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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
3-12 SECTION 3 - OPERATING INSTRUCTIONS
synchronization, resulting in the exact ionogram running time of 2m 27s 870ms. Without multiplexing, CIT
time is 64 x 10 ms = 640 ms, so that Doppler resolution is 1/0.64s = 1.5625 Hz.
3:15. Choice of the upper frequency for the ionogram sweep is determined by the maximum expected fre-
quency at which echoes can still be observed. The upper frequency shall be high enough to accommodate unu-
sual increases in the ionospheric plasma density. It is recommended to program daytime and nighttime iono-
grams separately and setup nighttime ionograms with a lower upper frequency but finer frequency steps to im-
prove quality of the ARTIST ionogram autoscaling.
3:16. Choice of the lower frequency for the ionogram sweep is influenced by new capability of the
Digisonde-4D to mitigate powerful RF interference from broadcasting stations that operate below 1.5 MHz.
Depending on Digisonde
®
location and RFI environment, it may be possible to observe echoes at frequencies as
low as 0.5 MHz.
3:17. Interpulse phase switching is another RFI mitigation technique in which phase of odd pulses is flipped
by 180º on both transmission and reception, so that when integration of multiple pulses is performed, coherent
interferers are suppressed while signals are still enhanced.
3:18. specifies a fixed-frequency drift measurement with 4 multiplexed frequencies separated by 100 kHz.
The set of 4 frequencies 100 kHz apart is positioned at 2 MHz, unless the latest data from the ionogram au-
toscaling is available to move the set to a more appropriate anchor frequency. With 6 repeats of 4 frequency
sets, total number of frequency [operations] is 24. On each of these 24 frequencies, N = 128 integrated repeti-
tions are made, each repetition consisting of 2 complementary codes (and only one polarization is recorded to
reduce data volume). This, total number of pulses needed to complete this measurement is 24 x 256 = 6144.
Although 256 pulses are sent on each frequency, frequency multiplexing provides 4x improvement in Doppler
resolution, raising CIT from 256 x 10 ms = 2.56 sec (non-multiplexed) to 256 x 4 x 10 ms = 10 sec 240 ms.
With this CIT duration, Doppler resolution is 0.098 Hz, and Doppler spectra cover 128 x 0.098 = +/- 6.3 Hz.
3:19. While range sampling is configured identically in both ionogram and drift mode, motivation for these
choices is different. The ionogram measurement sets upper range to 1357.5 km to make sure that echoes from
the raised F2 layer, whose virtual ranges can go well above 800 km during storm conditions, are still observed.
Setting the interpulse period to 5 ms (thus cutting ionogram running time in half) and recording virtual ranges
from 80 to 717.5 km might suffice in majority of the geophysical conditions, except perhaps the most interest-
ing cases of the storm activity. It is therefore recommended to always use 2.5 x 512 range sampling mode dur-
ing ionogram measurements. For the drift mode, choice of 10 ms for the interpulse period is driven primarily
by the need to increase CIT time.
Autogain Evaluation and Control
3:20. The Digisonde-4D design takes into account operation in noisy RF environments where strong interfer-
ers could saturate the receiver inputs rendering the receivers insensitive to smaller message signals. For iono-
spheric sounding, AM radio stations in the 0.55-1.65 MHz band and the 6-24 MHz HF communications bands
can pose serious problems for ground-based receivers. At some locations we have measured interference levels
of >1 V at the receive antennas compared to <1 μV message signals. Digital receivers that directly digitize RF
signals are especially prone to this problem since ADCs have a limited dynamic range. The ADCs need to be
protected from powerful out-of-band interference as well as from saturating in-band signals. In the Digisonde-
4D, a combination of gain controls in the wideband input amplifiers and the analog tracking bandpass filters
limit the input voltages at the ADCs to the maximum allowed values.
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