ALM2TLE is a DOS utility to create TLE's (Two Line Elements) for the Active GPS Satellites directly from the GPS Almanac. Amateurs wanting TLE's have had to rely historically on access to USSPACECOM derived TLE's - this utility allows a user to generate their own TLE's directly from their own GPS Receivers or Almanac files posted on the Internet.
What is the Almanac The GPS Almanac is a set of data to describe the orbits of the complete active fleet of Satellites. GPS receivers use the almanac to determine "approximately" where the satellites are relative to the local sky. It then uses this information to determine what satellites it should track (no point in devoting resources to satellites below the horizon!). Alm2tle converts the almanac data into the correct format and units required by software using TLE's.
What are TLE's TLE's (Two Line Elements) describe the shape, position and reference time of the elliptical orbit of a satellite orbiting the earth. The data when feed into computers running SGP (Standard General Perturbation) theory, will locate a satellite position anytime in the near future or past. See end of page for a sample of a TLE.
Who would use ALM2TLE Anyone wanting to know the position of GPS satellites in Space. Amateur Satellite Observers wanting pointing information for their Telescopes. Anyone wanting to calculate the rise/set times of the GPS satellites to show how local horizon and antennae directionality affects reception. Educators and students wanting to display and understand satellite orbits. Show recent satellite positions when you may have detected a high DOP (Dilution of Precision) and want to know what the geometry of the satellites was at the time. Anyone who has not got Full access to the Internet or are located at a remote site with NO Internet (no access to USSPACECOM derived TLE's).
Who would NOT use it? GPS users only wanting to know "their own" position on or near earth. ALM2TLE is a utility for locating the satellites in space, the opposite of what is normally done with GPS - using satellites to find where YOU are.
Accuracy of resultant TLE's? Testing the TLE's produced from ALM2TLE against "cm" accuracy position data from the IGS, gives an accuracy of better than 6 minutes of arc (1/10th of a degree) for up to 3 days following the TLE epoch. This is adequate for most amateur "studies". Persons wanting greater precision than this (for Geodesy studies etc) would use their GPS units (and some high school maths) to get position data down to a few meters accuracy of satellite position in space. Because the Almanac is updated daily, one can always expect better than 6 minute of arc accuracy from ALM2TLE.
Cost of ALM2TLE Because I wrote the program as a "hobby" to learn about GPS, the program is FREE. However it did take much researching and testing, so I do request if anyone uses it more than a few times, they may like to say "thankyou" to me by sending a nice picture postcard of where they live. The program is not restricted or crippled.
Requirements: The programs was compiled as a 16bit DOS
application, it should run using DOS 3.0 or above, or as a DOS
window in the various Windows environments. The program needs
the GPS Almanac as the raw data, this can be obtained if your
have a GPS receiver by 3 possibilities. Sample formats shown at
end of page.
- Your GPS receiver can output a YUMA format
almanac.
- Your GPS receiver can output a NMEA format
almanac.
- You have a MOTOROLA GPS receiver that outputs
the binary almanac.
- You have a MAGELLAN GPS receiver with
serial interface to a PC.
- If you DONT have a GPS receiver
but have access to the INTERNET - just having "email" access is
enough (using an email robot to obtain the required Almanac
WEBPAGE).
The program managed the Y2K period without a
problem, and should work fine until 2036 (where it decides it
should go into retirement - before the UNIX time functions lose
their bodily functions!). The time on your PC should be within a
few months of being correct to allow the program to calculate
the required epoch of the GPS time scale.
The TLE produced can be used on most Satellite tracking software available, recommended is Dave Ransom's STSPLUS which can be found (along with lots of other interesting Satellite Info) at Dave Ransom Home Page
Alm2tle written by Geoff Hitchcox Christchurch, New Zealand, South Pacific.
GPS LINKS - that provided me with all the information I needed for the
project.
Almanac Format Samples - so you can recognise what the formats look like, the data is all for the one SV and the same epoch.
Sample of the NMEA Almanac ($GPALM sentence) $GPALM,32,1,01,5,00,264A,4E,0A5E,FD3F,A11257,B8E036,536C67,2532C1,069,000*7B Sample of the YUMA Almanac **nmea2alm** Week 5 almanac for PRN-01 **nmea2alm** ID: 01 Health: 000 Eccentricity: 4.673957825E-03 Time of Applicability(s): 3.194880000E+05 Orbital Inclination(rad): 9.583808625E-01 Rate of Right Ascen(r/s): -8.057478483E-09 SQRT(A) (m^1/2): 5.154292480E+03 Right Ascen at TOA(rad): 2.047519431E+00 Argument of Perigee(rad): -1.745649913E+00 Mean Anom(rad): 9.129825962E-01 Af0(s): 1.001358032E-04 Af1(s/s): 0.000000000E+00 week: 005 Sample of TLE file GPS PRN=01 1 22231U 92079A 99272.69762732 .00000000 00000-0 00000-0 0 0001 2 22231 54.9112 121.6200 0046740 259.9816 52.3100 2.00491339 00009
Thanks to - Dave Ransom, Dave Martindale and Dave Checkett for their help in the project.