date/time is omitted, the  current  date/time  is used.   If  a  pass is already in progress at the starting date/time specified, orbital predictions are moved back to the beginning of AOS of the current pass, and data for the entire pass from AOS to LOS is provided. When either the -f or -p options are  used,  PREDICT  pro- duces  an  output consisting of the date/time in Unix for- mat, the date and time in ASCII (UTC),  the  elevation  of the  satellite in degrees, the azimuth of the satellite in degrees, the orbital phase (modulo 256), the latitude  (N) and  longitude (W) of the satellite's sub-satellite point, the slant range to  the  satellite  (in  kilometers),  the orbit  number,  and  the  spacecraft's sunlight visibility information.  For example: 1003611710 Sat 20Oct01 21:01:50   11    6  164    51    72 1389  16669 * The  output isn't annotated, but then again, it's meant to be read by other software. MODE PREDICT's network socket interface allows the  program  to operate as a server capable of providing tracking data and other information to client  applications  using  the  UDP protocol.   It is even possible to have the PREDICT server and client applications running on separate machines  pro- vided  the  clients  are connected to the server through a functioning network connection. The -s switch is used to start PREDICT in server mode: predict -s By default, PREDICT uses socket port 1210 for  communicat- ing  with  client  applications.  Therefore, the following line needs to be added to the end your /etc/services file: predict   1210/udp The port number (1210) can be changed to something else if desired.  There is no need to recompile the program if  it is  changed.   To run more than one instance of PREDICT in server mode on a single host, an alternate  port  must  be specified  when  invoking the additional instances of PRE- DICT.  This can be accomplished by using the -n switch: predict -n 1211 -t other_tle_file -s When invoked in server mode,  PREDICT  immediately  enters Multi-Satellite  Tracking  Mode,  and  makes live tracking data available to clients.  Clients may poll  PREDICT  for tracking  data  when  the program is running in either the Multi-Satellite or Single Satellite Tracking  Mode.   When in Multi-Satellite Tracking mode, tracking data for any of the  24  satellites  in  the  program's  database  may  be accessed by client applications.  When in Single-Satellite Tracking mode, only live  tracking  data  for  the  single satellite  being tracked may be accessed.  Either tracking mode may be ended at any time.  When this is done, PREDICT will  return  the  last calculated satellite tracking data until the program is again put into a  real-time  tracking mode.   This  allows  the user to return to the main menu, and use other features  of  the  program  without  sending potentially harmful data to client applications. The  best  way to write a client application is to use the demonstration program (demo.c) included in this  distribu- tion  of  PREDICT as a guide.  The sample program has com- ments to explain how each component operates.  It is  use- ful  to  pipe  the  output of this program through less to easily browse through the data returned (demo | less). In operation, a character array is filled with the command and  arguments to be sent to PREDICT.  A socket connection is then  opened,  the  request  is  sent,  a  response  is received,  and  the socket connection is closed.  The com- mand and arguments are in ASCII text format. Several excellent network client applications are included in  this  release  of  PREDICT, and may be found under the predict/clients directory. SATELLITES One of the most frequently asked questions is  how  satel- lites  in  PREDICT's  orbital database may be added, modi- fied, or replaced.  As it turns  out,  there  are  several ways  in  which this can be done.  Probably the easiest is to manually edit  your  ~/.predict/predict.tle  file,  and replace  an  existing satellite's entry with 2-line Keple- rian data for the new satellite.  If this method  is  cho- sen,  however, just make sure to include ONLY the two line data, and nothing else. Another way is to is select the Keyboard Edit option  from the  program's  Main  Menu, select a satellite you wish to replace.  Edit the name and object number  (replacing  the old  information  with  the  new  information).   Just hit ENTER, and accept all the other orbital parameters  shown. Get  back to PREDICT's Main Menu.  Select Auto Update, and then enter the filename containing the 2-line element data for  your  favorite new satellite.  The new satellite data should be detected by PREDICT, and the  orbital  data  for the old satellite will be overwritten by the new data. TRICKS In  addition  to  tracking and predicting passes of satel- lites, PREDICT may also be used to generate  a  NASA  two- line Keplerian element data set from data entered via key- board. For example, let's say you're  listening  to  Space Shuttle  audio re-broadcasts via WA3NAN and Keplerian ele- ments for the Space  Shuttle's  orbit  are  given  by  the announcer.  The  orbital data provided by WA3NAN in verbal form  may  be  manually  entered  into  PREDICT's  orbital database using option [E] of the program's main menu (Key- board Edit of Orbital Database). The orbital data for  the Space  Shuttle  in  NASA two-line element form can then be found in your orbital database file, and may  imported  to any other satellite tracking program that accepts two-line element files or distributed to others electronically. It is also possible to run PREDICT as a background process and  direct  its  display  to an unused virtual console by using the following command: predict < /dev/tty8 > /dev/tty8 & Switching to virtual console  number  8  (ALT-F8  in  text mode)  will  allow  PREDICT to be controlled and displayed even after you've logged out.  This  is  especially  handy when running PREDICT in server mode on a remote machine. OF TERMS The  following  terms  are  frequently used in association with satellite communications and space technology: AOS: Acquisition of Signal - the time at which a ground station first  acquires  radio  signals  from a satellite. PREDICT defines AOS as the time when the satellite  being  tracked comes  within  +/-  0.03  degrees  of  the  local horizon, although it may have to rise higher than this before  sig- nals are first heard. Apogee: Point  in a satellite's orbit when the satellite is at its farthest distance from the earth's surface. Period: A satellite orbital parameter specifying the time  between successive perigees. Node: Point  in a satellite's orbit when its sub-satellite point crosses the equator moving south to north. Azimuth: The compass direction measured clockwise from true  north. North = 0 degrees, East = 90 degrees, South = 180 degrees, and West = 270 degrees. Node: Point in a satellite's orbit when its sub-satellite  point crosses the equator moving north to south. Shift: The  motion  of a satellite in its orbit around the earth, and in many cases  the  rotational  motion  of  the  earth itself, causes radio signals generated by satellites to be received on Earth at frequencies slightly  different  than those upon which they were transmitted. PREDICT calculates what effect these  motions  have  upon  the  reception  of satellites transmitting on the 146 MHz and 435 MHz Amateur Radio bands. Elevation: The angle between the local horizon and  the  position  of the  satellite.  A satellite that appears directly above a particular location is said to be located at an  elevation of  90  degrees.  A  satellite located on the horizon of a particular location is said to be located at an  elevation of  0 degrees.  A satellite with an elevation of less than zero is positioned below the local horizon, and radio com- munication with a satellite in such a position is not pos- sible under normal circumstances. Footprint: Diameter of the Earth's surface visible from a  satellite. The  higher  the satellite's orbital altitude, the greater the footprint, and the wider  the  satellite's  communica- tions coverage. LOS: Loss  of Signal - the time at which a ground station loses radio contact with a satellite. PREDICT defines LOS as the time  when  the  satellite  being tracked comes within +/- 0.03 degrees of the local horizon. Phase: An orbital "clock" that describes  a  satellite's  orbital position  with  respect  to  perigee. Orbital Phase may be modulo 256, or modulo 360, and is sometimes referred to as mean  anomaly when speaking of amateur radio satellites in elliptical  orbits,  such  as  the  Phase  3   satellites. Orbital phase is zero at perigee. Loss: The  apparent  attenuation  a radio signal undergoes as it travels a given distance. This attenuation is  the  result of the dispersion radio waves experience as they propagate between transmitter and receiver using antennas of  finite gain.  Free  space  path  loss  is technically an oxymoron since free space is loss free. Perigee: Point in a satellite's orbit when the satellite is at  its closest distance to the earth's surface. Period: A  satellite orbital parameter specifying the time between successive ascending nodes. Range: The straight line distance between the ground station  and the satellite at a given time. Point: The  latitude and longitude specifying the location on the Earth that is directly below the satellite. INFORMATION Detailed information on the  operation  of  PREDICT's  UDP socket-based  interface as well as sample code for writing your own client applications  is  available  in  the  pre- dict/clients/samples  subdirectory.   The  latest  news is available through the official PREDICT software  web  page located at: <http://www.qsl.net/kd2bd/predict.html>. FILES ~/.predict/predict.tle Default database of orbital data ~/.predict/predict.db Satellite transponder database file ~/.predict/predict.qth Default ground station location information AUTHORS PREDICT   was   written   by  John  A.  Magliacane,  KD2BD <kd2bd@amsat.org>.  The socket server code was contributed by  Ivan  Galysh,  KD4HBO <galysh@juno.nrl.navy.mil>.  The PIC/TRACK serial port antenna rotator controller code  was contributed    by    Vittorio   Benvenuti,   I3VFJ   <ben- scosm@iol.it>.  SGP4/SDP4 code  was  derived  from  Pacsal routines  written  by Dr. T.S. Kelso, and converted to 'C' by Neoklis Kyriazis, 5B4AZ.   See  the  CREDITS  file  for additional information. Software             15 May 2006                 PREDICT(1)