VOACAP Method 30. A Long path / short path smoothing function. By George Lane and Hien Van Vo (International Broadcasting Bureau/USIA)

VOACAP Quick Guide: Home

[This document is part of the help files integrated into the ITS HFBC software package.]

INTRODUCTION: The Voice of America Coverage Analysis Program (VOACAP) has two different propagation models; namely a ray hop model for shorter distances and a ducted or forward scatter model for the longer distances. When operating the program using the complete system performance method, i.e. Method 20, the ray hop model is used for all distances less than 10,000 km. For paths of 10,000 km or greater the long path model is used. If the program user is interested in paths of nearly 10,000 km some ambiguity exists as to which model should be used. The models are NOT forced to yield similar results at the boundary distance so that discontinuities in predicted performance parameters can occur at distances just under 10,000 and right at 10,000 km. This is an artifact of the parent program, IONCAP. A smoothing function to eliminate the discontinuity in predicted signal level between the two propagation models has been incorporated in VOACAP and can be accessed by using the new Method 30.

BACKGROUND: Both IONCAP and VOACAP allow the user to force a particular propagation model to be used for paths at any distance. Method 21 forces the use of the long path model which simulates the ducted or forward scatter mechanisms that can prevail usually at distances having 3 or more hops. Method 22 forces the use of the conventional ray hop model. In VOACAP, the user can request area coverage plots using Methods 20, 21 or 22. Method 20 may produce "cliffs" or strange looking coverage plots at the discontinuity occurring at 10,000 km. Method 21, the long path method, produces unrealistic coverage plots at the shorter distances where the ray hops should occur. Significant errors occur in the regions of mode transitions (e.g. between the 1F2 and the 2F2). Method 22, the short path method, may produce overly pessimistic performance estimates at the distances beyond the third ionospheric hop.

APPROACH: Versions of VOACAP developed after April 1995 have Method 30 which allows the user to obtain smoothed signal power predictions for ranges of 7,000 km or greater. At these distances both Method 21 (long path model) and Method 22 (short path model) are run. Where appropriate, a distance-weighted smoothing function is applied. Method 30 was generated by making changes to Subroutine LUFFY taken from VOACAP version 93.04. The long path/short path smoothing function is applied if the path distance is equal to or greater than 7,000 km. The parameter which is smoothed is the predicted signal power which is expected to occur or be exceeded on 90 percent of the days of the month at that hour. This parameter is obtained from the median signal power prediction (S DBW) minus the dB range to the lower decile of the signal power (SIG LW) for the specified hour. The smoothing algorithm is as follows:

1) Run Method 22.
2) Is the great circle path distance >= 7,000 km?  If yes, run Method 21 also.
3) If Method 22 only, continue process using Method 22 and end.
4) If Method 21 and 22 exists, compute lower decile of signal power from median
   less range in dB to lower decile for both methods.
5) If lower decile of signal power for Method 22 >= the value from Method 21,
   continue the Method 22 process and end.
6) If the path distance >= 10,000 km, continue the Method 21 process and end.
7) If the path distance < 10,000 km, perform the following smoothing function:

Ssmooth .9 = 10log[ W * (10**(.1*SLP .9)  -  10**(.1*SSP .9)  )  +  10**(.1*SSP .9)  ]

   Where:

Ssmooth .9 = Smoothed signal power in dBW for 90% of the days

W	=  [(D - 7000)/ 3000], the weighting factor
D	=  great circle route distance in km
SLP .9	=  Signal power (dBW) at 90% reliability from Method 21, Long Path
SSP .9	=  Signal power (dBW) at 90% reliability from Method 22, Short Path.

8) Use the statistics obtained from Method 21 to compute performance factors for
   the smoothed case (i.e. range >= 7,000 but < 10,000 km and Method 21 signal power
   at 90% reliability is greater than the signal power at 90% reliability from Method 22).

DISCUSSION and CONCLUSIONS: The following discussion applies to the new Method 30 in VOACAP versions 95.0429 and later: