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

In summary, the speaker is seeking help with a problem they are having with SGP4 propagation. They acquired a TLE of the ISS and used a C++ SGP4 propagator to calculate future position and velocity vectors. However, they are unsure about some aspects of the results and have questions about the TLE data, settings used, and time calculations. After further experimentation, they discovered that the TLE they used was erroneous. They are seeking explanations and clarifications to properly interpret the results gained by the TLE and SGP4.
  • #1
thor36
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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 settingss 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 to Tend, once every 10 minutes ). It would look really messy to paste the results here, so I will only copy the last line.

R(x) = 4628.43055298
R(y) = 2891.92246591
R(z) = -3530.26416981
V(x) = -5.504313283
V(y) = 3.379764650
V(z) = -4.417716694
Time = 1995 12 61 23:31:53.501202​

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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  • #2
After doing more "experiments", it seems that the particular TLE I used was erroneous. This was my first testing of SGP4, and many things could be wrong with the code itself, so obviously, the TLE I got from online source wasn't the first thing I blamed for the errors. It seems that even generally reliable online TLE sources can occasionally produce an unfortunate bad instance of a TLE set.
 

1. Why do the results from TLE & SGP4 propagation seem incorrect?

There are a few potential reasons why the results from TLE & SGP4 propagation may seem incorrect. One possibility is that there may be errors in the input data, such as incorrect satellite parameters or inaccurate initial conditions. Another possibility is that the TLE and SGP4 models have limitations and may not accurately predict the satellite's orbit in all situations. It is also possible that there are external factors, such as atmospheric drag or perturbations from other objects, that are affecting the satellite's orbit and causing discrepancies in the results.

2. How can the results from TLE & SGP4 propagation be interpreted?

The results from TLE & SGP4 propagation can be interpreted as an estimation of the satellite's position and velocity at a given time. These models use simplified mathematical equations to predict the satellite's orbit, but they may not be entirely accurate in all cases. Therefore, the results should be considered as approximations rather than precise measurements.

3. What factors can affect the accuracy of TLE & SGP4 propagation results?

There are several factors that can affect the accuracy of TLE & SGP4 propagation results. These include errors in the input data, limitations of the TLE and SGP4 models, and external influences such as atmospheric drag and perturbations from other objects. Additionally, the accuracy of the results may also depend on the length of time that has passed since the TLE was generated, as the satellite's orbit may change over time due to various factors.

4. Can TLE & SGP4 propagation results be improved?

While TLE & SGP4 propagation results may not be entirely accurate, there are ways to improve their reliability. One way is to use more precise and up-to-date input data, such as satellite parameters and initial conditions. Another option is to use more advanced orbit prediction models that take into account additional factors, such as atmospheric drag and gravitational perturbations from other objects.

5. How can TLE & SGP4 propagation results be validated?

TLE & SGP4 propagation results can be validated by comparing them to other sources of satellite orbit data, such as ground observations or data from other models. Additionally, conducting multiple propagation runs with slightly different initial conditions and comparing the results can also help to validate the accuracy of the TLE & SGP4 predictions.

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