Retrieving Refractivity using Interferometry of Refracted Aircraft Radio Broadcasts

Water vapour is the key tropospheric constituent driving meteorological processes in the atmosphere of Earth.  However, its extreme spatial and temporal variability in the lower atmosphere presents an enormous challenge for existing observing systems.  Crucially, no single existing observing system can accurately capture the detailed four-dimensional distribution of water vapour in the troposphere.  There is a growing need for opportunistic remote-sensing technologies that can provide low-cost, high-volume humidity observations for use in numerical weather prediction (NWP) models.

Observations of refractivity, which has a strong dependence on water vapour in the lower atmosphere, provide an important indirect source of humidity information for use in NWP. An effective method of obtaining refractivity measurements is through the Global Navigation Satellite System radio occultation (GNSS-RO) technique, which uses the change in bending angle of radio signals emitted by GNSS satellites due to variations in the refractive index to construct vertical profiles of refractivity.  However, despite GNSS-RO proving to be an invaluable source of refractivity data, the horizontal resolution of such retrievals is limited to the order of hundreds of kilometres.  Other humidity-sounding techniques, such as lidar and ground-based GNSS receiver technologies, also suffer from limited horizontal resolution.  As the resolution of NWP models continues to increase, there is a clear need for observing systems that can resolve short spatial and temporal variations in tropospheric refractivity.      

We present a new way to obtain information on atmospheric refractivity structure by measuring the angle of arrival (AoA) of radio signals routinely broadcast by commercial aircraft.  The radio transmissions are the 1090 MHz Automatic Dependent Surveillance-Broadcast (ADS-B) transmissions which all commercial aircraft are mandated to broadcast for air traffic purposes.  As the radio transmissions propagate through the atmosphere, variations in refractivity induce bending in the ray path.  A prototype ADS-B interferometer was used to simultaneously measure the incident AoA of the signal and extract the aircraft positional information encoded in the ADS-B.  We show how the interferometrically derived AoA can be combined with the known aircraft position to obtain information concerning the refractivity structure of the atmosphere.  The rapid broadcast rate of ADS-B (approximately twice per second) and the high density of air traffic over Northwestern Europe allow for detailed sampling of the lower atmosphere.  Sensitivity tests indicate that measurements of AoAs below an elevation of approximately 2 deg. with an accuracy of 0.01 deg. should allow for meteorologically useful information to be extracted.  Recent experiments indicate that large-scale changes in refractivity are detectable and that measurements of individual refracted ADS-B transmissions are approaching the 0.01 deg. accuracy through improvements in the interferometer array.  The technique is analogous to the existing GNSS-RO technique and it is anticipated that data assimilation schemes could be adapted to use this new source of bending angle data.  
     

This work has been funded by the University of Exeter, the Met Office and the Harry Otten Foundation.