Assisted NMA Proof of Concept on Android Smartphones

Abstract

Navigation Message Authentication (NMA) is a technique to protect GNSS navigation messages by cryptographically signing them. Cell phone based GNSS receivers, which account for by far the largest segment of the GNSS market by user numbers, are typically unable to take advantage of NMA due to the their low rate of navigation message decoding. Assisted NMA (ANMA) turns the NMA concept on its head: the cell phone decodes only a subset of the navigation messages, but verifies that the cryptographic signatures in the decoded messages are authentic using an alternative internet-based source of data that provides the required information with some delay. In this paper we present a proof of concept implementation of ANMA, using the Android raw measurements API, the Galileo Open Service NMA (OSNMA) and the Galmon server to obtain the true navigation data. We test the implementation in a number of static and dynamic environments and demonstrate the feasibility of the concept. We also note some gaps that would need to be filled for the ANMA concept to be viable. Most notably the low and unpredictable rate of I/NAV message decoding within the cell phone is something that would need to be addressed.