Update on IF77 radio frequency propagation model ...



NAVIGATION SYSTEMS PANEL (NSP)

Working Group 1 Meeting

Montreal, Canada

December 11-15,, 2006

Agenda Item : Update of guidance material on VOR and DME service volumes

DME Power Availability Curves and Service Volumes

Presented by Bruce DeCleene

(Prepared by Robert Frazier, Ed Coleman)

SUMMARY

This Working paper is provided as documentation of the IF77 Radio Frequency Propagation Model currently being developed by the FAA. This new version runs under the Windows operating system and generates coverage plots of the type used by both ICAO and FAA.

1. INTRODUCTION

During 1960 – 1973, and air/ground propagation model applicable to irregular terrain was developed by the Institute for Telecommunication Sciences (ITS) for the FAA … [this model] has evolved into IF-77 … Conceptually, the model is very similar to the Longley-Rice propagation model”[1] Hardcopy plots generated by ITS via this model were used by the FAA in determining coverage plots for various systems as documented in FAA order 6050.32. Originally the “IF77” model was implemented on a mainframe computer but was eventually converted to a DOS executable. It is this executable that is currently being by the FAA

This working paper presents further information on the model and its operation.

2. DISCUSSION

As stated above, the propagation model is currently a compiled DOS executable and interaction with the program is limited to the original design. The DOS command line instruction for running the model is as follows:

“ata.exe atoa.out”

When this is executed, the program scans the text file “atoa.in” for the definitions of the input variables, executes the model, and produces an output text file named “atoa.out”. The input text file is analogous to a mainframe “card” in that the variables are defined dependent upon the text in a certain range of columns. For example, the sample file “atoa.in” contains the following:

[pic]

Figure 1: DOS model text input format

The first two rows of the above example are headings for the column number; the colored areas are the actual text of the file. By way of example, columns 24 thru 27 of row three (shown above, circled and in red) contain the entry “_301” which, according to the variable definitions, defines the surface refractivity field value. Similarly, all the other inputs to the model are encoded into this text file. The output file in also in plain text format and contains a header describing the input variables followed by a data block that contains power available at various ranges for that model’s run. Each run of the model is only applicable for one value of aircraft altitude. In order to generate the “contour” plots that are used by both ICAO and the FAA, it is necessary to run the model many times over a range of altitudes and examine the output for the desired power.

Using the DOS executable, with its text input and output, to generate the required data has proven to be quite cumbersome. The FAA has developed a windows interface that will allow a user to execute the model more efficiently and generate the desired output. Below is a screenshot of the variable declaration page of the windows interface:

[pic]

Figure 2: Initial Program Screen, variable declarations.[2]

From this screen, a user can define inputs to the DOS model by selecting options either from pull down menus or direct text entry. Most of the parameters shown above are self explanatory. The fields relating to “antenna tilt” and “half power beam width” apply only to a single type of antenna (a JTAC antenna) and for others will be ignored.[3] The JTAC antenna and its associated parameters have not been studied as part of this effort.

The buttons “Load Program Options” and “Save Program Options” give the user the ability to save and recall variable settings for later use. With the options shown above, pressing the “Run Model” button will result in the following display:

[pic]

Figure 3: Results of Single Run of DOS model

This plot shows the power available versus distance. The red trace is the power available based upon a simple free space path loss calculation, while the blue line is the “95% Available” data. The DOS program also calculates other statistics which can be shown on this graph, but for our purposes, we are only concerned with the 95% Available curve. At this point, the user has the option of saving the output file, returning to the main variable input screen, proceeding to a “contour” graph or quitting the program. Typically, at this point the user would select “Goto Contour Graph” will allow the user to produce a “contour graph” such as shown in the following figure:

[pic]

Figure 4: Contour Graph output

In the figure above, the red dots are the model’s calculated values, the solid blue lines are a “reference plot” that was derived from FAA Order 6050.32, Spectrum Management Regulations and Procedures the original version of that figure is shown below:[4]

[pic]

Figure 5: Limits of Coverage – VHF Communications – Gnd Ant=100 ft.

3. CONCLUSION

The windows implementation of the IF77 propagation model provides an effective interface for verifying existing contour graphs and developing new coverage plots. Currently the model is limited in the antenna patterns available. However the data space within the original code has been identified and it is thought that new antennas could be added. The FAA is considering porting the original code entirely to the windows environment which will allow for faster run times and easier updates, including addition of user’s antenna data.

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[1] Report No. FAA-RD-77-60 entitled “Applications Guide for Propagation and interference Analysis Computer Programs (0.1 to 20GHz). Page 3.

[2] Some of the input variables associated with the IF77 model are held constant for FAA’s applications and are not shown in figure 2.

[3] Report No. FAA-RD-77-60 entitled “Applications Guide for Propagation and interference Analysis Computer Programs (0.1 to 20GHz). Pages 82, 83, 88, 89.

[4] FAA Order 6050.32 Appendix 2 Figure 17 – Limits of Coverage – VHF – Antenna Height =100 ft.

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