Last May the Italian Space Agency and the CONAE (Comisión Nacional de Actividades Espaciales) signed an important agreement which will allow the two Countries to exploit the Italian COSMO-SkyMed (X-band) and Argentinian SAOCOM (L-band) constellation. The synergy between the two constellations will create new applications based on the joint employment of the two radar technologies.
Synthetic aperture Radar is the technology that in geoinformation and space sector is growing faster than the others. In fact thanks to the possibility to monitor targets in any light and weather conditions the archive of the RADAR – SAR constellation is serving the big data analytics companies (like URSA and Orbital Insight) and feed the e-GEOS platforms.
Today the integration between the large investments in the space sector made in the past from the Italian Government (ASI/MoD) on COSMO-SkyMed (4 X-band satellites) and today from the Argentinian side with the SAOCOM constellation (2 L-band Satellites) give us a new scenario in the RADAR sector.
SAOCOM, earth observation satellite constellation of the Argentine Space Agency (CONAE), whose launch is scheduled for August 2018 (SAOCOM 1A) and for an unprecised date in 2019 (SAOCOM 1B) , will be run in synergy with the italian COSMO-SkyMed satellite constellation. While COSMO-SkyMed operates with SAR technology in X-band, SAOCOM uses the same technology but in L-band. This will be a unique opportunity to leverage on the differences between the two instruments to have integrated applications covering a wider range of applications.
The main difference between the two sensors is the wavelength: 3 cm for COSMO-SkyMed X-band and 23 cm for SAOCOM L-Band. This translates in the fact that while X-band can detect even slight details, L-band is suitable to observe larger objects which would result unclear with X-band. Combining the two is possible to monitor both geometrical objects and small details.
The difference in wavelength has a number of consequences when possible imaged target are considered, with an impact on the main applications:
1) Complex targets: the two sensors respond to different "elemental" targets, i.e. backscattering objects. Let's take as an example a tree. Most of the signal in X-band comes from the canopy, where the single leaves are of the size of the wavelength. L-band microwaves are backscattered by the main structures of the tree, such as big branches and the trunk. Both bands can be exploited together to give a full representation of a forested area.
Agriculture is another field in which the size of the backscattering elements matters. Let's consider rice plants growing in a paddy. As soon as the young plant emerges from the water, a signal can be detected with X-band. L-band is much less sensitive at this stage of growth. Once the plant reaches a mature growing stage, X-band signal is saturated (i.e. no more proportional to growing stage) while L-band signal provides useful information about growing stage and health conditions of the plant.
Polarimetry is the discipline dealing with the difference in recorded signal, depending on orientation of the transmitted signal e.m. field. In practical terms, asymmetric objects can be detected and characterized with polarimetric analysis. As a matter of facts X-band typical backscattering objects are much more symmetrical than L-band ones (think about the example of the tree) and therefore polarimetry in L-band is a much more interesting and well developed discipline.
2) Surface backscattering: the SAR signal recorded from a surface is proportional to the surface roughness. X-band is very sensitive to even a small scale roughness (think about a grassland area). The same surface is seen as much flatter by the L-band SAR due to the difference in wavelength.
As a consequence of this most of the signal from a rough surface in X-band comes from the surface itself, thus concealing the signal from "3D" objects on the surface (e.g. trunks, walls....) ; on the other hand, these same objects are imaged very clearly in L-band images, due to the double bouncing of the signal over an almost perfectly flat surface.
3) Different penetration (volume backscattering): penetration of microwaves in terrain is directly dependent on wavelength. This means that longer wavelength SAR (L-band) has more signal/sensitivity to the volume content of terrain (e.g. soil moisture).
Combined X- and L-band measurements can support the disentanglement between surface backscattering (roughness) and soil moisture from a penetrated volume. Specific materials such as snow and sand can be penetrated for a certain amount by microwaves. Also in this case, penetration is more consistent for L-band, thus supporting applications such as underground archeology.
4) The last difference between the two bands is in the domain of phase measurements (i.e. interferometry). Phase difference measurements between two satellite passes are possible only if the two phase signals considered are correlated (coherent). Time decorrelation of phase signal is a strong limit to interferometric analysis and is much stronger in X-band than in L-Band. On the other hand phase measurements in X-band, when possible, provide a much higher accuracy due to the shorter phase cycle (half the wavelength).
Therefore again we can say that X- and L-band interferometric application are complementary in terms of the area of application: L-band interferometry is more robust and of wider application while X-Band provides more accurate results over selected areas.