  Automatic Speech Recognition (ASR), as the assistance of speech communication
between pilots and air-traffic controllers, can significantly reduce the
complexity of the task and increase the reliability of transmitted information.
ASR application can lead to a lower number of incidents caused by
misunderstanding and improve air traffic management (ATM) efficiency.
Evidently, high accuracy predictions, especially, of key information, i.e.,
callsigns and commands, are required to minimize the risk of errors. We prove
that combining the benefits of ASR and Natural Language Processing (NLP)
methods to make use of surveillance data (i.e. additional modality) helps to
considerably improve the recognition of callsigns (named entity). In this
paper, we investigate a two-step callsign boosting approach: (1) at the 1 step
(ASR), weights of probable callsign n-grams are reduced in G.fst and/or in the
decoding FST (lattices), (2) at the 2 step (NLP), callsigns extracted from the
improved recognition outputs with Named Entity Recognition (NER) are correlated
with the surveillance data to select the most suitable one. Boosting callsign
n-grams with the combination of ASR and NLP methods eventually leads up to
53.7% of an absolute, or 60.4% of a relative, improvement in callsign
recognition.
