The VOR (VHF Omnidirectional Range) is an aircraft navigation system that convoys the old concept of sea mark navigation into radio frequencies. It has been used since late ’40s and there are over 3000 VOR ground stations all over the world.
Each VOR ground station transmits a complex signal (in VHF band), with the ID of the ground station, a reference signal indicating the magnetic North and a directional sine wave, changing its phase towards the transmitting direction.
By comparing the directional and the reference signal, a receiver is able to compute its radial, which is the angle between the magnetic North and the line between the receiver and the ground station. The radial does not match with the course (angle between the true north and the motion direction) or with the heading (the pointing direction of the aircraft’s nose)!
The VOR has been used for decades as primary flight navigation system: the pilot only had to divide his optimal route into straight segments among VOR ground stations and to follow the sequence of radials while tuning his onboard radio to the ground stations’ frequencies.
The VOR service volume evaluation is based on calculations on the ground stations’ radiated power and it is approximated. The International Civil Aviation Organization (ICAO) prescribes strict radiated power rates to each kind of VOR stations, but it gives no indications on the service volume.
We know that an high altitude navigation VOR station has to be able to serve civil planes cruising at high altitude (10-12 km). The VOR service should be guaranteed until 18 km in height.
What happens then?
We aim to test some VOR receivers in the stratosphere, flying on the BEXUS balloon until 25-28 km of height and to check the functionality and accuracy of the VOR system from the ground to the stratosphere. The launch area (Kiruna, Sweden, 150 km above the Arctic Circle) is served by a lot of VOR ground stations and the BEXUS flight could signify an unique chance to discover the real service volume end of the VOR.
We will collect the receivers’ VOR data and compare it to the GPS ground track of the balloon. If we are able to compute the balloon’s radial and position within an acceptable precision, we could set a milestone for future stratospheric flight navigation systems.