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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
SECTION 1 - GENERAL SYSTEM DESCRIPTION 1-41
1:90. The following development of the two frequency ranging approach leads to a general theory (but not
expoused here) covering FM/CW ranging and stepped frequency radar ranging. Using Equation 1-30 a two
frequency measurement of allows the direct computation of R, by:
2
–
1
= 4R(f
1
– f
2
)/c = 4Rf/c
1:91. It is easy to see from Equation 1–29 that if the range is such that RΔf/c is greater than 1/2 then the
magnitude of
2
-
1
will exceed 2 which is usually not discernible in a phase measurement, and therefore caus-
es an ambiguity. This ambiguity interval (D for distance) is
R = D
A
= (1/2)c/f = c/2f
Example B.:
The measured phase is (
2
-
1
) =
/8 while
f = 1 kHz, then R = 9.375 km.
In the example above with
f = 1 kHz, the ambiguous range D
A
is 150 km. Since a 0 km reflection
height must certainly give the same phase for any two frequencies (i.e., 0
), then given that the am-
biguity interval is 150 km, then for this value of
f, the phase difference must again be zero at 150,
300, 450 km etc, since 0 km is one of the equal phase points, and all other ranges giving a phase dif-
ference of 0
are spaced from it by 150 km. If the phase measurements
2
and
1
were taken after
successive pulses at a time delay corresponding to a range of 160 km (at least one sample of the re-
ceived echo must be made during each pulse width, i.e., at a rate equal to or greater than the system
bandwidth, see Equation 1-4), one would conclude that there is an extra 2
in the phase difference
and that the true range is 159.375 km, not 9.375 km. Therefore, the measurement must be designed
such that the raw range resolution of the transmitted pulse is sufficient to resolve the ambiguity in
the d
/df measurement.
1:92. The validity of the two-frequency precision ranging technique is lost if there is more than one source of
reflection within the resolution of the radar pulse. The phase of the received pulse will be the complex vector
sum of the multiple overlapping echoes, and therefore any phase changes (
i
) will be partially influenced by
each of the multiple sources and will not correctly represent the range to any of them. Therefore, in the general
propagation environment where there may be multiple echo sources (objects producing a reflection of RF ener-
gy back to the transmitter), or for multipath propagation to and from one or more sources, many frequency
steps are needed to resolve the different components influencing f
i
. This “many step” approach can be per-
formed in discrete frequency steps as in the DPS’s HRR mode, or by a continuous linear sweep, as done in a
chirpsounder described in [Haines, 1994].
Passive RF Sensing Measurements
1:93. The concept of passive ionospheric RF sensing is to receive signals from remote transmitters of oppor-
tunity in order to infer characteristics of the ionospheric channel that received signals have traveled. Typical
area of interest to passive RF measurements is imaging of small-scale ionospheric irregularities, especially if
such observations can be made multi-static. Use of the Digisonde
®
in passive RF sensing has been constrained
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