Passive Ionospheric Sounding and Ranging
Chirp Sounding is a relatively new technique used to measure the ionosphere by bouncing signals off it,
in much the same way as a RADAR system. Indeed, older Ionosondes (equipment for sounding the ionosphere) worked in
exactly that way, by sending high powered pulses and listening for the response. The modern Chirp Sounder uses lower power
and a continuous signal which changes frequency at a steady rate.
Background
Peter Martinez G3PLX and others have used Digital Signal Processing (DSP) and
Doppler techniques to measure small differences in carrier frequency that result from
movements in the radio propagation path (see the Precision Carrier Analysis
page). While interesting for meteor scatter, aircraft
and satellite reflections, and more gross or localized ionospheric
effects, this technique gives no direct information
about reflection layer height, and it becomes difficult to infer
information about the propagation medium over more complex paths. What
was required
was a time domain - rather than frequency domain - technique, for
example measuring the propagation time of pulses. It was soon established that a wideband
technique, rather than a carrier based technique would be necessary, in order to achieve sufficient time resolution.
In searching for suitable pulse transmissions to use, preferably transmissions
available from all over the world on a 24 hour basis, Peter stumbled across a family
of transmitters that are used as swept frequency ionospheric sounders. In their
normal application, research, professional and military groups use these low power
devices to probe the ionosphere to measure propagation. The signal consists of a
single long 'chirp', sweeping up in frequency at a constant rate, and repeated periodically. These transmissions
are tracked by a companion receiver which is zero beat with the transmitter, and
so ionospheric reflections that are returned with short delays are heard as lower sideband audio
beats of a few hundred Hz. The equipment then builds an 'ionogram' or two dimensional
graphical representation of the ionosphere's reflection height or delay against
frequency. The adjacent picture illustrates a typical commercial 50W FM/CW (chirped) ionospheric
sounding transmitter.
The first step was to discover how these chirped signals could be used in a passive
manner, i.e. without reference to the transmitter oscillators or timing reference.
To do this, Peter developed a very clever chirped filter, which not only sweeps
in frequency at 100 kHz/second, but has properties not possible in a conventional
filter - a bandwidth of only 66 Hz, but with a pulse resolution of 0.66ms. This filter
and matching detection software formed the basis of the adventure to follow, and allowed
these chirped sounders to be analysed on a single frequency using a fixed receiver. The remaining
problem was to determine when the chirp transmissions started, in order to know when it should sweep past
the receiver, in order to calibrate the fixed frequency receiver for range.
Purpose
The purpose of this project has been to explore the use of publicly available chirped
ionospheric sounding
transmissions to study the ionosphere. We now know that these transmissions are made regularly from
most parts of the world, and cover much of the HF spectrum from about 3 to 30 MHz on a 24 hour basis.
The unique aspect of this project is that it is
wholly passive - it makes use of the sounding transmissions made by other agencies. This means that
anyone can potentially receive and interpret the transmissions.
The transmissions used by this project are chirped sounders which transmit
at a constant rate of 100 kHz/second, and have reliable chirp start times.
Most of these sounders are commercial and research transmissions for vertical or oblique
ionospheric sounding purposes. Some are probably military sounders with a similar purpose.
Once the capabilities of this passive sounding technique were understood, the next step was
to develop a solution which would enable anyone with an interest in HF propagation
to study it in real time with a minimum of equipment and expense - for example
using nothing more than a PC with sound card. In addition, one reference sounder might
be used to calibrate reception of others, avoiding the need for an expensive
high precision time reference. It was also discovered that many of the sounders,
especially those used for oblique sounding (transmitter and receiver at widely separated sites)
used GPS timing references in order to maintain calibration at multiple sites, and these have
been of the greatest interest.
History
The first phase of this project took place during the latter half of 1999, and proved that it is
possible to receive and accurately measure these sounding transmissions in a passive manner.
Assessment of the results identified a number of areas of worthwhile improvement. It was possible
during this phase to set and synchronise clocks on opposite sides of the earth, and to
measure arrival times of signals to ±1 ms. The system concentrated on sounders with
periods of 5 and 15 minutes.
Several sounder transmissions were identified
and their locations discovered by hyperbolic triangulation (measurement of arrival times at different
locations and plotting lines of equal delay). Improved tracking of sounders with different chirp
periods, improved time resolution and simpler setup, clock synchonisation and system
calibration were perceived as the main areas for improvement. Stations were in many cases able to receive the same signals,
thus making distance measurements possible. Some stations had high precision references, making single-station
distance measurements possible on some known sounders. Both long and short path transmissions
could be identified by timing, and on occasions it was possible to resolve long path
and short path signals simultaneously. On a few occasions round-the-world delays were
detected.
Transmission from Cyprus received in New Zealand,
showing long path (left) and short path (right).
Vertical scale is milliseconds.
In a later development, new software and tighter hardware requirements allowed measurements were made to ±125us
resolution, using sounders
with a wide range of periods from 5 to 30 minutes. High precision GPS time references were
used for the first time, providing ±1us clock accuracy and similar precision of synchronism
between sites.
Better data analysis allowed more accurate delay measurements to be made, making possible
the identification of individual propagation paths. At this point 10 or more stations were equipped
for passive sounding, but the number was limited by the non-availability of suitable DSP hardware.
More observers were needed, with a better geographic spread so that more accurate triangulation of the sounder sites
would be possible, but that meant finding a hardware independent receiver solution possible.
Some sounders were found to drift in time, jump in reference time, or could only be heard in some locations,
or were not available continuously. An army of 'chirp spotters' would be required to help solve these problems,
so the need for a PC sound card solution became very apparent.
PC Software
Thanks to Andy G0TJZ, we now have an excellent platform independent solution - the PC sound card
Chirpview
software. As a result
more stations have been able to explore these fascinating sounders. Some
really good information is coming to light on propagation over paths
that have been previously difficult to study. An email group and a
superb
Chirp-Sounder's Web Site has been put in place
to provide news, up-to-date statistics, software, and a database of known sounders.
The PC software requires only a modern Pentium™ class computer with a Sound Blaster™ compatible sound card,
a stable and accurate HF SSB receiver, and a GPS system with precise 1PPS pulse output.
Operating Principle
Given sufficiently accurate clocks, or the same clock reference used at the transmitter and
at the receiver, it is possible to measure the time it takes a radio signal to travel from
one place to another. At a speed of about 300,000 km/sec (3 x 108m.s-1),
a radio signal can travel right around the world in about 138 ms.
If the transmitter and receiver are in fixed locations, you would expect this delay to be constant.
However, it is not, and this variation is the principle on which this project is based. The
arrival time of the signal will depend on which way around the world it went, how many
times it bounced off the ionosphere and the earth, and which ionospheric layers were involved.
Ionogram of a UK sounder showing groundwave
(straight line) and skywave signals, range 50 km.
Most applications of ionospheric sounding are either vertical or
oblique, i.e. with the transmitter
and receiver either co-located or separated by up to a few thousand km.
This project takes oblique sounding
to the extreme - the receiver can be anywhere on the globe. This places
the highest demands on
stability of the receiver and especially on the time references used. As
the distance between transmitter and receiver increases,
of course the number of possible paths increases and complexity of the
returned signal is therefore increased.
An explanation of the above image is in order. This graph is called a 'waterfall', a type of ionogram
where the image axes are both time - horizontally in UTC hours (time of day), and vertically
in milliseconds, the delay time from some fixed reference point. Since it is not practical to
display more than a short period of time vertically with high resolution, the vertical size is
limited to ±40 ms (in this example) or up to +150ms (in other examples). The image displays the strength
of the signal during the receiving "window", using white for no signal, and black for
very strong signals. 256 grey levels are displayed, 0.25dB per step, over a range of 64 dB.
Imagine that a waterfall is set to a time of 2.5
seconds, with a period of 300 seconds (five minutes). Any signal that appears within
40ms of the UTC five minute points plus 2.5 seconds (00:02.5, 05:02.5 minutes:seconds etc)
will be displayed in the waterfall window. This technique is extremely sensitive, as no digital
detection process is involved - interpretation is left to the eye.
Unlike most examples shown, the image immediately above shows a constant
horizontal line - this is because the transmitter was within
groundwave range of the receiver. Much of the day, this is the only
signal received; however,
between 0600 and 2100 UTC, faint lines first with decreasing and later
increasing delay appear. These
are caused by scatter to the receiving site from an ionospheric skip
occuring to some
other part of the world. As the skip zone moves closer, the delay is
reduced.
The signal suddenly becomes very strong and with a stable short delay
between 1200 and 1900 UTC.
This is the F layer reflection which occurs during daytime, where the
transmitted signal is reflected
from the ionosphere and directly received at the observing receiver. The
additional delay
(i.e. the time later than the ground wave arrival time) is an indirect
measure
of the height of the reflective layer.
If you look closely, you can see that there are actually two separate
lines between 1200 and 1900 UTC, the ground wave signal and the F-layer
skip signal.
The fuzzy stuff with longer delays above these strong lines come about
because the
reflective layer is diffuse, causing some diffraction (scatter), which
occurs through a mechanism
not unlike the scattering of sunlight reflections from ripples on a
pond.
Timing
To measure differences between short and long paths around the world requires the ability to
measure delay times of about 1 to 140 ms with a resolution of about ±1 ms. To measure with sufficient
resolution to resolve individual reflection paths requires rather higher resolution.
However, resolution is only part of the story. The project involves measuring the delay times
of signals for hours on end, and if the clock was to wander off in that time, the accuracy of
the result would be lost. For the delay to be measured with an accuracy to match the resolution
would require an accumulated clock error of less than 125 ±us per day, or nearly one part in
10 7. Only the most expensive rubidium or caesium standards
can achieve this low order
of drift over long periods, so the decision was made to utilise the
timing provided by the GPS
(Global Positioning Satellite) system. We have discovered that many of
the
sounder transmissions are also controlled in this way.
The one second references pulses generated by a good GPS receiver
are accurate to easily ±1 us on a continuous basis, providing a high
quality time reference anywhere in the world (well, except near the
poles).
In addition to the precision second pulse, the system makes use of the GPS NMEA messages,
which allow the equipment to recognise which UTC second each pulse refers to. The NMEA information
on its own is not sufficiently accurate, since it suffers unreliable delays in serial
transmission and reception. The timing can also be affected by the actual data transmitted.
Receiving Chirps
Up to this point, the ionosonde signal has been described as though it was a simple
pulse. Thinking of it in this way makes understanding the process easier.
However, it is difficult to transmit a narrow enough pulse to provide good time resolution,
and at the same time provide sufficient energy in the pulse for good sensitivity. This
problem is shared with radar systems, and the solutions are similar. In addition,
the sounders require to measure the ionosphere throughout the HF spectrum, which is again
not so easy to achieve with a pulse.
The ionosonde transmitter in fact sends a continuous carrier, but with
smoothly changing frequency, at a fixed but accurate rate (in the case of most chirp
sounders we use, with increasing frequency at 100 kHz/second).
Peter's design uses the special chirped filter previously described, with properties not attainable with
a conventional filter, and so is able to detect the transmissions with 0.66 ms
time resolution, and with very narrow bandwidth that provides high sensitivity.
Knowing the chirp rate of the transmission, and what frequency
the chirp is being received on, one can work out the nominal chirp time at which
the received signal apparently started at zero frequency - by simply counting back at 100 kHz/second.
There are two useful advantages of this chirped filter technique:
- The receiver will have narrow bandwidth, so will work with low power
sounder transmissions.
- The pulse response of the filter provides high time resolution.
The disadvantage is that you can only look at the ionosphere on one frequency at a time, unless you have multiple receivers,
or a frequency agile receiver and appropriate control software.
In the case of conventional chirped ionosondes, the receiver is more conventional,
but follows (tracks) its matching transmitter
throughout the HF spectrum. In this passive sounding project, the receiver tracks
many different transmitters using the chirped filter, but only
over the width of an SSB receiver bandpass - about 2.4 kHz - since the receiver
frequency is fixed. This approach is more than sufficient for
sensitive single frequency measurements. You simply set the receiver frequency to
suit the band you wish to know about.
Listen to a typical chirp received in a 2.4 kHz bandwidth (44kB)
Equipment
There are two possible equipment solutions. The first system developed
was the G3PLX system, which uses the Motorola DSP56002 EVM development
kit. The new PC software system by Andrew Taylor G0TJZ requires only a
fast PC with sound card. This is good news for those not able to find
the (now discontinued) Motorola unit.
Requirements
To take part in chirp tracking, you will need the following components:
- Stable and accurate HF receiver, in USB mode
- 5m vertical antenna in a noise-free environment
- DSP unit, the Motorola DSP56002 EVM development kit (for G3PLX software)
- PC, 486 or better with Win3.1/95/98 and EVMCHIRP software by Peter Martinez G3PLX
- PC, Pentium™ or better with Win98 or later and sound card and Chirpview software by Andrew Taylor G0TJZ
- GPS receiver with seconds pulse and NMEA data
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The DSP software (in the DSP unit or the computer) is at the heart of the system - it receives the audio from the receiver,
filters out the chirps, and measures their amplitude and time. It also receives time information,
in the form of seconds pulses and NMEA format serial coded time data, which are
derived from the GPS unit. The NMEA (serial data) message allows the processor to decide which seconds pulse is which.
The time and amplitude data for each chirp is then analysed on the PC and displayed.
Since the data is sampled at 8 kHz, the system resolution is 125 us.
Calibration and setup tools allow for choice of serial port, setting a calibration delay to compensate
for the delay in the receiver, and also to compensate for when the NMEA message
arrives relative to the seconds pulse. The receiving frequency is entered (so the
software can extrapolate the chirptime),
and the signal level and UTC time (from the GPS) are displayed.
Three main products are provided by the PC:
- The Chirp Log
- A log of detected chirps, which can be saved to file. This contains UTC time, signal strength,
period and measured delay (chirptime), for every detected pulse. It does not contain
entries for signals too weak or distorted to be detected as a chirp, but will contain
occasional "hits" caused by strong noise pulses and other interference. Pulses as close as 1ms
are logged independently, so provided the signal is strong enough and not too distorted,
scatter and long path hits will also record.
- The Chirp Statistics
- A log of chirp statistics, which gives a summary of each known chirptime, with period,
number of detected pulses, and the time of the first and last detected pulses. The chirptime is
averaged over the pulses for the previous two hours, which enhances the precision.
Random noise hits are eliminated.
- The Waterfall Displays
- Multiple waterfalls can be set up, in order to monitor any
suitable period and chirptime. The waterfall gives a graphical display of the signal, and
is much more sensitive than the logs. The waterfall relies on visual interpretation,
not software detection of the chirp properties. The scale of the waterfall is 1 ms/pixel
vertically, and 5 minutes/pixel horizontally. Very complex reception conditions can be
displayed.
The Waterfall display (G3PLX software)
Chirp Software
All the software (both versions) are available from the Chirp-Sounder's Web Site.
There is also a range plotting utility, which
allows you to enter positions of receiving sites with their delay times, and will plot lines of equal
delay on which the transmitter must lie. It will also plot antipodal circles from long path - short
path differential delays. This software is ideal for locating unknown transmitters.
Chirp Statistics
It is not practical to distribute a definitive list of all known chirp sounders
on a web site, since many of them change from day to day, or week to week. Current observations are generally
distributed by email, by posting logs to the chirps mailing list. These logs are helpful in identifying or locating
new sounders.
In the log example below, the times quoted here are the source chirp time, i.e. the time that would
be measured by a receiver on the transmitter site. You can estimate your chirptime by adding 1 ms for every
300 km of range from the source. For example, for a chirp time of 300:245.000 and a range of
10,000 km, expect the signal to arrive at about 300:245.030.
Chirp times are typically quoted in the form period:chirp time, where the period is in seconds, and
the chirp time is the zero frequency extrapolated time of the first chirp transmitted each hour.
Period:Chirp Lat Long Approx location
-------------+--------+--------------------
900:178.5496 33S 149E Canberra, Australia
300:250.0000 35N 34E Cyprus
300:224.14595 52S 59W Stanley, Falkland Is
300:77.94247 54N 03W Inskip UK
Ionospheric Sounding Links
- Grahamstown Field Station (South Africa)
- Realtime ionograms and other ionospheric data (Australia)
- Canterbury University research ionograms (New Zealand)
More Info Here and
Here
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