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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
SECTION 3 - OPERATING INSTRUCTIONS 3-9
PROGRAMMING SCIENCE MEASUREMENTS
General Considerations
3:8. Fundamentally, ionogram and drift measurements use the same remote sensing approach described in
Section 1, in which Digisonde
®
transmits a sequence of pulses on an operating frequency, listens for the echoes
on its four antennas, performs signal processing (RFIM, cross-channel equalizing, pulse compression), and ob-
tains set of four Doppler spectra for each sampled range bin (one spectrum per receiver channel). Then, in the
drift measurement mode, all Doppler spectra are stored in the output product file for subsequent derivation of
the skymap and bulk drift velocity, whereas in the ionogram mode, only one echo is recorded per range bin by
reducing four Doppler spectra to one echo status (amplitude, Doppler shift, angle of arrival, etc.).
3:9. Practically, programming ionogram and drift measurements has several subtle differences.
a. Separation of the overlapping echoes by their Doppler frequency is instrumental to efficient skymap
construction. In order to accomplish high Doppler frequency resolution necessary for the drift/skymap
measurement, the coherent integration time (CIT) has to be selected as high as 20 or 40 sec, so that cor-
responding Doppler resolution is 0.05 or 0.025 Hz. Ionogram measurement at such high integration
times per frequency would take too much time to complete, and thus integration times in the ionogram
mode have to be selected much lower, generally in the order of 1 sec or less.
b. For ionogram measurements, number of pulses per frequency, N, is kept sufficiently small to reduce the
ionogram running time. Because smaller N means lower signal-to-noise ratio, such selection has to con-
sider implications of reducing the overall ionogram quality. While such selection is station-dependent,
it is generally recommended to have 16 pulses per frequency. Smaller N values are possible for cam-
paign periods requiring fast ionograms (below 30 sec). In the drift mode where Doppler frequency reso-
lution is high, number of pulses per frequency N (i.e., number of spectral lines in the Doppler spectrum)
has to be as high as possible to cover sufficient range of Doppler frequencies.
c. Drift measurements produce relatively large volumes of data for storage. A number of data reduction
measures are taken to keep drift file sizes reasonably small. In particular, it is customary to run drift
measurement on a small subset of fixed frequencies rather than sweep the whole frequency range as in
the ionogram mode. Digisonde-4D offers an automated frequency override mode, in which selected set
of fixed frequencies for drift measurement is shifted up and down in frequency using data from recent
autoscaled ionogram, so that these frequencies are optimally positioned to target the F region of iono-
sphere.
Frequency Multiplexing
3:10. Frequency multiplexing is commonly used in the drift mode to effectively increase the integration time
without increasing overall measurement time. To illustrate the principle of frequency multiplexing, let’s consid-
er example where 8 pulses per frequency are transmitted, and the measurement includes 4 frequencies. Figure
3-3 shows progression of the non-multiplexed case in time. First, Digisonde
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sends and processes 8 pulses on
frequency f1, then another 8 for frequency f2, then frequency f3, and finally the last 8 pulses for frequency f4.
Figure 3-3 shows frequency multiplexing when frequency sequence f1-f2-f3-f4 is repeated 8 times. Both cases
involve transmission of 8x4 = 32 pulses using the same amount of time. However, coherent integration time for
each frequency in the multiplexed case is 4 times larger, resulting in 4 times better Doppler resolution of the
measurement. Further details on frequency multiplexing can be found in Section 1.
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