Radar Technology for Level Gauging - Krohne

8. Evaluation methods
8.1 Object discrimination
If two or more objects reflect the radar signal, the system will also receive a corresponding
number of pulses (Fig. 36, top). However, if the pulse length τ is too long, the two
reflections will not be separable (Fig. 36, bottom).
transmitted pulse
Object discrimination, i.e. the minimum difference in distance allowing two objects to be
distinguished, is given by:
∆a = c · τ / 2
By virtue of the analogy between pulse duration and bandwidth B (see Section 3.5.1)
the following general “uncertainty relation” can be set up between B and the object discrimination
∆a,which also applies to FMCW systems:
∆a = c / 2 · B
8.2 Unambiguity
If another pulse is emitted before the reflected signal from the previous pulse has been
received, the signals cannot be unambiguously assigned. Hence it is necessary to satisfy
the following relation (f i = pulse repetition rate):
f i ≤ c / 2 · a max
1st object
2nd object
For the relatively short distances involved in level measurements, however, this requirement
is normally not important (a max= 30 m → f i ≤ 5 MHz).
Fig. 36:
Reflected signal
at different pulse
lengths
Radar handbook 45
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t
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