5 March 2015

RTCE Sounders

The key to achieving significant benefits in the way that an operator or automated HF radio system controller uses the propagation medium for communication is to ensure that an adequate supply of  real-time data is available for decision-making purposes. Off-line propagation analysis is the older time-proven method for getting this information. More recently automated and adaptive systems have turned to real-time collection of information to be used in propagation analyses.
Sounding belongs to a general class of channel estimation or evaluation techniques (RTCE real-time-channel evaluation).
Sounding is the process of monitoring or testing the transmission medium for real-time propagation information. Soundings provide up-to-date indications of propagation characteristics over vertical (directly overhead) paths and oblique paths (along the actual communication route direction). It is not practical to sound all possible paths in a large communication network, but some benefits from sounding may still be achieved if selected paths are probed and the results are extrapolated to geographically nearby paths. 

Sounding can be divided into three subgroups for purposes of distinguishing the significance of each type. The subgroups consist of ionospheric pulse sounding, linear sweep sounding, and channel evaluation sounding

Ionospheric pulse sounding
Ionospheric pulse sounding is used to test the propagation medium caracteristics for such things as channel unit impulse response, signal propagation delay, and signal amplitude. Pulse sounding consists of emitting a pulse sweep over a portion or all of the HF band for a period of a few seconds to several minutes. The received signal is then analysed. The results of a frequency sweep of a sounder will indicate to the user, or automatically to the equipment the range of frequencies that will propagate. Vertical-incidence-sounder (VIS) where the soundings are emitted vertically and the reflected returns are received by a nearby receiver and oblique incidence backscatter sounding where the soundings are emitted in the direction of the actual communication, and the returns scattered from a distance are gathered by a co-located receiver very near the transmitter, are general techniques which require interpretation before being of direct use to an adaptive link.
Oblique-incidence-sounder (OIS), where the sounding is emitted in the direction of the actual communications path, and the receiver is located at the remote location, is of more direct application, subject to the antennas and system parameters in use

continuous-sweep sounder leaving a gap between (more ore less) 16170 and 16200 KHz
Linear swept frequency sounding (i.e. chirp sounding)
Linear FM modulation or chirp sounding consists of sending at low power 2-30 MHz. This method can be linear FM/CW test signal over the communication path used over either a vertical or an oblique path. The data received from the chirp sounding equipment is similar to the pulse sounding equipment, but has the advantage of causing less interference to nearby equipment.
Oblique incidence sounding technology offers benefits for adaptive HF communications systems using the 2-30 MHz bands. In addition, the frequency modulated continuous wave (FMCW) swept-frequency “chirp” method is shown to offer adaptive HF system engineers with more options in the design of HF networks. Moreover, it is found that FMCW “chirp” sounding provides the communicator with a relatively unobtrusive waveform for establishing optimum network connectivities, if the sounding is carried out in near-real time and the network consists of frequency-adaptive radio.
Recommendation ITU-R F.1337 outlines the case for frequency management of adaptive HF radio systems and networks using FMCW oblique incidence sounding. Specifically it recommends that automatic and adaptive management schemes beconsidered for adaptive HF networks to include dynamic selection of optimum frequencies, the sharing of frequencies within a network, and adaptive selection of alternate network paths; that FMCW “chirp” sounding be considered for use in dynamic frequency management schemes including:
- as a real-time input data source for updating resource management and propagation prediction programmes;
- as a means for updating the frequency scan lists of adaptive HF systems;
- for modification and enhancement of the link quality analysis (LQA) matrices for adaptive HF systems;
- as a complement to the exclusive use of in-band channel sounding, thereby increasing network communication capacity and reducing interference introduced by channel sounding.
Channel evaluation sounding
Channel evaluation sounding consists of probing only frequencies that are allocated to this system, rather than a broadband approach of the other two methods. Channel evaluation provides information used in evaluation of signal-to-noise performance such as: data error rate, speech intelligibility, and noise levels.

Something about sounder-tech products may be looked at here:
but, as usual, Google is your friend...

Below a single-sweep sounder, up-chirped, observed around 18 Mhz (you may observe these sweeps several times in just one hour or so):

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