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1990-03-24
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195 lines
THE N3EMO ORBIT SIMULATOR
VERSION 3.7
Robert W. Berger, N3EMO
March 7, 1990
1 Introduction
The N3EMO orbit program simulates the motions of earth satellites. The
program was written for use by amateur radio operators, but is useful to
others, such as astronomers interested in observing artificial satellites,
space enthusiasts tracking shuttle missions, and meteorologists using weather
satellites. The program is distributed in source form in the C language, and
has been used on a wide variety of computers, from micros to mainframes.
2 Changes
The following changes have been made since the last public release (version
2.3):
- The internal calculation routines have been rewritten to increase
performance, readability, and modularity. This facilitates use of
orbit's routines from other programs.
- A new sidereal time reference is generated for each run. This allows
the program to maintain its accuracy without having to be updated
every year.
- Predicted doppler shifts are much more accurate. The instantaneous
range-rate is now calculated directly instead of by differencing
successive range samples. The doppler shift displayed is now the
correct one for the sample time, instead of an average from the
preceding sample interval.
- Eclipses are optionally reported.
- Data for 0-180 degree elevation rotators is optionally provided.
- Satellite names may contain spaces.
- Single character abbreviations are provided for up to 62 satellites
(up from 26).
- A program is provided to convert NASA two-line keplerians to the
AMSAT format used by the program.
3 Compiling Orbit
The details of how to compile the program vary with the host machine. The
main program is in orbit.c, and the calculation subroutines are in orbitr.c.
These two files should be compiled and linked together to form the orbit
executable.
4 Data Files
Two data files are required to run orbit; a third is optional. The first
required data file, called "kepler.dat", contains the database of satellites.
"kepler.dat" is distributed by the Amateur Satellite Corporation (AMSAT), a non
profit organization that designs and operates amateur radio satellites. The
following is a sample entry from "kepler.dat":
Satellite: AO-13
Catalog number: 19216
Epoch time: 90054.30232176
Element set: 77
Inclination: 57.0658 deg
RA of node: 167.4162 deg
Eccentricity: 0.6899473
Arg of perigee: 221.7453 deg
Mean anomaly: 57.5228 deg
Mean motion: 2.09701313 rev/day
Decay rate: -9.10e-07 rev/day^2
Epoch rev: 1301
Satellite keplerians are also distributed by NASA in a format called the NASA
two-line format. The program in nasa.c converts a file from NASA format to
AMSAT format for use with orbit.
Keplerian sets for various interesting satellites are regularly posted to the
rec.ham-radio and sci.space usenet newsgroups. The Datalink RBBS at (214)
394-7438 has keplerians, as well as lots of other information of interest to
satellite and space enthusiasts.
The second required data file is a site file which describes the observers
position and other station related data. Multiple site files are supported,
with each one named after the corresponding observation site. The following
file is "pgh.sit", a site file for station W3VC in Pittsburgh:
W3VC Pittsburgh
40.45361 Latitude
79.94417 Longitude
300 Height (Meters)
0 Min Elevation (Degrees)
Eclipse
Flip
The first five lines are required, and give the name of the site, its
geographic location, and the minimum satellite elevation above the horizon
which is to be considered visible.
The last two lines contain optional keywords which enable features of the
orbit program. "Eclipse" enables reporting of eclipse times for the satellites.
"Flip" enables alternative bearing/elevations for 0-180 degree antenna
rotators, such as the Kenpro rotators. By offering a choice of two settings for
any sky position, such rotators allow a satellite to be tracked through a pass
without requiring the bearing rotor to be reoriented after hitting a rotation
stop.
The optional data file, "mode.dat", allows orbit to provide scheduling and
operation data for a satellite. The following is an example of a "mode.dat":
Satellite: UO-11
Beacon: 145.8260 MHz
Satellite: AO-13
Beacon: 145.8260 MHz
Mode: B from 0 to 165
Mode: JL from 165 to 195
Mode: S from 195 to 200
Mode: BS from 200 to 205
Mode: B from 205 to 256
Satellite: MIR
Beacon: 143.6250 MHz
Satellite: RS-10/11
Beacon: 29.357 MHz
Mode.dat may contain data for multiple satellites. The most common entry is
"Beacon", which is the frequency orbit uses for calculating doppler shifts.
5 Running Orbit
When run, orbit will ask which satellite in "kepler.dat" you wish to
simulate. Type the full satellite name, or the single character next to the
satellite name. Orbit will then ask for the name of the site. If your site file
is "foo.sit", type "foo". Next enter the UTC date of the start of the
simulation period. Years may be 2 or 4 digits. For example, 1990 may be entered
as "1990" or simply "90". The UTC hour of the start of the simulation period is
then entered. Next comes the duration in days, followed by the time between
samples in minutes. The final input is the name of the file for the output;
hitting the "Return" key places the output on the terminal. Here is the start
of a sample run:
N3EMO Orbit Simulator v3.7
Available satellites:
a) AO-10
b) UO-11
c) RS-10/11
d) AO-13
e) UO-14
f) UO-15
g) AO-16
h) DO-17
i) WO-18
j) LO-19
k) FO-20
l) NOAA n) MET-2/16
o) MET-2/17
p) MET-3/2
q) NOAA-11
r) MET-2/18
s) MET-3/3
t) SALYUT 7
u) MIR
Letter or satellite name :d
Site name :pgh
Start date (UTC) (Month Day Year) :3 10 90
Starting Hour (UTC) :0
Duration (Days) :1
Time Step (Minutes) :10
Output file (RETURN for TTY) :
AO-13 Element Set 77
W3VC Pittsburgh
Doppler calculated for freq = 145.826000 MHz
Saturday 10 Mar 1990 ----Orbit # 1332-----
U.T.C. Az El Az' El' Doppler Range Height Lat Long Phase(256)
0120:00 130 1 310 179 -1199 19383 14156 -15 32 24 B
0130:00 127 6 307 174 -1152 20834 16051 -10 31 28 B
0140:00 124 10 304 170 -1098 22222 17833 -6 31 32 B Eclipse
The output contains a line for each sample time at which the satellite's
elevation above the horizon is equal to or greater than the "Min Elevation"
entry in the site file. The first two columns are the bearing and elevation of
the satellite from the observer's site. If "Flip" is enabled, the next two
columns are the alternate form of the bearing and elevation, with the elevation
greater than 90 degrees. The next column is the doppler shift of the satellites
beacon frequency, followed by the range from the satellite to the observer, in
meters, and the satellites height above the earth. Next is the latitude and
longitude of the point on earth directly underneath the satellite. The phase is
then printed, followed by any applicable operating modes from the satellites
"mode.dat" entry. If "Eclipse" is enabled in the site file, and the satellite
is in the earth's shadow, the word "Eclipse" is printed.
If the satellite is in a highly elliptical orbit, a separate line is printed
for the exact time of the apogee.