Introduction to Meteor Scatter Operation by Marc C. Tarplee, Ph.D. N4UFP
Introduction • The earth is bombarded 24/7 by small meteors • They generally arrive from a variety of directions • They arrive with random velocities reaching up to 70 km/sec (45 mi/sec) Leonid Meteor Shower from Space
Introduction • The rate at which meteors the Earth’s atmosphere peaks at around 6:00 AM local time • Between midnight and 6:00 AM the Earth is turning into the path of oncoming meteors • Meteors strike with higher velocity during this time.
Introduction • The meteor strike rate also varies annually. • Approximately twice as many meteors strike the Earth’s atmosphere per hour during the summer as during the winter.
Because the Earth rotates and moves around the Sun, random meteors appear to cluster around one of two “hot spots” approximately +/- 10 degrees off the great circle bearing between transmitter and receiver Which hot spot has the greatest meteor concentration depends on the time of day Meteor Scatter Path Geometry“Hot Spots”
Meteor Trails • As meteors are vaporized in the upper atmosphere, they leave behind ionized trails at heights of 60 – 70 miles that are sufficiently dense to reflect radio waves in the HF and VHF range. • A long trail lasts only 15 seconds – most trails are less than 1 second long • Meteor trails are classified according to electron line density: • Underdense (n < 2*1014 m-1) • Overdense (n >= 2*1014 m-1) • Most meteor trails are underdense
P0 = RF power scattered by meteor PT = transmitter power GT, GR transmit and receive antenna gains L = RF wavelength re = classical electron radius Signal Strength Equation • q = line electron density • g = angle between E-field vector an the direction to the receiver • f = scatter half-angle • b = angle between meteor trail and propagation plane • RR, RT = distance from transmitter and receiver to scatter point
Meteor Trails and RF • The electron density within an ionized meteor trail is often high enough to reflect RF • The amount of scattering decreases with increasing frequency.
Meteor Trails and RF • After the trail has been created, electrons and ions slowly recombine, reducing the ionization and scattering ability of the trail • The length of time over which a trail can scatter RF decreases rapidly with increasing frequency.
Meteor Showers • In additon to the random meteors that continuously strike the Earth, there meteor showers that occur throughout the year, during which the meteor rate climbs above 100 meteors per hour. • Meteor showers occur when the Earth passes through a region of space rich in debris, perhaps the leftovers of a destroyed comet. • Meteor showers are generally named after the constellation from which the meteors apparently come. • The most famous meteor shower is the Perseid meteor shower which occurs around 12 August.
Meteor Scatter Communications • Under normal circumstances, at least 5 – 10 large meteors per hour enter the Earth’s atmosphere and leave behind ionized trails, so it is possible to communicate by scattering RF off of meteor trails • Since individual trails last for only a short time, information must be sent in small packets • The exact time a trail will occur is unknown, so the information must be sent repeatedly. • To keep the SNR high, the signal bandwidth should be as narrow as possible.
Meteor Scatter Modes • The earliest experiments with MS communications used CW and AM phone. • Today, the following modes are used: • HSCW • SSB • FSK-441
HSCW • CW sent at high speeds (200 wpm or more) • Used primarily for meteor scatter • Is being phased out in favor of new digital modes • Most QSO’s are made via sked using defined protocols • HSCW activity occurs primarily in the following band segments: • 50.250 – 50.300 MHz • 144.100 – 144.150 MHz • 222.000 – 222.200 MHz
HSCW Protocols • Messages sent in 30 sec intervals – westernmost station transmits during first 30 seconds of each minute. • CQ’s are sent in 60 second intervals. • In North America a speed of 6000 lpm (1200 wpm) is the standard • A complete HSCW QSO consists of exhange of call signs, signal reports and rogers by both stations • Message sequence • 1. both call signs (ex: N0ABC N4UFP) • 2. call signs plus signal report (ex: N0ABC 26 N4UFP 2626) • 3. roger + report (ex R27 R27) • 4. final roger (RRR RRR) • Both stations begin by sending both call signs. • First station to copy both call signs sends calls plus signal report. • When other station receives calls + signal report, he sends roger + his report • When first station receives roger + report, he sends his roger. • Final exchange of “73” is optional
SSB • Used primarily on 6 m • Can be used with random meteors, but generally works better during a meteor shower • SSB activity occurs primarily in the following band segments: • 50.125 – 50.250 MHz • 144.200 – 144.250 MHz
SSB Protocols • Messages sent in 15 sec intervals – westernmost station transmits during first and third 15 second periods of each minute. • Exact sequencing is not followed – break in operation is used • A complete HSCW QSO consists of exhange of call signs, signal reports and or grid squares and rogers by both stations • Signal reporting on SSB is generally the burst length: S1 - ping with no information S2 - ping up to 5 seconds long S3 - ping 5 – 15 seconds long S4 - burst 16 – 60 seconds long S5 - burst over 60 seconds • Four character grid squares are used for reports (ex: EM94)
FSK-441 • FSK441 • Uses triplets of 4 tones to transmit data • 882, 1323, 1764, 2205 Hz • Each character is sent as a 3 tone sequence • 43 Character alphabet (letters, numbers . , / ? # $ <sp>) • Single tone characters used for shorthand messages: • 882 Hz - R26 1764 Hz - RRR • 1323 Hz – R27 2205 Hz - 73 • Data rate = 147 characters per second (3 tones/char) • Used for meteor scatter communications • Most activity takes place near 50.270 MHz
FSK-441 Protocols • Messages sent in 30 sec intervals – westernmost station transmits during first 30 seconds of each minute. • A complete FSK441 QSO consists of exhange of call signs, signal reports and rogers by both stations • Message sequence • 1. both call signs (ex: N0ABC N4UFP) • 2. call signs plus signal report (ex: N0ABC 26 N4UFP 2626) • 3. roger + report (ex R27 R27) • 4. final roger (RRR RRR) • Both stations begin by sending both call signs. • First station to copy both call signs sends calls plus signal report. • When other station receives calls + signal report, he sends roger + his report • When first station receives roger + report, he sends his roger. • Final exchange of “73” is optional • Time to complete a QSO ranges from 2 minutes to over an hour.
VHF Station Requirements • RF output at least 100 W • Good VHF transceiver or transverter with MDS of -125 dBm (3 KHz BW) • Mast-mounted pre-amp, gain > 10 dB • Horizontally polarized beam antenna, gain > 10 dB
Antennas for MS • Antennas do not need to have high gain • Lower-gain antennas illuminate more trails • Higher-gain antennas illuminate weaker trails • MS contacts on 6m can be made with a 2 element yagi or quad • Higher gain antennas often result in quicker QSO’s
Operating Notes • Time synchronization is important – be sure to set your PC’s clock to WWV before beginning a MS contact. • MS contacts may be made at any time, but the best time is in the early morning hours (around 6:00 AM) • Links to MS QSO scheduling sites • http://www.pingjockey.net/ • http://www.dxworld.com/hsms.html
Useful Tools • General Meteor Scatter Info • http://www.qsl.net/w8wn/hscw/hscw.html • Aurora Info • http://aurora.n1bug.net/ • Tropospheric propagation prediction tools: • VHF Propagation from APRS data • William Hepburn’s Tropo Forecasts