Chapter 3: Vehicle-Mounted GPR System for Landmine Detection

32 3. Vehicle-Mounted GPR System for Landmine Detection
The second problem is interference of the signals. Use of some VNAs
simultaneously means the same signals are emitted and measured by each VNA,
yielding a signal radiated by the transmitter #1 may be received by not only the
receiver #1 but also the receiver #2 and #3, for instance. To fix this problem, slightly
different frequency ranges are used in each of the VNA as shown in Fig. 3.5. The
offset frequencies are defined by trial and error. Fig. 3.6 shows the power spectra of
100 ground surface reflections measured by the system with offset frequencies of
100 kHz and 300 kHz and an IF bandwidth 1 of 30 kHz. The obvious fluctuations
can be observed, and those for VNA #2 are more than 10 dB. Fig. 3.7 shows the
spectra with offsets of 500 kHz and 1.25 MHz and an IF bandwidth of 3 kHz. They
have almost no fluctuations. From the trials, it is found that an offset frequency must
not be a multiple of the other offset, because the harmonics may be radiated by the
transmitter and it can be measured by a receiver of another pair. Moreover, a sweep
of a VNA can catch up with another sweep since each VNA has its own individual
characteristics. In SAR-GPR, the offset frequencies of 500 kHz and 1.25 MHz are
used.
Table 3.1: Specification of the VNA, Anritsu MS6223 (commercial) 2 .
Measurement mode
Frequency sweep range
Amplitude accuracy
Frequncy accuracy
Output level
Dynamic range
S21
100 MHz to 4 GHz
± 1 dB (at -20 dB)
± 5 kHz (at 2GHz)
-5 dBm to +10 dBm
70 dB
Number of frequencies 69/137/275/551
Sweep Speed
Frequency resolution
Weight
Size
0.86 s (551 points)
< 100 kHz
< 3.5 kg (with case, battery)
313× 211× 77 mm (with case)
Operating temperature -10 to +50 ºC
1 IF (intermediate frequency) bandwidth is the filter bandwidth for an input signal of a VNA.
2 Actual installed VNA is modified for the system.