Results from TLE & SGP4 propagation don't seem right & need help with interpretation

thor36

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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 :

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​


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(y), R(z), V(x), V(y), 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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