Hi guys!

I am happy to announce that I finished the first version of the SGP4/SDP4 implementation 100% written in Julia. The source code is part of SatelliteToolbox.jl. Notice that I did not tag yet, so you need to use `master`

.

As for the correctness of the algorithm, **all** the test TLEs available in [1] has been added into the test set, and everything passes. Thus, I am not expecting any major problems. But I will be very happy if you can help me to test in other scenarios and report weird behaviors

Notice that I have not yet implemented the verifications for things like the satellite crashed or numerical errors arising in veeeery long integrations (this is easy and will be done soon).

Furthermore, I decided to make a â€śreadableâ€ť code instead of the most performant possible. Because this will be used to teach students about orbit propagators. So, for example, the FORTRAN code:

```
SING=SIN(OMEGAO)
COSG=COS(OMEGAO)
TSI=1./(AODP-S4)
ETA=AODP*EO*TSI
ETASQ=ETA*ETA
EETA=EO*ETA
PSISQ=ABS(1.-ETASQ)
COEF=QOMS24*TSI**4
COEF1=COEF/PSISQ**3.5
C2=COEF1*XNODP*(AODP*(1.+1.5*ETASQ+EETA*(4.+ETASQ))+.75*
1 CK2*TSI/PSISQ*X3THM1*(8.+3.*ETASQ*(8.+ETASQ)))
C1=BSTAR*C2
```

was translated into

```
aux0 = abs(1-Î·^2)
aux1 = aux0^(-7/2)
aux2 = Îľ^4*all_0*Î˛_0^2*aux1
C2 = QOMS2T*Îľ^4*nll_0*aux1*
( all_0*( 1 + (3/2)Î·^2 + 4e_0*Î· + e_0*Î·^3) +
3/2*(k_2*Îľ)/aux0*(-1/2 + (3/2)Î¸Â˛)*(8 + 24Î·^2 + 3Î·^4) )
C1 = bstar*C2
```

(give or take some lines of codeâ€¦)

I know that FORTRAN version should be faster, but the Julia one is more easily related to the equations we have in the books.

Finally, I want the code to be very documented about the theory. So that, in the far far far away future, when someone needs to rewrite in another language, he/she will not have the problems I hadâ€¦ Hence, if you are an export in Astrodynamics and want to add comments about the equations in the code (specially the deep space bitsâ€¦), be my guest

[1]: Vallado, D. A., Crawford, P., Hujsak, R., Kelso, T. S (2006). Revisiting Spacetrack Report #3: Rev1. AIAA.