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The ALSE (Apollo Lunar Sounder Experiment) (also known as Scientific Experiment S-209, according to NASA designations) was a
ground-penetrating radar Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. It is a non-intrusive method of surveying the sub-surface to investigate underground utilities such as concrete, asphalt, metals, pipes, cables ...
(subsurface sounder) experiment that flew on the
Apollo 17 Apollo 17 (December 7–19, 1972) was the eleventh and final mission of NASA's Apollo program, the sixth and most recent time humans have set foot on the Moon. Commander Gene Cernan and Lunar Module Pilot Harrison Schmitt walked on the Moon, ...
mission.


Mission and science

This experiment used radar to study the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
's surface and interior.
Radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
waves with wavelengths between 2 and 60 meters (frequencies of 5, 15, and 150 MHz) were transmitted through a series of
antennas In radio-frequency engineering, an antenna (American English) or aerial (British English) is an electronic device that converts an alternating electric current into radio waves (transmitting), or radio waves into an electric current (receivi ...
near the back of the
Apollo Service Module The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother ship ...
. 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 The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...
has been similarly used to map ancient river valleys beneath the
Sahara The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the list of deserts by area, third-largest desert overall, smaller only than the deserts of Antarctica and the northern Ar ...
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 The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
. This experiment revealed structures beneath the surface in
Mare Crisium Mare Crisium (Latin ''crisium'', the "Sea of Crises") is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. Mare Crisium is a basin of Nectarian age. It was formed by the flooding of basaltic lava that fill ...
,
Mare Serenitatis Mare Serenitatis (Latin ''serēnitātis'', the "Sea of Serenity") is a lunar mare located to the east of Mare Imbrium on the Moon. Its diameter is . Geology Mare Serenitatis is located within the Serenitatis basin, which is of the Nectarian ...
,
Oceanus Procellarum Oceanus Procellarum ( ; from ) is a vast lunar mare on the western edge of the near side of the Moon. It is the only one of the lunar maria to be called an "Oceanus" (ocean), due to its size: Oceanus Procellarum is the largest of the maria ("s ...
, and many other areas. 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.


Instrument design

The ALSE instrument operated in two HF 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 A chirp is a signal in which the frequency increases (''up-chirp'') or decreases (''down-chirp'') with time. In some sources, the term ''chirp'' is used interchangeably with sweep signal. It is commonly applied to sonar, radar, and laser syste ...
ed signal). The two HF bands shared the same center-feed
dipole antenna In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely used antenna types, types of antenna; the other is the monopole antenna, monopole. The dipole is any one of a class of antennas producin ...
, while a 7-element
Yagi antenna Yagi may refer to: Places *Yagi, Kyoto, in Japan * Yagi (Kashihara), in Nara Prefecture, Japan * Yagi Ridge, a mountain ridge in British Columbia, Canada * Yagi-nishiguchi Station, in Kashihara, Nara, Japan * Kami-Yagi Station, a JR-West Kabe Line ...
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 weighed 43 kg and required 103 W of power. The electronics was located inside the
Apollo Service Module The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother ship ...
. The two halves of the dipole antenna were 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
sidelobe In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the ''main lobe''. The radiation pattern of most antennas shows a pattern of "''lobes'' ...
s of the compressed
chirp A chirp is a signal in which the frequency increases (''up-chirp'') or decreases (''down-chirp'') with time. In some sources, the term ''chirp'' is used interchangeably with sweep signal. It is commonly applied to sonar, radar, and laser syste ...
: 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 Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation and ...
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 (AGC) is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the inpu ...
(AGC) feature was included in all the channels to optimize the signal allocation within the receiver
dynamic range Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...
. The AGC update rate was 30 s. In both the HF and VHF transceivers, the chirp signal was generated by a swept
oscillator Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
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 CRT or Crt most commonly refers to: * Cathode-ray tube, a display * Critical race theory, an academic framework of analysis CRT may also refer to: Law * Charitable remainder trust, United States * Civil Resolution Tribunal, Canada * Columbia ...
(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 μs of echoes immediately following it. Additionally, on this channel, the receiver gain was increased 13 μs after the arrival of the main surface echo to best exploit the dynamic range on weak subsurface returns. Since the recorder was located in the service module, astronaut Ron Evans performed an
Extra-Vehicular Activity Extravehicular activity (EVA) is any activity done by an astronaut in outer space outside a spacecraft. In the absence of a breathable Earthlike atmosphere, the astronaut is completely reliant on a space suit for environmental support. EVA i ...
(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 Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...
, demonstrating the capabilities of the system. The main parameters of the ALSE radar are summarized in the table below:Porcello et al. - "The Apollo Lunar Sounder Radar System" - Proceedings of the
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is an American 501(c)(3) organization, 501(c)(3) public charity professional organization for electrical engineering, electronics engineering, and other related disciplines. The IEEE ...
, June 1974


References


External links


Apollo 17 Mission
at the Lunar and Planetary Institute {{DEFAULTSORT:ALSE Space radars Apollo 17