# VHF Omnidirectional Range (VOR) Experimental Positioning for Stratospheric Vehicles
## 2 Materials and Methods
### STRATONAV Experiment and Data Collection Processes  
STRATONAV is a student stratospheric experiment developed in 2016 by the S5 Lab (Sapienza Space Systems and Space Surveillance Laboratory) research team at Sapienza University of Rome. The project aim was to verify the accuracy of VOR while flying in stratosphere, since the Standard Service Volume (SSV) of VOR does not reach beyond 18 kmof height above ground, while link budget calculations suggest that a broader utilization is possible well above the predicted SSV end [19].  
The experiment was selected in late 2015 for the ninth cycle of the REXUS/BEXUS Programme, managed by SNSA (Swedish National Space Agency), DLR (German Aerospace Center), and ESA (European Space Agency) for providing launch opportunities on-board sounding rockets and stratospheric balloons to European University students. The experiment was developed throughout 2016 at SSLab by the student team, which was profiting from the lessons learned and the experience from previous space systems manufacturing projects carried out at the laboratory [20; 21; 22; 23; 24; 25; 26; 27]. The experiment was launched on-board the BEXUS 22 stratospheric balloon from the Esrange Space Center in Kiruna (Sweden), on 5 October 2016 at 13.35 UTC. The balloon overflew the Swedish and Finnish Lapland with a ground track of approximately 240 km and a total flight time of approximately four hours [16].  
The experiment was equipped with two different Radio-Frequency receivers in order to collect and detect VOR data:  
* A COTS (Commercial Off The Shelf) portable VOR receiver, able to autonomously receive and decode the signal and to show the calculated radial on the screen. This receiver was modified by the team in order to be able to tune in the different VOR frequencies and to externally log the detected radials. This receiver will be referred as \"COTS receiver\" from now on;
* A Software Defined Radio (SDR) able to scan and record the frequencies of interest (from 108 MHz to 118 MHz). The recorded raw data was saved on a Solid State Drive (SSD) and post-processed after the experiment recovery after landing. This device will be referred from now on as \"SDR receiver\" in the paper.  
In order to be able to compare the collected data accuracy to real positioning data, a commercial GPS (Global Positioning System) receiver with native interconnectivity with the selected On-Board Data Handling System [28] was included in the experiment box. After the flight, the GPS data retrieved by the experiment were compared and validated with the navigation and positioning data made available by the launch base team for the balloon.  
STRATONAV managed to acquire a large amount of data throughout the flight. The VOR radials were proven to be within the previous ICAO standard accuracy rates for the 99% of the collected samples. The experiment profited by a very high number of VOR ground stations installed in the flight area (i.e., Swedish and Finnish Lapland) [29; 30; 31].  
Concerning the positioning data, the samples collected by the COTS receiver are much less than the ones collected by the SDR. This is justified by the data collection processes: while the SDR scans the VHF band, recording all the VOR signals in the band, thus accessing at many VOR radials at a time, the COTS receiver can be tuned to a single frequency. The positioning samples, requiring two (or more) radials from different stations, can be collected only when the COTS receiver changes its tuning frequency between two active VOR stations.