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/
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:
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.