Antispoofing Measure by Optimizing and Switching Radiation Pattern through Phase Control on QHA Monofilars
Shifa Nadeem, Muhammad Amin, Abdur Rehman Maud, Institute of Space Technology, Pakistan
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Security of GNSS, Global Navigation Satellite Systems, is important due to the dependency of GPS on navigation, positioning and time synchronization. GNSS security is threatened by spoofing attacks which is why GPS spoofing has caught much attention recently. In this paper a simple anti-spoofing technique of L1 band GPS signals, using phase control of Quadrifilar Helix Antennas (QHA), mono filars is presented. Assuming that the spoofing signals are directed from ground or low elevation angles; GPS signals antenna reception is made significantly lower at these angles. For this application Quadrifilar Helix Antenna has been optimized to give lowest upper hemisphere to lower hemisphere ratio to reject the spoofing signals originating from ground. On the other hand the hemispherical radiation pattern of QHA is also periodically switched between zenith and nadir angles and the difference in the received signals in these two states is used to differentiate the spoofing signal from authentic one. It is assumed that the spoofing signals are coming from a point source therefore signal power and phase difference are the two parameters used to point out the difference between spoofing signal and authentic signals. It is seen that the spoofing signals are high power signals with constant phase difference which allows the receiver to distinguish between authentic and fake signals. Spoofing signals are then mitigated using spatial nulling effect.
QHAs are one popular candidate for GPS antenna because of its light weight, wider 3 dB beamwidth and because it can also be designed without the requirement of a ground plane. A good QHA design gives unidirectional hemispherical pattern with maximum front to back ratio. However used as a good GPS antenna on aerial platforms in antispoofing role, a higher front to back ratio is essential only within angle 0 / 180 deg +_ 45 deg (depending on the height of the aerial platform) excluding low elevation angles. In this paper we therefore optimize the QHA antenna radiation pattern so that we have a minimum front to back ratio within a cone angle that is most likely to receive signals from spoofing ground station.
We also demonstrate that switching of QHA radiation pattern from upper (satellite communication) hemisphere to lower hemisphere (ground communication) by controlling phase of QHA monofilars is useful to prevent spoofing of GPS system.