# VHF Omnidirectional Range (VOR) Experimental Positioning for Stratospheric Vehicles
## 4 Discussion  
The results from the conducted analysis highlight how VOR-based positioning can be adopted in stratospheric flight to support the GNSS receivers, thus improving the reliability of the on-board navigation and station keeping system for stratospheric missions.  
Between the two analyzed sensors, the SDR has been demonstrated as capable of reaching higher accuracy and precision values. The lower performances of the COTS receivers are mainly determined by the data acquisition strategy, forcing to receive radials from one only station at a time, thus allowing to achieve positioning when switching frequencies from one station to the other. On the other hand, the SDR spectrum recording allows to acquire many VOR radial simultaneously, permitting to perform an optimization among the collected radials to achieve more accurate positioning. Although these analyses were performed after the stratospheric flight, an automatic, real-time, data analysis from SDR data can be achieved through airborne computing units and low cost SDRs. Software Defined Radio architectures for stratospheric or even suborbital tracking offer extremely interesting perspectives in terms of ease of implementation, flexibility, and low costs. After STRATONAV, other experiments have been conceived and developed at S5 Lab to perform SDR-based tracking of stratospheric balloons. For example, a further experiment developed by S5 Lab for the REXUS/BEXUS Programme, named TARDIS (Tracking and Attitude Radio-based Determination In Stratosphere [32]), was aimed at real-time determination of the position and attitude of the BEXUS gondola while in stratospheric flight through SDRs. New concept experiments on innovative tracking systems are the aim of STRAINS (Strataspheric Tracking Innovative Systems, [33]), a new experiment aimed at demonstrating large baseline Time Difference of Arrival (TDOA) for stratospheric and suborbital vehicles, in the perspective of a future implementation for satellite tracking and Space Traffic Management. For the adopted technologies and for their implementation on-board the flight unit, STRATONAV can be considered a precursor of the mentioned new projects that have profited from the lessons learned from this successful experimentation. As for the future applicability of such experimental units, SDRs can be easily implemented on all stratospheric vehicles for real-time navigation purposes: an independent unit with batteries, computing unit and RF hardware can easily present a mass below 0.5 kg and very limited volume occupation (even less than 50 \(\times\) 100 \(\times\) 100 mm, as demonstrated for small satellites implementing the same technologies [34]).  
If implementing VOR or VOR-based signal processing techniques as secondary navigation method, it shall be remarked how VOR can be available only above land and it finds no applications when overlying the ocean. Although this limitation can anyway fit with several mission profiles (e.g., temporary probing of populated areas or telecommunication links for areas caught by natural disasters), the service cannot be assured worldwide as for the GNSS. Furthermore, the implementation costs of SDR technology for exploitation of radio-navigation systems have a comparable cost to low-cost GNSS receivers on redundant constellations. Finally, some of the NavAids, including VOR, are progressively undergoing discontinuation by some national civil aviation authorities, which can limit the future applications of the investigated system [35], although proposing a promising secondary positioning method.