# A Controlled-Environment Quality Assessment of Android GNSS Raw Measurements
## 3 Methodology and Methods
### Controlled Environment Setup  
An anechoic chamber is a testing facility designed to completely absorb Radio-Frequency (RF) waves thanks to the particular shape and material of the walls. Therefore, it can be used to simulate the ideal propagation condition. Being isolated from the surroundings, the chamber also reduces the impact of external RF impairments. In addition, the signals transmitted inside are not propagated through the walls, thus avoiding any jamming or spoofing situation while replaying a GNSS signal.  
Figure 5: Example of second order derivative of the raw and carrier-smoothed pseudorange measurements for a single Global Positioning System (GPS) satellite and related positioning solutions.  
The anechoic chamber of the Department of Electronics and Telecommunications (DET) of Politecnico di Torino has been used for the test. Different Android devices and the GNSS antenna transmitting simulated GNSS signals are clearly visible in the left pictures reported in Figure 6.  
The transmission and reception of GNSS signals was pursued according to the Record and Replay paradigm [11]. A block scheme of the setup is shown in Figure 7. The original transmitted GNSS signals were obtained in a controlled setup as well, exploiting an IFEN(tm)NavX (IFEN GmbH, Poing, Germany) professional constellation and hardware signal generator, simulating a static position according to a set of given coordinates. L1 C/A signals from all visible satellites belonging to GPS constellation were simulated at RF and then digitized by means of a front end, connected to the simulator through a wired link. No receiving antenna was used for the record step, thus avoiding multipath effects due to RF propagation. Signals were recorded as digitalized raw samples by means of Ettus ResearchTMUniversal Software Radio Peripheral (USRP) N210 front end, equipped with an RFX OS364-13 (RFX Ltd., Livingstone, UK) Oven Controlled Crystal Oscillator (OCXO). The signal acquisition was performed at 5 Msps, at Intermediate Frequency (IF), and at 8 bit/sample for In-phase and Quadrature components to guarantee an adequate replication of the stream. Binary files were hence stored in an external memory, and then replayed through a second USRP, used in transmitting mode, directly in the anechoic chamber. The system was equipped with a Rubidium atomic clock to provide precise timing for the digital-to-analog conversion of the samples. Analog signals emitted by the USRP were amplified by a Low Noise Amplifier (LNA) to feed a passive Novatel hemispherical antenna, installed in the anechoic chamber for the transmitting test. The devices were located on a support aligned to the main axis of the nominal pattern of the transmitting antenna inside the anechoic chamber. The patch was connected to a u-Blox EVK-M8 QCAM (u-blox Holding AG, Thalwil, Switzerland) benchmark receiver, used to verify the adequacy of power calibration of the transmitting front end and the actual visibility of the simulated signals (Figure 8).  
Figure 6: Anechoic chamber setup: on the left, receivers in face-up orientation; on the right, the transmitting GNSS antenna.