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Quite generally, ionosondes are radars. In fact, the idea to use radio pulses for 'detection and ranging' was conceived for the purpose of measuring the height of the ionosphere in 1925, by Gregory Breit and Merle Tuve. The principal components of any radar must include circuits or devices which (a) define the signal to be transmitted; (b) amplify the signal to a useful power level; (c) radiate (as antennas) the signal generally upward and accept the downcoming echoes; (d) 'receive' the signal (or 'echo'), by appropriate amplification, filtering and noise- rejection; (e) record the echo information in some suitable form. Unlike more familiar radars, the ionosonde does not attempt to direct its "beam" to locate its "target". On the practical side, forming such a beam at medium and high frequencies (1 - 20 MHz) would be prohibitivly expensive; it would also be ineffective, since (like the sea surface) the ionosphere is continuously tilted, wavy and irregular: these properties, and not the direction of a beam, determine the location (or locations)from which reflections occur. Ionosonde antennas illuminate the "whole sky".
Pulsed radio waves of up to ~ 20 MHz (15m wavelength) may be totally reflected in the ionosphere, giving strong echoes even with rather low transmitted power (a few kW). This is the fundamental principle of the 'ionosonde'. The ionospheric reflection process is strongly 'dispersive', or frequency-dependent, each frequency being reflected by one, two, or even three specific ionization densities, because the ionized plasma is 'magnetoactive'. Classical 'ionograms', simple graphs of pulse travel time (a few milliseconds) vs radio frequency, directly display details of ionospheric structure. They require only a little interpretive skill to recognize the usual stratification of the ionosphere, and to estimate prevailing layer peak ionization densities. Estimation of the actual height of the ionosphere is much more difficult, requiring extensive 'inversion' calculations. Some 200 million analog (film) ionograms were produced by some 200 ionosondes, until the arrival of digital technology, around 1975.
Most of the hardware components of an ionosonde, and all of the data processing applicable to its measurements, benefit from digital technology. At present there are six digital ionosonde designs known to us. They have in common the integration with a Personal Computer for programmable scheduling, observing-mode parameter settings, system control and data management. They differ considerably in ideology, ranging from analog-ionosonde emulation to highly specialized or highly generalized data acquisition concepts. The following WebSites provide source information for these six, listed in approximate order of design and performance sophistication:
Many properties of an echo (its travel time, phase-path, polarization, direction of arrival, amplitude and Doppler ... and even the distribution of these properties within the echo) are significantly modified by ionospheric properties near the reflection level and, to a progressively lesser extent, by the entire ionospheric path below reflection. Among the ionospheric properties are varying solar, auroral, meteoric and other ionization production sources, chemical reactions by which ionization returns to the neutral state, motions induced by winds, turbulence and electric fields, all resulting in very wide spectra of spatial irregularities and motions. Thus, by careful and complete measurement of radio echo properties, a wealth of information may be available for diagnostics of ionospheric processes.
Digital ionograms are defined by the ideology followed in the design of the digital ionosonde. Digitization (analog-to-digital conversion) is applied to the high-level receiver output(s). Almost without exception, the output represents the signal complex amplitude; the two quadrature channels may be combined to express echo amplitude and phase. Some method of echo recognition and/or noise rejection may be applied at this stage, or later. Whether the digital data at this point are considered to comprise an "ionogram" is arguable, and unimportant. Usually, (and at least incidently) the various digital ionosonde systems emulate the ionogram's graphical presentation of echo "virtual height" vs sounding frequency, because this compact and informative display involves few, if any, arbitrary selection rules, and is uniquely sensitive to many of of the physical influences affecting ionospheric structure. Usually also, there is a great deal of additional information that cannot be included in one graphical display. Furthermore, in some cases, the information may posess levels of accuracy, resolution, and dynamic range which are impossible to convey graphically in any simple way. Multi-parameter data tables and graphics tools are usually provided with proprietary software for analysis and presentation of digital ionogram data.
The Dynasonde evolved at NOAA between 1970 - 1980 as "an ionosonde competent to measure the dynamics of the ionosphere". A few hardware upgrades and continuing software development have maintained the several operating Dynasondes at a 'state of art' functionality.
The distinguishing ideology of the Dynasonde is to provide the hardware and software resources to accomplish complete characterizations of the radio echoes available by sounding throughout the LF/MF/HF bands, 0.1 - 30 MHz. Specifically, just following the transmission of a spectrally-pure (cos^2 envelope) radio pulse, signal complex amplitude is digitized (12 bits) every 10 microseconds of "Time of Arrival", ToA, within a 467 - 5333 microsecond (70 - 800 km) listening window; in addition, a "noise sample" is obtained outside this window, where echoes are unlikely. Pulses are transmitted in groups, or 'pulsets', of 4 or 8 in carefully designed patterns of receiving-antenna selection and small (~1 kHz) frequency offsets. Echoes are recognized in real time by one or another of several algorithims which effectively discard false echoes (impulsive noise from broadcast signals, rain or electrostatic noise, vehicle ignition, etc.) and which identify the ToA values at which complex amplitudes are to be retained. It is the collection of these complex amplitudes (plus the independent variables of radio frequency, time, antenna location, and a few system parameters, which constitute a Dynasonde Echo.
Subsequently, in near real-time or deferred analysis, this information is transformed into physical quantities which even more explicitly define useful echo parameters and their estimation accuracies. The principal parameters are 'echolocations' (or the apparent direction-of-arrival, and the precise 'stationary-phase-group-range'), a line-of-sight Doppler estimate, one or more echo polarization parameters, an average phase value, the echo peak amplitude, and (depending on the pulset design) several other echo characteristics. It is the collective assembly of these echoes with all of their physical attributes, that is considered to comprise the Dynasonde Digital Ionogram.
It is important to the distinction between the dynasonde and other digital-ionosonde concepts, to recognize that these steps in data acquisition are made with minimal model assumptions and with no data pre-processing whatever. An echo is an echo; it is not a selected Doppler peak following FFT processing (as by one of the popular digital ionosonde designs); it is not even simply an amplitude peak: when amplitude is used to recognize echoes, it is the required ToA consistency among amplitude peaks within the pulset which rejects false echoes and identifies true echoes.
These established data acquisition strategies exercise most, but not yet all, of the hardware resources of the dynasonde, particularly those of radio-frequency resolution and spectral purity, system timing, pulse waveform, antenna performance, echo recognition and parameterization, and, finally, of data processing capabilities ... all in addition to fully programmable control of of each of these features.
For the results usually presented on this Web site, the dynasondes operate in a fairly stable monitoring mode, and much of the potential flexibility of operation is not evident.