Passive radar or radiometer is a data compilation device which integrates various passive sensors combined with navigation sensors and movement adjustment sensors.

The main system includes: three different microwave narrow band radiometric antennas, a thermal camera, a Global Positioning System (GPS) and an inertial triaxial system (IMU).

The lower of the microwave frequencies is used to detect the presence of metallic particles of minerals and compounds that alter the dielectric properties of subsoil. The other two antennas of higher frequency are used in order to detect changes in the most superficial layers.

Finally, a thermal camera compensates the distribution of soul temperature due to sun radiation in the total of radiated emissions.

This document is presented in three different sections:

1- Introduction to remote sensing.
2- Physical properties of subsoil detectable with microwave.
3- Characteristics and technical note of M2 Radar.

1. Introduction to remote sensing

Remote sensing is the capability to measure something without touching it. Before the development of remote sensing techniques, the only way to take any measurement required direct contact.

By physics laws: “Every substance above the absolute zero (-273.15 degrees Celsius) emits some form of EM radiation.” This means that with the suitable device, you could detect every object and differentiate them from another one.

Regarding Earth Science, remote sensing refers to the ability of satellites to detect Electromagnetic (EM) radiation from the surface of the Earth or in the atmosphere1.

Taking as example the Sun, it dissipates a huge amount of energy that is radiated directly to the space. When it reaches the Earth, this energy is observed in the visible spectrum, like the light generated by the Sun, but there are other types of radiation like microwaves, X-ray, infra-red, etc. which can also be measured. In order to be capable to measure by remote sensing, the mean through the EM waves are propagates has to be transparent

Figure 1 shows atmospheric opacity through all wavelength spectrum:

radar m2
Figure 1: Electromagnetic opacity, of the Earth's atmosphere. Credits: NASA - public domain.

As it can be appreciated in Figure 1, the radio waves region of the spectrum is one of the most permeable in the atmosphere. This region it the microwave band, and it is where applications like WiFi, mobile phones, radio, television, Bluetooth, radar, remote control, and others.

The other two nearby transparent regions are the visible region of the spectrum and the infrared region both visible with thermography.

Speaking of microwaves, frequency is strongly related to penetration Depth and element detection capabilities. Each of the frequencies is associated to an application.

Below are shown the frequency bands commonly used to detect anomalies of subsoil layers (see table 1).

Band Wavelength (cm) Frequency (MHz) Soil application simples
VHF 1000 - 100 30 - 300 Geology, deep geophysics.
P (UHF) 100 - 30 300 - 1000 Archaeology, pipe detection.
L 30 - 15 1000 - 2000 Topsoil (Canopy) and soil moisture.

Table 1: Microwave bands and soil applications. Active vs passive.

Remote sensors can be classified regarding if they are active or passive. Its measurement principle is schematized in Figure 2.

Active sensors are designed to actively create a signal or stimulus in hardware that is propagated to the Earth. The sensor detects the response that is reflected from the Earth and processes the reflected signal to extract useful information. Through the reflex of the signal sent, it is possible to retrieve the distance (the longer the signal takes to return, the further away the measured object is) or speed (by means of the doppler effect). Sonar, Lidar and Active Radar are examples of active sensors.

Passive sensors do not create nor radiate any signal or stimulus. Passive sensors detect radiated energy from natural sources like what is emitted by the landscape, radiated by artificial sources (like the Wi-Fi, TV, remote door openers, etc.) or reflected by another source, like the Sun. Microphones, optical cameras and passive radars are examples of a passive sensor.

Its operation is outlined in Figure 2.

Figure 2: Passive (left) vs Active (right) remote sensing.
Figure 2: Passive (left) vs Active (right) remote sensing.

2. Physical properties of subsoil detectable with microwave

As per usual, relative permittivity changes in subsoil can be detected with microwaves. Relative permittivity is a material property of materials relating to the capability of its molecules to polarize when under the effects of an electromagnetic radiation field.

The most immediate consequence of a change in the relative permittivity in a particular media is the modification in wave propagation velocity for such media. Una de las consecuencias más directas del cambio de la constante dieléctrica de un medio es el cambio en la velocidad de propagación de una onda por dicho medio. As per Maxwell equations, wave propagation velocity is faster the smaller the relative permittivity (see figure 3).

Figure 3: Relation between relative permittivity and wave propagation speed.
Figure 3: Relation between relative permittivity and wave propagation speed.

All values are between limits 1 (air) and 81 (water). In table 2 some values of relative permittivity for some mineral compounds and rocks where the presence of gold, zinc and copper, between others are shown.

Compound Constante dieléctrica
Pyrite 10,5 - 11,5
Quartz 4,2 - 5,5
Galena 18
Hematite 25
Calcite 8,8 – 8,5
Beryllium 5,5 a 7,8
Feldspar 3 a 5,8
Gneiss 8,5
Basalt 12

Table 2: Relative permittivity of some rocks and compounds

As radiation is propagated through earth’s crust, it continuously receives interferences which affect the relative permittivity of subsoil. Some simples are the presence of water, presence of salts with electrical or thermal conductivity properties.

There are spectral filters applied to remote sensing with radar that allow the characterization of such effects. Some of the most known are:

a) European Space Agency SMOS satellite sensor subsoil electrical properties measurement.

b) NASA Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) hyperspectral sensor measurements of subsoil effects of temperature.

In figure 4 is displayed the work operation of a passive radar where changes due to very different physical properties of a material compared to its surroundings, produce substantial modifications to the subsoil emissions and allow its detection:

Figure 4: Passive radar receiving subsoil radiation emissions.
Figure 4: Passive radar receiving subsoil radiation emissions.

Another of the most relevant effects over received data signals are interferences due to the effect of external to the media emissions, usually man-made.

With the objective to mitigate interference bands, M2 radar employs narrow bands focalized in protected frequencies by International Telecommunication Union (ITU1). In this sense, the applied technology is based on the devices first used in the radiometer MIRAS2 Of the European Space Agency (ESA).

Protected bands are frequency bands which allow the detection, through radio astronomy, elements of interest on the atmosphere and earth’s surface. The use of those bands in remote sensing have the advantage they can’t be mistaken with man-sourced radiation emissions so any radiating source proceeds from a natural source.

In table 3 are shown ITU protected M2 radar used bands:

Band Wavelength (cm) Frecuency(MHz)
VHF 411-402 73,0-74,6
P (UHF) 92-91 327,0 – 327,7
L 22-21 1.370,0-1427,0

Tabla 3: Bandas protegidas por la ITU utilizadas por el radar M2.

On the other side, one of the characteristics of using narrowband antennas in front of using broadband antennas is the focalization of the sensor received microwave energy that allows data acquisition at greater depths1. Broadband antennas are usually applied by ground penetrating radar (GPR) devices while narrowband antennas are used in radiometers.

In figure 5 is shown as an illustrative form, the reception of void spectrum of broadband antennas and narrowband antennas. Central frequencies are the three ITU protected ones used by the M2 Radar:

Radar M2

Figure 5: Broad and narrow bands centered in protected frequencies used by M2 Radar.

M2 RADAR workings

The component schematics of the measurement, correction and filtering system is as shown in figure 6:

RADAR M2 Funcionamiento
Figure 6: M2 radar components.

In figure 7 it’s shown as an illustrative picture of the return corresponding to a deep mineralized layer and a shallow wáter one. The VHF antenna detects the mineralization in low frequencies, while UHF remove higher frequencies due to wáter presence. L-band identify shallower layers.

RADAR M2 Funcionamiento
Figure 7: Broad and narrow bands focalized in protected frequencies used by M2 Radar.

The resulting image is a wavelength diagram corresponding to greater mineralization:

RADAR M2 Funcionamiento

In figure 8 the various components of M2 Radar are shown while in flight:

Figure 8: M2 Radar components.
Figure 8: M2 Radar components.

RADAR M2 operational capabilities

Metal detection
Three different qualitative measurement (HIGH, MID, LOW).
Low limits::
 ‑ Au, from 0.1-0.5 g/Tn
 ‑ Cu, from 0.1-0.5 %
 ‑ Zn, from 0.5 %
Pixel size  ‑ 70 m natural pixel size
 ‑ 5 m pixel using an overlaped flight plan and postprocessing
Area covered 100 to 2000 ha for a 10 to 20 days prospection project.