NAME
predict - Track and predict passes of satellites in Earth orbit
SYNOPSIS
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]
DESCRIPTION
PREDICT is a multi-user satellite tracking and orbital prediction
program written under the Linux operating system by John A. Magliacane,
KD2BD. PREDICT 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 any
later version.
PREDICT 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.
FIRST TIME USE
PREDICT tracks and predicts passes of satellites based on the
geographical location of the ground station, the current 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 PREDICT 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 program 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 satellites 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/.
PROGRAM 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.
PREDICTING 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 previous passes to help to confirm or
identify past observations.
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 calculations are started and
prediction information is displayed on the screen.
The date and time in UTC, along with the satellite’s elevation 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
spacecraft attitude parameters (ALAT, ALON) are included in PREDICT’s
transponder database file, then spacecraft antenna squint angles are
displayed instead of orbit numbers 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 Shuttles, 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
directory. 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 underscore (_) 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 predictions 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 provides the
option of breaking out of visual orbital prediction 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 geostationary 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
prediction screen displays the first pass in its entirety from start to
finish.
SINGLE SATELLITE TRACKING MODE
In addition to predicting satellite passes, PREDICT allows satellites
to be tracked in real-time using PREDICT’s Single 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 positions 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 experienced on uplink and downlink
frequencies, path loss, propagation 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 displayed 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 displayed 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 coordinates in real-time. Announcements such as:
"This is PREDICT. Satellite is at fifty six degrees azimuth and forty
five degrees elevation, and is approaching. 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 satellites 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 announcements 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
heading, elevation, sub-satellite point latitude (in degrees North) and
longitude (in degrees West) positions are provided, 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 indicates 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 sunlight 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 lettering. 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 geostationary orbits or for
satellites so low in inclination and/or altitude that they can never
rise above the horizon of the ground station.
SOLAR 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 especially valuable to spacecraft designers and
satellite ground station controllers who must monitor spacecraft power
budgets or thermal conditions on-board their spacecraft due to sunlight
and eclipse periods. It can even be used to predict the optimum times
for astronauts to perform extra-vehicular activities in space. Solar
illumination predictions may be logged to a file in the same manner
that orbital predictions may be logged (by pressing L).
SOLAR 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 ascension, 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 corresponds to
the speed and direction the object is traveling 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.
OPERATION UNDER THE X-WINDOW SYSTEM
PREDICT may be run under the X-Window System by invoking it through the
xpredict script contained with this software. xpredict can invoke rxvt,
xterm, Eterm, gnome-terminal, 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.
COMMAND 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 predict.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/predict.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.
QUIET 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. PREDICT then quietly exits without
displaying anything to the screen, thereby eliminating the need for
entering the program and selecting the appropriate menu options. This
option is invoked using the -u command line switch as follows:
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 follows:
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.
AUTOMATIC 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/pictrack.html>, TAPR’s EasyTrak Jr.
(currently under development), and Suding Associates Incorporated’s
Dish Controllers
<http://www.ultimatecharger.com/Dish_Controllers.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 automatically
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
ADDITIONAL OPTIONS
The -f command-line option, when followed by a satellite name or object
number and starting date/time, allows PREDICT 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 location 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 January 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 produced. 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 following 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 produces an output
consisting of the date/time in Unix format, 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.
SERVER 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 provided 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 communicating 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 PREDICT. 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 distribution of PREDICT as a guide.
The sample program has comments to explain how each component operates.
It is useful 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 command 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.
ADDING SATELLITES
One of the most frequently asked questions is how satellites in
PREDICT’s orbital database may be added, modified, 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 Keplerian data for
the new satellite. If this method is chosen, 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.
NEAT TRICKS
In addition to tracking and predicting passes of satellites, PREDICT
may also be used to generate a NASA two-line Keplerian element data set
from data entered via keyboard. For example, let’s say you’re listening
to Space Shuttle audio re-broadcasts via WA3NAN and Keplerian elements
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
(Keyboard 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.
GLOSSARY 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
signals are first heard.
Apogee:
Point in a satellite’s orbit when the satellite is at its farthest
distance from the earth’s surface.
Anomalistic Period:
A satellite orbital parameter specifying the time between successive
perigees.
Ascending 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.
Descending Node:
Point in a satellite’s orbit when its sub-satellite point crosses the
equator moving north to south.
Doppler 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 communication with a
satellite in such a position is not possible 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 communications 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.
Orbital 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.
Path 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.
Nodal Period:
A satellite orbital parameter specifying the time between successive
ascending nodes.
Slant Range:
The straight line distance between the ground station and the satellite
at a given time.
Sub-Satellite Point:
The latitude and longitude specifying the location on the Earth that is
directly below the satellite.
ADDITIONAL 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 predict/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
SEE ALSO
predict-g1yhh(1)
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
<benscosm@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.