Using Predict
PREDICT(1) KD2BD Software PREDICT(1)
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 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.
FIRST 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/.
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 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.
SINGLE 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.
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 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).
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 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.
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 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.
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 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.
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. 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.
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/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
ADDITIONAL 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.
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 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.
ADDING 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.
NEAT 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.
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 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.
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 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.
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 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.
KD2BD Software 15 May 2006 PREDICT(1)