LDI Intellectual Property.
Not for secondary distribution or replication, in part or entirety.
DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
1-40 SECTION 1 - GENERAL SYSTEM DESCRIPTION
1. the absolute phase of the transmitted signal;
2. the transmitted frequency (or free space wavelength); and
3. the phase distance, d, where:
is the line integral over the propagation path, scaled by the refractive index if the medium is not free space. If
the first two factors, the transmitted phase and frequency, can be controlled very precisely, then measuring the
received phase at two different frequencies makes it possible to solve for the propagation distance with an accu-
racy proportional to the accuracy of the phase measurement, which in turn is proportional to the received SNR.
This is often referred to as the d/df technique. The two measurements form a set of linear equations with two
equations and two unknowns, the absolute transmitted phase and the phase distance. If there are several “prop-
agation path distances” as is the case in a multipath environment, then measurement at several wavelengths can
provide a measure of each separate distance. However, instead of using a large set of linear equations, the
phase of the echoes have chosen to be analyzed as a function of frequency, which can be done very efficiently
with a Fast Fourier Transform. The basic relations describing the phase of an echo signal are:
(f) = –2f
p
= –2d/ = –2(f/c)d
where d is the propagation path length in meters (the phase path described in Equation 1-28, f in Hz, in radi-
ans, in meters and
p
is the propagation delay in seconds. Note that the first expression casts the propagation
delay in terms of time delay (# of cycles of RF), the second in terms of distance (# of wavelengths of RF), and
the third relates frequency and distance using c.
1:88. For monostatic radar measurements the distance, d is twice the range, R, so Equation 1-29 becomes:
(f) = –4R/ = –4(f/c)R
If a series of N RF pulses is transmitted, each changed in frequency by f, one can measure the phases of the
echoes received from a reflecting surface at range R. It is clear from Equation 1-30 that the received phase
will change linearly with frequency at a rate directly determined by the magnitude of R. Using Equation 1-30
one can express the received phase from each pulse (indexed by i) in this stepped frequency pulse train:
i
(f
i
) = –4f
i
p
= –4f
i
(R/c)
where the transmitted frequency f
i
can be represented as:
a start frequency plus some number of incremental steps.
1:89. This measurement forms the basis of the DPS’s Precision Group Height mode. By making use of the
simultaneous (multiplexed) operation at multiple frequencies (i.e., multiplexing or interlacing the frequency of
operation during a coherent integration time (CIT) it is possible to measure the phases of echoes from a particu-
lar height at two different frequencies. If these frequencies are close enough that they are reflected at the same
height then the phase difference between the two frequencies determines the height of the echo.
To Next Page
Loading...