ALM2POS Is a program that uses a Yuma GPS almanac file to calculate the positions of the GPS satellites relative to a user location. It can also compare the almanac calculated position with "precise" position info from IGS (+/- 50 cm accuracy). This shows how accurate an almanac predicts position, if an almanac has any problems, and allows experiments on how the almanac accuracy changes with time etc.
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 "approx" 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!). ALM2POS uses the almanac data to predict the GPS satellite positions relative to your location and allows accuracy tests on the almanac.
Who would use ALM2POS Anyone wanting to know the position of GPS satellites in Space. Amateur Satellite Observers wanting pointing information for their Telescopes. Anyone wanting to know GPS satellite positions to show how local horizon and antennae directionality affects reception. Educators and students wanting to display and understand the almanac in terms of accuracy and ageing effects.
Who would NOT use it? GPS users only wanting to know "their own" position on or near earth. ALM2POS 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.
Cost of ALM2POS 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 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 seconds of being correct to allow the program to calculate the GPS Satellite positions for your location.
ALM2POS 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