Evaluation of Code-Carrier Divergence Threat for DFMC GBAS in Urban Area

The Dual-Frequency Multi-Constellation Ground Based Augmentation System (DFMC GBAS) holds significant potential for enabling safe and precise urban air mobility (UAM) operations, such as electric vertical takeoff and landing (eVTOL). A new DFMC GBAS architecture, known as GBAS Approach Service Type (GAST) E, has been proposed by International Civil Aviation Organization (ICAO) Navigation System Panel (NSP) GBAS Working Group (GWG). However, urban environments introduce severe code-carrier divergence (CCD) threats due to significant multipath effect including non-line-of-sight (NLOS) signals and dynamic multipath interference (MI). This challenges the integrity of traditional 30s filtering GAST-D-like CCD monitors. This paper evaluates multipath-induced CCD threats in DFMC GBAS under real-world urban conditions through real field tests. Two strategy of DF CCD test metric using L1/B1C and L5/B2a signals, raw CCD test metrics and 30 seconds filtered GAST-D like test metrics, are used to quantify the CCD threats induced by MI and NLOS. In the real field test, pseudorange multipath errors in urban canyons exceed 100 meters, dominating CCD anomalies. Experimental results reveal that raw CCD DF1 and DF5 test metrics can detect rapid MI-induced pseudorange variations, however, they fail to identify constant NLOS biases, leading to delayed or false alerts. Meanwhile, the proposed GAST-D MOPS defined threshold for CCD monitor cannot satisfy the requirement of multipath-induced CCD identification. The threshold should be change for real operation of DFMC GBAS in urban area.