## 2 Rationale and Requirements  
The objective of this work is the design and prototyping of the on-board sensor to collect measurements of GNSS reflected signals suitable to enable the estimate of some soil parameters, in particular the soil moisture, using the GNSS sensor mounted on-board a small, possibly unmanned, aircraft. To implement the radar capabilities, the direct signal coming from the satellite is received for positioning purposes, in order to evaluate and geo-reference the specular reflection point on the ground,as described in Section 4; furthermore, the characteristics of the direct signal are used as a reference for the processing of the reflected one, to enable the remote sensing of the soil features.  
The design flow of the on-board sensor has been distributed over three main layers, depicted in Figure 1:  
* the hardware platform,
* the GNSS signal processing and
* the signal processing for soil parameter retrieval.  
The first step is the definition of the hardware architecture, _i.e._, the RF front-end, the microprocessor board and the antennas. Then, the design of proper GNSS signal processing techniques follows, to detect and estimate the relative delay and the amplitude of the reflected GNSS signals. The major concern at this stage is the extreme weakness of the reflected signals, which may lose around \(13\) dB for the LHCP and \(23\) dB for the RHCP with respect to the direct signal [18], received at a nominal power on the order of \(-160\) dBW. Finally, proper remote sensing algorithms post-process the raw GNSS data.  
The focus of this paper is explicitly on the hardware architecture of the prototype. For this reason, only a few details are given about the signal processing and soil parameter retrieval algorithms; the interested reader may refer to [22, 23]. Nonetheless, Section 5 shows some examples of the results obtained from processing the data recorded by the prototype, during one of the test flights.  
The design of the GNSS-based bistatic radar was guided by a set of requirements dictated by the target application. The subset of functional requirements applicable to the hardware platform discussed here is constituted by six propositions:  
**Advanced antenna configuration.**The sensor shall be able to handle three antennas, implying three RF chains and three digital streams, as depicted in Figure 1.  
**Storage capability.** The sensor shall be able to store raw measurements, observed during a flight mission.  
**Direct and reflected signals' synchronization.** The three signal streams (one direct, two reflected) shall be synchronized in sampling, storage and processing. Since the reflected signals are in general very weak, their processing can obtain significant benefits if aided by the direct signal, but aiding procedures require the synchronization among the three streams.  
**Flexibility.** The radar shall be programmable and reconfigurable, at least in terms of the receiving bandwidth, signal conditioning and digital signal processing parameters (e.g., the number of correlation points).  
Figure 1: High-level information flow and the three layers of the prototype design.  
**Processing capability.** The radar should be equipped with enough computing resources to allow the implementation of some on-board digital signal processing algorithms.  
**Size and weight.** The sensor must be lightweight, _i.e._, <3 kg, and small, _i.e._, \(\leq\)200 mm \(\times\) 250 mm \(\times\) 250 mm (length \(\times\) width \(\times\) height), to be mounted on-board UAVs and light aircraft.