Examining the Trajectory in Detail

There are several ways of examining a trajectory.  The ground trace of all sensors and targets is plotted in the main window.  A user can change the values in the Ground Trace Control section of the main window to allow more or less of the ground trace to be viewed.

For some purposes it is necessary to get a better understanding of the engagement geometry between a sensor and target than can be found from the Ground Trace Map alone.  This is certainly the case when one wishes to spend an evening viewing satellites.  A knowlege of the azimuth, elevation, and range to a target from your location is essential.  To this end, a panel can be displayed which shows this information.  Here is an example where Rendezvous has been set up as follows:

(see the section "Opening Sensors and Targets" if you don't know how to perform these steps):

o Load the Sensor browser with the state vector file USCities.gnd.
o Load the Targets browser with the file sat329.tle.
o Select CapeCanaveral from the Sensor Browser
o Select MIR from the Target Browser
o Set the Epoch Date to December 25, 1998
o Set the Epoch Time to 18 : 25 : 00 GMT
o Enable Satellite Coverage button in Map Inspector

Note:  A common mistake is to load the USCities file as an orbiting sensor rather than a ground station.  When loading USCities, make sure you select either the AutoDetect or the GroundStation selections under the Platform button of the Open Panel.

Your main window should now look like this:

The little grey icon represents MIR, the red line is the trajectory MIR follows for the next 1500 seconds, and the little yellow dot indicates Cape Canaveral.  The azimuth, elevation, and range time histories can be displayed by clicking on the Trajectory Plots button in the main menu.  When you do this, the following panel appears:

The top three plots in this panel show the longitude, latitude, and altitude of the target MIR throughout the time interval selected in the Ground Trace Control of the main window.  In this example, the plots range from an offset of 0 seconds to 1500 seconds from the Epoch Date.  The plots are evaluated between these endpoints using an interval whose value is located in the Delta T textfield of the main window.  In this example, that value is 100 seconds.  The bottom three plots show the azimuth, elevation, and range of MIR from the sensor located at Cape Canaveral.  Very quickly we can see that MIR approaches from about -50 degrees in azimuth, and recedes at about 130 degrees.  Its peak elevation is only 15 degrees, and the minimum range is approximately 1000 km.  If our goal were to locate MIR visually, this information tells us where to look.  Of course, we can see from the Ground Trace that Cape Canaveral is on the daylight side of the solar terminator during this pass, and we wouldn't expect to be able to actually see MIR for this reason.

Sensor State Vector

There are other times when it is necessary to know the numerical values of the state vector.  To show the state vector for a sensor, click the Inspectors : Sensor... button in the main menu.  This brings up the Sensor Inspector panel:

The state vector information is displayed by selecting the Platform button in the Detail section of this panel. Selecting the Platform button brings up the state vector panel:

The state vector can be displayed in any of seven different coordinate systems.  A description of each coordinate system can be found in the "Coordinate Systems" section.  Notice this sensor is a ground station and is not in a ballistic trajectory.  It is fixed with respect to the earth.  Consequently, the parameters specifying the ECR velocity (speed, ascent, and course) are essentially meaningless.

Notice if several sensors are selected in the browser of the main window, they can each be viewed in turn by pressing the up or down buttons in the Sensor Index area of the Sensor Inspector panel.  When a new sensor is selected in this way, all inspectors are updated to show the current information associated with that sensor.

Target State Vector

We can view the target state vector by clicking on the Inspectors : Target...  button from the main menu.  The following panel is displayed:



To get information about the target's platform state vector, select the Platform button in the Detail area of the Target Inspector panel.  You'll see a panel which looks like this:



The state vector can be displayed using any one of seven coordinate systems selected by the Coordinate System pull-down menu.  The state vector of all the targets selected in the main menu can be sequentially viewed by clicking on the up or down arrows next to the target name.  More information about each of the coordinate systems can be found in the "Coordinate Systems" section.