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1
Lunar Surface Propagation Modeling and Effects on
Communications
Dr. Shian U. Hwu1
Barrios Technology Inc., Houston, TX 77058
Matthew Upanavage 2
ERC Inc., Houston, TX 77058
and
Catherine C. Sham3
NASA Johnson Space Center, Houston, TX 77058
This paper analyzes the lunar terrain effects on the signal propagation of the planned
NASA lunar wireless communication and sensor systems. It is observed that the propagation
characteristics are significantly affected by the presence of the lunar terrain. The obtained
results indicate that the terrain geometry, signal frequency, antenna location, and lunar
surface material are important factors determining the propagation characteristics of the
lunar wireless communication systems. The path loss can be much more severe than the free
space propagation and is greatly affected by the antenna height, operating frequency, and
surface material. Signal delay could be a concern in a crater environment. The signal delay
due to multiple reflections from the lunar terrain can put a limit on the maximum data rate
that can be achieved in the lunar environment. The analysis results from this paper are
important for the lunar communication link margin analysis in determining the limits on the
reliable communication range and radio frequency coverage performance at planned lunar
base worksites.
I. Introduction
n recent years, many radio wave propagation studies were conducted using both experimental and theoretical
techniques. However, most of these studies were in support of commercial cellular phone wireless applications.
The signal frequencies are mostly at the commercial cellular and personal communications services (PCS) bands of
900 and 1800 MHz. The antenna configurations are, primarily, one on a high tower and one near the ground to
simulate communications between a cellular base station and a mobile unit.
There is great interest in wireless communication and sensor systems for lunar missions by NASA because of the
emerging importance of establishing permanent lunar human exploration bases, as shown in Figure 1. Since the
specific lunar terrain geometries and radio frequencies (RFs) are of interest to the NASA missions, much of the
published literature for the commercial cellular and PCS bands of 900 and 1800 MHz may not directly apply to the
lunar base environment. Test data from Earth terrain may not be applicable due to foliage/vegetation effects. There
are various communication and sensor configurations in a lunar base, including the communications between
astronauts, between astronauts and the lunar vehicles, and between lunar vehicles and satellites on the lunar orbits.
Also, wireless sensor systems exist among various scientific, experimental sensors, and data collection ground
stations, as shown in Figure 2.
Lunar communication systems operate in the ultra-high frequency (UHF), S-, X-, and Ka-bands. They are
different from the commercial cellular and PCS bands. This study performs multipath and propagation analysis in
the lunar environment for the wireless communication and sensor systems at NASA frequency bands. Due to the
1Sr. Engineering Specialist, Avionics Systems Analysis Section, JE-6WA, and AIAA Senior Member.
2Engineer, Avionics Systems Analysis Section, JE-6WA.
3Branch Chief, Systems Evaluation & Verification Branch, EV7.
I
26th International Communications Satellite Systems Conference (ICSSC)
10 - 12 June 2008, San Diego, CA AIAA 2008-5495
Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes.
All other rights are reserved by the copyright owner.
2
reflections and diffractions off the surrounding terrain environment, RF signals arrive at a receiver from different
directions with different strengths, time delays, and polarization. Thus, a receiver at one location may have a signal
strength that is quite different from a similar receiver located only a short distance away. When an astronaut or a
lunar vehicle moves from one location to another, significant signal fluctuations may occur depending on the
surrounding environment.
Figure 1. A typical lunar base worksite. Figure 2. Wireless communications on lunar base.
Various propagation models were developed to characterize the terrain and environment effects. There are many
empirical propagation models for predicting propagation characteristics in Earth rural and urban environments 1-6.
Most of the models are based on fitting regression curves to limited measured data, which were collected for specific
ranges of various system parameters. These models are not valid for systems with different parameters, such as
terrain type, frequencies, and antenna height and configurations, from which the data were collected. No one
propagation model can account for all parameter variations for a practical system. The limitations of the models
must be considered in applications to achieve a valid design of a wireless system.
The reflections and diffractions are dominating mechanisms in the specific terrain effects at UHF and above
frequencies. The Geometrical Theory of Diffraction (GTD) 7-12 is capable of taking into account the reflections and
diffractions off three-dimensional (3-D) lunar terrain. For detail and specific terrains and objects around the antenna,
such as astronauts and the lunar vehicle, we propose using GTD to provide more accurate and reliable results than
from an empirical propagation model. This paper presents the propagation characteristic analysis of the NASA lunar
wireless communication and sensor systems, taking into account the 3-D terrain multipath effects.
II. Computational Method
To determine the signal strength and distribution, including the lunar terrain effects, computer simulations were
performed using the computational electromagnetic technique – GTD7-12. The RF coverage may be determined from
computed signal strength results by comparing against the receiver thresholds. At high frequencies, the scattering
fields depend on the electrical and geometrical properties of the scatterers in the immediate neighborhood of the
point of reflection and diffraction. In the field computation, the incident, reflected, and diffracted fields are
determined by the field incident on the reflection or diffraction point multiplied by a dyadic reflection or diffraction
coefficient, a spreading factor, and a phase term. The reflected and diffracted fields at a field point r’, Er,d(r’), in
general, have the following form:
Er,d (r’) = Ei(r) Dr,d Ar,d(s) e-jks . (1)
where Ei(r) is the field incident on the reflection or diffraction point r, Dr,d is a dyadic reflection (Dr) or
diffraction (Dd) coefficient, Ar,d(s) is a spreading factor for reflection or diffraction, and s is the distance from the
reflection or diffraction point r to the field point r’. Dr,d and Ar,d can be found from the geometry of the structure at
reflection or diffraction point r and the properties of the incident wave there. Thus, the total fields (Etot) can be
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obtained by summing up the individual contributions of the direct field (Edir), reflected field (Eref), and diffracted
field (Edif) along the propagation paths, as shown in Figure 3,
E E E E
tot dir n
ref
n
N
m
dif
m
M
1 1
.
(2)
Etot : total field at the observation point,
Edir : direct fields from antennas,
Eref : reflected fields from plates and curve surfaces,
Edif : diffracted fields from plates and curve surfaces,
where n is the nth reflection and N is the total number of reflections. The m is the mth diffraction and M is the
total number of diffractions.
Reflector
(Plate or
Cylinder)
Transmitting
Antenna
E
d
E
r
E
i
E=E
i
+E
r
+E
d
Figure 3. The GTD field computation
-20
-15
-10
-5
0
5
10
-90 -60 -30 0 30 60 90
Angle (Degrees)
Gain (dB)
Free Space
h=2m
Figure 4. The lunar ground effects on a dipole
antenna pattern at 401 MHz.
III. Flat Ground Surface
In this section, a simple flat lunar ground surface is investigated. Various RF parameters, such as ground
material, operating frequencies, and antenna heights, are investigated for the effects on wireless signal propagation.
The signal variations due to various antenna height and lunar surface conductivity were investigated. A lossless (or
low-loss) and a lossy (or high-loss) lunar surface material were compared with free space and perfect electric
conducting (PEC) ground. For the low loss lunar material, the following parameters are assumed: Relative
Permittivity=3, Conductivity=1.E-11 S/m. For the high-loss lunar material, the Permittivity=3 and
Conductivity=1.E-1 S/m. Both the transmitter and receiver antennas are assumed to be the typical vertical half-wave
dipole. The transmitting power is normalized to 1 Watt. Two signals of UHF at 401 MHz and S-band at 2.1 GHz are
investigated. Three antenna heights are analyzed. The 2 m above ground is used for astronaut personnel
communications. The 6 m and 10 m of antenna height are assumed for the lunar vehicle wireless communication
systems.
A. Antenna Pattern
Figure 4 shows the lunar ground effects on a dipole antenna pattern at 401 MHz. The lunar ground is assumed
with Permittivity=3 and Conductivity=1.E-1 S/m. The ground reflections cause the antenna pattern’s many ripples
with peaks and valleys. Figs. 5 and 6 show the 3-D patterns with and without the lunar ground effects. The ground
effects on the antenna pattern are very significant. The lunar ground acts as a partial reflector and partial absorber to
the RF signals.
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B. Lunar Ground Material
Figs. 7 and 8 show the comparison of the computed signal strengths versus various lunar ground materials at 401
MHz. The half-wave dipole antenna and vertical polarization are assumed for transmitter and receiver. The transmit
power is normalized to 1 Watt without circuit loss. Both of the transmitting and receiving antennas are placed at the
same height of 6 m and 2 m above the ground. The free space case is without a ground effect. The PEC case is with
a perfect electric conducting ground.
The ground reflections cause the signal oscillations. The strong reflections from the conducting ground give deep
nulls. The signal signatures are similar and only slightly shift between low-loss (or lossless) and high-loss (or lossy)
lunar ground. The signal strength is about 1.5 dB lower for the high-loss ground at a range distance of 100 m. The
loss will increase at long-range distance. Note that the averaging signal strengths at long range with lunar ground
effects are lower than the free space, as shown in Figure 8. The lunar ground acts as a partial reflector and partial
absorber to the RF signals.
Figure 5. The free space dipole antenna pattern. Figure 6. The dipole antenna pattern with lunar
ground effects
Ant Height=6m, Various Ground
Material
-65
-60
-55
-50
-45
-40
-35
10 30 50 70 90 110
Distance (meters)
Signal Power (dBm)
Free Space
PEC
Lossless
Lossy
Figure 7. The received UHF signal power
oscillates with various lunar ground materials at
an antenna height of 6 m.
Ant Height=2m
-65
-60
-55
-50
-45
-40
-35
10 30 50 70 90 110
Distance (meters)
Signal Power (dBm)
Free Space
PEC
Lossless
Lossy
Figure 8. The received UHF signal power
oscillates with various lunar ground materials at
an antenna height of 2 m.
C. Antenna Height
Figure 9 shows the comparison of the signal strengths for antenna at various heights above the lunar ground
surface. The averaging signal strengths increase with increasing antenna heights. As the curves show, higher
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antennas lead to smaller path loss since less blockage and better ground clearance produce closer to a line-of-sight
free space condition. Therefore, there are advantages to mounting the antenna at a higher position to reduce the path
loss due to the lunar ground effects.
Various Ant Height, Lossy Ground,
Permittivity=3 Conductivity=1E-1
-65
-60
-55
-50
-45
-40
-35
10 30 50 70 90 110
Distance (meters)
Signal Power (dBm)
Free Space
h=10m
h=6m
h=2m
Figure 9. The received UHF signal power
oscillates due to lunar ground effects with various
antenna heights.
Path Loss at f=401MHz
Permittivity=3, Conductivity=0.1
40
50
60
70
80
90
0 40 80 120 160 200
Range Distance (Meters)
Path Loss (dB)
Free Space
Height=2m
Figure 10. The path loss of the UHF signal is
greater than free space at range >50 m.
D. Frequency Effects
Figure 10 shows the comparison of the computed UHF signal path loss with and without the lunar ground
effects. The ground effects cause a path loss that is more severe than in the free space at a range greater than 50 m.
The transmitter and receiver antennas are vertical dipoles at 2 m above ground. The propagation loss is 10 dB more
than free space at 200 m. Figure 11 shows the comparison of the computed S-band signal path loss with and without
the lunar ground effects. The ground effects cause a path loss that is 5 dB less than in the free space at a range of
100 m due to the constructive interferometer effects from the ground reflection. However, the path loss can be worse
than the free space at long-range distance, as shown in the following section.
Propagation Loss at f=2.1GHz
Permittivity=3, Conductivity=0.1
40
50
60
70
80
90
0 40 80 120 160 200
Range Distance (Meters)
Path Loss (dB)
Free Space
Height=2m
Figure 11. The path loss of the S-band signal is
less than free space due to the lunar ground effects
at 80-200 m.
Lunar Path Loss
Permittivity=3, Conductivity=0.1
40
60
80
100
120
140
0 100 200 300
Range Distance (Meters)
Path Loss (dB)
UHF
S-band
Figure 12. The path losses of UHF and S-band
signals approach each other at distances great
than 180 m.
6
Figure 12 shows the comparison of the computed path loss between UHF and S-band signals. The path losses of
UHF and S-band signals approach each other at distances great than 180 m. The path loss is independent of
frequencies at long-range distances in lunar environment when the antenna height is relative smaller than the range
distance. The path loss in the lunar environment is contradicted to the free space path loss. In free space, path loss is
a strong function of frequency.
E. Path Loss
The computed results indicate that when the range distance is small, the received signal strengths oscillate with
peaks and nulls due to the interferometer from the ground reflection. In the short-range region, path loss increases in
proportion to 1/R2, as in free space. However, when the range distance is large, path loss increases in proportion to
1/R4, as shown in Figs. 13 and 14. The break point is at 75m for the UHF frequency and is at 365m for the S-band
frequency. This is because the reflection coefficiency is approaching -1 at an angle of near-grating incidence that
causes cancelling between direct field and reflected field. The breakpoint is where the path loss transitions from
square law to fourth law.
Lunar Path Loss at f=401MHz
Permittivity=3, Conductivity=0.1
40
60
80
100
120
140
10 100 1000 10000
Range Distance (Meters)
Path Loss (dB)
Free Space
Height=2m
1/R^4
Figure 13. The UHF signal path loss increases in
proportion to 1/R2(blue line) in short range and to
1/R4(green line) in long range.
Lunar Path Loss at f=2.1GHz
Permittivity=3, Conductivity=0.1
40
60
80
100
120
140
10 100 1000 10000
Range Distance (Meters)
Path Loss (dB)
Free Space
Height=2m
1/R^4
Figure 14. The S-band signal path loss increases in
proportion to 1/R2(blue line) in short range and to
1/R4(green line) in long range.
The computed results indicate that the breakpoint is a function of frequency and antenna height, as shown in
Figs. 15 and 16. For the UHF systems, with antenna height of 2 m, the breakpoint is taking place at about 50 m. For
the S-band signals, it is at about 320 m. The computed data indicate that the pass loss decreases with increasing
antenna height, as shown in Figs. 15 and 16, at a long-range distance that is greater than 60 m for UHF and 300 m
for S-band. The breakpoint, where the path loss turns to fourth power, is also moved farther with a higher antenna
mounting location. However, deep nulls are observed in Figure 16 for higher antenna height due to the signal
cancelling from the ground reflections.
The computed path loss data also show the independence of the frequency at long-range distances, as shown in
Figure 17. Note that the free space path loss is a strong function of frequency. Thus, in the circuit margin
calculations for the lunar wireless systems, the commonly used free space loss formula cannot be used in the long-
range communication link calculations due to the lunar ground effects. Otherwise, significant errors will occur in the
computed link margin.
7
Lunar Path Loss at f=401MHz
Permittivity=3, Conductivity=0.0001
40
60
80
100
120
140
10 100 1000 10000
Range Distance (Meters)
Path Loss (dB)
Free Space
H=2m
H=6m
H=10m
Figure 15. The UHF signal path losses vs. various
antenna heights.
Lunar Path Loss at f=2.1GHz
Permittivity=3, Conductivity=0.0001
60
80
100
120
140
10 100 1000 10000
Range Distance (Meters)
Path Loss (dB)
Free Space
H=2m
H=6m
H=10m
Figure 16. The S-band signal path losses vs.
various antenna heights.
Lunar Path Loss
Permittivity=3, Conductivity=0.001
40
60
80
100
120
140
10 100 1000 10000
Range Distance (Meters)
Path Loss (dB)
UHF
S-band
Figure 17. The path loss is independent of
frequency at long-range distances. Fig. 18. The Meteor Crater modeled in the
analysis.
IV. 3-D Lunar Crater Terrain
Craters are common in lunar terrain. The propagation in a crater environment was investigated. Signal strengths
were computed, including reflections and diffractions from the 3-D terrain model. A typical crater was used to
simulate a lunar crater, as shown in Figure 18. Terrain material was modeled with permeability and conductivity that
resemble lunar soil. The GTD modeling technique is computationally efficient for electrical large and complex 3-D
terrain models. The lunar terrain was modeled with complex dielectric constant for typical lunar soil material. Lunar
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vehicle and astronauts can be included in the model. Various types of antennas can be modeled. Multiple reflections
and diffractions are included in the signal strength computations, as shown in Figure 19. Signal strength can be
mapped in a specified region, including the shadow region, for RF coverage analysis13.
A. Signal Strengths
As shown in Figure 19, significant reflections exist in a crater environment. Figure 20 shows sample results in
the crater environment. The transmitter was placed at the edge of the crater ring. A receiver grid covered the bottom
of the crater’s surface. The receiver grids were placed in reference to terrain elevations. The shadowing and
interferometer effects are observed in the simulation results. If the transmitting antenna is located away from the
ring, the crater terrain can cause significant signal drop due to shadowing, as illustrated in the blue color region in
Figure 20. The most challenges wireless network applications will be a video link in crater environment. The higher
data rates wireless channels will deteriorate with non-line-of-sight (NLOS) fading. Simulation results indicate that
large throughput variations are possible in crater environment with NLOS channel conditions. These variations may
severely affect the quality of delay-sensitive real-time applications.
Figure 19. Propagation paths for receiver points
at various locations. Figure 20. Signal strength in horizontal receiver
grid through crater.
B. Signal Delay
Reflection and diffraction of the radio signal result in multiple copies of the transmission signal being received
with different delays and strengths. The signals arriving at the receiver consists of components from the direct path,
multiple reflected paths, scattered energy, and diffracted propagation paths, as illustrated in Figure 19. These signals
have different delay spreads, attenuation, polarizations, and stability relative to the direct path. This is known as a
multipath channel.
The lunar terrain propagation environment can be significantly more delay than in office environment. Craters
are common in lunar terrain and with large spatial separation. This causes strong reflective and diffractive multipath
effects with long delays 8, 9. The resulting delay spread typically is quite significant in long range propagations.
The signal delay is a major concern from the multipath for a high data rate wireless system. Many field tests
identify multipath propagation, rather than lack of received signal strength, to be the system limiting factor. This
degradation is a direct result of self-interference arising from signal reflection and diffraction in multipath
environments. Due to the reflections off the lunar terrain, many indirect rays reach the receiver at longer travel time
via longer indirect paths than the direct ray via shorted direct path, as shown in Fig. 21. The receiver may see a mix
of delayed previous symbol and current symbol. This leads to intersymbol interference (ISI) at the receiver which
can cause bit errors.
9
Figure 21. The reflected signals can be delayed
and cause ISI for high data rate systems.
-100
-90
-80
3650 3700 3750 3800 3850 3900
Arrival Time (ns)
Signal Power (dBm)
Direct Rays
Reflected Rays
Figure 22. The reflected signals are delayed by
about 160 ns.
To decrease the probability of ISI, the symbol length can not be shorter than the signal delay. A long symbol
length will limit the maximum data rate can be transmit by the wireless systems. Fig. 22 shows the signal delays are
about 160 nano seconds (ns) for the selected case. The delay specification for many commercial 802.11g products is
70ns or less. This delay may put a limit on the maximum data rate can be achieved in the lunar crater environment.
Emerging standards such as IEEE 802.16e and 802.20 are designed for larger range coverage and may tolerate
longer delay.
V. Conclusion
Propagation characteristics and signal distribution are the essential parameters for wireless network planning and
systems performance analysis. This paper presents the propagation analysis of the lunar wireless communication and
sensor systems taking into account the 3-D terrain multipath effects. It is observed that the propagation
characteristics are significantly affected by the presence of the lunar terrain.
The obtained results indicate that terrain geometry, signal frequency, antenna location and lunar surface material
are the important factors affecting the propagation characteristics of the lunar wireless systems. The antenna pattern
is distorted due to lunar ground effects. Lunar ground also causes higher propagation loss and lower signal strength
than in free space. Raising antenna height improves signal levels. Data show a higher path loss for higher frequency
signals at short range. However, the path loss is frequency independent at long range due to the ground effects. The
crater terrain is common on the lunar surface, which can cause significant signal drop due to shadowing. Signal
delay could be a concern in a crater environment. The signal delay due to the lunar terrain can put a limit on the
maximum data rate that can be achieved in the lunar environment.
The path loss in the lunar environment can be much more severe than the free space propagation and is greatly
affected by the antenna height, operating frequency, and surface material. Test data from Earth terrain may not be
applicable due to foliage/vegetation effects. The results from this paper are important for the lunar wireless system
link margin analysis to determine the limits on the reliable communication range, achievable data rate, and RF
coverage performance at planned lunar base worksites.
Acknowledgment
The study described in this paper was carried out under contracts with Johnson Space Center of the National
Aeronautics and Space Administration (NASA).
10
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