STK Exercise and Training Session #4
Rev 1.1 Sept 14, 2003
Objective: Practice the skill of placing a satellite in orbit using the “Initial State” tool within the
Astrogator propagator.
We’re going to use an IJK coordinate system with the origin located at the center of our central body
(Earth). The I direction is taken as the direction in space from Earth to the center of the Sun on the first
day of spring (in the northern hemisphere). See figure 4-25 scanned in from Understanding Space.
The K direction is taken to point up through the Earth’s North pole. The J direction is in the plane of the
Earth’s equator and is at a right angle to I and K (using the right hand rule – using your right hand, curling
your fingers from I to J causes your thumb to point toward K). See figure 5-3.
You are to use the physics equation v = [GM/r]^(1/2) to calculate the velocity of a satellite in a circular
orbit for a given orbital radius.
To get the orbital radius, add the height of the satellite to the radius of the central body (Earth). The
radius of Earth is 6378 km.
G = 6.672 x 10^(-11) N-m^2/kg^2
Mass of Earth = 5.98 x 10^24 kg
Exercise 1: Place a satellite in a circular equatorial orbit at a height of 300 km above the surface of the
Earth.
Procedure:
Launch STK.
Dismiss (cancel) the Scenario Manager.
Under File, choose to create a new scenario. Name it training4.
In the STK browser window select “Create New Satellite.”
Dismiss (cancel) the Orbit Wizard.
More 
Click on the VO window.
STK Session 4 page 2
Rev 1.1
Click on the “3D Graphics properties” window along the top menu bar.
Select the “Vector” tab.
Choose to show vectors along Earth’s Inertial Axis. Hit okay to close the dialog window.
You should now see the IJK vectors sticking out of the Earth. Note K goes up through the North pole.
Click on the STK browser window.
Click on “satellite1.”
Under the Properties menu, select “basic.”
Change the propagator to Astrogator.
A new dialog window will come up.
Click on “Initial state.”
A window will come up showing the initial xyz values of your satellite, and the initial vx, vy, and vz
values of your satellite.
Calculate the orbital radius of a satellite located 300 km above Earth. r = _____________
Calculate the orbital velocity of this satellite. v = ________________
We’ll take the position of the satellite to be along the x axis. The velocity will be at a right angle to its
position, along the y axis. Enter your value of r for the x position of the satellite (in correct units). Enter
your value of v for the initial vy value of your satellite. Set the other four variables to zero (y, z, vx, vz).
Close the dialog window. On the VO window you should see your satellite orbiting Earth along the
equator.
Exercise 2: Add a second satellite at an altitude of 300 km in a polar orbit.
In a polar orbit the satellite goes directly over the North and South poles. Create a second satellite,
dismiss the orbit wizard, change the satellite’s propagator to Astrogator, and enter the initial state of the
second satellite. Use the same xyz values you had used for satellite1. Change the vx, vy, and vz values
so the satellite now goes over the poles instead of along the equator.
Exercise 3: Add a third satellite that orbits the Earth at an angle of 45 degrees to the equator. (Satellite 1
had an inclination of zero degrees, while satellite2 had an inclination of 90 degrees). Again use a satellite
height of 300 km, starting along the x axis. Change the initial velocity vector so the satellite’s inclination
is now 45 degrees. (Hint: sketch a 45-45-90 right triangle. The satellite’s velocity is the hypotenuse.)
Extra exercise 4. Add a fourth satellite that starts above South America instead of Africa. This means
you’ll be changing the initial xyz values of the satellite. Make the inclination of the satellite 90 degrees.
To get the initial xyz values you’ll have to convert r and theta from polar coordinates to Cartesian
coordinates. Theta is the longitude of a point measured in degrees east (+) or west (-) from the prime
meridian. South America is around –100 degrees of longitude. Try this and show me if you get it.