ACP-WGF/16-WP17
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 LongleyRice 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.in> atoa.out”
Report No. FAA-RD-77-60 entitled “Applications Guide for Propagation and interference Analysis
Computer Programs (0.1 to 20GHz). Page 3.
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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:
1
2
3
4
5
6
7
8
1 234 567 890 123 456 789 012 345 678 901 234 567 890 123 456 789 012 345 67 890 123 456 789 012 345 678 90
3 2 0 0 2 2 1 V OR
26 0 0 0
16
4 0 1
0 0
1 0
5 2
1 2 7
300 00
1 0 1
3 01
1 13
22 . 2
0 3
0
0
0
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:
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
Some of the input variables associated with the IF77 model are held constant for FAA’s applications and
are not shown in figure 2.
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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:
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:
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.
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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
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.
4
FAA Order 6050.32 Appendix 2 Figure 17 – Limits of Coverage – VHF – Antenna Height =100 ft.
ACP-WGF/16-WP17
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.