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Car 64 where are you?

By: Richard DeYoung

Hi, I’m Rich. I’m a full-time instructor for AGI. I’ve been wracking my brain to come up with something to write about. There are questions that often come up in my classroom, so I thought I’d start answering some right here. Two lessons taught in the classroom are on setting up communication links and setting up radars. Communications and radars have something in common: antennas. OK, you know that already. Well, did you know that in STK, you need to ensure your antenna is at the proper height? It doesn’t just happen. You have to set that yourself. If you don’t know what you’re doing, your link budget or radar analysis will be wrong…big time.

Mountain law enforcement communications can be tricky. Let’s create a small scenario in order to show you what I’m talking about. Turn off Terrain Server and use a local terrain file for analysis. The file is a United States Geological Survey Digital Elevation Model that’s been converted into a STK Terrain File (pdtt), and it’s located in the beautiful state of Colorado. Keeping the scenario as realistic as possible, turn on the Terrain Integrated Rough Earth Model (TIREM). TIREM adds fidelity to the calculation and dynamic modeling of point-to-point line-of-sight effects for link performance in STK Communications, by taking into account the effect of irregular terrain, sea water, and non-line-of-sight effects. Place an antenna at the actual spot that it’s located (911 Dispatch). Using Bing maps, a Place object, and the 3D Object Editing tool, put the antenna at its exact location.

The local sheriff dispatch Transmitter object is attached to the Place object called “Antenna”. The Place object center point is located on the ground. Therefore, so is the Transmitter object which is a subordinate object. It is transmitting on a frequency of 800 MHz using an output power of 250 Watts. The antenna type is a dipole. A Ground Vehicle object is moving through the mountains along a highway. Its center point is also on the terrain. The Ground Vehicle object is within 10.5 kilometers of the police dispatch office at all times.

The Ground Vehicle object has an attached Receiver object and is using a dipole antenna. Just like the Place object, the Ground Vehicle object’s antenna is at ground height. The Receiver object is set up to analyze a link margin based on carrier to noise ratio (C/N). For this analysis, we desire a link margin of 10 decibels (dB) or higher. Creating a custom graph the first analysis shows a link margin of -10000 dB! Message Viewer, which is located at the bottom of STK, tells the story. Double clicking on the error message it states: STK Comm: RF TIREM Model; receiver/transmitter antennas should be at least half a meter above ground. (X11).

The first problem is that both antennas need to be at least one half meter above the terrain. Turning on the Place object’s body axis, you can see that positive Z points to the Earth’s center point. Therefore, if you want to move the antenna up (the antenna is embedded in the Transmitter object), move the Transmitter object along the parent object’s Z body. Since the positive Z body points to Earths center point, move the Transmitter object in a negative Z direction to make it go up. The Ground Vehicle object’s positive Z body points up, so move the Receiver object (the antenna is embedded in the Receiver object) along the parent object’s Z body which is up and is in a positive direction.

After moving both antennas to the minimum height of one half meter, reanalyze the link margin. Now we’re getting somewhere.

Although the link margin is below the C/N threshold of 10 dB for a portion of the route, the one half meter height difference made a huge improvement in the analysis. The values range from negative 24 dB through 86 dB which is when the Ground Vehicle object is closest to the Place object. Next, put the antennas at their proper height. The Transmitter object (police dispatch) is at a height of 100 feet. The Receiver object (police vehicle) is at a height of 6 feet. Reanalyze the link margin.

Again, there is a significant difference. The values now range from negative 9 dB though 91 dB. Based on the police vehicle's route, the analysis shows a C/N link margin value of 10 dB or higher for the majority of the analysis time.

Using an object’s body axis and vectors provide great visual clues when moving subordinate objects in STK. Using Message Viewer helps you to understand errors that occur during your analysis. To help you out, most of the “how to steps” in this blog can be found in the tutorial “Using the Terrain Integrated Rough Earth Model (TIREM)”.

Posted: 8/20/2018 7:00:00 AM


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