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Chen et al. Satell Navig (2021) 2:12 Page 3 of 7

authentication message is transmitted frequently, it developed the EGNOS Authentication Security Test-
needs signifcant bandwidth and potentially degrades the bed (EAST) (Chiara et al. 2017), preliminarily designed
performance; on the other hand, when authentication the authentication protocol, the authentication message
message is transmitted infrequently, it forces the receiver broadcasting scheme and the key performance indica-
to coast during a longer time using non-authenticated tors, and continuously evaluated the authentication
information (Figs. 1, 2). method. Alternatives for SBAS authentication include
AL, understood as the maximum time between the ECDSA digital signature and TESLA protocols (Chiara
reception of a message and its authentication, is also a et al. 2017; Neish et al. 2018), in which ECDSA adopts the
relevant parameter given that, unlike GNSS ephemerides, Elliptic Curve Schnorr (EC-Schnorr) standard.
SBAS messages are continuously changing. AL is directly Te United States has not yet explicitly proposed the
related to Time To Alert (TTA). Te ideal authentication Wide Area Augmentation System (WAAS) authenti-
delay should not exceed 6 s, because the TTA is 6 s. AL cation service plan, while a team from Stanford Uni-
and TBA are interrelated and their relationship depends versity has been actively promoting the formulation of
on the scheme, as shown in Fig. 3. SBAS signal authentication standard. Tey adopted the
Considering the channel (I/Q) and the authentication same alternatives as those used in Europe, including the
protocols (TESLA/ECDSA), four schemes were devel- ECDSA and TESLA protocols (Neish et al. 2019a, 2019b),
oped, as shown in Fig. 3. in which ECDSA adopted the National Institute of Stand-
ards and Technology (NIST) standard.
Status of SBAS signal authentication Compared with Europe and the US, China is at early
In 2016 the European Union (EU) proposed the European stage in the development of the SBAS signal authen-
Geostationary Navigation Overlay Service (EGNOS) tication technology. Te Civil Aviation University of
signal authentication plan (Chiara et al. 2016), then China and the China Academy of Sciences Institute of

Privatekey Public key

Public key/Privatekey server

Send message
and signature
Receiver
Sender
Mess Decryptwith In contrast, if
age Encryptwith Signature public key Message thetwo are
from
privatekey Signature received thesame, the
(Generate (Authenticate signature authentication
signature)
Private signature) Public is successful
key key Message

Fig. 1 Schematic diagram of authentication based on digital signature

Fig. 2 message authentication code principle