VOACAP Method 30. A Longpath/shortpath smoothing function
[This document is part of the help files integrated into the ITS HFBC software package.]
By George Lane and Hien Van Vo (International Broadcasting Bureau/USIA)
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 distanceweighted 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:

The short path model (Method 22) is the more rigorous solution
using a quasiray trace model for multiple ionospheric
reflections. It includes all of the ionospheric and earth
bounce losses; therefore, Method 22 should represent the worst
case transmission loss for any path length.

The long path model (Method 21) may predict that higher signal
powers are possible via ducted or forward scatter mechanisms
at distances normally associated with 3 or more ionospheric
hops (e.g. around 7,000 km). It is assumed that the weighting
factor is zero at 7,000 km and is one at 10,000 km. The short
path signal power in Watts is linearly increased by the
weighting factor times the difference in Watts of the signal
levels from the long and short path methods. This smoothing
function is applied in the transition regions from 7,000 to
10,000 km.

Under certain conditions the long path model may predict lower
signal powers than the short path model. This is usually due
to the different ionospheric control points used by the
models. The short path model is considered to be more rigorous
and its values are used at all distances when the long path
model provides lower signal power values.

The VOACAP prediction is more accurate at the lower decile of
the signal power distribution than at the median value. This
is true because the signal power distributions are often
nonGaussian and/or bimodal. The lower decile value is based
on actual measurements and not on an assumed distribution
function. Consequently, the smoothing function is applied to
the lower decile of the signal power distribution. The median
and upper decile values of the signal power are computed using
the signal distribution from the method providing for the
largest lower decile value. Also the most reliable mode
information, given in the output, is taken for the method
providing the higher signal power on at least 90 percent of
the days (i.e. lower decile value). In the case where Method
21 is controlling, the takeoff and arrival angles at the
transmit and receive terminals are given as ANGLE and ANGLER
(soon to be renamed TANGLE and RANGLE for transmit and receive
angle, respectively). When Method 22 prevails these angles are
the same value.
 Other parameters in addition to the signal power are also smoothed. The smoothed signal power is used to compute the median field strength (DBU), the median signaltonoise ratio (SNR), the required power gain (RPWRG), reliability (REL), lower and upper decile ranges of the SNR (SNR LW and SNR UP) and the signaltonoise ratio at the reliability specified by the user (SNRxx).