The S-Band radio frequency (RF) link between the Merritt Island (MILA) Tracking Station and the Space Shuttle launch pads is a critical communication path for prelaunch and launch operations. The proposed siting of the Center for Space Education (CSE) at the Visitor Center required a study to avoid RF line-of-sight blockage and reflection paths. The study revealed the trees near MILA's 9-meter (9-M) antennas are obstructing the optical line-of-sight. The studies found diffraction is the main propagation mechanism. This paper describes a link model based on the Geometric Theory of Diffraction

Caroglanian, Armen

Pellerano, Fernando A.

Shama, Dale D.

English

NASA Technical Reports Server

N94-2i340
AN IN-DEPTH ANALYSIS AND MODELLING OF THE SHUTTLE TO MILA
S-BAND TELEMETRY LINK
Armen Caroglanian
Fernando A. Pellerano
NASA Goddard Space Flight Center
Telecommunication Systems Branch
Greenbelt, MD 20771
Dale D. Shama
Loral Aerosys
Beltsville, MD 20705
Summary- The S-Band radio frequency (RF) link between the Merritt Island (MILA)
Tracking Station and the Space Shuttle launch pads is a critical communication path for pre-
launch and launch operations. The proposed siting of the Center for Space Education (CSE)
at the Visitor Center required a study to avoid RF line-of-sight blockage and reflection paths.
The study revealed the trees near MILA's 9-meter (9-M) antennas are obstructing the optical
line-of-sight. The studies found diffraction is the main propagation mechanism. This paper
describes a link model based on the Geometric Theory of Diffraction.
1.0 Introduction
The S-Band radio frequency (RF) link between the Merritt Island (MILA) Tracking
Station at the Kennedy Space Center (KSC) and the Space ShuttIe launch pads is a critical
communication path for pre-launch and launch operations. The Visitor's Information Center
(VIC) is located near the lines-of-sight. As new buildings are added to the VIC, care has
been taken to avoid RF line-of-sight blockage and reflection paths. The proposed siting of
the Center for Space Education (CSE) at the VIC required extensive theoretical and
experimental studies [1], [2], [3].
The work, performed by NASA, Bendix, Lincom and ECAC, identified scattering from
buildings, vegetation, and the launch pad as probable causes of link degradation. In addition,
weather was found to be a factor in altering the RF propagation. Field probes in front of the
MILA 9-M antennas determined the trees in the near-field were placing a severe illumination
taper on the antenna aperture. The modeling of the RF perturbation caused by the trees is the
focus of this report.
PI_CCliNN@ PAGE BLAt'CK. NOT FtLMtED
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https://ntrs.nasa.gov/search.jsp?R=19940016867 2020-06-16T18:51:05+00:00Z
2.0 RF measurements
Currently, an optical line-of-sight does not exist between the 9-M antennas and the
launch pads. Maximum signal strength at MILA is achieved when the 9-M antennas are
pointed at the top edge of the forest. The maximum signal pointing angle is above the line-
of-sight angle, meaning the top edge of the trees appears to be a diffraction source. Previous
studies 1"4] have modelled this effect as knife-edge diffraction. Multipath, however, has also
been detected. Improved test procedures to measure the influence of diffraction and multipath
individually were conducted during a test trip to MILA in May 1992. Data analysis showed
that, although multipath was detected, diffraction was a stronger mechanism [5]. A
theoretical forest attenuation model developed George Washington University was also
employed [6]. The model determined the forest was "hard". That is, the average attenuation
is high, therefore multipath effects will be minimal. Using the measurements and the model,
it was determined that link maintenance analysis would be based on diffraction models.
3.0 Diffraction Theory and Modelling
The MILA diffraction model is based on the Geometric Theory of Diffraction [7].
The model has two modes of operation. One mode calculates RF energy incident at a given
point on an antenna's surface in the presence of blockage. The second mode calculates the
total received RF power of a reflector antenna due to either direct (free-space), diffracted, or
the combined RF energy.
The RF energy originates from an omnidirectional power source at the launch pad and
may be partially blocked by a perfectly-conducting knife-edge obstacle (see Figure 1). This
set-up closly parallels the actual conditions at MILA. In the MILA link, the polarization of
the transmitted signal is circular. The MILA diffraction model, however, utilizes linear
polarization. Circular polarization can be modeled by properly phasing and summing the two
orthogonal linear received electric fields.
218
MILA 9 m REFLECTOR
SHUTTLE 1")( ANTENNA PHASED ARRAY EQUIVALENT
"_]_ : #' I z
' ......
x _ D' ;rl-_ O ,-1
Figure 1 MILA diffraction model geometry.
The received electric field at the output of the 9-M antenna may be calculated on the
basis of the following development. The incident (i.e. non integrated) direct E field is found
as,
Eo(h ) =. Z(h) j" e. -Jt'R (1)
4nR
where h = vertical height on the 9-M antenna aperture
R = distance from the transmit antenna to the 9-M antenna
k = the wavenumber = 2n/'A
Z(h) = 0, blocked
= 1, unblocked
Function Z(h) determines, by the given geometry, whether the transmitted energy is blocked
by the tree-line or not. The incident diffracted E field at the 9-M antenna is found as,
219
E_h) = _ * e-j_'* CDCH *A e-p'P
4xp /
where p' = distance from the transmit antenna to diffraction edge
p = distance from diffraction edge to the 9-M antenna aperture
CDCH = diffraction coefficient for vertical polarization
(or CDCS = diffraction coefficient for horizontal polarization)
A = spatial attenuation function for diffracted power.
The diffraction coefficient CDCH, a complex quantity, is generated from an expression
dependant upon the incidence angle _', the reflected angle of diffraction (_, the distance from
the diffracting edge to the source, D', the distance from the diffracting edge to the receive
antenna, D, and the assumption that the diffracting edge is a perfectly-conducting half-plane.
The function A, represents the spatial attenuation for the diffracted field, and is given by,
,4 = (3)
P (P/+P)
The received E field is found as a function of the incident signal integrated over the
gain function of the receive antenna aperture. The model assumes no E field variation in the
horizontal direction of the aperture. This is a valid assumption because the direct and
diffracted fields incident on the antenna are planar and cylindrical fields, respectively. The
gain function of the receive antenna is generated by a separate reflector antenna model [8].
The model breaks up the aperture into horizontal segments of width ES _. and calculates the
complex values for the electric field (far field) on the aperture. An equivalent field for the
segment is generated, thus the 9-M antenna gain model becomes a phased array with vertical
dependance only. This simplification greatly reduces the computation time for the model.
For the present study, an optimal value of ES=I was used. The antenna may also be pointed
at an angle 0 with respect to the horizontal plane. The received (integrated) field, E, can be
found as,
220
-- [Eo,+Ed,]
k
where Hto _ = height of the forest edge
D = distance of the forest edge to the 9-M antenna
G(h) = gain function of the 9-M antenna
Eok -- incident direct field (equation 1)
E,_ = incident diffracted field (equation 2)
(4)
It is mathematically possible to analyze the contributions of the direct and diffracted
fields independently. All cases can be analyzed as a function of blockage height and distance
from the antenna, signal frequency, or antenna pointing angle. Both the magnitude and phase
relationships are calculated for each case and can be plotted in terms of electric field or
power.
4.0 Simulation Results and Analysis
The first check of the GTD software was the calculation of the 9-M antenna receive
elevation pattern. Figure 2 shows the comparison of the experimental and theoretical results.
The peak receive signal, as expected, occurs when the 9-M antenna is pointed to an elevation
angle corresponding to the closest tree-line. The experimental and calculated patterns show
good agreement out to the first sidelobe. The slightly higher than expected receive energy at
low elevation angles may be due to forest multipath and/or ground reflections.
Figure 3 shows the MILA 9-M received energy for a simulated forest being trimmed
in height. The 9-M received energy contributions for direct, diffracted, and total signal are
shown. For the current blockage case, the trees near the 9-M are at a height of 45 ft. The
trees would have to be reduced in height to 30 feet, all the way out to the VIC, to recover the
direct signal. Figure 4 shows the results of trimming the forest back and leaving the closest
trees at a height of 45 feet. The trees would have to be trimmed back nearly 5,000 feet for
221
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....................i ............i ...i .............i........i
°, ,. _ ..._ .... . .... r ......... , ......... •......... _, ........ J ....
" :lvleasuced Doto. i !
: : > • . : :
i ! .-"i i ! : : :
•, ,' ......... :.../ ..... : . ! ......... : ......... :......... _ ........ . ....
.... ......... .. ............ : ......... :......... - ........ -;....
C: ' ! ::
i " : / : "
: • .g • . ,
...................... ; ................................................................. - ........ _ ....
, _ 1 , i I .... _ J
0 2 3
MIL.A 9-meter Antenno Pointing Angle (degrees)
Figure 2 MILA 9-M antenna pattern.
1
4
,,. ......... ; ........ ; ......... , ......... , ......... ; ........ . ......... , ................................
..: __ "__: _, _--, . ..... • ........ ; ......... :......... .......... , ........ , ....
.._°
: _ Direct
":- _ Diffrocted
: _ Direct + Diffracted
I ; I ;
0 10
Figure 3
.................................................. i ..............
, = Ii I
20 30 40 50
MILA Forest Height (feet)
MILA 9-M received power vs. forest height
222
f13
'ID
O
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r"
ID3
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t'V"
0
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..... v .-..i
i
i
[ • r i
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.......i !iiiiiii, liiiiiii
, _ _ , , _ I
0000 20000 30000 40000 50000
Forest Edge Distance from MILA g-meter antenna (feet)
Figure 4 MILA 9-M received power vs. forest edge distance.
the direct signal to match the diffracted signal level. This approach would result in a mile
long deforested path.
5.0 Forest Characterization
To apply the model to the MILA forest, the forest height and tree locations are
needed. The bulk of the trees are located along the swampy mile-long path between MILA
and the VIC. During the MILA link studies, surveyors targeted selected tree-lines and
obtained a few height/position measurements.
A technique known as stereoscopic photography is being used to map out the heights
and positions of the trees. Photos are taken of the trees from the vantage point of a cherry
picker located at MILA. Two pictures, separated 50 to 100 feet, are taken at precisely known
locations. The photos are then scanned into a computer. The location of the Vehicle
Assembly Building and rockets at the VIC are used as reference objects for the analysis. The
223
tree imagepositionandimageshift betweentheright andleft photosis usedby a spreadsheet
programto calculatethe tree'sheightandpositionon map.
6.0 Conclusions
The GTD Diffraction Model is a valuable tool for modelling the MILA/Shuttle S-Band
RF Link. Tree cutting at MILA is extremely difficult due to strict environmental regulations.
The diffraction model is currently being used to evaluate various tree cutting scenarios at
MILA. The preferred approach is selective cutting. With the help of the GTD Diffraction
Model, the benefits of a potential tree cutting can be evaluated and justified.
7.0 References
[1] W.C. Peng, "CLASS MILA Interference Analysis", NASA/GSFC Technical
Report TR-8707-03, 1991.
[2] A. Caroglanian, editor, "MILA/Pad 39 S-Band Shuttle Support Link RF Testing
and Launch Monitoring Study Report", NASA/GSFC Technical Memorandum,
1991.
[3] D.Kenney, et al, "Analysis of MILA 9M Antenna to Orbiter S-Band PM
Subsystem", ECAC Engineering Memorandum, EM-91-DN-001.
[4] R.F.White, et al, "Engineering Considerations for Microwave Communications
Systems", GTE Lenkurt, 1970, p.35.
[5] F.A.Pellerano, "A Study of the Effects of Vegetation on the Performance of S-
Band Radio Frequency Links", NASA/GSFC Professional Intern Program
Report, May 1991.
[6] S.S. Seker, "Determination of an Electromagnetic Model for Vegetation in
Support of the MILA/Shuttle RF Link Study", GWU Technical Report, 1991.
[7] C.A.Balanis, "Advanced Engineering Electromagnetics", Wiley & Sons
Publishers, New York, 1989.
[8] D.D.Shama, "MILA Diffraction Model", NASA/GSFC Internal Memorandum,
March 1993
224


An in-depth analysis and modelling of the Shuttle to MILA S-band telemetry link

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