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The L-Band Digital Aeronautical Communication System (LDACS) is a new terrestrial aviation data link providing higher data rates and network capacity than existing terrestrial systems, while reusing existing L-Band aviation spectrum allocations. Its approximately 498 kHz bandwidth enables LDACS to make accurate air-ground ranging measurements that can support Alternative Positioning, Navigation and Timing (APNT) applications when GNSS reception is degraded. This paper examines the baseline LDACS communication system design, primarily as documented in the current air-ground signal-in-space (SIS) specification, to determine constraints that an APNT system based on LDACS would have to consider. In addition, gaps in the current LDACS specifications that might have implications for LDACS APNT are identified and proposals for additions and/or modifications to the specifications are made. Different APNT modes of operation, specifically ground-to-air pseudoranging (PR), two-way timing and ranging (TWTR), and hybrid modes are assessed. A primary difference between the PR and TWTR modes is that PR requires tight synchronization between ground stations in the network, whereas TWTR has relaxed timing requirements at the expense of slightly increased data bandwidth needed specifically for APNT. Furthermore, the ground-to-air forward link (FL) ranging rates are much higher the air-to-ground reverse link (RL) ranging rates. These asymmetric measurement rates complicate TWTR processing. Alternative approaches for dealing with this are examined. Detailed PR/TWTR measurement processing equations are derived and the effects of radio system calibration errors are highlighted.