Introduction: Digital L-Band (1.4 GHz) Polarimeter for UAVs

Skaha's L-Band polarimeter measures fractional polarization values and polarization angles, setting it apart from conventional dual-polarized radiometers.

Skaha's L-Band polarimeter consists of a microstrip patch panel antenna, an RF amplifier chain, a digital processor, and a controller unit. Moreover, the polarimeter has a separate ground station that remains on the ground. The ground station connects to the polarimeter controller via WiFi, providing a user interface and data storage.

The system consists of three components: the antenna, which houses the RF amplifier and digital correlator, the control unit, and the ground station. Both the control unit and the antenna are payloads to the UAV. During flight, the control unit sends data to the ground station, which is also used to set up the sensor.

The difference between Skaha's digital polarimeter and a traditional dual-polarized radiometer can be explained as follows: While a conventional radiometer can measure signal power independently in horizontal and vertical polarization, it cannot determine the polarization angle of partially polarized transmission if it deviates from precisely horizontal or vertical. To explain this further, assuming the received signal is a completely polarized signal with a brightness temperature of 50 Kelvin and a polarization of zero degrees relative to vertical (left graph):

Example polarization

Both the digital polarimeter and the dual-polarized radiometer would register a brightness temperature of 50 K in the vertical polarization, while no signal would be detected in the horizontal polarization. Based on this information, it can be concluded that the input signal is fully polarized in the vertical direction.

Assuming a polarization angle of 30 degrees (as shown in the graph on the right), a conventional dual-polarized radiometer would measure brightness temperatures of 43 K in vertical polarization and 25 K in horizontal polarization. However, it is impossible to deduce the correct polarization angle and percentage polarization under these circumstances. One possible conclusion could be that the input signal is composed of an unpolarized component with a brightness temperature of 25 K and a vertically polarized signal with a brightness temperature of 18 K.

A digital polarimeter is capable of determining fractional polarization values and polarization angles of the polarized component. For instance, in the given example, it could correctly identify 100% fractional polarization at an angle of 30 degrees relative to the vertical. A conventional, dual-polarized radiometer can measure the Stokes parameters I and Q. However, Skaha's polarimeter can also measure Stokes parameters U and V. From these measurements, the fractional polarization $\mathrm{PP} = \sqrt{Q^2+U^2} / \mathrm{I}$ and the polarization angle $\mathrm{PA} = 0.5\cdot\arctan(Q/U)$ can be calculated, along with the total power (Stokes I).