太空|TAIKONG DISCOVER THE SKY BY LONGEST WAVELENGTH - WITH SMALL SATELLITE CONSTELLATION - The International Space Science Institute
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
国际空间科学研究所 - 北京
太空|TAIKONG ISSI-BJ Magazine
No. 14 July 2019
DISCOVER THE SKY BY LONGEST
WAVELENGTH
WITH SMALL SATELLITE CONSTELLATION
IMPRINT FOREWORD
太空 | TAIKONG On January 23-25, 2019, the three- with a short introduction to ISSI-BJ
ISSI-BJ Magazine day Forum on “Discover the Sky by and to CAS Strategic Priority Program
License: CC BY-NC-ND 4.0
the Longest Wavelength with Small on Space Science. Following that, the
Satellite Constellation” was successfully future low frequency space missions
organized by the International Space were discussed, such as SUNRISE and
Science Institute in Beijing (ISSI-BJ). DSL Mission at Lunar Orbit. The rest of
Address: No.1 Nanertiao, ISSI-BJ Forums are informal, free the first day, as well as the beginning
Zhongguancun, debates, and brainstorming meetings of the second day of the Forum were
Haidian District,
among high-level participants on open dedicated to the expert presentations
Beijing, China
questions of scientific nature. About on various science cases related to
Postcode: 100190
Phone: +86-10-62582811 30 leading scientists from 10 countries the low frequency radio. Then, global
Website: www.issibj.ac.cn participated in this forum, which was spectrum experiments were introduced
convened by Xuelei Chen (NAOC, CAS, and discussed, followed by the talks
China), Ji Wu (NSSC, CAS, China), on past and current space missions,
Jack Burns (Colorado University, USA), such as CE-4 Longjiang, CE-4 NCLE,
Authors
Joe Silk (JHU/IAP, USA/France), Leon and CE-4 LFRS. The last day of the
Koopmans (Groningen University, The Forum was dedicated to the topic of the
See the list on the back cover
Netherlands), Hanna Rothkaehl (SRC, instruments and technology for space
PAS, Poland), and Maurizio Falanga missions. and an open discussion on
(ISSI-BJ, China). science and technology of the missions,
Editor the international collaboration, as well as
The aim of this Forum was to discuss on the future work plan, including outline
Anna Yang,
the low frequency radio observation, of the forum report, as it appears in this
International Space Science
Institute - Beijing, China
which is hampered on the ground by issue of the TAIKONG magazine.
the ionosphere and man-made radio
frequency interferences, and so far I wish to thank the conveners and
our knowledge about the sky in this organizers of the Forum, as well as the
part of the electromagnetic spectrum ISSI-BJ staff, Lijuan En, Anna Yang,
is very limited. During the Forum, the and Xiaolong Dong, for actively and
participants discussed various science cheerfully supporting the organization
FRONT themes related to the low frequency
radio, such as the signature of cosmic
of the Forum. In particular, I wish to
thank the authors, who with dedication,
COVER dark age and dawn, the solar system, enthusiasm, and seriousness,
galactic and extragalactic sources, conducted the editing of this report. Let
the propagation effects, and data me also thank all those who participated
DSL mission concept.
analysis methods. The Forum started actively in this stimulating Forum.
Image Credit: NAOC
Maurizio Falanga,
Executive Director
ISSI-BJ
2 太空|TAIKONG
1. INTRODUCTION
Over the last century, part of the spectrum might low frequency radio
astronomical observations provide unique probes observation and data
have expanded from the for the dark ages after processing. In particular,
optical to the radio, infrared, the Big Bang and Cosmic it focused on a possible
ultraviolet, X-ray and Dawn when first stars and future lunar orbit array
gamma-ray bands of the galaxies formed. It may mission of Discovering
electromagnetic spectrum, also shed light on many Sky at the Longest
and further supplemented astrophysical phenomena, wavelengths (DSL). The
by non-electromagnetic from active processes in proposed array is made up
observations such as the Sun and planets, through of satellites flying in linear
cosmic ray, neutrino, and exoplanets, interstellar formation, making both
gravitational waves. These medium and galactic , to interferometric and single
new observational domains radio galaxies, quasars, antenna observations on
brought many unexpected clusters and intergalactic the orbit behind the Moon,
discoveries, which greatly medium. They might also shielded from the Earth-
changed our view of the reveal previously unknown originated radio frequency
Universe, and gave deep objects or phenomena. interferences. This mission
insight on the fundamental This Forum is dedicated concept is under intensive
laws of Nature. However, to the exploration of this study by the Chinese
at the longest wavelengths new observational window Academy of Sciences
of the electromagnetic of the electromagnetic (CAS) in collaboration with
spectrum, our view is spectrum. national and international
still incomplete, as the partners.
observations at frequencies The Forum reviewed our
below ~30 MHz are current understanding on
strongly hampered by the various science subjects
ionosphere and human- related to the low frequency
made radio frequency radio window, previous
interferences (RFIs). Even and ongoing observations,
at ~100 MHz high precision recent progress, and key
observations can still be science problems to be
affected by these factors. solved. It then discussed
Observations at this concepts and technologies
unexplored low frequency related to space-based
太空|TAIKONG 3
2. THE EXPLORATION OF SKY AT LOW
FREQUENCY
2.1. Early ground and space experiments
Radio astronomy was born up to ~40 m (Braude et al. Europe, the LWA2 (Long
at what we would call now 1978). At about the same Wavelength Array) in
long wavelengths: the time decametric radio the USA, and the MWA
pioneering observations telescopes were put in 3
(Murchison Widefield
conducted by Karl Jansky operation in the USA at Clark Array) in Australia, They
in 1932-33 were performed Lake (Erickson et al. 1982) all are considered to be
at the wavelength of about and in France at Nançay pathfinders for the next
15 m. Further development (Nançay Decameter Array, generation large radio
of radio astronomy was NDA, Boischot et al. astronomy facility, the SKA
carried out with a strong 1980). Some observations (Square Kilometre Array 4).
emphasis on shorter below 30MHz were made However, these facilities
wavelengths. This was from ground facilities at will not address the
dictated by both the Tasmania in the south and strengthening science case
astrophysical research Canada in the north (Reber for radio astronomy studies
agenda and difficulties of 1994, Bridle & Purton 1968, at ultra-long-wavelengths,
astronomy observations Caswell 1976, Cane & longer than ~10 m. The
at decametric and longer Whitham 1977, Cane 1978. only solution for addressing
wavelengths due to Roger et al. 1999). Around the science case of ultra-
ionosphere opacity. There the turn of the century, long wavelength radio
were only few noticeable the interest to cosmology astronomy is in placing
exceptions of radio and astrophysics in the a telescope beyond the
astronomy facilities that long wavelength spectrum ionosphere, in Space
operated at frequencies domain stimulated (Jester & Falcke 2009,
below ~60 MHz deployment of several Boonstra et al. 2016).
(wavelengths longer than large meter-wavelength
~5 m). One of them was the facilities such as LOFAR1 In the 1970s, the IMP-6
Ukrainian T-shaped Radio (Low Frequency Array) (Brown 1973), the Radio
Telescope (UTR) able to centered in the Netherlands Astronomy Explore (RAE)-1
observe at wavelengths and spread throughout (Alexander & Novaco 1974)
1 http://www.lofar.org
2 http://lwa.unm.edu
3 http://mwatelescope.org
4 http://astronomers.skatelescope.org
4 太空|TAIKONG
Figure 1: The RAE-2 sky map at 4.70 MHz. Credit: Novico & Brown 1978.
and RAE-2 (Alexander et observation. However, wavelengths, (ii) the ever
al. 1975) satellites made these single antenna rising level of human-
low frequency radio observations had poor produced radio frequency
observations from space. angular resolution despite interference (RFI), and
The data collected by a remarkably long antenna (iii) the astrophysics-
these satellites showed deployed in space (Novaco driven demand for higher
that the Earth have strong & Brown 1978). angular resolution, which
natural radio emissions at in turn requires aperture
the kilometric wavelengths, Space-borne radio sizes (i.e., interferometric
and that man-made radio astronomy is a logical baselines) larger than the
frequency interferences and inevitable step in the Earth diameter. The latter
are visible from space. overall development of necessitates extension
The Moon can shield astronomical science. It of the global very long
the spacecraft from is driven by three factors: interferometer baselines
these emissions of the (i) the afore-mentioned (VLBI) to orbital dimensions,
Earth, so the far side opacity of the ionosphere creating Space VLBI
of the Moon provides at long and atmosphere (SVLBI) systems. The
an ideal environment at short (millimeter history of SVLBI began
for low frequency radio and sub-millimetre) almost simultaneously
太空|TAIKONG 5
with the invention of the TDRSS (Levy et al. wavelengths shorter than
VLBI technique as such in 1986), and two first- those under consideration
the middle of the 1960s. generation dedicated in this Report, lessons
To date, interferometric SVLBI missions, the learned from their design,
baselines longer than Japan-led VSOP/HALCA construction, tests and
the Earth diameter (1997–2003, Hirabayashi operations (Gurvits
produced astrophysical et al. 1998) and Russia-led 2018, 2019) might be
results in the first SVLBI RadioAstron (2011-2019, relevant for a prospective
demonstration experiment Kardashev et al. 2013). ULW spaceborne radio
with the geostationary While all three implemented interferometer.
communication satellite, SVLBI systems operated at
2.2. The exploration of the Solar System with space radio
astronomy instruments
The main low frequency PWS, Cassini/RPWS and range of the major radio
radio sources in the Solar Juno/Waves instruments. astronomy space probes.
System are related to Solar observations at low The space radio astronomy
Solar activity on one hand, frequency have been instruments are limited in
and planetary aurora on also studied on the long sensitivity (single antenna,
the other hand. Many run with the ISEE3, WIND bright sky background,
space probe including or STEREO missions. limited power and downlink
radio astronomy receivers Terrestrial kilometric rate, limited antenna
explored our Solar System. radiation was observed gain…), but they were
The outer planets (Jupiter, by many space missions developed included so-
Saturn, Uranus and (RAE, Swedish Viking, called “Direction-Finding”
Neptune) low frequency FAST, Cluster, Interball, or “Goniopolarimetric”
radio emissions have Geotail…). capabilities, which allows
been discovered by the to retrieve the flux, direction
Voyager PRA (Planetary Figure 2 shows the of arrival and polarization of
Radio Astronomy) normalized power spectral any dominant radio source
experiment. The Jupiter densities for low frequency in the sky (see Cecconi
and Saturn radio emissions Solar and Planetary radio 2011).
have been extensively emissions, as well as the
studied by the Gallileo/ observation frequency
6 太空|TAIKONG
The analysis of data
acquired with space-
borne radio astronomy
instruments with
goniopolarimetric
capabilities have provided
the Solar system science
communities with many
observational evidences
of relativistic accelerated
particles populations
interacting with a colder
ambient plasma, through
plasma instabilities (e.g.,
in the Solar Wind or in the
auroral regions of planetary
magnetospheres). Low
frequency radio astronomy
is thus a powerful remote
sensing tool for probing
energetic distant plasma.
Figure 2: Planetary radio emissions. Credit: Terrestrial in
red, Jupiter in black, Saturn in green, Uranus and Neptune
in blue, extracted from Cecconi, et al (2018); and space
probes observation spectral range.
2.3. The various mission concepts
Single co-located low- physics phenomena. as is shown for example
frequency antenna systems Strong celestial signals can in Chang'e 4 (CE4) lunar
are well suited for space even be spatially located, lander (see Sec.2.4) or
science and for studying at least for one or a few Cassini. As at frequencies
solar system plasma strong dominating sources below about 15 MHz the
太空|TAIKONG 7
antenna patterns are science usually requires separate signals coming
spatially symmetrical, a high spatial resolution to from the front of the array
rough indication is needed isolate a particular source versus coming from the
of the source’s origin. for research on transient back, a planar array needs
For planetary sciences behavior, morphology, to change it orientation over
this often is the case. The polarization, or for spectral time. In the DSL concept
enabling factor for being analysis. A space-based (Boonstra et al. 2016,
able to observe in full constellation of small Huang et al. 2018) this is
polarization and to localize satellites operating in achieved by making use of
sources is the number of aperture synthesis mode lunar orbit precession.
degrees of freedom of the or operating as a phased-
system. For this reason, array can provide this. Different deployment
ideally a spacecraft would Creating sky images are locations of satellite
have three orthogonal done either by forming aperture synthesis ‘clouds’
dipoles. If space is limited several (parallel) broad- in space have their pro’s
and only one outer wall of band beams, or by and con’s. One could
a spacecraft can be used, creating a full narrow-band deploy them far from Earth’s
such as for CE4-RS, then cross correlation matrix, transmitters such as in the
monopoles can be used as and sending these to Earth Sun-Earth L2 Lagrangian
well. Spatial nulling of radio for further processing and point (e.g. FIRST, Bergman
interference, either external analysis. et al., 2009; SURO, Blott et
or self-generated, can in al. 2013). At that location
principle also be applied. The constellation can the satellites would only
However, this consumes have fixed relative satellite slowly drift, which would
at least one degree of positions, or the satellites allow longer satellite cross-
freedom, so systems can be slowly drifting, correlation integration
should be made flexible provided their relative times and lower downlink
in terms of online and positions are known. As data rates. However, the
offline processing. In this with ground-based radio aperture filling would be
way one could exchange interferometry, the celestial very sparse resulting
for example optimal radio signals observed by in a somewhat limited
polarization performance the satellites are mutually instantaneous source
for interference correlated, filling the so- separation capability.
suppression. called aperture plane,
or aperture sphere for In relatively low altitude
Galactic and extra- three-dimensional array. Lunar orbit a dynamic two
galactic radio astronomy As a planar array cannot or three-dimensional array
8 太空|TAIKONG
of satellites such as OLFAR data rates. Low altitude orbit as proposed in DARIS
(Engelen et al. 2013), and Lunar orbits are unstable, (Saks et al. 2010). Another
DSL would have excellent but putting satellites in the concept is the deployment
aperture coverage. But same orbit, as a linear array of a rotating tethered string
due to the fast changing as in the DSL concept, this of antennas as proposed in
satellite relative positions, issue is circumvented. Kruithof et al. (2017). Such
the correlation integration a constellation could be
times need to be split in very A solution that optimizes combined with one or more
short intervals, resulting in aperture filling and downlink free-flyers to provide the
relatively large downlink rates is a constellation in third aperture dimension.
an Earth leading or trailing
3. THE CE-4 MISSION
The Chang'e project is an Earth, a communication projects are carried out:
ongoing series of Chinese relay satellite Queqiao the Longjiang satellites on
robotic missions to the (Magpie Bridge) is lunar orbit; the Netherland-
Moon. The Chang'e 4 launched before the lander China Low-Frequency
mission put an lander on to a halo orbit around the Explorer on board the
far side of the Moon for the Earth-Moon Lagrangian Queqiao satellite, and the
first time, the landing site is point L2. Taking the very low radio frequency
at the Aitken Basin near the opportunity to go to the spectrometer on the lander.
lunar south pole. To provide far side of the Moon, three
communication link with ultralong wavelength radio
3.1. Longjiang satellites
The Longjiang 1 and 2 planned to reach the Moon 30MHz. These two satellites
satellites were launched by their own thruster, then were made by the Harbin
into space on May 21, 2018 form a two element lunar Institution of Technology.
together with the CE-4 interferometer on orbit Longjiang 2 successfully
lunar probe’s relay satellite. to do the interferometric reached its destination
These two satellites were measurement on 1MHz to near the Moon on May 25,
太空|TAIKONG 9
2018, and entered a lunar The Upper antenna is made were scheduled with the
orbit with the perilune at by the Polish Academy of aim of studying radiation
350km and the apolune Sciences, and the lower characteristic of celestial
at 13700km. However, antenna is made by the sources. The Earth
Longjiang 1 suffered an National Space Science occultation experiments
anomaly and failed to enter Center, Chinese Academy and other planets
lunar orbit. The objective of Sciences. The low occultation experiments
for Longjiang 2 was to do frequency interferometer have been scheduled.
spectrum measurement (LFI) is installed on the However, due to limitation
at 1MHz~30MHz, Longjiang 1 and 2. The of the battery power, the
corresponding to the specifications are listed in payload worked only 10 to
wavelength of 300m Table 1. 20 minutes per orbit.
to 10m. As illustrated
in Figure 3, the low Some key technologies Up to Deccember 2018,
frequency interferometer of low frequency the total observing time
(LFI), was installed on interferometer have been was more than 1000
the Longjiang 1 and 2. validated in this mission, min, the number of Earth
LFI included deployable including deployable occultation is more than
antenna, digital receiver, Antenna, low frequency 20, and the number of
communication ranging data receiver, internal Jupiter occultation more
and timing synchronization calibration and so on. than 6. Preliminary analysis
unit. The observational plan of the data had showed
significant RFIs from the
Earth, and the shielding by
the Moon is clearly seen.
As illustrated in Figure 4,
three channels of LFI had
observed almost identical
Earth RFI suppression
phenomenon by the Moon.
Figure 3: Longjiang satellite and LWF. Credit: NSSC.
10 太空|TAIKONGItems Value
Longjiang 2 Weight 50kg
Lifetime 1year
Orbit apolune 13700km
perilune 350km
inclination 36.248deg
duration 21hour
Low Frequency Interferometer Weight 2kg
Power 15w
Antenna Triple dipole
Length 1m
Frequency range 1MHz~30MHz
Channel number 3
Spectrum resolution3.2. The CE-4 lander
The far side of the on the far side of the moon, components of the electric
Moon is recognized as a Very Low Frequency field of the waves from
the best place for low Radio Spectrometer is solar burst or cosmic
frequency radio astronomy installed on the Chang'e space. According to the
observations. The Moon 4 Lander (Figure 5). Its theory of electromagnetic
can effectively shield radio scientific mission is mainly wave propagation, the
waves from the Earth, as to explore the radiating intensity and polarization
well as those from the characteristics of the characteristics of the
Sun at night. Therefore, electric fields from radio total electric field can be
low-frequency radio bursts during the lunar obtained by processing
astronomical observation day, and to study the of the three electric
at 10KHz~40MHz offers ionospheric characteristics field components. The
the opportunity to discover over the landing area. frequency spectrum and
new phenomena and time-varying information
laws in the evolution of The Very Low-Frequency of the electric field can be
celestial bodies. Using the Radio Spectrometer uses obtained too. In addition,
opportunity of Chang'e-4 three orthogonal active using the amplitude
exploration probe landing antennas to receive three and phase of the three-
component electric field,
the direction of arrival of the
wave can also be obtained
after data processing.
The composition of the
very low frequency radio
Spectrometer system is
shown in Figure 6. As
shown in Figure 6, the
very low-frequency radio
Spectrometer is mainly
composed of electronic
unit, preamplifier, three
receiving antennas and
Figure 5: Tripole antennas of the very low frequency radio
cable s. The electronic unit
spectrometer. Credit: Institute of Elecotrnics, CAS.
12 太空|TAIKONGFigure 6: System of the very low frequency radio spectrometer. Credit: Institute of Elecotrnics,
CAS.
is composed of controller, landed on the far side of internal calibration and data
distributor, clock module, the moon successfully. acquisition modes were
multi-channel receiver, At 00:40 on January 4, carried out respectively.
internal calibration the receiving antennas A, The equipment worked
module, and interfaces B and C of VLFRS were normally, the telemetry
with the Lander. The main deployed smoothly under data were normal and the
performance requirements the control of ground scientific data were correct.
of low frequency radio commands, and their
spectrum analyzer are deployment lengths were
shown in Table 2. 5 m. At 9:20 on January 5,
2019, the low-frequency
On January 3, 2019, radio spectrum analyzer
Chang'e-4 lunar probe was powered up. The
太空|TAIKONG 13Frequency 100kHz~40MHz
Sensitivity 7.5nV/√Hz
Dynamic Range ≥ 95dB
Frequency ≤ 5KHz (100KHz~2.0MHz)
Resolution ≤ 200KHz (1.0MHz~40MHz)
Receive Antenna Three 5m orthogonal monopole antennas
Bit-Rate ≤5Mbps
Power 24W
Table 2: Technical Specifications of VLFRS
3.3. The NCLE experiment
The Netherlands-China signal, NCLE also aims cross-calibration with Earth
Low-Frequency Explorer at observing the Sun and based radio telescopes,
(NCLE) is a low-frequency large solar system planets. and the band above 80
radio astronomy and space kHz MHz suitable for space
science payload on-board The payload is outside the science, and planetary and
the CE4 relay satellite Earth’s ionosphere and plasma physics. Although
orbiting the Earth-Moon L2 relatively far away from the digital electronics is
Lagrangian point. NCLE terrestrial interference capable to cover the entire
is a pathfinder mission for although that will still band in one go, the band
a much larger future array be detectable. NCLE is was split in five sub-bands
of small satellites aimed equipped with three length for system linearity reasons
at observing the earliest 5m monopoles that can (Prinsloo et al. 2018).
phases of our universe, the be configured as (two) Currently NCLE is in the
Dark Ages period in which dipoles. It covers the commissioning phase.
the first stars and galaxies 1-30 MHz band virtually
were formed. Apart from inaccessible from Earth
constraining estimates for radio astronomers, the
of the global Dark Ages band 30-80 MHz allowing
14 太空|TAIKONG4. THE SCIENCE OPPORTUNITIES AT LOW FREQUENCY
4.1. Potential of Discovery
Radio astronomy began in tool for studying pulsars, – transformational
1933 from the unexpected HI emission and discoveries, was built
discovery of a radio noise conducting planetary with at least one its main
of extraterrestrial origin radar experiments. specification significantly
detected by Karl Jansky. The Very Large Array exceeding any other
Throughout its history over (VLA) and Westerbork instrument. Such the
more than eight decades, Synthesis Radio Telescope specification might deal
radio astronomy offers significantly outperformed with sensitivity, spectral,
numerous examples of original expectations as time or angular resolution,
unexpected discoveries. radio imaging instruments spectrum coverage, etc.
These include detections in continuum and spectral In other words, broadening
of radio emission from line regimes. The latest up coverage of at least
celestial radio sources in example is the Canadian one major parameter of a
continuum (e.g., pulsars, Hydrogen Intensity new experimental facility
Fast Radio Bursts) and Mapping Experiment, practically guarantees
spectral lines (e.g., OH CHIME, which has outstanding discoveries.
masers). Moreover, become recently the most
almost all major radio productive discoverer of In this sense, there is every
astronomy facilities Fast Radio Bursts (FRB’s). reason to expect that a
built in the past nearly Even from the name of this prospective space-based
60 years demonstrated experiment it is obvious ultra-long wavelength
their superb potential in that the original prime task facility will continue
making discoveries not of the CHIME observatory the trend of delivering
in the areas for which was unrelated to FRB’s. unexpected pioneering
they had been built. For All these and many other discoveries. The reason
instance, the Arecibo radio examples have in common for such the expectation
telescope was conceived one major characteristic: is simple: just as many
as a facility for studying each radio astronomy other outstanding radio
back-scattering effects in instrument that proved astronomy facilities of the
the ionosphere, and not to carry a potential for past decades, the ULW
as a prime astronomical outstanding, sometimes radio observatory will open
太空|TAIKONG 15up a large unexplored area spectrum of cosmic in the large unchartered
of a major parameter of a emission. While all declared spectrum domain have
telescope, its spectrum science tasks of the ULW a potential of becoming
coverage. The ULW domain facility do deserve careful dominant in the facility’s
is the last unexplored area observational studies, operational agenda.
of the electromagnetic unexpected discoveries Expect unexpected!
4.2. Cosmic Dawn and Dark Ages
The Big Bang left its elusive sky that have been by all inflationary models,
fingerprints on the cosmos, detected and mapped as requires a thousand fold
but the mystery remains. millions of tiny ripples, the increase in precision
The scientific consensus is seeds of all large-scale in measuring the PNG
that all we see in the visible structure in the universe. parameter fNL. The Planck
universe emerged from However, this approach satellite experiment,
what is called “inflation”, limits cosmology to at most and any future CMB
an immensely rapid 0.1 percent precision. This experiments, are limited
expansion that occurred falls well short of what we at best to fNL ~ 10. Future
some trillionth of a trillionth need to search for the surveys of billions of
of a second after the Big fingerprints of inflation. The galaxies will yield fNL~1.
Bang. The rapid expansion only guaranteed signal, To improve precision by
left a pattern of scattered primordial non-Gaussianity another 100-fold, we need
photons on the microwave (PNG), robustly predicted to go beyond galaxies, to
Figure 7: The Dark age and Cosmic Dawn. Credit: NAOC.
16 太空|TAIKONGFigure 8: The global signal (left) and the corresponding power spectrum (right) for different
model parameters. The colored dots on the right panel mark the different stages of evolution:
Lyman alpha coupling (red dot), heating transition (green dot) and mid point of reionization (blue
dot). The black solid curve is the standard model. Credit: Cohen et al. 2018.
their millions of gas cloud formation, when the signal energies above 13.6 eV
precursors, detectable by during the dark ages ionizes neutral hydrogen
searching in the so-called (the period prior to star in the intergalactic
“dark ages” before there formation) is fully dictated medium. These processes
were any galaxies, via by atomic physics. As soon are responsible for the
highly redshifted (z~50, as the first stars form, their deep absorption trough
the sweet spot for the lights dramatically change (centered at the frequency
predicted signal) 21 cm the environment strongly of about 60-90 MHz)
absorption against the affecting the 21-cm signal. and the emission peak
CMB. Sensitivity at such During cosmic dawn (at 100-150 MHz) in the
low frequencies (~30 MHz) (Figure 7), ultraviolet (Lyα) global signal (Figure 8).
may only be achievable radiation produced by first Owing to the patchiness
by the far-side lunar radio stars couples the 21-cm of primordial star formation
arrays (Jester & Falcke signal to the thermal state and the finite distance
2009). of the gas, which is cooled from each source out to
by the expansion of the which the photons can
At lower redshift, the shape Universe and heated by propagate before being
and the features of the X-ray radiation of first black absorbed or scattered,
21-cm signal are tied to holes. In addition, radiation the radiative backgrounds
the astrophysics of galaxy produced by stars at (Lyα, X-ray, ionizing) are
太空|TAIKONG 17fluctuations of the dark age
and cosmic dawn requires
very high sensitivity and
stability, which could only
be achieved with a very
large area array on the far
side of the Moon in the
distant future. However, a
small array such as the DSL
could map the foreground
radiation, which would be a
useful and necessary first
step toward that direction.
The signal-to-noise ratio
of a global spectrum
detection is however
independent of the receiver
Figure 9: The number of cross dipoles required for high-z collecting area (for a filling
21cm tomography. Credit: Jester & Falcke 2009. factor of unity, as is the
case for three orthogonal
dipoles) and hence could
not uniform across the sky. pattern of Baryon Acoustic be carried out with a
These fluctuations result Oscillations in the 21- modest mission. The global
in variability of the 21- cm power spectrum from spectrum can be measured
cm across the sky. Exotic cosmic dawn (Fialkov to a high precision of
processes, such as baryon- 2014). However, in most interest to cosmology
dark matter scattering cases, star formation study with a single
(Barkana 2018) or neutrino “contaminates” the signal antenna in a short time, as
decay, might affect thermal from cosmic dawn and already demonstrated by
and ionization histories of reionization, and the dark experiments such as the
the gas, modifying the 21- ages remain the optimal EDGES, SCI-HI, SARAS,
cm signal from the dark period in cosmic history to and so on. The sensitivity
ages, cosmic dawn and search for the imprints of however depends on
reionization. For instance, the exotic physics. effective control of the
velocity-dependent systematics. In some
scattering cross-section Due to the large foreground aspects, a space-borne
results in enhanced radiation, mapping the experiment can be very
18 太空|TAIKONGadvantageous as the ground reflection artifact frequency-dependent
problem of ionosphere which may generate fake beam effect must be tamed
distortion can be largely absorption signals (Bradley for the measurement to be
avoided. A lunar orbit et al. 2019). However, successful.
mission also helps to the RFIs generated by
reduce the effect of the satellite itself and any
4.3. Helio Physics and Space Weather
The active Sun exercises the most violent of which are observational capabilities
a fundamental influence identified above the solar CME in particular to
on the Earth's geosystem surface as coronal mass describe 3D structures
thereby affecting the ejections (CME), clouds evolution in time.
quality of life on Earth of highly ionized plasma
and the performance of ejected into interplanetary Three main types of radio
technological systems. space. Despite their great bursts are observed from
Plasma instabilities importance to life on Earth, the Sun particularly in its
below the solar surface the physical mechanisms active state, both related
(chromosphere) governing such events are to flares and CMEs.
sporadically generate poorly understood. This Type II bursts have a
bursty releases of energy is in part due to limited frequency drift with time
Figure 10: An example of solar burst detected by PL610 single LOFAR station P. Credit: Space.
太空|TAIKONG 19at rates consistent with
the speed of the shock
through the solar corona
and interplanetary medium
(~1000-2000 km/s). Type
III bursts are emitted by
mildly relativistic (~0.1
- 0.3 c) electron beams
propagating through the
corona and interplanetary
space that excite plasma
waves at the local plasma
frequency. Their frequency
drift rate is much higher Figure 11: The IPS measurement. Credit: Space Research
than that of Type II bursts. Center, Polish Academy of Sciences.
Type IV bursts are emitted
by energetic electrons in subsystem is strongly emissions, future missions
the coronal magnetic field coupled via the electric such as DSL will allow
structure field, particle precipitation, monitoring and modelling
heat flows and small scale of plasma instabilities in
To develop a quantitative interaction. The wave- the solar corona and wave-
model of energy transfer particle interactions in particle interactions in
from Sun to the ionosphere- radiation belts region are the activity centres of the
magnetosphere system it is one of the key parameters Sun. DSL will offer great
necessary to consider the in understanding the opportunities for radio
plasma wave interaction. global physical processes studies of the solar wind
It is very hard to judge which govern the near- and the heliosphere. It
which physical process Earth environment. The will permit observations of
dominates during a efficiency of the solar driver solar radio bursts at low
geomagnetic disturbance, depends on the prevailing frequencies with much
in particular it seems specific properties and higher spatial resolution
necessary to bind the preconditioning of the than possible from any
observations from different near-Earth environment. current space mission. It
region of Sun-Earth system. will also allow observations
By providing dynamic much further out from the
The magnetosphere- spectra and detailed solar surface than possible
ionosphere-thermosphere imaging of the solar radio from the ground, where
20 太空|TAIKONGFigure 12: The LOFAR4SW experiment. Credit: Space Research Center, Polish Academy of
Sciences.
the ionosphere confines relative contributions for all-sky astronomical
the field of view to within of shocks and ejecta in measurements.
a few solar radii. DSL will interplanetary CMEs and Complementary terrestrial
dynamically image the track the evolution of these LOFAR observations will
evolution of CME structures structures separately. The provide the heliospheric
as they propagate out into combined in situ and remote context for DSL
interplanetary space and solar observations could in diagnostics. The ongoing
potentially impacts on the the next decade give us a project LOFAR4SW in the
Earth's magnetosphere much better understanding frame of European H2020
of heliophysics, solar- program will deliver the full
In combination with terrestrial physics, and conceptual and technical
interstellar plasma space weather. DSL design for creating a new
scintillation (IPS) will also provide an ideal leading-edge European
measurements from low-noise facility for IPS research facility for space
terrestrial radio telescopes, observations in its own weather science. In a major
particularly LOFAR, it right, with the additional innovation, LOFAR4SW
should be possible to virtue of providing will prepare for a large
distinguish between the heliospheric calibration scale high-end research
太空|TAIKONG 21facility in which completely signal paths provide radio astronomy and space
simultaneous, independent continuous access to two weather research.
observing modes and research communities:
4.4. The Planets, Exoplanets
The Earth and the four giant instability (CMI), operating via similar mechanisms.
planets in the solar system at the local electron If detected, they would
have magnetospheres, cyclotron frequency in the provide many parameters
where energized keV-MeV magnetospheres of these on the exoplanetary
electrons produce intense radio-planets, is confirmed system. The observation
non-thermal low frequency as the general mechanism would however be very
radio emissions in the for radio emission at the difficult, many spacecraft
auroral regions near and polar regions. Planetary and integration will be
above the magnetic poles, lightning is another source required.
and these affected by the of low frequency radio
solar wind and satellite emission. Ground and space
interactions. The solar measurement are
system radio sources are The radio waves may also complementary. In
very bright, Jupiter and Sun be produced by exoplanets particular, space-
have equivalent brightness
in decametric range. The
planetary radio emissions
are intense but sporadic
and strongly anisotropic.
The Jupiter is alway active,
and emits up to 40 MHz
which can be observed
from the ground. The other
planetary radio sources are
emitting primarily below 1
MHz. which puts them far
below the cutoff frequency
of the Earth ionosphere.
Figure 13: The flux density of various low frequency sources.
The Cyclotron Maser
Credit: B. Cecconi et al. (2018).
22 太空|TAIKONGborne observation and sensing capability for the planetary emission on
monitoring may provide auroral plasma study. The hourly time scale. In future,
useful information at the DSL is a near-linear array more systematic lunar orbit
lower frequencies. The and has limited snapshot- observation may also be
fine structure of the radio imaging capability, achieved with a swarm of
emission provides remote nevertheless it can observe satellites
4.5. Cosmic Ray and Neutrinos
The research on nature particle physics. Since the cosmic ray and neutrinos.
and sources for the highest neutrinos are chargeless When a high-energy
energy particles is one of weakly interacting, particle interaction occurs
the major topics of modern ultrahigh energy neutrinos in a dense medium like ice,
high-energy astrophysics (UHEN) can propagate rock salt, lunar regolith,
and particle physics. To unaffected over cosmic and the atmosphere, it
date a couple of tens of distances, and therefore draw electrons off the
cosmic ray events have their arrival directions carry surrounding medium and
been observed with direct information on their transferring them into
energies in excess of sources. Observations of the shower disk. With the
1020 eV, these are the so- UHEN would open up a annihilation of shower
called ultra-high-energy new window on the highest- positrons in flight, there
cosmic rays (UHECR). energy astrophysical will be a net excess of
According to our current process. electrons. These fast
understanding of the excess electrons can emit
Universe, such particles The big problems of both radio waves through the
should not exist because UHECR and UHEN event Cherenkov mechanism.
of the spectrum cut-off ~6 detection are not from the Using this mechanism
× 1019eV caused by the character of the event, but many experiments have
Greisen-Zatsepin-Kuzmin from their extreme rarity. been performed or
(GZK) effect. In any case, The Moon offers a very huge proposed to detect high-
these particles need further natural detector volume, energy cosmic particles,
experimental investigation. and is first proposed by as GLUE (Gorham et al.
Detection of UHE neutrinos G.A. Askaryan (Askaryan 2004), LUNASKA (James
is also of great importance 1962) to be a target to et al. 2010), and NuMoon
for both astrophysics and detect showers initiated by (Scholten et al., 2009) etc.,
太空|TAIKONG 23which are all based on terrestrial radio telescopes, UHEN events per year. the terrestrial telescopes. a lunar orbit radio telescope With the decreasing However, the radio signal will be more competitive of the orbit height, the attenuation over the long since it is close to the lunar UHECR detections will distance between the Moon surface and lack of the be increased greatly. The and the Earth severely atmosphere. Preliminary joint observations between limits the detections, and studies show that a DSL- multi elements of the array the refracting of radio like array operating at the will further provide the waves by the ionosphere frequencies (
observations of DSL will new multi-frequency all- will reveal the distribution
provide an excellent means sky map at the ultralong of the ISM, showing the
of tracing the ISM in this wavelength where ISM ionized gas around the
phase. Besides information absorption become solar system in detail
dervied from the pulsar significant and highly
dispersion measure, the sensitive to frequency
4.7. Extragalactic Radio Sources
One of the major goals of an synchrotron self-absorption However, the particle
ultra-low frequency survey spectrum if the source were acceleration mechanisms
would be building the first sufficiently compact and in galaxy clusters are
catalog of extragalactic optically thick (Jester & unknown. Low frequency
radio sources at Falcke 2009). The strength observations could probe
frequencies lower than 30 of magnetic field and the giant radio halos in
MHz. Most sources in the the free-free absorption merging clusters, or mini
radio sky are extragalactic by the local electrons halos in cool-core clusters.
sources, mainly including can also modify the The steep spectra of
active galactic nuclei, observed spectral energy cluster halos imply that
radio galaxies, and galaxy distribution. In addition, lower frequencies are more
clusters. However, there is due to the spectral aging promising to detect the
till barely any observational effect (Alexander and cluster halos, specifically
constraints on the spectral Leahy, 1987; Blundell and for low mass and high-
energy distribution of them Rawlings, 2001), lower- redshift objects (Cassano
at frequencies lower than frequency observations et al. 2006, 2008). Savini
20 MHz. Any observations probe the older parts of a et al. (2018) have found
of the spectral indices source and can therefore the first evidence of diffuse
of these sources will be used to constrain the ultra-steep-spectrum radio
constrain the mechanisms age of a source. emission surrounding the
of radiation and absorption, cool core of a cluster, which
and shed light on the Galaxy clusters are means that under particular
nature and environments extended radio sources, circumstances, both a mini
of the sources. In deed, where the plasma in the and giant halo could co-
the typical synchrotron intracluster medium emits exist in a single cluster.
spectrum would change to synchrotron radiation. Observational constraints
太空|TAIKONG 25on the spectral properties could further constrain the formation history and the
and the sizes of them magnetic fields of clusters
4.8. SETI
The Search for Extra- interference (RFI) and the requirements of high
terrestrial Intelligence other astrophysical signals. time and frequency
(SETI) is always important Using the Moon to shield the resolution can be quite a
for humans’ curiosity, and RFI from the Earth, the DSL challenge with the current
aims to test the hypothesis project could significantly technologies, the DSL array
that extraterrestrial reduce the number of false observing at the far side
civilizations emit detectable positives from terrestrial of the Moon will be a very
signals using a technique transmissions, and increase promising complementary
similar to what we have, the confidence in the to the bands covered
preferentially in radio band detection. The advantages by the ground-based
(e.g. Croft et al. 2018). of using an interferometer experiments, placing
A major issue with the array with a wide field of extra constraints on the
detection of SETI signals is view in searching for a prevalence of civilizations
to distinguish the signal from SETI signal are highlighted in the Universe.
terrestrial radio frequency in Garrett (2018). Although
5. THE DSL MISSION
The Discovering Sky at mission is to map the sky cosmic dawn, as it is not
the Longest wavelength below 30 MHz using the affect by the ionosphere
(DSL) concept consists constellation of satellites disturbance and RFIs
of a constellation of as an interferometer in ground observation.
satellites circling the Moon array. Another goal of the The linear configuration
on nearly-identical orbit, mission is to make high allows the relative
forming a linear array while precision global spectrum positions of each satellite
making interferometric measurement over the to be measured with very
observations of the frequency range of interest limited instrumentation-
sky. A major goal of the to study of the dark age and -star sensor camera for
26 太空|TAIKONGFigure 14: An artist’s concept of the DSL mission. Credit: NAOC.
angular measurement, and are each equipped the high frequency band
microwave line for distance with electrically short spectrometer which shall
measurement, the latter antennas and receivers make the global spectrum
also serves as inter-satellite to make interferometric measurement in the
data communication and observations, while the frequency above 30 MHz.
synchronization system. mother satellite will collect At present we consider
The constellation includes the digital signals from a circular orbit of 300 km
one mother satellite and a the daughter satellites for height which is sufficiently
number of (tentatively set interferometry correlation, stable against lunar gravity
as 8) daughter satellites. and transmit the data perturbations.
The daughter satellites back to Earth. It also has
5.1. Array Configuration
The constellation observations with a range satellites will be docked in
is designed to be of different baselines the mother satellite during
reconfigurable in orbit, formed between the launch and lunar transfer,
allowing interferometric satellites. The daughter then sequentially released
太空|TAIKONG 27after entering into the lunar The gaps between the all directions, and the Moon
orbit to form the linear rings can be partially only shields a small fraction
array. An artist impression filled by using bandwidth of the sky, there is a mirror
is shown in Figure 14. synthesis and by varying symmetry between the two
the relative distance sides of the plane which
The daughter satellites between the satellites. It cannot be distinguished
form multiple baselines, should be emphasized using the interferometry
Figure 15 shows the that the satellites do data of a single orbit
evolution of the baseline not need to have fixed alone. However, the orbital
vectors between the relative positions, as long plane processes a full 360
daughter satellites as as the positions can be degrees in 1.29 years,
the array circles around, determined the array could so after a few orbits, the
generating concentric rings work. aperture plane will be tilted.
in the so called uv plane (u, Sources in both halves of
v are Cartesian coordinates As the short dipole antenna the hemispheres will have
in units of wavelength). are sensitive more or less in different projections on
Figure 15: Evolution of the spatial aperture plane filling after one orbit of DSL (down) as it orbits
the Moon (up). Credit: NSSC.
28 太空|TAIKONGof the maximum baseline
would not improve the
imaging resolution, but the
technological difficulty in
position measurement and
data communication would
increase significantly.
Most of the known
radiation mechanisms at
low frequency such as
the synchrotron and free-
free radiation produce
continuum radiation,
for which high spectral
resolution is not needed.
The foreground subtraction
Figure 16: Cumulative filling of the 3D aperture over of the redshifted 21cm
respectively 90, 180, 270 and 360 degrees of orbit precession. line also only requires
Credit: NSSC. a moderate spectral
resolution. An exception
the two aperture planes The angular resolution of to this is found in the radio
and can in principle be the array is determined recombination lines, which
separated. Figure 16 by the maximum baseline. requires a sub-kHz spectral
shows the cumulative However, interstellar resolution. However, as the
filling over time (many medium (ISM) scattering satellite array is circulating
orbits) of the aperture can broaden the point the moon and the baselines
with visibility sample source to about 0.5 degree are moving, fine spectral
points. After 360 degree at such low frequency, resolutions are required to
precession, the visibility and the broadening by keep coherence.
filled a 3D “doughnut” interplanetary medium
shaped structure. This (IPM) may be even larger Detection sensitivity
dataset is similar to a 3D (Jester & Falcke 2009). The at long wavelengths is
hologram, and a sky image maximum baseline for DSL strongly influenced by
can be reconstructed by is set as 100 km, which the confusion of multiple
linear inversion (Huang at 1MHz corresponds sources nominally above
et al. 2018), as shown in to a resolution of 0.17 the sensitivity. Confusion
Figure 17. degree. Further increase limits identification and
太空|TAIKONG 29measurement of individual
sources, and occurs
when there is more than
one source in every 30
synthesized beam areas.
Extrapolation of source
count statistics of the VLA
74 MHz and the Parkes
80/178 MHz Catalog lead
to an estimated one sigma
confusion noise limit of 5.6
Jy and 65 mJy for 1 MHz
Figure 17: A reconstructed sky map with simulated DSL
(10’ beam size) and 10 MHz
observation. A few bright spots are added to the lower left part
(1’ beam), respectively.
of the map the to illustrate the reconstruction for point sources.
After observing with 8
Credit: Q. Huang et al. (2018).
antennas at 1 MHz (1
MHz bandwidth) for a few
would allow a detection MHz) to cover the full 20-
weeks, DSL will reach
of the dark age signal at 100 MHz range
the confusion noise limit
SNR=10. Reaching this 5
above, which effectively
mK accuracy requires >60 A summary of the DSL
sets the limit to imaging
dB pass-band calibration. requirements is listed
sensitivity of DSL. At 10
For foreground subtraction, in Table 3. It basically
MHz mapping sensitivity is
the full band needs to be covers all requirements the
limited by integration time
observed with a spectral science cases described
primarily, not by confusion.
resolution of 1 MHz. We above, appended with
expect that these need to a few requirements
For the Dark Ages science
be combined with higher- stemming from technical
case, the goal is to reach 5
frequency measurement considerations as will be
mK error on the brightness
of the global sky from the explained in the following
temperature estimate at 20
ground and possible from sections.
MHz, within eight months
of mission duration: this space (i.e. DARE at >40
30 太空|TAIKONGNo. Requirement Value
1 Frequency range 1~30MHz for imaging
30~120MHz for high precision
spectrum
2 Spectral resolution 1MHz for imaging
10kHz for high precision
spectrum
3 Total integration time 1.29y for imaging
4 Baselines Maximum length 100km
Minimum length 100m
5 Mission lifetime 5y
Table 3: Summary of DSL requirements
5.2. Satellite and Payloads
The mother and daughter satellite carries the ring of the first consideration
satellites are launched daughter satellites on its for antenna design,
together to lunar orbit. As external sides. This combo and thermal effect and
illustrated in Figure 18, the sits above a propulsion mechanical strength will
upper part of the mother module, which propels also be taken into account.
satellite has deployable the satellites during the A deployable ground plate
solar panels, high gain Earth to Moon transfer and will be mounted right under
antenna (for ground separated after entering the the antenna to ensure the
communication), high gain lunar orbit. The daughter beam pattern not affected
ISL antenna and other satellites are then released by the satellite itself. A
sensors and antennas, one by one. precise calibration system
and on top of these sit will be embedded into
the cone-shaped high To measure the 21cm global the receiver as a core
frequency spectrometer spectrum in the frequency module, and differential
(HFS) antenna and its range 30-120MHz, a measurement will be
ground plane which dedicated spectrometer used in the receiving
shield it from the rest of is placed on the mother system.
the mother satellite. The satellite. A frequency-
lower part of the mother independent beam is
太空|TAIKONG 31bands (1 kHz) within the
1-30 MHz range are used
for interferometry.
The satellites will naturally
have some velocity
differences under the
irregular gravity field of the
Moon, and without control
they may run away. The
formation is automatically
controlled to keep the
linear array stable. A
Figure 18: A split diagram of the Mother satellite. Credit:
reconfiguration strategy
NAOC.
which balances the fuel
consumption among all
daughter satellites include
two steps, step 1: keep S1
stationary, S2-S8 shrink;
step2: keep S8 station,
S1-S7 expand, as shown
in Figure 20. We estimate
the reconfiguration period
is about 20 days, each
satellites needs 80mm/s
Figure 19: Daughter satellite with LFIS antenna deployed.
delta-V in each formation
Credit: NSSC.
reconfiguration control
Each daughter satellite better than 1 ̊ , attitude period.
is an identical cuboid, stability better than 0.1 ̊/s,
The inter-satellite
which carries a low and attitude measurement
communication, ranging,
frequency interferometer better than 0.01 ̊. Each
clock synchronization and
and spectrometer (LFIS), has three orthogonal
angular measurement
as shown in Figure 19. short dipole antenna, a
is provided by the inter-
The daughter satellites receiver, and a digitizer.
satellite dynamic baseline
have three axis stabilized Limited by the inter-satellite
apparatus (ISDBA). It uses
with respect to the Moon, communication bandwidth,
microwave link (Ka band)
with pointing accuracy a selection of 1000 narrow
32 太空|TAIKONGNo Item Value
1 Antenna 2.5m three-dipoles
2 Frequency range 1MHz-30MHz
3 Spectral resolution points 8192
4 Receiver gain stability 0.02dB
5 Sensitivity 1000K@30MHz (1MHZ,
10minutes integrated time)
Table 4: Summary of LFIS parameter
for data communication (40 mentioned science determined by comparing
MHz) between the mother requirements. For the with star map (Figure 22).
and daughter satellites. angular measurement, The baseline positioning
The same microwave line an LED light array is put precision is set to be
is also used for ranging on the mother satellite 1m each direction (1/10
and synchronization by for identification, and the wavelength at 30 MHz).
using the dual one-way star sensor cameras on This requires a ranging
ranging (DOWR) principle the daughter satellites will precision of 1 m and an
(Figure 21). The scope take photos in the direction angular precision of 10urad
of inter-satellite baseline of the mother satellite at 100km. The parameters
is from 100m to 100km in against background stars, are summarized in
order to meet the above- the baseline direction is Table 5.
Figure 20: Formation reconfiguration strategy. Credit: NSSC.
太空|TAIKONG 33Figure 21: The principle diagram of dual one way ranging measurement. Credit: NSSC.
Figure 22: Angular position determination. Credit: NSSC.
No Item Value
1 Communication distance 1km-100km
2 Ranging accuracy 1m@100km
3 Clock synchronization 3.3ns@100km
4 Inter-satellite data transmission Adjustable, up to 40Mbps for
rate each daughter satellite
5 Angular accuracy 10urad@100km
Table 5: ISDBA parameters
34 太空|TAIKONG6. SYNERGIES
A number of both ground- science objectives, from approaches in their design,
based and space-borne the detailed study of the so that they provide both
low frequency radio Sun and planets and independent checks and
experiments are currently space weather, to the also complementary to
operating or being planned. exploration of the dark each other.
These experiments are ages and cosmic dawn,
aimed for many different and adopted different
6.1. Ground Global Spectrum Experiments
Examples of current Hidrógeno Neutro (SCI- the exception of EDGES.
ground-based Global 21- HI, Voytek et al. 2014), the These include:
cm experiments operating Large-aperture Experiment
between 50 and 200 MHz to detect the Dark Ages • An upper limit for the
include: the Experiment (LEDA, Price et al. 2018), absorption amplitude of
to Detect the Global Probing Radio Intensity 0.89 K at 95% confidence
Epoch of Reionization at high-z from Marion by LEDA (Bernardi et al.
(EoR) Signature (EDGES, (PRIzM, Philip et al. 2019), 2016);
Bowman et al. 2018a), and the Cosmic Twilight
the Shaped Antenna Polarimeter (CTP, Nhan • Upper limits of 1-10 K by
measurement of the et al. 2019). Monsalve et SCI-HI (Voytek et al. 2014);
background RAdio al. (2019) summarized the
Spectrum (SARAS, Singh results from these single- • Upper limits on the power
et al. 2018), the Sonda antenna experiments and spectrum by MWA (Ewall-
Cosmológica de las Islas several radio arrays, all of Wice et al. 2016) and by
para la Detección de which are upper limits with LOFAR (Gehlot et al. 2018).
6.1.1. The EDGES experiment
The Experiment to Detect (EDGES) is a pioneering measured the sky-averaged
the Global EoR Signature experiment that has radio spectrum since
太空|TAIKONG 35You can also read
