Denny K0LGI - SNOTEL
Receiving Meteor Reflections Using SNOTEL/SCAN Transmitters
Are you interested in the possibility of monitoring earth detected
asteroids such as 2012 DA14 near earth flyby or
the one that struck the atmosphere on the morning of February 15th
over the city of Chelyabinsk Russia
recently where it became a meteor that exploded causing personal injuries and
property damage?
As unlikely as that may be, it does bring about an interesting question.
Just how can one observe something similar but not as massive as those objects
using modest radio equipment and what does it takes to do that?
There are far more typical smaller alien debris events that occur daily
which are arriving as meteors that are also easily radio detectable.
The hobby of receiving radio reflections from meteors arriving within the
earth’s outer atmosphere that
often create an ionized path that allows some reflected radio signals to
be propagated a considerable distance from an operating transmitter signal
origin to a user’s receiver.
Drawing from: International Meteor Organization Introduction to Forward
Scattering Radio Techniques
During most major meteor shower events that occur during a typical year
there often occurs an exceptional high number of
meteors that are observed visually and by radio. There are also continuing but
lessor meteor ionization trails occurring each day of the year.
Less daily meteor numbers usually occur in the later afternoon and
early evening hours, with the prevalence of higher occurrences during the
remaining hours, with a preference being the local early morning hours.
This has been known historically for many decades by radio experimenters;
hams included and are currently use in by the US government and other commercial
entities. The frequencies used are varied as is the intended purpose.
In this example using the SNOTEL (SNOpack TELemetry) operated by the
National Research Conservation Service and SCAN (Soil Climate Analysis Network)
master transmitters located in several states including
Idaho, Utah, Missouri, Ohio, Mississippi and Alaska are used to provide
the signal source for meteor reflections. These operate primarily for the
purpose of collecting a variety of weather related data including, snowpack, and
soil moisture from remote measuring sites throughout the US.
The master station transmitting sites obtain multiple weather related
datasets from several hundred remote locations throughout the US by
interrogating them by use of its signal.
The master stations operate continuously to provide a high power and
constant reliable source of low band VHF signals operating on a frequency of
40.670 MHz that cover the majority of the United States. They operate on what is
generally considered to be low VHF band frequencies largely due to maximizing
the meteor trail reflection characteristics and proven resulting performance
reliability at these lower VHF frequencies.
Reflected meteor reception occurs at an
atmospheric height of approximately 50 to 80 miles is common. Exact
meteor detection range will vary considerably dependant on the receiving end
overall sensitivity and ambient noise level present including meteor size,
height, meteor path location, orientation and reflected signal levels from it.
It is not uncommon to also see aircraft reflections also
dependent on the aircraft position and size or RCS (Radar Cross Section). The
typical meteor position for optimum detection is usually considered to be near
the midpoint between the transmitter and receiver location. The resulting trace
indicated on the Argo software that is covered later will show a longer duration
image by an aircraft than what a typical meteor leaves and will indicate the
actual Doppler shift returned from the it.
The detection method used is commonly known as forward backscatter from
the object whether of a meteor or aircraft that is very straight forward and
within the capability of a very modest setup, consisting of a communications SSB
receiver and antenna that covers the desired frequency range.
This would be adequate for most to begin receiving meteor returns
audibly, but with the addition of a PC and some additional software designed for
very low signal levels will dramatically enhance the experience and provide a
very good record of intercepts along with a better understanding of meteor
reflected trail signal propagation mode.
The first consideration is the receiver used to hear these mysterious
sounds. A good unit that has a SSB voice bandwidth mode is necessary as it will
provide the reference from which the resulting sounds will be heard and analyzed
when using the low level signal recording PC software mentioned before.
Good receiver sensitivity, frequency accuracy and stability are required
for consistent, repeatable results. USB (upper sideband) detection mode is
typically used. A line level output of the receiver to the PC input is
desirable, but is not essential for use with the software used for recording and
analysis. If a line level output is
not available, a means of attenuating the speaker output may be required to
drive the PC software.
The receiver antenna preference is a Yagi designed or constructed for the
40.670 MHz operating frequency and should be rotated toward the nearest
SNOTEL/SCAN transmitter. It could be a simple two element unit or more will
improve the end result by an increase of signal levels. A good starting point is
to use a horizontal dipole to see if the received signal is strong enough for
use as is, then decide if a higher gain antenna such as a Yagi is needed to
optimize reception.
Usually it is desirable to use low loss coaxial 50Ω cable for this
project, however for cost reduction reasons, some versions of good, fully
shielded 75Ω coax used for CATV can also be used at this frequency and is
readily available almost anywhere. F to BNC connector adapters are also usually
available at modest cost to adapt to the more commonly used BNC antenna and
receiver connections.
To see weaker meteor returns that often are not heard on the receiver
because of the 2 to 3 kHz bandwidth typically used in SSB mode, there is
available a free program that can be run on a Windows PC OS called Argo that
will provide a better detection method and record it visually for later
analysis.
Argo has several setup options for keeping a log of received signals that
are sent for record keeping on the PC of meteor intercepts. It is easy to setup
and use to see extremely weak signals that often occur from some smaller meteor
returns by using very narrow FFT bandwidths that can be set to less than 1 Hz.
It can be downloaded at:
http://www.weaksignals.com/
Example of a SNOTEL-SCAN meteor return using the Argo software
In this example of a SNOTEL-SCAN meteor return, there appears four images
of the resulting reflection from a meteor entry
into the upper atmosphere. The multiple images are caused by the data PRF rate
of the SNOTEL-SCAN transmitted signal and the recording span bandwidth of the
Argo setting. In reality any one of the traces are a valid representation of the
meteor returns and the appearance of four are only due to an artifact of the
Argo software setting.
By narrowing the
ARGO software span bandwidth even further and setting the frequency to where
only one of the traces shown in this image will provide a single meteor return
detail.
Below: ARGO image indicating both meteor and aircraft returns on 40.670 MHz
Below: 40.670 MHz Yagi Antenna Example
This antenna was used successfully for receiving meteor returns from the
SNOTEL Master stations. It can be slightly tilted upward from the horizon for
improvement of meteor midpoint reception dependent on the distance of your
receive site from the SCANSNOTEL master. The gain is estimated at 8-9 dBd and is
connected to the receiver using Belden 9913 low loss 50Ω coax. It could be
constructed of aluminum tubing mounted on a boom for rotary use or in this case,
is made from wire that is supported at both ends at a modest average height of
8’ in a fixed heading from this location.
Initially, a test dipole antenna of the same length stated as DE (driven
element) in the antenna drawing can be used to see if it is a sufficient antenna
for receiving meteor reflection returns. Either way the antenna whether of a
dipole or Yagi design should be aligned facing in the direction most likely to
receive returns from the nearest SNOTEL-SCAN master station transmitter for
meteor reflections.
Below: Picture From: NRCS USDA Fall 2011 SnowNews
Previous SNOTEL Master Station Site – Utah
This picture is of the previous Ogden Utah Master station site that has
since been removed in 2010 and relocated to the nearby Utah Dugway Proving
Grounds location.
Note the numerous multi element Yagi antennas that are used for connecting to
US Western states SNOTEL remote data transmission sites. There are eight
receiving antennas and one transmit antenna at this site for nearly complete
western US coverage)
If you would like to see more of what is being done in radio monitoring
of meteors, check out Stan Nelsons KB5VL Meteor Monitoring website at
http://www.roswellmeteor.com/default.htm
or join the Google RadioMeteors Group for the latest radio meteor observations
and discussion at
https://groups.google.com/forum/?fromgroups#!forum/radiometeors
Enjoy the fascinating sights and sounds of radio detected meteors !
Dennis Condron - K0LGI
Credits and references:
http://www.wcc.nrcs.usda.gov/ftpref/downloads/factpub/soils/SNOTEL-SCAN.pdf
http://www.wcc.nrcs.usda.gov/snotel/SNOTEL-brochure.pdf
http://www.wcc.nrcs.usda.gov/scan/SCAN-brochure.pdf
http://www.wcc.nrcs.usda.gov/ftpref/publications/SnowNews/PastIssues/2011Fall_SnowNews.pdf
http://www.amsmeteors.org/radio/ams203.txt
http://www.imo.net/radio/intro
Tony Tolsdorf, USDA Hydrologist Portland, OR – SNOTEL-SCAN System Information
Alberto di Bene, I2PHD - ARGO software
Rev 006 06-29-13 DC
Note: All
information provided in this document are subject to changes, additions, or
deletions without notice or obligation.