ALSE
Encyclopedia
The ALSE (also known as Scientific Experiment S-209, according to NASA designations) was a ground-penetrating radar
(subsurface sounder) experiment that flew on the Apollo 17
mission.
's surface and interior. Radar
waves with wavelengths between 2 and 60 meters (frequencies of 5, 15, and 150 MHz) were transmitted through a series of antenna
s near the back of the Apollo Service Module. After the waves were reflected by the Moon
, they were received using the same antennas and the data was recorded on film for analysis on Earth. The primary purpose of this experiment was to "see" into the upper 2 kilometers of the Moon's crust in a manner somewhat analogous to using seismic waves to study the internal structure of the Moon
. This was possible because very long radar wavelengths were used and because the Moon is very dry, which allowed the radar waves to penetrate much deeper into the Moon than would have been possible if water were present in lunar rocks. (A radar experiment on the space shuttle
has been similarly used to map ancient river valleys beneath the Sahara
Desert.) This experiment also provided very precise information about the Moon's topography. In addition to studying the Moon, the experiment also measured radio emissions from the Milky Way
Galaxy
.
This experiment revealed structures beneath the surface in Mare Crisium
, Mare Serenitatis
, Oceanus Procellarum
, and many other areas [3]. In the mare areas, layers were observed in several different parts of the basins and are therefore believed to be widespread features. Based on the properties of the reflected radar waves, the structures are believed to be layering within the basalt that fills both of these mare basins. In Mare Serenitatis, layers were detected at depths of 0.9 and 1.6 kilometers below the surface. In Mare Crisium, a layer was detected at a depth of 1.4 kilometers below the surface. The bottom of the mare basalts were apparently not detected by this experiment. However, in Mare Crisium
the Lunar Sounder Experiment results were combined with other observations to estimate a total basalt thickness of between 2.4 and 3.4 kilometers.
The Lunar Sounder Experiment also contributed to our understanding of wrinkle ridges on the Moon. These long, low ridges are found in many of the lunar maria. Most lunar geologists believe that these ridges formed when the Moon's surface was deformed by motion along faults ("moonquakes") in the Moon's crust more than 3 billion years ago. The weight of several kilometers of mare basalt in these areas caused the Moon's surface to sag somewhat, and this motion caused the surface to buckle in some places, forming the wrinkle ridges. However, other scientists suggested that these ridges are volcanic features, formed by the flow of magma either on the Moon's surface or within the crust. The Lunar Sounder Experiment studied several wrinkle ridges in southern Mare Serenitatis
in detail, providing information about both the topography of these ridges and about structures in the crust below these ridges. These results support the idea that wrinkle ridges formed primarily by motions along faults.[1]
bands (5 MHz - HF1 - and 15 MHz - HF2) center frequencies and one VHF band (150 MHz), each with a bandwidth of 10% (using a chirp
ed signal). The two HF bands shared the same center-feed dipole antenna
, while a 7 elements Yagi antenna
was used for the VHF channel. Two different transceiver were used for the HF (alternating operation between HF1 and HF2 on a PRF
-by-PRF
basis) and VHF, sharing a common optical recorder. It was not possible to operate in VHF and HF simultaneously.
The whole system weighted 43 kg and required 103 W of power.
The electronics was located inside the Apollo Service Module. The two halves of the dipole antenna where retractable, on the two sides of the service module itself, while the Yagi used for VHF was stowed close to the main engine and then deployed into position after launch.
Being the primary objective of the experiment the mapping of subsurface layers, the most critical trade-off in the design was that of penetration depth vs resolution: lower frequencies penetrates more, but allowed a smaller signal bandwidth and, therefore, a worst resolution which, in turn, affected the capability to discriminate subsurface echoes close to the surface.
The sounding capability was also affected by:
Clutter from across-track scatterers had instead to be inferred from the knowledge of the surface topography.
An Automatic gain control
(AGC) feature was included in all the channels to optimize the signal allocation within the receiver dynamic range
. The AGC update rate was 30 sec.
In both the HF and VHF transceivers, the chirp signal was generated by a swept oscillator synchronized with a STAble Local Oscillator (STALO) in order to preserve the phase coherency for the SAR
processing. The received signal was converted at IF and the signal amplitude was used to amplitude-modulate a CRT
(swept at PRF rate), in turn impressing a 70 mm film for optical recording of the data. Due to the high recording speed required for the wider bandwidth VHF channel, to minimize the amount of recorded data, this channel used an echo tracking system to acquire and record only the main surface return and the 70 usec of echoes immediately following it. Additionally, on this channel, the receiver gain was increased 13 usec after the arrival of the main surface echo to best exploit the dynamic range on weak subsurface returns.
Being the recorder located in the service module, one of the astronauts (Ron Evans) had to perform an Extra-Vehicular Activity
(EVA) during the return flight from the Moon
to collect the recorded films.
The processing facility on ground allowed for both full optical processing (at that time, the standard approach for SAR
processing) performing azimuth and/or range compression, or digitization of rough or azimuth-compressed data for later digital processing.
During the developments phase, a modified ALSE prototype was installed on-board a KC-135 aircraft to perform sounding tests over the southeast US and over Greenland
, demonstrating the capabilities of the system.
The main parameters of the ALSE radar are summarized in the table below [2]:
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HF1
HF2
VHF
Frequency (MHz)
5.266
15.8
158
Estimated Depth of Penetration(m)
1300
800
160
Chirp bandwidth (MHz)
0.5333
1.6
16.0
Pulsewidth (usec)
240
80
8.0
Time(bandwidth product
128
128
128
Range resolution, free space (m)
300
100
10
Transmitter peak power (W)
130
118
95
Antenna effective Gain (dB
one-way)
-0.8
-0.7
+7.3
Noise Figure (dB)
11.4
11.4
10.0
Pulse Repetition Frequency (Hz)
397
397
1984
Acquisition window length (usec)
600
600
70
AGC Gain Range (dB)
12.1
12.1
13.9
Echo tracker
No
No
Yes
Ground-penetrating radar
Ground-penetrating radar is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band of the radio spectrum, and detects the reflected signals from subsurface structures...
(subsurface sounder) experiment that flew on the Apollo 17
Apollo 17
Apollo 17 was the eleventh and final manned mission in the American Apollo space program. Launched at 12:33 a.m. EST on December 7, 1972, with a three-member crew consisting of Commander Eugene Cernan, Command Module Pilot Ronald Evans, and Lunar Module Pilot Harrison Schmitt, Apollo 17 remains the...
mission.
Mission and Science
This experiment used radar to study the MoonMoon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
's surface and interior. Radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
waves with wavelengths between 2 and 60 meters (frequencies of 5, 15, and 150 MHz) were transmitted through a series of antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
s near the back of the Apollo Service Module. After the waves were reflected by the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, they were received using the same antennas and the data was recorded on film for analysis on Earth. The primary purpose of this experiment was to "see" into the upper 2 kilometers of the Moon's crust in a manner somewhat analogous to using seismic waves to study the internal structure of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
. This was possible because very long radar wavelengths were used and because the Moon is very dry, which allowed the radar waves to penetrate much deeper into the Moon than would have been possible if water were present in lunar rocks. (A radar experiment on the space shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
has been similarly used to map ancient river valleys beneath the Sahara
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...
Desert.) This experiment also provided very precise information about the Moon's topography. In addition to studying the Moon, the experiment also measured radio emissions from the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
Galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
.
Mare Crisium
Mare Crisium is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. This basin is of the Pre-Imbrian period, 4.55 to 3.85 billion years ago. This mare is in diameter, and 176,000 km2 in area. It has a very flat floor, with a ring of wrinkled ridges...
, Mare Serenitatis
Mare Serenitatis
Mare Serenitatis is a lunar mare that sits just to the east of Mare Imbrium on the Moon.It is located within the Serenitatis basin, which is of the Nectarian epoch. The material surrounding the mare is of the Lower Imbrian epoch, while the mare material is of the Upper Imbrian epoch...
, Oceanus Procellarum
Oceanus Procellarum
Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
, and many other areas [3]. In the mare areas, layers were observed in several different parts of the basins and are therefore believed to be widespread features. Based on the properties of the reflected radar waves, the structures are believed to be layering within the basalt that fills both of these mare basins. In Mare Serenitatis, layers were detected at depths of 0.9 and 1.6 kilometers below the surface. In Mare Crisium, a layer was detected at a depth of 1.4 kilometers below the surface. The bottom of the mare basalts were apparently not detected by this experiment. However, in Mare Crisium
Mare Crisium
Mare Crisium is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. This basin is of the Pre-Imbrian period, 4.55 to 3.85 billion years ago. This mare is in diameter, and 176,000 km2 in area. It has a very flat floor, with a ring of wrinkled ridges...
the Lunar Sounder Experiment results were combined with other observations to estimate a total basalt thickness of between 2.4 and 3.4 kilometers.
The Lunar Sounder Experiment also contributed to our understanding of wrinkle ridges on the Moon. These long, low ridges are found in many of the lunar maria. Most lunar geologists believe that these ridges formed when the Moon's surface was deformed by motion along faults ("moonquakes") in the Moon's crust more than 3 billion years ago. The weight of several kilometers of mare basalt in these areas caused the Moon's surface to sag somewhat, and this motion caused the surface to buckle in some places, forming the wrinkle ridges. However, other scientists suggested that these ridges are volcanic features, formed by the flow of magma either on the Moon's surface or within the crust. The Lunar Sounder Experiment studied several wrinkle ridges in southern Mare Serenitatis
Mare Serenitatis
Mare Serenitatis is a lunar mare that sits just to the east of Mare Imbrium on the Moon.It is located within the Serenitatis basin, which is of the Nectarian epoch. The material surrounding the mare is of the Lower Imbrian epoch, while the mare material is of the Upper Imbrian epoch...
in detail, providing information about both the topography of these ridges and about structures in the crust below these ridges. These results support the idea that wrinkle ridges formed primarily by motions along faults.[1]
Instrument Design
The ALSE instrument operated in two HFHigh frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
bands (5 MHz - HF1 - and 15 MHz - HF2) center frequencies and one VHF band (150 MHz), each with a bandwidth of 10% (using a chirp
Chirp
A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...
ed signal). The two HF bands shared the same center-feed dipole antenna
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
, while a 7 elements Yagi antenna
Yagi antenna
A Yagi-Uda array, commonly known simply as a Yagi antenna, is a directional antenna consisting of a driven element and additional parasitic elements...
was used for the VHF channel. Two different transceiver were used for the HF (alternating operation between HF1 and HF2 on a PRF
Pulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
-by-PRF
Pulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
basis) and VHF, sharing a common optical recorder. It was not possible to operate in VHF and HF simultaneously.
The whole system weighted 43 kg and required 103 W of power.
The electronics was located inside the Apollo Service Module. The two halves of the dipole antenna where retractable, on the two sides of the service module itself, while the Yagi used for VHF was stowed close to the main engine and then deployed into position after launch.
Being the primary objective of the experiment the mapping of subsurface layers, the most critical trade-off in the design was that of penetration depth vs resolution: lower frequencies penetrates more, but allowed a smaller signal bandwidth and, therefore, a worst resolution which, in turn, affected the capability to discriminate subsurface echoes close to the surface.
The sounding capability was also affected by:
- the range sidelobes of the compressed chirpChirpA chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...
: they can mask weak subsurface echoes if not properly controlled. ALSE was designed to have a minimum peak-to-sidelobes ratio of 45 dB after the 3rd lobe. - the surface off-nadir clutter return, which can be confused with the subsurface echo with the same delay. To reduce the along-track clutter, a synthetic aperture is generated in the ground processing, thus narrowing the effective antenna footprint.
Clutter from across-track scatterers had instead to be inferred from the knowledge of the surface topography.
An Automatic gain control
Automatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
(AGC) feature was included in all the channels to optimize the signal allocation within the receiver dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
. The AGC update rate was 30 sec.
In both the HF and VHF transceivers, the chirp signal was generated by a swept oscillator synchronized with a STAble Local Oscillator (STALO) in order to preserve the phase coherency for the SAR
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...
processing. The received signal was converted at IF and the signal amplitude was used to amplitude-modulate a CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
(swept at PRF rate), in turn impressing a 70 mm film for optical recording of the data. Due to the high recording speed required for the wider bandwidth VHF channel, to minimize the amount of recorded data, this channel used an echo tracking system to acquire and record only the main surface return and the 70 usec of echoes immediately following it. Additionally, on this channel, the receiver gain was increased 13 usec after the arrival of the main surface echo to best exploit the dynamic range on weak subsurface returns.
Being the recorder located in the service module, one of the astronauts (Ron Evans) had to perform an Extra-Vehicular Activity
Extra-vehicular activity
Extra-vehicular activity is work done by an astronaut away from the Earth, and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth , but also applies to an EVA made on the surface of the Moon...
(EVA) during the return flight from the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
to collect the recorded films.
The processing facility on ground allowed for both full optical processing (at that time, the standard approach for SAR
Synthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...
processing) performing azimuth and/or range compression, or digitization of rough or azimuth-compressed data for later digital processing.
During the developments phase, a modified ALSE prototype was installed on-board a KC-135 aircraft to perform sounding tests over the southeast US and over Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
, demonstrating the capabilities of the system.
The main parameters of the ALSE radar are summarized in the table below [2]:
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one-way)
External links
- http://www.lpi.usra.edu/expmoon/Apollo17/A17_Orbital_sounder.html