| Introduction |
This method of observation takes advantage of signals from several powerful NATO transmitters operating in the VLF band between 16kHz and 24kHz. Some of these sources operate more or less continuously and offer good stable point source illumination of the ionosphere.
This particular receiver uses a simple ferrite rod antenna and a broadly tuned preamplifier to collect the VLF signals, which are then delivered to a PC soundcard. The rest of the work is carried out by software which separates the incoming signals, measures their amplitudes, and records the timestamped results to the hard disk.
This approach differs slightly from many existing SID receivers, in that they tend to use external receivers (such as a gyrator) to filter and demodulate the signals. From there, they go into an A/D convertor, and the amplitude samples reach the PC via an RS232 serial connection. However the modern PC soundcard offers some irresistible advantages, some of which are:-
| Receiver |
The circuit diagram of the receiver is shown on the right. The antenna coil L1 is made from the EHT overwinding rescued from an old TV set. This resonates at about 23kHz with a Q of around 10 when loaded with the capacitance C1 of the short screened lead which connects the antenna coil to the rest of the circuit.
The signal is amplified by the JFET Q1, which is a BF245B. L2 is another small ex-TV inductor in the range 2mH-4mH and C2 resonates this coil at about 18kHz. Resistors R3 and R4 are chosen to set the Q factor to about 12, giving a voltage gain of about 5-10 from this stage. R3 and R4 also set the bias point of IC1 to about 6 volts.
The combination of the two tuned circuits gives a band pass response, with peaks at 18kHz and 23kHz, dipping a few dB in the middle.
IC1 is a NE5534, R6 and R5 are chosen for a gain of about 10. C4 sets a
low frequency cutoff, and C5 limits the gain at high frequencies so that
the stage does not oscillate.
The small coupling transformer T1 is yet another component pulled from the
scanning circuitry of a TV set. It is a 1:1 transformer with a few mH
of inductance in each winding and a high coupling coefficient. This isolates
the coax which runs to the PC, preventing a ground loop which would otherwise
inject a lot of mains interference.
Zener diode Z1 regulates the first stage supply at about 8 volts and was put in to make sure the gain stayed constant as the battery voltage varied, but it is probably not necessary. The 12V supply comes from a sealed lead-acid battery via an inline fuse. The zero volt rail is not grounded, and it has not been found necessary to use any electrostatic shielding.
The electronics is located a couple of metres from the antenna, in a separate box, and both are floating with respect to both the local ground near the antenna and the domestic mains ground of the house. The battery lives in a third box. All three are waterproof plastic boxes fitted with cable glands. The antenna and electronics boxes are buried just below ground to prevent disturbance by weather and animals.
Overall, the noise floor of the receiver is about 10dB below the natural VLF background, and the received signals are typically 20dB to 50dB above the noise. Earlier experiments delivered reasonable results by just connecting a tuned antenna coil directly to the PC microphone input via a short length of screened lead, but the noise floor was set by the level of domestic mains and computer interference. The main reason for making the external preamp was to allow the antenna to be placed at the far end of a long coax.
It is not essential to use a ferrite rod antenna. A loop antenna wound on a
wooden frame also worked satisfactorily, but after some trials I chose to use the ferrite rod
antenna because it is small and compact. An E-field antenna would not be as good for
this kind of application, because its sensitivity can easily be affected by the weather. For
example a rain shower might well create a leakage path to earth, reducing signals by a couple
of dB, which might be hard to distinguish from a genuine ionospheric disturbance.
| Software |
The PC clock is synchronised by nptd over the Internet and remains within 10mS of UT as long as the PC is online. When the net is unavailable, the clock is slewed by a drift compensation program working from a measured clock drift, which gives a maximum drift of 50mS per day. If undisciplined, this particular PC clock would drift by 750mS per day.
The software runs under Linux as a non-interactive daemon program, and consumes only about 3% of the CPU capacity of a 500Mhz Pentium. Each day's output file is about 100Mbytes, and compresses down to about 30Mbytes for archival.
The source code, along with some notes and a sample configuration file can be downloaded in sidd-1.0.tgz. You will also need to install the FFTW3 package from http://www.fftw.org/, which is a very easy job.
The program can be operated in stereo mode which effectively gives you two independent SID monitors. These can be used, for example, for orthogonal H-field antennas, or, as in my case, another receiver which monitors the E-field.
| Signals Monitored |
Some of the available signals are listed in the table. Others can be received, but as yet they are unidentified. The receiver is located in central UK at 53:42:08N, 2:04:20W.
| Frequency band, kHz | Transmitter | Bearing and Range |
|---|---|---|
| 18.20 - 18.40 | Le Blanc, France, 46:37N 1:05E | 162.8 deg 508.6 miles |
| 19.48 - 19.68 | Anthorn, UK 54:54n 3:18W | 329.6 deg 96.4 miles |
| 22.05 - 22.15 | Skelton, UK, 54:42:24N 2:53:06W | 335.0 deg 76.7 miles |
| 23.30 - 23.50 | Burlage, Germany, 53:05N 7:37E | 92.2 deg 400.9 miles |
| 20.19 - 20.34 | Tavolara, Italy, 40:55N 9:45E | 143.4 deg 1038.8 miles |
| 16.30 - 16.50 | Novik(en), Norway, 66:58N 13:54E | 24.0 deg 1059.0 miles |
Only the first four signals are suitable for SID monitoring. The others are either too weak, or don't
transmit continuously enough. However, they are all recorded, along with a couple of other empty channels
to measure the background VLF noise level.
There is some doubt about Anthorn and Skelton - they may need to be swapped when my DF bearings are improved.
| Normal Daily Variation |
| Scintillation |
The reflecting under-surface of the ionosphere is uneven, which means that
the signal arrives at the receiver via multiple paths - each of which is
continually varying in amplitude and phase due to the constant fluctuation of
the ionosphere. The result is chaotic random interference, and the
received signal amplitude therefore constantly varies around its mean level, producing
the slow fading well known to HF operators, as well as more rapid variations
which can readily be observed when the signal amplitude is sampled
more rapidly than about once per second. The standard deviation of
the signal amplitude can be several times the deviation expected due to
the VLF background noise, and gives an indication of how turbulent the
reflecting and absorbing layers are.
A typical example is shown in
the graph on the left. The amount of scintillation is portrayed by the thickness
of the signal trace, and varies constantly - slowly waxing and waning, presumably as
patches of turbulent ionosphere drift across the path of the signal.
This variation of scintillation does not repeat from one day to the next,
and the turbulent regions must be quite small because pairs of signals
crossing similar paths, eg Anthorn and Skelton, tend to be affected differently.
Sometimes, bursts of scintillation appear abruptly, and are often seen to be
coincident with a strong sferic. An example is shown on
the right, which is one of a series of similar events during that morning, in
which sferics and step changes in mean level and deviation were occuring simultaneously.
There is less than 0.1 second delay between the sferic and the step change,
suggesting that it is the direct effect of the electromagnetic pulse of the sferic
which is causing the upset, rather than the more delayed effects of charged particles.
These bursts of scintillation never seem to occur on more than one signal simultaneously,
so they are probably not large area disturbances, such as the LEP events (below).
| Lightning-Induced Electron Precipitation |
The electromagnetic pulse from a lightning discharge can upset the flow
of electrons as they spiral along the earth's magnetic field lines.
These upsets travel polewards
from the lightning source, precipitating electrons down into the D-layer
as they do so. These appear in the VLF record as a sudden change in signal
amplitudes, occuring about a second after the lightning discharge. These
events can be observed when the VLF path is polewards displaced from the
lightning source. For the paths to Le Blanc and Burlage, these events tend to
occur when lightning is present in the Mediterranean west of Sardinia.
A typical series of LEP events is shown on the left, one of which is shown on an expanded scale. The sferic from the causitive lightning is often seen as a spike preceeding the disturbance by about a second.
LEP events often affect more than one path, indicating that they involve a large area of the ionosphere, and there is a small time delay (a second or less) between the onsets on different paths, presumably due to the time taken for the spiralling electrons in the radiation belts to make their way polewards. From the onset, the time taken to reach maximum disturbance is around 2 seconds - varying somewhat from path to path. Often the disturbance can be an enhancement rather than a dropout, and the ionosphere typically takes from 20-100 seconds to recover. LEP events seem to be present on several nights per month.
For more info on LEP events, see http://www-star.stanford.edu/~vlf/LEP/LEP.html.
| Solar Flares |
During a solar flare, the upper layers of the ionosphere are bathed in X-rays,
causing extra ionisation, and thus affecting radio propagation.
The graph shows the effect of an M-class flare on several VLF signals. It takes about 15 minutes
for the extra ionisation to have maximum effect on the D-layer, and almost an hour
for propagation to return to normal.
Various extra-solar sources of radiation can also noticeably increase the ionisation. In particular, some strong gamma ray bursts (GRBs), and the hard X-rays from soft gamma ray repeaters (SGRs), can produce a clear signature in the record of VLF signal amplitudes. An excellent example is the reception of GRB030329 by Peter Schnoor in Kiel, Germany.
The ionosphere can be quite a
sensitive detector of high energy events, and the ionisation process can be responsive enough for
the rotation period of the SGR to be detected in the tail of the SGR burst, as in
this VLF record of SGR 1900+14
from the HAIL VLF receiver array.
| Good Links |
The most obvious signal variations are those caused by the Sun's radiation. During daylight,
the Sun's ionising radiation penetrates deep into the ionosphere, producing enough free electrons at
altitudes down to 40km to allow radio waves to reflect from that region. But the air density is
still relatively high at that height, so the electrons soon lose energy through collisions with
air molecules, and consequently there is a lot of absorption too.