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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
1-14 SECTION 1 - GENERAL SYSTEM DESCRIPTION
1:23. The accurate measurement of all of the parameters, except frequency (it being precisely set by the sys-
tem and need not be measured) depends heavily on the signal to noise ratio of the received signal. Therefore
vertical incidence ionospheric sounders capable of acquiring high quality scientific data have historically uti-
lized powerful pulse transmitters in the 2 to 30 kW range. The necessity for an extremely good signal to noise
ratio is demanded by the sensitivity of the phase measurements to the random noise component added to the
signal level. For instance, to measure phase to 1 degree accuracy requires a signal to noise ratio better than 40
dB (assuming a Gaussian noise distribution which is actually a best case), and measurement of amplitude to
10% accuracy requires over 20 dB signal to noise ratio. Of course, is it desirable that these measurements be
immune to degradation from noise and interference and maintain their high quality over a large frequency band.
This requires that at the lower end of the HF band the system’s design has to overcome absorption, noise and
interference, and poor antenna performance and still provide at least a 20 to 40 dB signal to noise ratio.
SIGNAL PROCESSING IN DIGISONDE-4D
General Considerations
1:24. Several advances in ionospheric sounding were made over the past four decades to move significantly
beyond the basic pulse techniques developed in the 1930’s. Introduced techniques include:
• Coherent integration of several pulses transmitted at the same frequency
• Spectral pulse integration applicable to moving reflectors
• Pulse compression for improved signal-to-noise ratio
• Multiple receiver arrays
• Transmission and reception of circularly polarized signals
• Frequency multiplexing for improved Doppler resolution
• Precision ranging on two closely separated frequencies
• Mitigation of RF Interference (RFI)
• Pulse modulation for twin frequency sounding
1:25. Like its Digisonde
®
predecessors, the Digisonde-4D simultaneously measures seven observable param-
eters of reflected (or in oblique incidence, refracted) signals received from the ionosphere:
1) Frequency
2) Range (or height for vertical incidence measurements)
3) Amplitude
4) Phase
5) Doppler Shift and Spread
6) Angle of Arrival
7) Wave Polarization
1:26. Because the physical parameters of the ionospheric plasma affect the way radio waves reflect from or
pass through the ionosphere, it is possible by measuring all of these observable parameters at a number of dis-
crete heights and discrete frequencies to map out and characterize the structure of the plasma in the ionosphere.
Both the height and frequency dimensions of this measurement require hundreds of individual measurements to
approximate the underlying continuous functions. The resulting measurement is called an ionogram and com-
prises a six dimensional measurement of signal amplitude vs. frequency and vs. height as shown in Figure 1-8.
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