In the real world, radio transmissions are subject to propagation effects which affect the received signal in a variety of different ways. For many applications, it is important that simulated radios exhibit the same sort of behavior.

The ASTi simulated radio environment currently provides a number of capabilities to make its radios as realistic as possible. "Out-of- the-box" ASTi radios simulate propagation over a smooth, WGS-84 earth, by including the effects of free-space (1/r^2) propagation, as well as first fresnel zone effects and occulting by the earth's surface. These effects are computed and applied automatically within the Model Builder environment.

Model Builder also provides the capability to access a terrain server, to simulate the effects of propagation over a realistic, irregular terrain, rather than a smooth model of the earth. When a transmitter and receiver are in-tune and in-range of each other, Model Builder periodically sends a DIS path loss request packet over its host interface. All the information needed to compute the signal propagation effects from the transmitter to the receiver, including world positions and transmit frequency, is in the packet.

A "terrain server" on the network can receive the packet, parse the needed information, and use any algorithm at its disposal to compute a path loss factor (a measure of the signal degradation caused by propagation effects). The path loss factor is then returned to Model Builder in a path loss response packet, and Model Builder applies the path loss factor to the received signal level of the radio.

Note that the algorithm used to compute the signal degradation can be any level of complexity, from a simple Line- Of-Sight determination (somewhat useful) to a more complicated algorithm which includes frequency-dependent diffraction effects (more useful).

In addition to the capabilities described above, which are built into the Model Builder environment, ASTi is now offering a standalone terrain server as a product. This terrain server is deployed on the host network, and will interact with the Model Builder virtual radio environment to provide realistic terrain-related propagation effects.

Here are some of the issues related to terrain capabilities, so that you can better compare our terrain product to others on the market.

Terrain "data" consists of elevation data over an area of coverage, and is available from a number of different sources.

One of the most popular sources of terrain data is the US National Imaging and Mapping Agency (NIMA), which pubishes terrain elevation data through the United States Geological Service (USGS). This is the so-called DTED data. DTED data is offered in a number of different resolutions, as characterized by the average distance between elavation data points, or "grid-post spacing." This resolution is designated by the DTED Level number, and is summarized in the table below.

DTED Level	Nominal Grid Post Spacing (m)
0	1000
1	100
2	10
3	3
4	1

Now, just because Level 3 or Level 4 DTED data exists, doesn't necessarily mean it exists for the area of the world you are interested in, or that it is available. Level 0 DTED for the entire world is publicly available for download through NIMA/USGS. Level 1 DTED data for the US is publicly available from the USGS. Some customers may have access to high resolution terrain elevation data for certain areas of interest.

One of the more challenging aspects of a terrain product is dealing with the huge amounts of data inherent in covering areas of the earth with a grid. Let's consider the size of the data for a second. Let's imagine that we wanted to store DTED data for the entire world on a hard disk. DTED data is typically organized into sections which are 1 degree latitude by 1 degree longitude. For Level 1 data, this turns out to be a grid consisting of 1200x1200 data points per section. At 2 bytes per sample, that's 2.88 MB of data per one degree by one degree section. To cover the entire land mass of the earth, you would need approximately 18,500 of these sections, totaling over 50,000 MB of data! For other resolutions, see the table below.

DTED Level	Approximate Data Size for whole earth coverage (MB)
0	530
1	50,000
2	50,000,000
3	500,000,000

The point here is not that you can't put a Level 2 or Level 3 dataset encompassing the entire world on a single platform -- it's obvious that you can't. The point is that we are talking about large amounts of data, even in smaller operational areas at Level 1 resolution. This has implications in the processing resources needed to read and query these data sets. Would you want the computer that is taking care of critical voice comms tasks to be the same computer that is sorting through and querying huge datasets, extracting terrain profiles, and computing the propagation characteristics for a (large) number of transmitter-receiver pairs?

Also, consider the resolution of the data. What resolution data do we need to accurately capture affect of the terrain features on radio communications? For UHF and VHF communications, wavelengths are on the order of tenths of meters to meters. Variations in terrain over 3 meter grid post spacing, or ten meter grid post spacing, are not going to have significant effects on RF propagation at these wavelengths. Elevation data at 30 m to 100 m grid post spacing is most likely highest resolution you would ever consider using for radio propagation purposes.

ASTi is interested in providing real solutions that customers can apply to large simulation applications, rather that just checking off the "we provide terrain capabilities" box on some checklist. Our terrain product is a product that the customer can actually use in real applications to improve the fidelity of their simulation. The ASTi terrain product has the following features.

• The product is a terrain server, where all the terrain data and functions are located on an optimized, separate platform that resides on the host network. This architecture offloads the large processing and storage burden from the platform hosting the communications environment. Also, a single terrain server provides terrain capabilities to multiple platforms on the network. There is no need to duplicate the large amounts of terrain data, or the necessary processing and storage resources on each and every communications platform.
  
  
• DTED Level 0 world-wide coverage data is used as an underlying baseline blanket coverage. Multiple, arbitrary, user-defined operational areas at higher resolution can be defined. All terrain functions are resolution- agnostic, so any resolution data that can be provided, can be used, if desired.
  
  
• Sophisticated propagation algorithms that go beyond simple Line-Of- Sight determination, to provide realistic, frequency dependent diffraction effects.
  
  
• Utilities to import terrain data in multiple popular formats, and to visualize terrain datasets. Compatibility with future, high quality dataset formats, such as SRTM data.
  
  
• Tight integration with the ASTi Telestra Remote Management System (RMS)