Ionospheric sounding (detecting the presence, properties and location of different refractive layers in the ionosphere) can be achieved using several techniques. The most common are:
Chirped sounders are widely used, and they measure propagation over a wide frequency range. Using high power phase modulated pulses, Over the Horizon Radar is also able to measure and account for ionospheric effects. PN-sequence phase modulated data transmission sounders operate only on a single frequency. The ZL2AFP PSK Sounder uses a similar (single tone fixed frequency) approach, which is more suited to low power Amateur operation.
- Very high power pulsed radar sounders
- Medium power chirped sounders
- Medium power PN-sequence phase modulated sounders (often incorporated in data transmissions)
The single tone approach has three principal advantages:
Because the transmissions are continuous, operation of transmitter and receiver at the same site (as is common with pulsed sounders) is not practical.
- Continuous transmission gives better higher average transmitter power, higher sensitivity, and so less power is required
- It is a narrow-band technique (well, far narrower than pulses or chirps)
- Doppler frequency measurement can be achieved at the same time as range measurement
PSK Sounder uses a modulation technique which is widely used by high speed HF radio modems. Using BPSK modulation, a pseudo-random binary (PN) sequence is used to identify an exact point in the transmission from which the data can be synchronized. A cross-correlator is used in the receiver to locate the one point in the whole message where the sequence matches up with the local copy of the sequence. As opposed to an auto-correlator, which tries to find repeated patterns in an unknown message, the cross-correlator works with a known pattern to look for, and is a very much more powerful tool.
These radio modems use the PN sequence technique to enable complex high speed data to be decoded accurately - the ranging information is in this case not generally used as a tool for propagation study. However, in PSK Sounder the ranging information is primary, and data transmission of secondary importance.
PSK Sounder uses a 31-bit PN sequence borrowed from STANAG 4285, with one chance in two billion of a perfect score being caused by noise. It uses 80 symbols (modulation time slots) of this sequence in a 256 symbol frame. Because the program uses the standrad STANAG 4285 waveform, we can practice with the software on the many military transmissions we hear on HF. We can represent the cross-correlation of the signal over 256 symbols (512 samples) by drawing a graph where horizontal position represents sample time starting at an arbitrary point, and vertical position represents the cross-correlator score measured at that each sampling time:
Graphical representation of a real cross-correlation
PSK Sounder (and STANAG 4285) always use messages of the same length, 256 symbols. The cross-correlator allows us to locate each message block (which we call a frame) exactly in time, making decoding so much simpler. By comparing the time of the message block with a clock in the computer, we can also measure changes in the propagation delay of the received signal over time.
As an example, imagine we are receiving a signal from an Amateur Radio station some distance away, which is providing both ground wave and F-layer ionospheric signals - the classic NVIS situation. The ground wave will be delayed by just a millisecond or two (300km per millisecond at the speed of light), while the F-layer signal, travelling a further 300km up to the ionosphere and another 300km back, will be delayed an extra 2ms. Both these signals arrive at the receiver, and the cross-correlator has to work with the combined signal. So, it will in fact find two peaks, about 2ms apart, and we can display or plot this information, and measure the delay. Very often the propagation is even more complex, especially at greater distances, and of course very often the ground wave signal is not received at all.
Multiple cross-correlation responses with multi-path reception
Like the STANAG 4285 system, PSK Sounder can operate at 2400 baud, using a sub-carrier frequency of 1800Hz. However, to fit the signal into a normal amateur transceiver IF, it is usually operated at 2000 baud using a 1500Hz sub-carrier.
The receiver software displays the correlation trace, a Dopplergram and a really superb Correlogram, which is a representation of propagation delay (vertically), elapsed time (horizontally) and correlation significance (relative signal strength of each ray) as brightness. The display range is 13ms, with a resolution of 500µs. You can use the mouse to read off the delay between responses.
The PSK Sounder Correlogram
Using this display, you can easily resolve different refraction layers, the ground wave signal (if available), E-layer, O&E F-layer, multi-bounce F-layer and spread F, as well as sporadic E and other sporadic and unusual effects. Identifying them is another matter entirely!
The PSK Sounder message is very simple. It is an ID message in standard 8-bit ASCII. There are 176 data bits, so the message will accomodate 18 ASCII characters and a CCIR16 CRC (error check). The user can set any suitable message, for example 'ZL1BPU RF72ku 1W', and the CRC is coded automatically. The same frame of data is repeated over and over, at about eight or nine frames per second, depending on the baud rate.
You can't use the system to send free-form text or make a QSO. It just sends the one message over and over again. The receiver makes use of the multiple repeats of the message to be sure it has the message correct, using a combination of bit averaging and a CRC check.
The receiver will decode the ID message reliably with 10 second averaging at -10dB S/N, and the correlation peaks can just be detected at -20dB S/N.
PSK Sounder consists of two separate programs, one for transmit (TX_data_CRC.exe), the other for receive (PSK_sounder.exe). By using both of the programs at the same time, you can test the performance and become familiar with the receiver controls, all inside the same computer. Normally, however, a user would run only one program or the other, one station acting as transmitting station the other (or others) receiving.
Another small program (PTT toggle.EXE) is provided in order to control the transmitter where VOX is not available.
Unzip the archives into a common directory and read the readme.txt file. The program parameters are controlled by a setup file. There are two modes, 'military' and 'amateur'. The normal operating settings are:
- PSK Sounder Programs V5 85kb
- PSK Sounder Help archive V5 196kb
Mode Baud Rate Sample Rate Sub-carrier Amateur 2000 8000 1500 Military 2400 9600 1800
Computer and Hardware Requirements
- A Pentium 750MHz computer or better, with Windows 98, 2000 or XP software. For Equalizer settings greater than 10, use a 2GHz computer. One serial port (for transmit control), a good quality 16 bit or better sound card or system.
- An HF receiver or transceiver with 10Hz step synthesis or better, and high stability. Fine tune to 1Hz (e.g. using RIT) is required. For 2000 baud, use a conventional 2.4kHz SSB filter. For 2400 baud, use a 3.3kHz filter. Use USB, or (amateur only) LSB below 9MHz. Set AGC to fast.
- Radio sound interface with isolation transformers and isolated PTT control.
The STANAG 4285 waveform on which this program is based is widely used by military, government and non-government organizations. The software is not equipped to demodulate or decode any data from non-Amateur transmissions. All it does is recognise the cross-correlation sequence, frame size, modulation technique and scrambler, all public domain information from the STANAG 4285 documentation. It is not possible to read any traffic from these non-Amateur signals.