# Dual-Probe Near-Field Phaseless Antenna Measurement System on Board a UAV
## 2 System Description
### GNSS-RTK Unit: Features and Integration in the System  
The GNSS-RTK unit on board the UAV [23] is a dual-band multiconstellation GNSS receiver, which is more robust to the multipath and can keep centimeter accuracy even in challenging environments (e.g., with limited sky). However, the convergence time (time required by the GNSS-RTK module to resolve carrier phase ambiguities to integer number, i.e., to achieve a \"fix\" status) is smaller and it can recover faster from temporary loss of accuracy than single-band GNSS-RTK modules. Therefore, the proposed positioning system provides better accuracy and overall performance compared to the single-band GNSS-RTK unit used in the previous versions of the system [9; 12]. Quantitatively, it has been found that the average time to reach a fix status (i.e., to obtain cm-level accuracy coordinates) was reduced from more than 2-3 minutes to less than 10 s. The assessment of the GNSS-RTK systems was done in the same scenario in all the cases (airfield of the University of Oviedo, [[https://goo.gl/maps/smXzpN1](https://goo.gl/maps/smXzpN1) jmTjELpCj6]([https://goo.gl/maps/smXzpN1](https://goo.gl/maps/smXzpN1) jmTjELpCj6)).  
Concerning the integration of the GNSS-RTK unit [23], a customized dual-band RTK receiver was integrated into the UAV. an ad-hoc driver for the RTK system that runs on a single board computer (SBC) on board the UAV was developed. This driver is in charge of (i) receiving Radio Technical Commission for Maritime Services (RTCM) corrections via the internet using a transmission control protocol (TCP) from an RTK base station (at a fixed position on the ground); (ii) forwarding the RTCM corrections to the RTK rover beacon (on board the UAV); (iii) receiving binary positioning information from the RTK rover beacon and transforming it into human-readable data (obtaining the enhanced coordinates); (iv) forwarding the enhanced coordinates to the flight control software and to the RF measurement software using a user datagram protocol (UDP) connection. A scheme of how these hardware devices and software modules are interconnected is depicted in Figure 2.  
Furthermore, the accuracy was also improved by a factor of approximately 3 in all directions of space, with the current dual-band system having an accuracy of 0.5 cm in the horizontal plane and 1 cm in the vertical direction. It must also be noticed that its ambiguity resolution is more robust so it is less likely to lose the fix status.  
As a result, these improvements enable a better UAV navigation and measurement geo-referring accuracies. The former helps to reduce the deviation of the UAV flight path from the desired path defined with waypoints. The latter contributes to improve the accuracy of the antenna diagnostics information and the radiation pattern.  
In the configuration presented in this contribution, the GNSS corrections were taken from a fixed based station of the Spanish Instituto Geodesico Nacional [24]. This allows the deployment of a fixed RTK beacon in the ground (to act as base station for the RTK system) to be avoided, which results in less hardware complexity and faster deployment of the system. The connection to the fixed base station is done by providing 3G/4G connectivity to the WLAN.