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.
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.
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.