LDACS Navigation System Design Considerations

Abstract

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 500 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 looks at different operational APNT use cases; the positioning requirements associated with Automatic Dependent Surveillance Broadcast (ADS-B) in the terminal area are the most stressing in terms of accuracy. Alternative 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 pseudoranging 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. Air and ground architectures supporting these modes are discussed and a preliminary ranging accuracy budget is developed. An example ground station (GS) network laydown for Munich Airport is used to assess the coverage difference between PR and TWTR; it is found that TWTR/Hybrid provide roughly a factor of two improvement in horizontal dilution of precision (HDOP). This result is combined with the ranging error budget to determine the required GS network synchronization to support pseudoranging. The preliminary analysis suggests that roughly 30 ns RMS synchronization will be required. While this is attainable with GNSS, high accuracy ground synchronization would be needed to support operations in long-term GNSS outages. For this and other factors, it is recommended that a TWTR-based LDACS APNT architecture be considered by industry standards groups.