Hi!

In this post, I would like to introduce the SatelliteToolbox.jl, which is a package for Julia language with many options to analyze space missions. It is used on a daily basis at the Brazilian National Institute for Space Research (INPE). First, it is presented a brief history about the package, and then I show some interesting analysis that can be done with it.

## History

In 2013, I joined INPE as a Junior Space Systems Engineer. I was assigned to the space systems division in which I had to work with the attitude and orbit control subsystem (AOCS). Since I had only an intermediate knowledge about orbits, I decided to go into this subject by coding algorithms and comparing the results to the INPE heritage and to the literature in my spare time.

The very first thing was to select the language! In my Ph.D., I used MATLAB to simulate an inertial navigation system, but the Monte Carlo simulations were so slow that I had to rewrite many parts in C using CMEX. On the other hand, in my post-doctoral research where I studied estimation in distributed, non-linear systems, I decided to go with FORTRAN (using the 2008 standards to have at least a readable code…) so that the execution speed is not an issue. Yeah, the performance was very good, but it took me too much time to code. Then I heard about a new language that was promising the best of both worlds: something that resembles an interpreted language with speed of a compiled one! And that’s how I met Julia.

By that time (using v0.2 I think), Julia was a really new language. But I decided to accept the rough edges and try to code my algorithms using it. Anyway, it was just a personal side project to learn more about orbits. I did face many bugs, I had to use `master`

(pre-v0.3) due to some bugs and missing features, but it was fun 🙂

After some years (and huge rewrites due to breaking changes), Julia released its v0.4. In this time, given the amount of code I had and the state of the language, I started to see that this bunch of algorithms can indeed be used for something at INPE to help in my activities. Hence, I decided to create a private package, which was called **SatToolbox.jl**, to organize everything I have done.

After some time, this little side project turned out to be a very important core for a simulator, called Forplan, of the mission operational concept we started to develop at INPE’s Space Mission Integrated Design Center (CPRIME). Given the good feedback I received, I decided to rename the toolbox to SatelliteToolbox.jl and it was released as an official Julia package in March 2018.

In this post, I would like to briefly describe **SatelliteToolbox.jl** and how it can be used to analyze space missions. The entire set of the features can be seen in the documentation. A brief, non-exhaustive list of the algorithms implemented by the time of this post (in v0.5.0) is:

**Earth atmospheric models:**- Exponential atmospheric model;
- Jacchia-Roberts 1971;
- Jacchia-Bowman 2008; and
- NRLMSISE-00.

**Earth geomagnetic field models:****Space indices:**- Capability to automatically fetch many space indices,
*e.g.*`F10.7`

,`Ap`

,`Kp`

, etc.

- Capability to automatically fetch many space indices,
**Functions to perform general analysis related to orbits,***e.g.*converting anomalies, computing perturbations, etc.**Orbit propagators:**- Two body;
- J2; and
- SGP4/SDP4.

**Functions to convert between ECI and ECEF references frames:**- All the IAU-76/FK5 theory is supported. Hence, the conversion between any of the following frames is available:
**ITRF**: International Terrestrial Reference Frame;**PEF**: Pseudo-Earth Fixed reference frame;**MOD**: Mean-Of-Date reference frame;**TOD**: True-Of-Date reference frame;**GCRF**: Geocentric Celestial Reference Frame;**J2000**: J2000 reference frame;**TEME**: True Equator, Mean Equinox reference frame.

- All the IAU-76/FK5 theory is supported. Hence, the conversion between any of the following frames is available:
**Functions to convert between Geocentric and Geodetic (WGS-84) references.**

In the following, I provide a few examples of how **SatelliteToolbox.jl** can be used to analyze space missions.

## Installation

The very first thing (provided that you have Julia installed, which can be obtained here) is to install the package. This can be done by typing:

```
julia> using Pkg
julia> Pkg.add("SatelliteToolbox")
```

To load the package, which must be done every time Julia is restarted, type:

`julia> using SatelliteToolbox `

Information

Here and in the following, the command you have to type is what is presented after `julia>`

, as you will see in the Julia REPL. Everything else is what you should see printed on the screen.

## Examples

Now, I will show some analysis that can be done with the functions that are available.

Necessary background

To keep this post short, I will assume that you have knowledge about Julia and a little background in satellite and orbits.

### New Year in ISS

Let’s see how can we calculate where the astronauts onboard the ISS were during the New Year in Greenwich! The first thing we need to do is to get information about the ISS orbit. In this case, we must obtain the TLE (Two-Line Element) file, which is a data format consisting of two lines with 70 characters each that contain all information related to the orbit. The following TLE was obtained from Celestrak at January 4, 2019, 12:25 +0200.

ISS (ZARYA)

1 25544U 98067A 19004.25252738 .00000914 00000-0 21302-4 0 9994

2 25544 51.6417 96.7089 0002460 235.6509 215.6919 15.5373082014978

This TLE must be loaded into a variable inside Julia. There are a number of methods to do this using **SatelliteToolbox.jl**. Here, we will use a special string type:

```
julia> iss_tle = tle"""
ISS (ZARYA)
1 25544U 98067A 19004.25252738 .00000914 00000-0 21302-4 0 9994
2 25544 51.6417 96.7089 0002460 235.6509 215.6919 15.53730820149783
"""[1]
TLE
==========================================================
Name: ISS (ZARYA)
Satellite number: 25544
International designator: 98067A
Epoch (Year): 19
Epoch (Day): 4.25252738
Epoch (Julian Day): 2458487.75253
Element set number: 999
Inclination: 51.64170000 deg
RAAN: 96.70890000 deg
Argument of perigee: 235.65090000 deg
Mean anomaly: 215.69190000 deg
Mean motion (n): 15.53730820 revs/day
Revolution number: 14978
B*: 0.000021 1/[er]
1 d
---.--- n: 0.000009 rev/day²
2 dt
1 d²
---.--- n: 0.000000 rev/day³
6 dt²
==========================================================
```

This code loads the first TLE specified inside the string enclosed by `tle"""..."""`

into the variable `iss_tle`

.

Now, we have to initialize an orbit propagator using the loaded TLE. In this case, we will use the SGP4:

```
julia> orbp = init_orbit_propagator(Val{:sgp4}, iss_tle)
OrbitPropagatorSGP4{Float64}(Orbit{Float64,Float64,Float64,Float64,Float64,Float64,Float64}(2.45848775252738e6, 6784.486114511487, 0.000246, 0.9013176963271557, 1.687888720981944, 4.112884090287905, 3.764246850766715), SGP4_GravCte{Float64}
R0: Float64 6378.137
XKE: Float64 0.07436685316871385
J2: Float64 0.00108262998905
J3: Float64 -2.53215306e-6
J4: Float64 -1.61098761e-6
, SGP4_Structure{Float64}
epoch: Float64 2.45848775252738e6
n_0: Float64 0.06779429624677841
e_0: Float64 0.000246
i_0: Float64 0.9013176963271557
Ω_0: Float64 1.687888720981944
ω_0: Float64 4.112884090287905
M_0: Float64 3.764533824882357
bstar: Float64 2.1302e-5
Δt: Float64 0.0
a_k: Float64 1.0637096874073868
e_k: Float64 0.000246
i_k: Float64 0.9013176963271557
Ω_k: Float64 1.687888720981944
ω_k: Float64 4.112884090287905
M_k: Float64 3.764533824882357
n_k: Float64 0.06778673761247853
all_0: Float64 1.0637096874073868
nll_0: Float64 0.06778673761247853
AE: Float64 1.0
QOMS2T: Float64 1.880276800610929e-9
β_0: Float64 0.9999999697419996
ξ: Float64 19.424864323113187
η: Float64 0.005082954423839129
sin_i_0: Float64 0.7841453225081564
θ: Float64 0.6205772419196336
θ²: Float64 0.3851161131885794
A_30: Float64 2.53215306e-6
k_2: Float64 0.000541314994525
k_4: Float64 6.0412035375e-7
C1: Float64 4.150340425449004e-10
C3: Float64 0.0052560300138783985
C4: Float64 7.530189312128724e-7
C5: Float64 0.0005696111334271365
D2: Float64 1.4236674016273006e-17
D3: Float64 7.305590907411524e-25
D4: Float64 4.371134237708994e-32
dotM: Float64 0.06779430410299993
dotω: Float64 4.494429738092806e-5
dotΩ1: Float64 -6.037619137612582e-5
dotΩ: Float64 -6.0409004140717795e-5
algorithm: Symbol sgp4
sgp4_gc: SGP4_GravCte{Float64}
sgp4_ds: SatelliteToolbox.SGP4_DeepSpace{Float64}
)
```

The variable `orbp`

now holds the orbit propagator structure of type SGP4 with the orbit specified by the TLE `iss_tle`

. This TLE was generated at the Julian day 2458487.75253 (2019-01-04 06:03:38.592 +0000). Thus, we have to backpropagate the orbit to the desired instant 2019-01-01 00:00:00.000 +0000 (New Year in Greenwich). This can be accomplished by the function `propagate!`

. However, this function requires a number of seconds **from the orbital elements epoch** (in this case, the TLE epoch) to which you want to propagate the orbit. Thus, the following code can be used to accomplish this:

```
julia> o,r_teme,v_teme = propagate!(orbp, (DatetoJD(2019,1,1,0,0,0)-iss_tle.epoch)*24*60*60)
(Orbit{Float64,Float64,Float64,Float64,Float64,Float64,Float64}(2.4584845e6, 6.784512486615914e6, 0.00024606332568975184, 0.9010786845520161, 1.9711454719429669, 3.9021657307355526, 0.4001307036976068), [4.61152e6, -9.76729e5, -4.88282e6], [-998.41, 7209.56, -2387.48])
```

The function `propagate!`

returns three values. The first one `o`

is the osculating orbit elements at the propagation instant, the second one `r_teme`

is the position vector, and the last one `v_teme`

is the velocity vector. Those vectors are represented on the same reference frame that was used to describe the orbit elements when the propagator was initialized. Since we are using the TLE, those vectors are represented in the TEME (True Equator, Mean Equinox) reference frame.

TEME is an Earth-Centered Inertial (ECI) reference frame. Hence, we must convert the position vector to an Earth-Centered, Earth-Fixed (ECEF) reference frame so that we can compute what was the ISS position (latitude, longitude, and altitude) at the desired instant. SatelliteToolbox.jl has the entire IAU-76/FK5 theory related to the conversion between reference frames. For this example, we will convert TEME into the International Terrestrial Reference Frame (ITRF) for more accurate computation. This kind of conversion requires the Earth Orientation Data (EOP) that is provided by IERS. **SatelliteToolbox.jl** can easily load and use this data as follows:

```
julia> eop = get_iers_eop()
[ Info: Downloading file 'EOP_IAU1980.TXT' from 'https://datacenter.iers.org/data/latestVersion/223_EOP_C04_14.62-NOW.IAU1980223.txt'.
```

```
julia> eop = get_iers_eop()
[ Info: Downloading file 'EOP_IAU1980.TXT' from 'https://datacenter.iers.org/data/latestVersion/223_EOP_C04_14.62-NOW.IAU1980223.txt'.
```

Warning

The EOP data is measured and published by IERS. This means that the ITRF cannot be used here to predict what will be the ISS position in the future since the EOP data will not be available. In this scenario, if high accuracy is not needed, then the Pseudo-Earth Fixed (PEF) reference frame can be used. The conversion between TEME and PEF using IAU-76/FK5 does not require external data.

The DCM (Direction Cosine Matrix) that rotates TEME into alignment with ITRF is computed by:

```
julia> D_ITRF_TEME = rECItoECEF(TEME(), ITRF(), DatetoJD(2019,1,1,0,0,0), eop)
3×3 StaticArrays.SArray{Tuple{3,3},Float64,2,9}:
-0.17984 0.983696 6.88009e-7
-0.983696 -0.17984 -1.32013e-6
-1.17487e-6 -9.14204e-7 1.0
```

Thus, the position vector represented in ITRF is:

```
julia> r_itrf = D_ITRF_TEME*r_teme
3-element StaticArrays.SArray{Tuple{3},Float64,1,3}:
-1.7901449017554987e6
-4.360669445952034e6
-4.882825969847775e6
```

Finally, considering the WGS-84 reference ellipsoid, the latitude, longitude, and altitude of the ISS during the New Year in Greenwich can be obtained by the function `ECEFtoGeodetic`

as follows:

```
julia> lat,lon,h = ECEFtoGeodetic(r_itrf)
(-0.8061561400806859, -1.960339209305694, 419859.07126381807)
julia> rad2deg(lat)
-46.18934445518049
julia> rad2deg(lon)
-112.31916310722917
julia> h/1000
419.85907126381807
```

*i.e.*, latitude 46.189° S, longitude 112.319° W, altitude 419.859 km. This is in agreement with the historical information on I.S.S. Tracker website:

Information

The minor difference can be explained by the TLE we used to computed the ISS position and the one used by the historical data of I.S.S. Tracker. Since a TLE has errors and the propagation increases them, it is desirable to obtain a TLE as closest as possible to the desired instant, which was not pursued here.

## Atmospheric density profile

In this second example, we will use the built-in functions of **SatelliteToolbox.jl** to compute the atmospheric density profile. There are many models available in the literature. **SatelliteToolbox.jl** implements four of them: **Exponential atmospheric model**, **Jacchia-Roberts 1971**, **Jacchia-Bowman 2008**, and **NRLMSISE-00**. All of them but the former requires as input some space indices, like the `F10.7`

that measures the Sun activity and the `Ap`

that measures the geomagnetic activity. **SatelliteToolbox.jl** is prepared to download all the required files from the Internet so that those indices can be easily obtained. This can be accomplished by:

```
julia> init_space_indices(wdcfiles_newest_year = 2018)
[ Info: Downloading file 'DTCFILE.TXT' from 'http://sol.spacenvironment.net/jb2008/indices/DTCFILE.TXT'.
[ Info: Downloading file 'fluxtable.txt' from 'ftp://ftp.geolab.nrcan.gc.ca/data/solar_flux/daily_flux_values/fluxtable.txt'.
[ Info: Downloading file 'SOLFSMY.TXT' from 'http://sol.spacenvironment.net/jb2008/indices/SOLFSMY.TXT'.
[ Info: Downloading file 'kp2017.wdc' from 'ftp://ftp.gfz-potsdam.de/pub/home/obs/kp-ap/wdc/kp2017.wdc'.
[ Info: Downloading file 'kp2016.wdc' from 'ftp://ftp.gfz-potsdam.de/pub/home/obs/kp-ap/wdc/kp2016.wdc'.
[ Info: Downloading file 'kp2018.wdc' from 'ftp://ftp.gfz-potsdam.de/pub/home/obs/kp-ap/wdc/kp2018.wdc'.
```

Information

The keyword `wdcfiles_newest_year`

is not necessary, but it is added to avoid an error since the `kp2019.wdc`

file was not available by the time this tutorial was written. For more information, see the documentation.

We will compute the atmospheric density profile from 100 km to 1000 km (steps of 1 km) using all the four models at 2018-11-1 00:00:00+0000 over São José dos Campos, SP, Brazil (Latitude 23.2237° S, Longitude 45.9009° W).

The exponential atmospheric model, the simpler one, depends neither on space indices nor on the location, only on the altitude. Thus, the atmospheric profile is computed by:

```
julia> at_exp = expatmosphere.(100e3:1e3:1000e3)
901-element Array{Float64,1}:
5.297e-7
4.4682006197154693e-7
3.7690800034030025e-7
3.1793478585921236e-7
2.6818886298003355e-7
2.2622647607479199e-7
1.9082976790512217e-7
1.6097143424840973e-7
1.357849088663833e-7
1.1453921350666081e-7
9.661e-8
8.418342953216043e-8
7.335524073901479e-8
6.391983997068123e-8
5.5698078292918074e-8
4.8533850005678735e-8
4.229112868106303e-8
3.685138444423763e-8
3.211133345951797e-8
⋮
3.316262334792707e-15
3.2979959953424034e-15
3.279830268908571e-15
3.2617646013035953e-15
3.2437984413923887e-15
3.225931241075577e-15
3.2081624552727763e-15
3.190491541905968e-15
3.172917961882957e-15
3.155441179080928e-15
3.1380606603300916e-15
3.1207758753974133e-15
3.1035862969704427e-15
3.086491400641224e-15
3.069490664890299e-15
3.0525835710707956e-15
3.0357696033926073e-15
3.019e-15
```

Each element is the atmospheric density [kg/m³] related to one altitude.

Information

Here we use the Julia broadcast operator `.`

to compute for the entire altitude interval using only one line. For more information, see the documentation.

For the Jacchia-Robert 2008, we must specify the geodetic latitude [rad], longitude [rad], and altitude. Notice that, since we have already initialized the space indices, all the required information will be gathered automatically:

```
julia> at_jb2008 = jb2008.(DatetoJD(2018,11,1,0,0,0), deg2rad(-23.2237), deg2rad(-45.9009), 100e3:1e3:1000e3)
901-element Array{JB2008_Output{Float64},1}:
JB2008_Output{Float64}
nN2: Float64 8.646783160590725e18
nO2: Float64 2.002012321529147e18
nO: Float64 6.354156624129256e17
nAr: Float64 1.0339387366523766e17
nHe: Float64 1.4269240166433797e14
nH: Float64 0.9667293309147541
rho: Float64 5.323059860159937e-7
T_exo: Float64 677.6952552196058
Tz: Float64 193.02497135461914
JB2008_Output{Float64}
nN2: Float64 7.196278316404481e18
nO2: Float64 1.6373098215597222e18
nO: Float64 5.865508118864621e17
nAr: Float64 8.604946802614022e16
nHe: Float64 1.1875563628018653e14
nH: Float64 0.9651359622807867
rho: Float64 4.430112278422747e-7
T_exo: Float64 677.6952552196058
Tz: Float64 195.66319833921864
...
JB2008_Output{Float64}
nN2: Float64 7.63695901782906
nO2: Float64 0.005854548029499879
nO: Float64 1.9770627164533935e7
nAr: Float64 4.496252494092111e-9
nHe: Float64 9.905504449767151e10
nH: Float64 2.2106945558924915e11
rho: Float64 1.0288388821434309e-15
T_exo: Float64 677.6952552196058
Tz: Float64 677.6523510137463
```

Each element is an instance of the structure `JB2008_Output`

that contains the density of the atmospheric species in [kg/m³] related to one altitude.

The NRLMSISE-00 requires the same information, but in a different order. One more time, since we have already initialized the space indices, all the required information is fetched automatically:

```
julia> at_nrlmsise00 = nrlmsise00.(DatetoJD(2018,11,1,0,0,0), 100e3:1e3:1000e3, deg2rad(-23.2237), deg2rad(-45.9009))
901-element Array{NRLMSISE00_Output{Float64},1}:
NRLMSISE00_Output{Float64}
den_N: Float64 3.225647667164233e11
den_N2: Float64 1.1558415665482785e19
den_O: Float64 4.649965500403523e17
den_aO: Float64 4.631659520454273e-37
den_O2: Float64 2.6263326718789934e18
den_H: Float64 2.533162671436194e13
den_He: Float64 1.2320073447340945e14
den_Ar: Float64 1.160809744681819e17
den_Total: Float64 6.968049043353933e-7
T_exo: Float64 1027.3184649
T_alt: Float64 215.25904311781903
flags: NRLMSISE00_Flags
NRLMSISE00_Output{Float64}
den_N: Float64 3.5691748620576013e11
den_N2: Float64 1.0012594360559639e19
den_O: Float64 4.6065649467733504e17
den_aO: Float64 1.8947221849916303e-36
den_O2: Float64 2.2358683099960123e18
den_H: Float64 2.35052621777078e13
den_He: Float64 1.121318459050076e14
den_Ar: Float64 9.845632305957742e16
den_Total: Float64 6.029280387245405e-7
T_exo: Float64 1027.3184649
T_alt: Float64 213.7198609515809
flags: NRLMSISE00_Flags
...
NRLMSISE00_Output{Float64}
den_N: Float64 5.097414213511122e6
den_N2: Float64 106.44260908421272
den_O: Float64 7.592117961201309e7
den_aO: Float64 2.0421422183370042e9
den_O2: Float64 0.07891050457188623
den_H: Float64 1.3014187084557657e11
den_He: Float64 8.255445331597499e10
den_Ar: Float64 1.442732462296107e-7
den_Total: Float64 8.205713083292234e-16
T_exo: Float64 724.4998315669409
T_alt: Float64 724.4998315398782
flags: NRLMSISE00_Flags
```

Each element is an instance of the structure `NRLMSISE00_Output`

that contains the density of the atmospheric species in [kg/m³] related to one altitude.

Online NRLMSISE-00

The values computed here can be compared to the online NRLMSISE-00 model. Notice, however, that the online version by the time this tutorial was written only allows computation between 1960/02/14 and 2018/03/17.

The Jacchia-Roberts 1971 model does not support fetching the space indices automatically yet. Hence, we will need to do this manually. It requires three indices: the daily `F10.7`

, the averaged `F10.7`

(81-day window, centered on input time), and the `Kp`

geomagnetic index (with a delay of 3 hours). That information can be fetched by:

```
julia> F107 = get_space_index(F10(), DatetoJD(2018,11,1,0,0,0))
65.8
julia> F107m = get_space_index(F10M(), DatetoJD(2018,11,1,0,0,0); window = 81)
68.29135802469136
julia> kp = get_space_index(Kp(), DatetoJD(2018,11,1,0,0,0)-3/24)
0.875
```

Thus, the atmospheric profile computed by JR1971 is obtained by:

```
julia> at_jr1971 = jr1971.(DatetoJD(2018,11,1,0,0,0), deg2rad(-23.2237), deg2rad(-45.9009), 100e3:1e3:1000e3, F107, F107m, kp)
901-element Array{JR1971_Output{Float64},1}:
JR1971_Output{Float64}
nN2: Float64 5.7192521805880885e19
nO2: Float64 1.0370130013611293e19
nO: Float64 1.2248930040184422e19
nAr: Float64 4.797323831280962e17
nHe: Float64 3.150115795435398e15
nH: Float64 0.0
rho: Float64 3.4060767884871413e-6
T_exo: Float64 657.1677377132266
Tz: Float64 191.2125970557249
JR1971_Output{Float64}
nN2: Float64 8.080629730659746e18
nO2: Float64 1.6344357654348047e18
nO: Float64 1.0616020508369499e18
nAr: Float64 9.003667794720594e16
nHe: Float64 7.366460213846506e13
nH: Float64 0.0
rho: Float64 4.96920849944813e-7
T_exo: Float64 657.1677377132266
Tz: Float64 193.37386958205954
...
JR1971_Output{Float64}
nN2: Float64 4.812412406159134
nO2: Float64 0.00284582346906836
nO: Float64 2.7544317744479913e7
nAr: Float64 1.3825250583995572e-9
nHe: Float64 1.0969262471305798e11
nH: Float64 3.6326221208226086e11
rho: Float64 1.3378408900651963e-15
T_exo: Float64 669.4417264661663
Tz: Float64 669.441725137747
```

Each element is an instance of the structure `JR1971_Output`

that contains the density of the atmospheric species in [kg/m³] related to one altitude.

Finally, using the PyPlot.jl package, the atmospheric profiles (altitude vs. density) in semi-log scale can be plotted using:

```
julia> using PyPlot
julia> figure()
julia> h = 100:1:1000
julia> semilogx(at_exp, h, map(x->x.rho, at_jb2008), h, map(x->x.den_Total,at_nrlmsise00), h, map(x->x.rho,at_jr1971), h)
julia> legend(["Exp.", "JB2008", "NRLMSISE-00", "JR1971"])
julia> xlabel("Density [kg/m^3]")
julia> ylabel("Altitude [km]")
julia> title("Atmospheric Density, 2018-11-01 00:00:00+0000")
julia> grid()
```

which leads to:

For more information about the many options to compute the atmospheric density, please see the documentation.

Warning

If the desired date is not available on the space indices files, then an error will be thrown when computing the atmospheric density.

## Conclusion

I hope that this tutorial has helped you to understand a little bit how **SatelliteToolbox.jl** can be used to perform analysis related to satellites and orbits. If you have any question, please, feel free to leave a comment below!

**Update 2018-01-08:** Fix algorithm that plots the data, because the variable `h`

was not defined (thanks Bernard_GODARD).