Tag Archives: STK

Systems Tool Kit (STK) intro

Note 1: STK only works on Windows (sad but true), well to be fair there is actually an old version of STK that technically works on Linx but it’s just the core engine, not really the Graphics version.

Note 2: this tutorial is a follow up of the Astronautics 101 post:

  1. Go to the AGI website and get registered to download the Free version of STK: http://www.agi.com/products/stk/modules/default.aspx/id/stk-free
  2. Follow the instructions to install STK
  3. Create a new scenario, give it a name and use the defaults start and stop dates
  4. insert and STK object: satellite: From standard object database
  5. search the name ‘iss (zarya)’ – that is the official designator for the ISS. Select the result and click insert. You should be connected to the internet to allow the software to download the right database for the ISS. Then close all the insert windows untill you’re left with just the main STK views. At this point you should see a 3D window and a 2D window showing the orbit of the ISS
  6. Click the blue ‘start’ button on the control toolbar to see the ISS move. Then click pause.
  7. Now click the yellow ‘Real-time animation mode’ button and then the ‘play’ button again. Now you are seing the actual position of the ISS at this exact moment. Is it almost over Honolulu? If you look at the time it’s in UTC instead of HST. But it doesn’t matter because it’s the real time.
  8. Now right click on the ISS icon on the Object browser on the left and select ‘Report & Graph Manager’. This will bring up a list of different types of reports you can get about the ISS. Explore the list of installed styles.
  9. Ok, now let’s double click on the ‘Classic Orbit Elements’ report (not the graph). This will bring up a new window with different columns: Time, Semi-major Axis, etc.
  10. Now let’s create a new custom report: click on the 3rd icon in the styles toolbar. Give it a name like ‘me419’
  11. Select ‘Classical Elements’ -> ‘J2000’ then double click on
    1. Time
    2. Apogee Altitude
    3. Perigee Altitude
    4. Apogee Radius
    5. Perigee Radius
    6. Semi-major axis
    7. Eccentricity
    8. Period
  12. Select ‘Cartesian velocity’ -> ‘J2000’ and double click on speed.
  13. Do the math with the given equations to confirm these results (for your own sake). Done!

Here is the STK scenario (zip) and report (txt) for download.

Astronautics 101

This tutorial will help you get started in simple Astronautics calculations using the Systems Tool Kit from AGI or Python. We will use the International Space Station as our orbiting body around the Earth.

The ISS is the largest man-made satellite ever made. It’s been orbiting the earth since 1998 and estimates say it has cost about $100 billion so far. It can be seen with the naked eye from ground. Since the year 2000 it has been continuously occupied by different astronauts from different nations. To read more about it go to: http://en.wikipedia.org/wiki/International_Space_Station

This tutorial will help you calculate some interesting orbital properties of the ISS like it’s orbital speed and period. I’ll give you some of the relevant variables and equations so you can easily compute everything. The main purpose of this tutorial is to get you started with powerful software tools that can help you solve real problems.

With a simple calculator go trough the given equations and get the relevant numbers. Just because I give you the numbers you should’t trust them blindly. Then you will verify your hand made numbers  using STK or Python.

Don’t worry if you don’t understand all the terms yet. Later you will learn more about these terms like the true anomaly and orbital energy (read this and this to get started right away) . These steps were made so you can easily compute the orbital speed and period of the ISS.

Here is the data and the relevant equations (make sure you use the right units):

Earth Radius $R_e =  6371 km$
Earth gravitational parameter $\mu = 3.986×10^5  km^3/s^2$
Perigee = 418 km
Apogee = 426 km

$R_p = 418 + Re$
$R_a = 426 + Re$

1) Find the eccentricity of the orbit (e)

$e = \frac{R_a – R_p}{R_a + R_p} = 0.000588841454438$

it’s very small!!! as expected, meaning the orbit is almost circular

2) Find the semi-major axis (a)

$2a = Ra + Rp => a = 6793.0 km$

3) Find the specific mechanical energy for the ISS orbit (espilon)

$\epsilon = – \frac{\mu}{2a} = -29.339025467392904 km^2/s^2$

4) Find the altitude (R) of the ISS when the true anomaly is 90deg (usually represented by the greek letter “nu”)

$R = \frac{a(1-e^2)}{1+e \times cos(\nu)} = 6792.9976446341825 km$

don’t forget to convert deg to radians when you use this equation.

5) Find the orbital speed (at nu = 90deg)

$V = \sqrt{2(\frac{\mu}{R} + \epsilon)} = 7.660163 km/s$

6) Find the orbiting period of the ISS

$P = 2\pi\sqrt{\frac{a^3}{\mu}} = 5571.90 sec = 92.87 min = 1.55 hr$

because the results are given in seconds I convert to minutes and hours to get a better feeling of it how long it really takes for the ISS to orbit the Earth once.

BTW, here’s a really cool video of the Earth taken from the ISS at 7.6 km/s

Now follow these links to check if the above equations are correct:

Systems Tool Kit (STK) intro

go to http://www.spacemig.com/stk-intro/

Python intro

go to http://www.spacemig.com/python-intro/