Determining When Objects Enter and Exit the Field of View

Generalized Rise/Set Time

Rendezvous supports the notion of a generalized Rise and Set time.  The generalized Rise time for an object is the time at which it enters the field of view of a sensor.  The Set time is the time at which it exits the field of view. Rendezvous has a panel which allows you to find all the rise/set times of all the targets selected in the Target browser for all the sensors that have been selected in the Sensors browser.  This feature used in conjunction with the judicious setting of the sensor's field of view, can be used as a very powerful method to find the occurance of a wide variety of events.



Finding Rise/Set Times

As our first example, let's imagine we're interested in finding all the times when the cosmonauts aboard MIR will be able to see the astronauts manning the new International Space Station Zarya module during the first week of July 1999.  If you're not sure how to do perform the following four steps, see the section "Opening Sensors and Targets".

o Load the Sensor browser with the file sat329.tle
o Double-click MIR in the Sensor browser
o Load the Target browser with the file sat329.tle
o Double-click Zarya in the Target browser

Bring up the Rise-Set Selection panel like this

o Click the Event Prediction : Rise-Set button in the main menu.

A panel comes up which looks like this:

Using the Calendar and Clock, set the Start time:

o Select the Start button just above the Calendar
o Set the date to July 1, 1999
o Set the time to 0 : 00 :00

Now set the End time

o Select the End button just above the Calendar
o Set the date to July 7, 1999
o Set the time to 0 : 00 :00

Now we are ready to find all the instances that MIR will see Zarya:

o Click the Find button at the bottom of the Rise-Set Selection panel

When the process is finished, the scrollview in the Rise-Set Selection panel has all the times when Zarya is in MIR's field of view.  The panel looks like this:

Double clicking on any entry in the scrollview will automatically load the main window with the Begin date and time for that event.  Let's look at the first entry in the table.

o Double-click on the first entry in the scrollview in the Rise-Set Selection panel

The main window is loaded with the Begin date and time of July 1, 1999 at 0 : 37 :20, and it looks like this:

I have zoomed the map shown above by click-dragging a box around the area of interest, and enabled the coverage area circles.  Remember that the coverage area circles define the boundary of the orbiting objects' horizon line, and then notice how the coverage area circles osculate, just barely touching each other.  This indicates that the objects are sitting right on top of each other's horizon.  We can show this another way by bringing up the MIR's Telescope Inspector.

o Click the Inspectors : Sensor button in the main menu
o Select the Telescope button from the Detail area of the Sensor Inspector panel which just popped up

We want to display the earth's horizon line in the Telescope Inspector.  We can do that two ways.  The first way:

o Select the Outline button in the Display Options area of the Telescope Inspector panel

A purple line appears indicating where the earth's horizon is located in MIR's field of view.  The Telescope Inspector panel now looks like this:



The purple line shows the horizon as MIR would see it, and the red dot is the Zarya module just barely peeking up above the horizon as we thought it should be.  If you increment the clock in the main window by a minute at a time, you can watch this panel display Zarya as it moves above the horizon.

And here is the second way to display a rendition of the horizon line.  If you have the High Resolution databases, you can show the earth itself like this:

o Select the Earth button in the Display Options area of the Telescope Inspector

(On some slower machines, doing this may take a minute or so to complete)

If you select the Earth button, the Telescope Inspector should appear like this:

Again, we can see the little red dot representing Zarya just peeking up above the earth's horizon.



Finding Satellites Visible from the Ground

If you examine the Rise-Set Selection panel, you will notice an area labelled "Illumination Condition".  Sometimes it is quite useful to limit events to those in which the target is illuminated by the sun, but the sensor isn't, or vice versa.  Sometimes you need the events in which both are illuminated, other times when neither are.  The "Illumination Condition" area allows you to select these conditions.

For example, suppose we wish to find the times in July 1999 when we can see MIR from the ground in Albuquerque, New Mexico.  In order to see MIR from the ground, we need several condtions.  MIR must be illuminated by the sun at a time when Albuquerque is not (if the sun is above the horizon, we won't see MIR for the same reason we can't see the stars).  Also, we want to restrict the events we find to those above 45 degrees.  This is an arbitrary restriction for my own convenience.  Near a city which casts a significant amount of glow into the nearby late-evening horizon, it is difficult to see anything below 30 degrees or so.  If you are viewing from a rural area, this consideration is less important.

Let's set up Rendezvous like this:

o Load the USCities.gnd file into the Sensors browser
o Double-click Abq,NM in the Sensors browser
o Load the sat329.tle file into the Targets browser
o Double-click MIR in the Target browser

We need to restrict the Albuquerque sensor field-of-view to elevations above 45 degrees.  Bring up the Telescope Inspector:

o Click the Inspectors : Sensor button in the main menu
o Select the Telescope button from the Detail area of the Sensor Inspector panel
o Enter the value 45.0 in the El Min: text field in the Field of View Extent area of the Telescope Inspector and press the Tab key to move to the next text field
o Enter the value 90.0 in the El Max: text field, and press the Carriage Return key.

These actions restrict the Albuquerque sensor field of view to angles above 45 degrees in elevation.  After doing this, your Telescope Inspector will look like this:

MIR may or may not be in your field of view display (we haven't set the Epoch Date to a particular value yet, so MIR's position is indeterminate, it might be above 45 degrees, it might not be).

Bring up the Rise-Set panel, and set the Calendar as follows:

o Click the Event Prediction : Rise-Set button in the main menu
o Select the Start button above the Calendar in the Rise-Set Selection panel
o Set the date to July 1, 1999
o Set the time to 0 : 00 :00
o Select the End button above the Calendar
o Set the date to July 31, 1999
o Set the time to 0 : 00 : 00

Now we need to set the restrictions on solar illumination.  Remember, we want Albuquerque (the "source") not to be illuminated, while we want MIR (the "destination") to be illuminated.  We set this condition like this:

o Select the Not Illuminated button in the Source section of the Illumination Condition are in the
Rise-Set panel
o Select the Illuminated button in the Destination section of the  Illumination Condition are in the Rise-Set panel

Now, we're ready to find the events:

o Click the Find button at the bottom of the Rise-Set panel

Rendezvous finds three events that match our selection criteria.  The Rise-Set panel now looks like this:

Let's look at the first event.

o Double-click on the first entry in the scrollview of the Rise-Set panel

This loads the main window with the Date and Time of this event.  The main window looks like this:

Can we verify that this engagement satisfies our requirements?  Let's first check the illumination conditions. Notice Albuquerque is in the dark area of the solar terminator, so Albuquerque is not illuminated, so far, so good. To check the illumination of MIR we need to use the Map Inspector to enable the Illumination Condition for the Ground Traces:

o Click the Inspectors : Map button in the main menu
o Select the Satellite Illumination button in the Options area of the MapInspector panel

We check that MIR is within the dark red region of the ground trace indicating that MIR is illuminated by the sun. Both our illumination conditions are met.

Now let's check the restrictions on elevation.  Remember, we only wanted cases where MIR was above 45 degrees.  Bring up the Telescope Inspector for Albuquerque:

o Click the Inspectors : Sensor... button in the main menu
o Select the Telescope button in the Detail area of the Sensor Inspector panel

Here is what the Telescope Inspector panel looks like:



Look closely at the very bottom of the Boresight Field of View.  You can see a red dot at the bottom near the graticule line.  That red dot represents MIR, and the position at the bottom shows that it is a tiny bit abover 45 degrees.  If you increment the Clock in the main window by a minute at a time, you can watch as MIR rises above 45 degrees.

So our elevation condition is satisfied.  Since all our criterial have been met, we have a reasonably good chance of actually seeing MIR during this pass.

Useful Other Events

There are often restrictions on azimuth and elevation imposed on an engagement when performing telemetry transmissions.  Rendezvous can find the times during which telemetry may be performed.   Many times it is necessary to determine when the sun is within a certain region to re-charge on-board batteries.  Or perhaps you must find when the sun is getting close to an exclusion angle to avoid damage to cameras and focal-plane detectors.  Rendezvous can find those events too, using the same type of procedure we used above.  In all these cases, you set the field of view to the area of interest, set the illumination condition, and hit the Find button. Rendezvous does the work and finds all the events.