Development of an ultra-wideband ground penetrating radar (Izh-Terra ground penetrating radar)

Introduction

Ground penetrating radars (GPRs) on the market are mostly pulse radars. However, there have been a number of recent reports [1, 2, 3] on the development of continuous wave ground penetrating radars (hereinafter referred to as CW ground penetrating radars). The advantages of CW ground penetrating radars are shown both theoretically and practically [1, 4]:

1 dynamic range of CW ground penetrating radar exceeds the dynamic range of pulse analogues by more than 20 dB (all other characteristics being equal). In practice, this may mean an increase in detection depth by 3 times for point targets and by 4-5 times for linear extended targets;

2 In pulsed ground penetrating radars, the researcher is exposed to short high-power pulses, especially when working with an unshielded antenna (e.g., the Loza ground penetrating radar or its analogs). The effect of such pulses on humans has been poorly studied or, due to commercial interests, is not openly published. In any case, the health of the user of a pulsed ground penetrating radar is at serious risk. In the case of a CW ground penetrating radar, the average emitted power is several milliwatts (which is 1-2 orders of magnitude less than the radiation power of a mobile phone in talk mode), and with a shielded antenna, it has no noticeable effect on the user.

With all the above advantages, CW ground penetrating radars are not available for purchase, which was a good incentive to start developing our own ground penetrating radar. The developed ground penetrating radar was named “Izh-Terra”. Now we can say that the achieved technical characteristics of the developed ground penetrating radar are competitive. Table 1 presents a comparison of the radio frequency characteristics of the Izh-Terra ground penetrating radar and radars with similar operating principles. RIMFAX ground penetrating radars were selected for comparison.[3] (from the Perseverance rover) and ORFEUS [2]Both georadars are described in detail in available publications and, in my opinion, represent the level of modern technology (State of the Art).

Table 1 – Comparison of radio frequency characteristics of the Izh-Terra ground penetrating radar with analogues

The frequency range of the ground penetrating radar was chosen as a compromise between the probing depth and the requirement for mobility, related to the antenna sizes, as well as the depth resolution. The lower the frequency, the better.Ogreater probing depth is provided, but the larger the antenna size is required for efficient use of transmitter energy. For example, the FMCW RIMFAX ground penetrating radar as part of the Perseverance rover operates in the frequency range (150-1200) MHz and provides a scanning depth of over 10 m [3]. Orfeus ground penetrating radar [2] has a frequency range of (100-1000) MHz with acceptable (for ease of use and mobility) antenna dimensions of 0.52×1.1 m2.

All the considered georadars have a signal band of about 1000 MHz. In this case, a depth resolution of 5 cm is provided (for typical soils with a permittivity of 9). In georadars with continuous radiation, it is advisable to use linear frequency modulation (LFM) as the simplest to implement and meeting modern requirements for radars in terms of noise immunity.

The operating principle of the FMCW georadar is shown in Figure 1. The transmitter generates a signal with a continuous linear change in frequency over time, which after amplification enters the transmitting antenna. The signal reflected from the target from the receiving antenna enters the receiver, is mixed in the mixer with the signal for transmission. At the output of the mixer, a signal is extracted corresponding to the difference in frequencies of the transmitted and received signals (the frequency difference is also called the beat frequency). After digitizing such a signal, its spectrum is estimated, usually using the fast Fourier transform. The spectrum is a reflection of the surrounding environment.

Description of the transmitter operation

When forming the requirement for the transmitter power, it is necessary to take into account the amount of power leakage from the transmitting antenna to the receiving one, which is usually minus (30-40) dB, and the amount of limitation of the receiver amplitude characteristic (usually minus (5-10) dBm for a classic receiver circuit containing a low-noise amplifier and mixer). In this case, the transmitter power should not exceed 20 dBm.

The transmitter is based on a phase-locked loop (PLL) and a frequency converter (Figure 2). The PLL consists of a phase detector, a charge pump unit, a loop filter, a frequency divider, and a voltage-controlled oscillator (VCO).

Using PLL, a chirp signal is formed with frequencies from Fн to Fв such that Fв>Fн>1100 MHz, (Fв-Fн)=1000 MHz. The sweep period is 1 ms by default and can be changed by the user.

The frequency converter, consisting of a low-pass filter, mixer, constant frequency signal generator, amplifiers, is designed to transfer the frequency to the frequency range of the georadar operation. The heterodyne is a generator at a frequency of Fв+100 MHz, and also has a PLL circuit in its composition.

Figure 3 shows a graph of frequency change over time at the transmitter output, obtained by circuit simulation of the transmitter operation. According to the simulation data, the peak frequency deviation from the linear dependence is 460 Hz (RMS – 150 Hz). Such frequency nonlinearity is acceptable.

The transmitter phase noise is shown in Figure 4. It is possible to attempt to reduce the transmitter phase noise (which will increase the cost of the ground penetrating radar), but the following circumstance should be taken into account. A characteristic feature of FMCW ground penetrating radars is their operation with objects located in close proximity to the antennas. In this case, the phase noise of the reference and received signals (see Figure 1) are correlated and are subtracted during conversion in the mixer, which leads to their significant reduction (up to 30 dB) [5]. Therefore, I will assume that measures to further reduce phase noise are ineffective and inappropriate. The transmitter power is selected in the range (0-10) dBm, based on the current legislative restrictions in a specific territory.

Thus, the following transmitter characteristics are achieved:

• frequency range – (100-1100) MHz;

• transmitter power – (0-10) dBm;

• sweep period – 1 ms;

• maximum frequency deviation from the linear characteristic is 460 Hz.

Description of the receiver operation

The receiver contains a low-noise amplifier (LNA) and a mixer. The LNA provides a receiver noise figure of no more than 2 dB, but at the same time reduces the dynamic range of the radar by an amount equal to the LNA gain (15 dB), limiting the permissible transmitter power. Therefore, the use of LNA in ground penetrating radars is debatable, and in some FMCW ground penetrating radar schemes it is not used. [1]However, in practice it turned out that its use is necessary for matching the receiving antenna with the mixer, and abandoning the LNA in the circuit under consideration turned out to be impossible.

The signal is digitized using an analog-to-digital converter (ADC) with a sampling frequency of 1 MHz and a bit grid of 16 bits. The digitized signal is processed in a computing module based on the BF706 DSP processor, then the data is transferred to a laptop with Windows 7, 10 OS via a USB cable up to 5 m long. It is also possible to transfer the radargram image in real time via WiFi to any Android device.

To evaluate the dynamic range, the receiver was connected to the transmitter via a 45 dB attenuator (the isolation value between the receiving and transmitting antennas) and a 1 m long RG402 cable. The result is shown in Figure 7.

The following receiver characteristics have been achieved:

• minimum detectable signal (receiver sensitivity) – minus 135 dBm;

• maximum signal at a signal compression level of 1 dB – minus 5 dBm;

• receiver noise figure – 2 dB;

• The receiver's dynamic range is 130 dB.

Description of antennas. During the work on the georadar, the following antennas were developed, manufactured and tested (Figure 7):

• circular resistively loaded bow tie antenna with a diameter of 0.5 m;

• rectangular resistively loaded bow tie antenna measuring 0.52 x 0.2 m2;

• resistively loaded Archimedes spiral with a diameter of 0.3 m.

All antennas are shielded. Each antenna has its own advantages and disadvantages. If readers are interested, a comparative analysis of antennas can be made into a separate article. This article will consider a round bow-tie antenna (Figure 7, upper left corner) and the operation of a ground penetrating radar with it. The radiating surface is shown in Figure 8. To reduce the antenna quality factor (reduce ringing and increase the bandwidth), the electrical conductivity of the antenna wings is forced to decrease toward the edges according to the Wu-King distribution known in antenna technology. The efficiency of such antennas is about 10% over most of the frequency range.

Figures 9 and 10 show the S-parameters of a pair of such antennas. The frequency range at the S11 level minus 10 dB (VSWR no more than 2) is (90 – 1200) MHz, the decoupling between the antennas does not exceed minus 42 dB. Figure 11 shows the general view of the directional diagram at the central frequency of the range. Such a directional diagram ensures high selectivity of the georadar – insensitivity to “airborne” interference, such as buildings, power lines, trees, etc.

User interface

The top-level software is a Python script using the SciPy, NumPy, PyQt4, PyQtGraph libraries. The software allows express analysis of radargrams during direct work with the georadar, or detailed analysis of recorded tracks using special processing methods. Data recording for subsequent processing is performed in HDF5 format. An example of the user window is shown in Figure 12.

Design

The winter version of the Izh-Terra georadar is shown in Figure 13. The dimensions of this version are: 1.43×0.73×0.25 m3, weight – 15 kg.

Practical test of the georadar operation

A typical radargram obtained when working on dry soil covered with snow is shown in Fig. 14. The distance along the horizontal axis was measured using GPS/GLONASS sensors, since the use of a classic measuring wheel (odometer) on snow is extremely difficult. It is worth noting that the radargram is ground penetrating radar data that has not been subjected to any special processing (except for subtracting the average). In Fig. 14, hyperbolas can be seen at a depth of up to (5-6) m.

Of interest is also the possibility of using a ground penetrating radar to measure the thickness of ice on water bodies. Typical results of such work on a fresh water body are shown in Figure 15 (without any special processing). The two red lines in the upper part of the radargram are signals from the upper and lower edges of the ice. The ice thickness is determined by the difference in the signal delay time. For the case shown in Figure 15, the time delay was 3 ns, which corresponds to an ice thickness of 0.24 m. This result coincides with the actual thickness of the ice cover with an accuracy of 0.01 m. Figure 15 also shows the bottom relief, which is also of considerable practical interest for various types of tasks. In practice, with an ice thickness of 0.24 m, the maximum depth at which a sandy bottom was observed was 1.5 m. I assume that in the absence of ice, the depth of bottom observation in a fresh water body will increase 3-4 times to 6 m with antennas located on the water surface.

Conclusion

Main characteristics of the developed Izh-Terra ground penetrating radar

1 frequency range: (100 – 1100) MHz, signal bandwidth (frequency deviation) B=1000 MHz;

2 measurement frequency: 1000 times per second with a time base of 500 ns;

3 depth resolution: 5 cm for typical material with ε=9 (asphalt, concrete, wet sand);

4 dynamic range: 130 dB (excluding antennas);

5 transmitter power: (0-10) dBm, radiated power taking into account the antenna efficiency (minus 10-0) dBm;

5 maximum detection depth (estimated) – 10 m for dry soils, (30 – 70) m for ice and snow cover;

6 operating temperature: (minus 40 – plus 40) degrees Celsius;

7 maximum speed of movement of the ground penetrating radar: 180 km/h;

8 continuous working time: 4 hours;

9 scanning mode with distance binding: GPS/GLONASS, by time;

10 noise immunity (primarily to “airborne” interference) – high.

Plans for further work

If there are customers, implement similar developments. For example, there is a desire to develop a radar for detecting people and animals buried alive as a result of explosions, earthquakes and other natural disasters. Detection is carried out by selecting moving targets at the slightest movements of the victim (including breathing, heartbeat, etc.). As far as I know, Sensors&Software has such a development and was successfully used after the earthquake in Nepal.

Bibliography

1 DJ Daniels Ground Penetrating Radar, 2nd edition. The Institution of Electrical Engineers. London. 2004. 752 p.

2 F. Parrini et. al., “ORFEUS GPR: a very large bandwidth and high dynamic range CWSF radar” Proceedings of the 13th International Conference on Ground Penetrating Radar, Lecce. Italy. 2010. pp. 1-5.

3 S. E. Hamran et al. Radar Imager for Mars' Subsurface Experiment—RIMFAX. Space Sci Rev 216, 128 (2020)

4 M. Pieraccini, “Noise performance Comparison Between Continuous Wave and Stroboscopic Pulse Ground Penetrating Radar” // IEEE Geoscience and Remote Sensing Letters. vol. 15, no. 2. Feb. 2018. pp. 222-226.

5 L. B. Ryazantsev, I. F. Kupryashkin, V. P. Likhachev. Analysis of the energy spectrum of phase noise of the signal at the output of a radar receiver with a continuous frequency-modulated signal // JOURNAL OF RADIO ELECTRONICS, ISSN 1684-1719, N6, 2018