Results from TLE & SGP4 propagation don't seem right & need help with interpretation
Greetings all !
I have this problem with SGP4 propagation, that I hope someone can help me out with.
I acquired a TLE of the ISS from internet and used the C++ SGP4 propagator to compute future position and velocity vectors of the Station. I am unsure about some aspects of results though and would like to get some explanations.
The TLE data looks like this :
The settings I use for the propagator are :
opsmode = i (improved)
typerun = m (manual)
typeinput = m (mfe - minutes from epoch)
I set SGP4 to propagate for 120 minutes after the initial moment in 10 minutes steps. The results are stored in 13 rows ( one for each computation from Tstart:10min:Tend ) and the columns represent this information, in same order :
minutes from initial moment, R(x), R, R(z), V(x), V, V(z), Time
So, my questions now :
1. Position
I reckoned calculating magnitude of R will result in distance of ISS from the Earth center ( or altitude, if you subtract Earth radius ). But calculating magnitude of this final set of data (posted above) results in a magnitude of approx. 6500 km, which is obviously too small value since we know ISS doesn't orbit the Earth at an altitude of about 100 km. Moreover, if I propagate just two more time steps ( +20 min ) I get error 6 - satellite has decayed. Why such a quick drop in altitude after propagating 140 minutes in future, which is a little more than an orbit and a half for ISS ?
2. Time
The last column is supposed to give the time of that particular computation. TLE epoch cell ( 14296.89020256 ) says the year is 2014 at 296th day of the year. But results seem to set the time at 61st December 1995 . Why such result ?
I may be missing something very fundamental, or cannot interpret results properly. I would like to ask for explanations / clarifications that will help me to make use of these ( and all subsequent ) results gained by TLE and SGP4.
Thank you very much and kind regards
T
Greetings all !
I have this problem with SGP4 propagation, that I hope someone can help me out with.
I acquired a TLE of the ISS from internet and used the C++ SGP4 propagator to compute future position and velocity vectors of the Station. I am unsure about some aspects of results though and would like to get some explanations.
The TLE data looks like this :
1 25544U 98067A 14296.89020256 .00021128 00000-0 36676-3 0 2914
2 25544 51.6464 181.6097 0002420 294.7068 172.8232 15.50795889911298
2 25544 51.6464 181.6097 0002420 294.7068 172.8232 15.50795889911298
The settings I use for the propagator are :
opsmode = i (improved)
typerun = m (manual)
typeinput = m (mfe - minutes from epoch)
I set SGP4 to propagate for 120 minutes after the initial moment in 10 minutes steps. The results are stored in 13 rows ( one for each computation from Tstart:10min:Tend ) and the columns represent this information, in same order :
minutes from initial moment, R(x), R, R(z), V(x), V, V(z), Time
Code:
0.00000000 2168.12732002 -3965.89285045 5083.39709989 7.248065746 1.634449716 -1.808768770
10.00000000 5718.06621406 -2187.87740091 2960.48303697 4.133199939 4.061427712 -4.990834499 1995 12 61 21:31:53.501188
20.00000000 6752.53317683 554.96048445 -471.34489062 -0.818580242 4.707421457 -5.979144785 1995 12 61 21:41:53.501193
30.00000000 4796.86402897 3050.71466683 -3692.16885499 -5.424525932 3.265426646 -4.308636879 1995 12 61 21:51:53.501197
40.00000000 714.92088401 4184.29809471 -5259.31492550 -7.628011028 0.365268332 -0.709424877 1995 12 61 22: 1:53.501202
50.00000000 -3673.94350416 3442.13282308 -4461.81655752 -6.430675696 -2.723102750 3.239086635 1995 12 61 22:11:53.501166
60.00000000 -6395.71253321 1134.73200770 -1629.93536931 -2.275086455 -4.614309985 5.763731448 1995 12 61 22:21:53.501170
70.00000000 -6173.96851133 -1678.95115636 1933.98379745 2.962489507 -4.383625334 5.631722374 1995 12 61 22:31:53.501175
80.00000000 -3090.01151193 -3702.92034359 4584.45558278 6.857113772 -2.129774567 2.894012182 1995 12 61 22:41:53.501179
90.00000000 1431.35294104 -3997.16258663 5087.27555839 7.570209291 1.142011952 -1.227896762 1995 12 61 22:51:53.501184
100.00000000 5252.89197563 -2406.18221773 3183.40970488 4.698437284 3.901772329 -4.803188390 1995 12 61 23: 1:53.501188
110.00000000 6541.10809520 331.08042759 -241.95701286 -0.478841787 4.820622266 -6.108464750 1995 12 61 23:11:53.501193
120.00000000 4628.43055298 2891.92246591 -3530.26416981 -5.504313283 3.379764650 -4.417716694 1995 12 61 23:21:53.501197
So, my questions now :
1. Position
I reckoned calculating magnitude of R will result in distance of ISS from the Earth center ( or altitude, if you subtract Earth radius ). But calculating magnitude of this final set of data (posted above) results in a magnitude of approx. 6500 km, which is obviously too small value since we know ISS doesn't orbit the Earth at an altitude of about 100 km. Moreover, if I propagate just two more time steps ( +20 min ) I get error 6 - satellite has decayed. Why such a quick drop in altitude after propagating 140 minutes in future, which is a little more than an orbit and a half for ISS ?
2. Time
The last column is supposed to give the time of that particular computation. TLE epoch cell ( 14296.89020256 ) says the year is 2014 at 296th day of the year. But results seem to set the time at 61st December 1995 . Why such result ?
I may be missing something very fundamental, or cannot interpret results properly. I would like to ask for explanations / clarifications that will help me to make use of these ( and all subsequent ) results gained by TLE and SGP4.
Thank you very much and kind regards
T
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