Radar M2TM. Technical note
- RADAR M2 is a system to measure the emissivity and reflectivity of the subsoil that integrates the combination of different passive sensors, as well as navigation sensors and correction of movement during measurements.
- The system fundamentally includes: three narrow band microwave radiometric antennas, a thermal camera, a global positioning system (GPS) and a triaxial inertial system (IMU).
- The combination of the intensities of the signals received by each sensor, duly processed and amplified, can give us a lot of information about the subsoil.
- On the other hand, the signal emitted by the GPS satellites, in addition to being used to geoposition the measurement system, can be exploited, using the reflectometry techniques, to measure the surface dielectric constant of the ground.
- Finally, a thermal camera compensates the contribution of the temperature of the surface of the ground produced by the Sun in the total emissivity radiated by ground surface.
1. Radiation
According to the physical laws: “Any substance above absolute zero (-273.15 ºC) emits some type of electromagnetic radiation.” This means that, with the right apparatus, each object can be detected and differentiated from any other.
Regarding Earth sciences, remote sensing refers to the ability of electromagnetic radiation (EM) detector satellites of the earth’s surface or atmosphere.
To be able to measure by remote sensing, the medium through which EM waves propagate must be transparent.
As it can be seen, the region of the spectrum corresponding to radio waves is the most permeable in the atmosphere. This region is the microwave band (EM), and is, together with the infrared (IR) in which the different M2 radar sensors operate.
Next, the atmospheric opacity is shown throughout the radiation spectrum:
In the universe of microwaves, the frequency is strongly related to the depth of penetration and its ability to detect the elements. The reason is that the ground behaves like a low-pass filter, which filters out high frequencies as it propagates through the ground at greater depths.
Below, the MW bands commonly used in remote sensing are shown:
| Band | Wavelength (cm) | Frequency (MHz) |
| Deep – VHF | 1000 – 100 | 30 – 300 |
| Intermediate – P (UHF) | 100 – 30 | 300 – 1000 |
| Superficial – (L) | 30 – 15 | 1000 – 2000 |
2. Types of sensors
In general, radar sensors can be active or passive:
- Active sensors. They generate a stimulus in hardware through an emitting dipole that can be detected by a receiver dipole sensor.
The sensor detects the reflected response. The active sensors measure distances, frequencies and speeds through the reflection of the signal sent. - Passive Sensors. They do not radiate any type of signal. They only detect radiated energy (emissivity) from natural sources (from the Sun or the Earth), or artificial sources (Wifi, TV or GPS).
Within the passive sensors there is a particular typology, the sensors that measure the energy radiated by the Earth. If the energy received by these sensors comes from natural sources, the sensor can be called a radiometer, while if the measured radiation is the reflectivity of a known artificial source, the sensor can be called a reflectometer.
M2 radar is a set of passive sensors that combine the functions of radiometers and reflectometers.
3. Physical properties of the subsoil detectable by MS
3.1 The dielectric constant
Since the dielectric constant is a parameter associated with the time domain, it is required to have a known radiation source; which can be, for example, the GPS signal.
One of the most direct consequences of changing the dielectric constant of a medium is the change in the velocity of propagation of a wave by said medium. According to Maxwell’s equations, the velocity of wave propagation is greater the lower the dielectric constant, as shown in the graph:
All values ?? Are between limits 1 (air) and 81 (water). The table shows some reference values ?? Of the dielectric constant of some mineral compounds and rocks where there may be the presence of gold, zinc or copper, among others.
| Mineral compound | Dielectric constant |
| Pyrite | 10,5 – 11,5 |
| Quartz | 4,5 – 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 |
The following figure outlines the operation of a reflectometer sensor:
3.2 The intensity
The intensity with which a signal is received from the subsoil depends on the changes produced by an element of physical properties very different from those of its environment, which substantially modify the emissivity of the subsoil.
However, one of the most relevant effects on the signals received is the interference caused by artificial emissions.
In order to mitigate the bands that produce interference, the M2 radar uses narrow bands centralized on frequencies protected by the International Telecommunication Union (ITU). In this sense, the technology used is based on other devices used, such as the MIRAS radiometer from the European Space Agency (ESA).
The protected bands are bands that allow detecting, by means of radio astronomy, elements of interest present in the atmosphere and the terrestrial surface. The use of these bands in remote sensing has the advantage that they cannot be confused with artificial sources of radiation, so any radiant source comes from a natural source.
The following table shows the bands protected by the ITU used by the M2 radar:
| Band | Wavelength (cm) | Frequencies (MHz) |
| VHF | 411-402 | 73,03 – 74,6 |
| P (UHF) | 92-91 | 327,0 – 327,7 |
| L | 22-21 | 1.370,0 – 1427,0 |
On the other hand, one of the characteristics of the use of narrowband antennas with respect to the use of broadband antennas is the focusing of the energy received by the sensor that allows the use of amplifiers, and, therefore, to collect deeper data.
The figure shows, in an illustrative way, the reception spectrum in the vacuum of wideband antennas and narrowband antennas, centralized on the 3 bands used by the M2 radar:
4. Characteristics and data sheet of RADAR M2
The following figure shows the different components of the M2 radar in flight:
RADAR M2 operational capabilities
| Metal detection Three different qualitative measures (HIGH, MID, 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. – 10 m pixel overlaying in post-processing. |
| Covered area | 100 to 2000 ha for every 10 to 20 days of prospecting project. |
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