for short-pulse and impulse waveforms. Another advantage of stepped-frequency radar is its
agility to skip over frequencies that could inter- Ž
fere with commercial broadcast stations Poirier, .
1993 . Ž
. Robinson et al.
1972 first proposed the
synthesized pulse or stepped-frequency radar method as a means of improving the penetration
performance of GPR without compromising res- Ž
. olution. Noon et al. 1994 have given a histori-
cal review and technical description of the Stepped-Frequency Ground Penetrating Radar
Ž .
SFGPR technique. SFGPR consists of measur- ing the complex reflection coefficient of the
ground at a number of discrete continuous-wave frequencies, and then transforming to the time
domain via a discrete Fourier transform. The main disadvantage with continuous-wave trans-
mission with SFGPR is that a strong signal either from leakage between the transmitter and
receiver antennas or from shallow reflectors can mask weaker signals from deep reflectors.
Ž A gated SFGPR Hamran et al., 1995; Stick-
. ley et al., 1996
combines the high average power and efficient sampling of stepped-
frequency radar with the ability of impulse GPR Ž
. to image deep weak targets in the presence of
Ž .
shallow strong reflectors. In such a system, the stepped-frequency transmissions are pulsed, and
a receiver ‘‘gate’’ is used to pass the reflections of targets from desirable depths and ‘‘blank’’
the unwanted reflections. Unlike the sampling head used by most impulse GPR systems, the
gated stepped-frequency receiver coherently samples the gated reflections from multiple
transmitter pulses, thereby improving the sys- tem performance. It is possible for the stepped-
frequency radar to coherently sample reflections from multiple receiver gate depths, although the
system described in Section 2 does not have this implemented.
Section 2 of this paper describes the newly developed SFGPR system that operates across
the 10–620 MHz frequency band. The gating concept for SFGPR is described in Section 3.
Results of the gated SFGPR system arising from field tests are presented and discussed in Sec-
tion 4, with conclusions stated in Section 5.
2. Stepped-frequency GPR system
A simplified block diagram of a gated SFGPR system is shown in Fig. 1. The main body of the
radar includes a synthesizer, quadrature re- ceiver, digital signal processor and real-time
display. The antenna subsystem contains sepa- rate transmit and receive antennas that are con-
nected to the main body of the radar via fibre optic links. The antennas used are modified
bowtie designs with shielding. The bandwidth and centre frequency of the antennas is approxi-
mately 1:1. Similarly to standard GPR systems, antenna subsystems with different centre fre-
quencies can be interchanged to suit the applica- tion.
The synthesizer in the current prototype gen- erates a single frequency signal that can be
stepped across a frequency band within 10–620 MHz. A portion of the synthesizer output is
coupled to the quadrature receiver to provide a phase reference. At each single frequency, the
transmitted signal can be rapidly gated on and off by the transmit gate in the antenna subsys-
tem. The duty cycle of the gating function is typically 33. A solid state transmit amplifier
Ž .
provides a high power signal up to 10 W to the transmitter antenna feed point. The received
Fig. 1. Simplified block diagram of the 10–620 MHz Ž
. SFGPR. The antenna subsystem shaded region is sepa-
rated from the main body of the radar. All interconnections between the antenna subsystem and the main body are by
fibre-optics links.
signal is amplified prior to being gated by the switch in the receiver antenna. The quadrature
receiver determines the amplitude of the re- ceived signal and its phase relative to the refer-
ence signal for each frequency step. The receiver system has a noise figure of 8 dB.
Frequency domain data from the quadrature receiver is transformed to the time domain using
a fast Fourier transform algorithm on a dedi- cated DSP. The time domain response is dis-
played on a PC in real-time.
The 10–620 MHz radar has been designed and built specifically for GPR applications. Un-
Ž like other SFGPR systems e.g., Hamran et al.,
. 1995 , this radar is not based around a commer-
cial network analyser. A fast switching synthe- sizer together with a fast settling quadrature
receiver is used to enable approximately 50 depth profiles to be collected per second. Inter-
changeable antenna subsystems can be used to change the operational frequency.
The main radar subsystem is housed in a shock-mounted, portable 19
Y
rack that can be accommodated in the back of a 4-wheel drive
vehicle. Electrical power can be supplied either at 12 VDC or 240 VAC. Rechargeable batteries
power the antenna subsystem providing suffi- cient capacity for a typical day of operation in
the field.
System performance is defined as the ratio of mean transmitted power to minimum detectable
Ž .
signal power Plumb et al., 1998 . It is typically used as a ‘‘figure-of-merit’’ for the penetration
performance of GPR systems. The minimum detectable signal power should be specified in
relation to some acceptable signal to noise ratio Ž
. SNR
and integration time. Throughout this paper we adopt the convention of assuming a 10
dB SNR and a 10 ms integration time unless otherwise stated.
Bench tests of the 10–620 MHz SFGPR Ž
measured through coaxial attenuators antennas .
bypassed have confirmed a system perfor-
mance of 175 dB in ungated mode and 170 dB in gated mode. In comparison, Wright et al.
Ž .
1994 estimated the system performances of conventional impulse GPR systems to be in the
range 100–130 dB without stacking. Impulse GPR system performance can be
improved significantly by using real-time digi- tisers and stacking successive waveforms, and
increasing the duty cycle of impulse transmit- ters. In addition, a fast sensitivity time control
Ž
. STC is required before the real-time digitiser
to utilise the improved system performance. Ž
. Wright et al. 1990 reported an experimental
impulse GPR system operating around 10 MHz with these improvements to obtain a theoretical
Ž system performance
for 100 ms integration .
time of 160 dB, although in practice it was about 145 dB. Coherently summing waveforms
during a 10 ms integration time should give a theoretical system performance of 180 dB.
However, these improvements to impulse GPR system performance become much more diffi-
cult as the operating frequency increases.
3. Gating technique for SFGPR
