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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
ABSTRACT
The Miniature Radio Frequency (Mini-RF) system is manifested on the
Lunar Reconnaissance Orbiter (LRO) as a technology demonstration and an extended
mission science instrument. Mini-RF represents a significant step forward in space borne
RF technology and architecture. It combines synthetic aperture radar (SAR) at two
wavelengths (S-band and X-band) and two resolutions (150 m and 30 m) with
interferometric and communications functionality in one lightweight (16 kg) package.
Previous radar observations (Earth-based, and one bistatic data set from Clementine) of the
permanently shadowed regions of the lunar poles seem to indicate areas of high circular
polarization ratio (CPR) consistent with volume scattering from volatile deposits (e.g.
water ice) buried at shallow (0.1-1 m) depth, but only at unfavorable viewing geometries,
and with inconclusive results. The LRO Mini-RF utilizes new wideband hybrid
polarization architecture to measure the Stokes parameters of the reflected signal. These
data will help to differentiate “true” volumetric ice reflections from “false” returns due to
angular surface regolith. Additional lunar science investigations (e.g. pyroclastic deposit
characterization) will also be attempted during the LRO extended mission. LRO’s lunar
operations will be contemporaneous with India’s Chandrayaan-1, which carries the
Forerunner Mini-SAR (S-band wavelength and 150-m resolution), and bistatic radar (S-
Band) measurements may be possible. On orbit calibration, procedures for LRO Mini-RF
have been validated using Chandrayaan 1 and ground-based facilities (Arecibo and
Greenbank Radio Observatories).
TABLE OF CONTENTS
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CHAPTER NO. TITLE PAGE NO
ABSTRACT iv
LIST OF FIGURES vi
LIST OF TABLES vii LIST OF ABBREVIATIONS viii
1. INTRODUCTION 1
2. Why we require Mini RF technology 2
3. What is Mini RF Technology 3
4. Comparison of Mini RF Technology
With other Radar Systems 4
5. Mini-RF Instrument Description 5
5.1Hybrid-polarity architecture 7
5.2 Antenna 10
5.3 Transmitter 11
5.4 Interconnect Module 14
5.5 Receiver 14
5.6 Control Processor 14
6. Calibration 15
7. Data analysis and Interpretation 16
8. Stokes Parameters as clear evidence of lunar ice 18
9. Applications 19
10. Conclusion 20
11. References 21
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LIST OF FIGURES
FIGURE NO. NAME PAGE NO
1 Dual scattering caused by ice or roughness 2
2 LRO Mini-RF system functional block diagram 6
3 Hybrid-polarity architecture 7
4 Mini-RF antenna design 10
5 Mini-RF transmitter block diagram 12
6 Mini-RF Microwave Power Module
Traveling Wave Tube (MPM/TWT) 13
7 Mini-RF calibration strategy 17
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
LIST OF TABLES
TABLE NO NAME PAGE NO
1 Radar System Comparisons with
Mini RF Technology 4
2 Communications System Comparison with
Mini RF Technology 4
3 Significant and Derivative of stock parameters 18
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
LIST OF ABBREVIATIONS
LRO Mini-RF- Lunar Reconnaissance Orbiter Miniature Radio Frequency
NASA- National aeronautics and space administration.
CPR- High Circular Polarization Ratio
SAR- Synthetic Aperture Radar
MPM- Microwave Power Module
FPGA - Field Programmable Gate Array
OC- Opposite Sense
SC- Same Sense
TWT - Traveling Wave Tube
IM- Interconnect Module
QDWS- Quadrature Digital Waveform Synthesizer
ART - Arecibo Radio Telescope
GBT - The Green Bank (Radio) Telescope
POC- Payload Operations Center
PRF- Pulse Repetition Frequency
UAV- Unmanned Airborne Vehicles
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 1
Introduction
The Miniature Radio Frequency (Mini-RF) system is manifested on the Lunar
Reconnaissance Orbiter (LRO) as a technology demonstration and an extended
mission science instrument. Mini-RF represents a significant step forward in space
borne RF technology and architecture. It combines synthetic aperture radar (SAR) at
two wavelengths (S-band and X-band) and two resolutions (150 m and 30 m) with
interferometric and communications functionality in one lightweight (16 kg) package.
Previous radar observations (Earth-based, and one bistatic data set from Clementine)
of the permanently shadowed regions of the lunar poles seem to indicate areas of high
circular polarization ratio (CPR) consistent with volume scattering from volatile
deposits (e.g. water ice) buried at shallow (0.1–1 m) depth, but only at unfavorable
viewing geometries, and with inconclusive results. The LRO Mini-RF utilizes new
wideband hybrid polarization architecture to measure the Stokes parameters of the
reflected signal. These data will help to differentiate “true” volumetric ice reflections
from “false” returns due to angular surface regolith. Additional lunar science
investigations (e.g. pyroclastic deposit characterization) will also be attempted during
the LRO extended mission. LRO’s lunar operations will be contemporaneous with
India’s Chandrayaan-1, which carries the Forerunner Mini-SAR (S-band wavelength
and 150-m resolution), and bistatic radar (S-Band) measurements may be possible.
Data quality and instrument characteristics suggest that hybrid polarity is highly
desirable for future exploratory radar missions in the Solar system. The new
technologies being qualified on LRO Mini- RF include: Microwave Power Module
(MPM) based multi-frequency transmitter, wideband dual-frequency panel antenna, all
digital receiver and waveform synthesizer incorporating field programmable gate array
(FPGA) and analog-to-digital conversion at 1 GHz sampling. The Mini-RF parts
qualification program, which included commercial technology, allowed innovative
components to gain space qualification.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 2
Why we require Mini RF technology
The LRO Mini-RF payload will address key science questions during the LRO
primary and extended missions. These include exploring the permanently shadowed
Polar Regions and probing the lunar regolith in other areas of scientific interest. The
nature and distribution of the permanently shadowed polar terrain of the Moon has
been the subject of considerable controversy. Previous earth based observations uses
that when an incident circularly polarized radar wave is backscattered off an interface,
the polarization state of the wave changes. For most surfaces, this leads to a return
with more of an “opposite sense” (OC) polarization than a “same sense” (SC)
polarization, so the circular polarization ratio (CPR=SC/OC) remains less than 1.
However, in weakly absorbing media (such as water ice) the radar signal can undergo
a series of forward scattering events off small imperfections in the material, each of
which preserves the polarization properties of the signal . Those portions of the wave
front that are scattered along the same path but in opposite directions combine
coherently to produce an increase in the SC radar backscatter .This coherent
backscatter effect leads to large returns in the same sense (SC) polarization and values
for CPR that can exceed unity. Note that CPR > 1, while diagnostic of water ice, is not
a unique signature for water ice. Very rough, dry surfaces have also been observed to
have CPRs that exceed unity. The CPR may increase in such regions due to double‐bounce scattering between the surface and rock faces. In order distinguish between
true ice and false ice mini RF technology will be helpful.
Figure .1 Dual scattering caused by ice or roughness
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 3
What is Mini RF Technology
The Mini-RF architecture is new. The hybrid-polarity design (transmitting circular
polarization and receiving coherently two orthogonal linear polarizations) provides
data sufficient to measure the 2 × 2 covariance matrix of the backscattered field, which
in turn lead to the four Stokes parameters. Analysis of these data, either by standard
radar astronomy methods or by applying matrix decomposition techniques, extracts all
information available in the radar reflections, thus providing a sharper tool than CPR
alone to help differentiate between “true” (ice) and “false” (regolith blockiness) lunar
returns. Mini-RF can operate as a dual-frequency, hybrid-polarity imaging radar
designed to collect information about the scattering properties of the permanently dark
areas near the lunar poles at optimum viewing geometry. As the LRO Mini-RF system
probes the lunar regolith at two frequencies (S-band and X-band) it will provide
additional information on the physical properties of the upper meter or two of lunar
surface. Under the proposed observational constraints, Mini-RF can identify areas of
high CPR (∼1), which could be caused by ice deposits. Areas that do show high CPR
can be analyzed with greater sensitivity through their backscattering features. It is
hypothesized that “ice” and “regolith” will have differentiable characteristics as seen
through their respective Stokes parameters at two wavelengths. When supported by
Chandrayaan-1 and other LRO data (e.g. neutron spectroscopy, shadow and lighting,
roughness and surface texture, thermal environment), the LRO Mini-RF measurements
should provide more conclusive evidence as to the likelihood that ice deposits occur in
permanently shadowed areas. Mini RF technology will be also be helpful to find the
several volatile deposits and lot of valuable minerals such as silver ,mercury, helium
on the portions of moon where there is no sunlight for several lakhs of years.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 4
Comparison of Mini RF Technology with other Radar
SystemsTable 1. Radar System Comparisons with Mini RF Technology
Table 2. Communications System Comparison with Mini RF Technology
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 5
Mini-RF Instrument Description
The Mini-RF observations are made possible within the mass and power constraints
imposed by LRO via application of a number of technologies. Two key technologies
are a wideband Microwave Power Module (MPM) based transmitter and a lightweight
broadband antenna and polarization design.
The Mini-RF Instrument is comprised of the following elements:
(1) Antenna,
(2) Transmitter,
(3) Digital receiver/quadrature detector waveform synthesizer,
(4) Analog RF receiver,
(5) Control Processor,
(6) Interconnection module
(7) Supporting harness, RF cabling, and structures.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Figure.2 The LRO Mini-RF system functional block diagram
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
5.1Hybrid-polarity architecture
Figure 3. Hybrid-polarity architecture
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
The mini RF architecture is unique in planetary radar; it transmits right circular
polarization radiation and receives the horizontal (H) and vertical (V) polarization
components coherently, which are then reconstructed as Stokes’ parameters during the
data processing step. Both the communications and the radar astronomical objectives
impose a requirement for circular polarization on the transmitted field. Conventional
radar that would measure CPR then would have to be dual-circularly polarized on
receiver. The hybrid-polarity approach provides weight savings by eliminating
circulator elements in the receiver paths, which reduces mass, increases RF efficiency,
and minimizes cross-talk and other self-noise aspects of the received data. The H and
V signals are passed directly to the ground-based processor. It is well known that the
Stokes parameters comprise a full characterization of the backscattered field. The
values of the four Stokes parameters do not depend on the choice of receiver
polarization, so this architecture minimizes hardware while maintaining full science
value. The result provides significant advantages over the conventional “CPR-
measuring” dual-circular-polarized approach, yet the radar is simpler. The use of
possible Stokes parameter-based products (e.g. CPR, degree of- depolarization,
degree-of-linear-polarization, and phase “double bounce”) have a number of
significant advantages over traditional radar systems: less hardware is needed,
resulting in fewer losses and a “cleaner,” simpler flight instrument. The signal levels
are comparable (within ∼2 dB) in both channels allowing relatively relaxed
specifications on channel-to channel cross-talk and more robust phase and amplitude
calibration. The processor has a direct view through the entire receiver chain;
including the antenna receives patterns and other radar parameters (e.g., gain and
phase). These parameters are applied in processing “Levels” (Level 0, 1) which
correspond to successive data processing stages. The design allows selective Doppler
weighting to maximize channel–channel coherence (e.g., reduce the H & V beam
mismatch). As CPR is less sensitive to channel imbalance by at least a factor of 2 with
respect to explicit RCP/LCP, Stokes parameter-based backscatter decomposition
strategies can help distinguish “false” from “true” high CPR areas. The hybrid-polarity
approach provides weight savings by eliminating circulator elements in the receiver
paths, which reduces mass, increases RF efficiency, and minimizes cross-talk and
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
other self-noise aspects of the received data. The H and V signals are passed directly
to the ground-based processor. It is well known that the Stokes parameters comprise a
full characterization of the backscattered field. The values of the four Stokes
parameters do not depend on the choice of receiver polarization, so this architecture
minimizes hardware while maintaining full science value.
The result provides significant advantages over the conventional “CPR measuring”
dual-circular-polarized approach, yet the radar is simpler. The use of possible Stokes
parameter-based products (e.g. CPR, degree of-depolarization, degree-of-linear-
polarization, phase “double bounce”) have a number of significant advantages over
traditional radar systems: less hardware is needed, resulting in Fewer losses and a
“cleaner,” simpler flight instrument. The signal levels are comparable within 2 dB)
in both channels allowing relatively relaxed specifications on channel-to channel
cross-talk and more robust phase and amplitude calibration. The processor has a direct
view through the entire receiver chain; including the antenna receives patterns and
other radar parameters (e.g., gain and phase). These parameters are applied in
processing “Levels” (Level 0, 1) which correspond to successive data processing
stages. The design allows selective Doppler weighting to maximize channel–channel
coherence (e.g., reduce the H & V beam mismatch). As CPR is less sensitive to
channel imbalance by at least a factor of 2 with respect to explicit RCP/LCP, Stokes
parameter-based backscatter Decomposition strategies can help distinguish “false”
from “true” high CPR areas. Compact polarimetric encompasses those options that fall
between dual-polarized and quad-pole SARs. Compact polarimetric radars transmit on
only one polarization, and receive on two orthogonal polarizations, retaining their
relative Phase. In the radar remote sensing world assumed to include only linearly
polarized systems, coherent dual-polarimetric imaging radar was disregarded.
However, if alternative transmit polarizations (such as circular or 45_ linear) are
considered, then compact polarimetric radars deserve recognition as a potentially
important SAR option. The major motivation for compact polarimetric is to strive for
quantitative backscatter classifications of the same finesse as those from a fully
polarized system, while avoiding the principal disadvantages (mass, power, and
limited coverage) associated with a quad-pole SAR.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
5.2 Antenna
An “egg crate” antenna allows a broadband, dual-frequency design with a single
antenna panel, without any deployable mechanisms (e.g. feeds) while also meeting
stringent weight and volume constraints. The elements are sized to allow a 3:1
frequency range. Each element incorporates radiators and physical phasing combines
their power. The thermal design, materials selection, manufacturing, and test
qualification heritage of single-frequency Chandrayaan Mini-SAR antenna was
applied to the dual frequency LRO Mini-RF unit. Because of this heritage, the Mini-
RF antenna is robust and lightweight (4 kg) while satisfying all technical
requirements.
Figure.4 Mini-RF antenna design
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
5.3 Transmitter
The LRO Mini-RF transmitter takes full advantage of the capabilities of the wideband
antenna. The transmitter is the first implementation of Microwave Power Module
(MPM) technology on a long-duration spaceflight, which affords a significant
breakthrough in available bandwidth and power efficiency with reduced mass as
compared to previous traveling wave tube (TWT) systems. The MPM combines a
solid state RF driver/preamplifier with a traveling wave tube amplifier, a hybrid
approach combining the advantages of both solid state and vacuum electronic
technology. Flight-testing the MPM technology is a major goal of the Mini-RF
demonstration. The MPM is enabling in giving Mini-RF its dual band capability
within the challenging mass, power, and volume constraints of the LRO spacecraft.
MPM technology has extensive heritage in airborne and other tactical systems but the
Mini-RF development program had to make significant efforts to qualify it for
spaceflight. The transmitter consists of a low-voltage CCA, high-voltage potted
assembly, solid-state driver amplifier, and a miniature helix TWT. A
compartmentalized EMI filter is also included in the transmitter, along with an RF
power monitor port with analog voltage output for power sensing and control. The
highly integrated nature of MPMs allowed for the implementation of several control
features to achieve the multi-mode, multi-band requirements for the Mini-RF
transmitter.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Figure.5 Mini-RF transmitter block diagram
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Figure.6 Mini-RF Microwave Power Module Traveling Wave Tube (MPM/TWT)
The primary Mini-RF transmitter challenge is adapting these proven airborne designs
for space application. These include materials and part selection, out gassing,
reliability, and radiation tolerance. The technical challenges overcome in the
transmitter included developing low out gassing, high voltage insulators and space
qualification parts screening for miniature high-voltage power supplies. The
transmitter complied with the overall Mini-RF parts screening program which
screened parts to a total dose of 20 kRad, no destructive latch-up, and tolerance of
non-destructive latch up at 75 MeV. Meeting the stringent mass and power
limitations required some parts to carry wavers but the overall parts’ program was
compatible with the Class S and LRO Class B requirement with de-rating criteria in
accordance with established procedures. Mini-RF uses PEMs (Plastic Encapsulated
Microcircuits) with the screening operable over the temperature range of −55 to
+125°C. The MPM thermal design necessitated integration with the LRO heat-pipe
system, which allowed for effective dissipation of heat generated by the transmitter.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
5.4 Interconnect Module
The Interconnect Module (IM) combines and splits the RF energy and serves as the
interface between the transmitter, receiver, and antenna. Its design specifically handles
issues such as multination using selected materials and geometry.
5.5 Receiver
LRO’s receiver section is consisting of analog RF receiver and digital
receiver/quadrature detector waveform synthesizer. Receiver section is responsible for
receiving signals transmitted from the Mini RF transmitter and also responsible to
carryout required operations. Analog receiver is responsible for down conversion of
RF to IF and provides gain control. Digital receiver is responsible for digitalize the IF.
The digital receiver and quadrature digital waveform synthesizer (QDWS), based on
airborne radar heritage, were adapted to lunar orbital requirements. These systems
enabled the flexibility and reprogrammability required by the mini-RF system in
LRO’s low-altitude lunar orbit.
5.6 Control Processor
Control Processor is responsible for coordinating the entire functions Mini RF
system. Functions of Control Processor are digitizing antenna temperature, accept
commands from bus electronics, control and configure payload electronics and
provide router interface from digital receiver to bus electronics for radar data.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 6
Calibration
Polari metric measurements imply more stringent calibration of the radar than required
by a single channel system; since quantitative Polari metric measurements depend on
the relative phase and magnitude between signals in the two receive channels. Hybrid–
polar metric architecture has the unique and appealing property that it is self calibrating.
Calibration of the mini-RF instruments proved challenging for several reasons. Prior to
launch no opportunities for end-to-end measurements of the complete radar system
were available; only standalone characterizations of the radar electronics and antenna
were possible. Once in flight, conventional calibration techniques used by Earth orbiters
.The mini-RF calibration campaign included direct and separate characterizations of the
transmit and receive portions of the radars via Earth-based resources, hence obviating
the assumption of perfect circularity of the transmitted signal from the calibration
analysis. The two mini-RF radars are the pioneers of this calibration strategy. The on-
orbit calibration experiments measured the mini-RF receive and transmit characteristics
on separate days, using different Earth-based antennas, as the corresponding transmitter
or receiver platforms. The Arecibo Radio Telescope (ART) in Puerto Rico acted as a
transmitter to the mini-RF receiver. The Green Bank (Radio) Telescope (GBT) in West
Virginia received mini-RF transmissions. For both sessions; the spacecraft was
maneuvered to generate cross sections of the elevation and azimuth antenna patterns.
Effects due to the Earth’s ionosphere proved to be negligible. In a separate operation,
the spacecraft was rolled to point the radar antenna towards the Moon at nadir. In this
orientation, the expected radar backscattering properties in the H and V polarizations
are known to be (on average) the reflection of the transmitted signal, comprising a
practical means of collecting an end-to-end data set of known characteristics.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 7
Data analysis and Interpretation
Following in-flight calibration, a set of coefficients will be derived and applied to the
radar data as part of the standard processing stream prior to computing the Stokes
parameters that in turn are used to calculate daughter products (e.g. CPR, degree of
linear and circular polarization). A joint Chandrayaan/LRO Mini-RF Payload
Operations Center (POC) is located at APL to support the Mini-RF experiments. The
POC will provide the following functions: forward data acquisition sequences to
GSFC, receive raw telemetry from GSFC, process raw telemetry, produce mosaics,
and catalog data for the PDS and other repositories. The POC team is comprised of the
POC lead engineers, Science Team representatives and the POC engineers. The
Calibration and Collect commands have embedded argument lists that configure the
radar with respect to waveform, pulse repetition frequency (PRF), pulse width, burst
time, burst duty factor, number of bursts, and position of receiving gate, bandwidth,
start frequency, and other supporting parameters. Once the acquired data and the
required ancillary data have been received by the POC, the data will be processed
according to the data type. This processing will use the ephemeris data supplied to the
POC to create products to be checked for completeness and quality. To calculate the
circular polarization ratio of the received signal (CPR = SC/OC), we must be able to
calculate the total power returned in the same polarization as was transmitted
(SC = ∣EH∣2) and the total power returned in the opposite polarization (OC = ∣EV∣2).
These values can be expressed in terms of the Stokes vector, which represents the time
averaged Polarization properties of the backscattered field .It is a straightforward
matter to determine the Stokes vector from the total power received in the H and V
channels and the real and imaginary parts of their complex conjugate, HV* Circular
polarization ratios can then be calculated from the Stokes vectors.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
For example, CPR = SC/OC = (S1 − S4)/ (S1 + S4), where S1 represents the total
received power, S1=[|E(H)|^2+ |E(V)|^2],
S2= =[|E(H)|^2-|E(V)|^2],S3=2Re[E(H)E(V)*] and
S4=-2Im [E (H) E (V)*] =.It is possible to derive lot daughter products from stock
parameters such as , , m, .where m is degree of polarization
m=sqrt(S2^2+S3^2+S4^2)/S1;
is circular polarization ratio = (S1-S4)/(S1+S4); is linear polarisation ratio
=(S1-S2)/(S1+S2); is relative phase =arctan(S1/S2).
Figure 7 Mini-RF calibration strategies
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 8
Stokes Parameters as clear evidence of lunar ice
Stokes parameters as described earlier, are useful tools to study the scattering
properties of various features on the lunar surface. The magnitude and sense of
polarization of the reflected signal associated with various lunar morphological
features as well as water-ice inside the craters were examined using Mini-SAR data.
Traditionally, the key parameter used to determine whether ice is present is the CPR.
CPR was used in the present study to find evidence of subsurface scattering due to
dielectric in homogeneities like water-ice. Apart from high CPR, parameters like
degree of polarization (m) and relative EH–EV phase (δ) are also important
parameters to study the scattering mechanisms associated with lunar ice. The m–δ
together indicate the type of scattering mechanism associated with the target, because
at higher m value (m > 0.5), δ values close to –90° and +90° indicate surface and
double bounce scatterings respectively, and all other values of δ indicate diffused
scattering mechanism. The problem with using CPR alone is that higher values can be
obtained from very rough surfaces, such as a rough, blocky lava flow, which has
angles that form many small corner reflectors. In this case, the radar signal could hit a
rock face (changing LCP into right circular polarization (RCP)), and then bounce over
to another rock face (changing RCP back into LCP) and hence to the receiver. This
double bounce effect also creates high CPR in that ‘same sense’ reflections could
mimic the enhanced CPR one gets from ice targets. Hence, the CPR values estimated
from Mini-SAR have been analyzed along with m and δ values for indicating the
presence of water-ice
Table 3.
Daughter Products Derivation Significance
Degree of polarisation (m)
m=sqrt(S2^2+S3^2+S4^2)/S1 Indicator of polarized and diffused scattering; related to entropy
Linear polarisation ratio ( )
=(S1-S2)/(S1+S2) Indicator of volume v/s sub surface scattering
Circular polarisation ratio (
)=(S1-S4)/(S1+S4) Indicator of scattering lunar ice deposits
Relative phase ( ) = arctan(S1/S2)Indicator of double bounce scattering
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 9
Applications
The Lunar Reconnaissance Orbiter (LRO) Mini-RF technology is the product of over
a decade of development. Its objectives are: (1) Flight verification of an advanced
lightweight RF technology for future NASA and DoD communications applications;
(2) Demonstration of a hybrid-polarity Synthetic Aperture Radar (SAR) architecture;
(3) Obtaining measurements of the lunar surface as a function of radar band (S and X)
and resolution (150 m, 30 m) which could identify water ice deposits in the
permanently shadowed polar regions; (4) Production of topographic data using
interferometry (S-band) and SAR stereo techniques; and (5) Mapping of areas of
interest identified by the Chandrayaan-1 Forerunner Mini-SAR experiment and other
lunar instruments. Because Mini-RF provides its own illumination and can penetrate
the near subsurface at meter scales, it will acquire data not obtained by any other LRO
payload. Over the previous decade, the Department of Defense (DoD) and commercial
industry made significant strides in developing advanced lightweight RF technology
for wireless communication, Unmanned Airborne Vehicles (UAVs), and tactical
missiles. The first, “Forerunner” Miniature SAR (Mini-SAR), was developed and
integrated into the Indian Space Research Organization which is Chandrayaan. The
ForerunnerMini-SAR had to operate in the lunar thermal and radiation environment,
yet was simpler in design and operation, providing significant experience and
reduction of risk for the more advanced LRO Mini-RF system. The LRO Mini-RF
affords NASA and the DoD an opportunity to flight-qualify lightweight technology
for a range of applications, including deep space communications. The flexibility,
reconfigurability, and capability of Mini-RF will be demonstrated by a
communications and radar mode utilizing the same hardware. The constraints of a
lunar mission (range, limited duty cycle over the poles) and the low mass of advanced
lightweight RF technology allows a technology demonstration which met the payload
constraints of both the Chandrayaan and LRO spacecraft, and provided an opportunity
to collect unique and valuable lunar science data.
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Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 10
Conclusion
The mini-RF radars on the LRO have proven to be very successful, meeting or
exceeding their design requirements and expectations. Their hybrid – polarity
architecture is the first demonstration of this design paradigm from orbit. They afford
the first polarimetric radar observations of the entire Moon, including especially the
lunar poles and far side. The results are providing new information and opening new
insights into the lunar surface. From a technical point of view, the mini-RF radars
have pioneered several innovations. They have been exercised in unique calibration
modes that do not depend on the usual in situ references of an extended distributed
backscattering feature (such as the Amazon rain forest) or a Calibrated point reference
(such as a corner reflector or active radar calibrator). For lunar or planetary
polarimetric radars, calibration methodologies are required that do not require known
references in the scene. By taking advantage of the polarization basis independence of
the Stokes parameters of the received data, the mini-RF systems are more capable
instruments than all previous radars that have ventured outside Earth orbit, yet their
implementation is relatively simple, an attribute that is especially appealing for
planetary deployments. Although for both radars the observed transmitted field is
elliptically polarized rather than purely circular, simple polarimetric analysis such as
estimation of CPR seems relatively robust to this imperfection. Other Stokes-based
measurements may be more sensitive to imperfect circularity of the illuminating field.
Al Ameen engg.college Dept. of E.C.E
26
Lunar Reconnaissance Orbiter Miniature RF Technology Demonstration
Chapter 11
References
1. www.spudislunarresources.com
2. www. lunar.gsfc.nasa.gov
3. www. ieeexplore.ieee.org
4. Ritchriya
5. NASA Fact
Al Ameen engg.college Dept. of E.C.E