A Testbed for Studying Automatic Dependent Surveillance Broadcast (ADS-B) Based Range and Positioning Performance to Support Alternative Position Navigation and Timing (APNT) (2013)

The Federal Aviation Administration (FAA) Alternative Position Navigation and Timing (APNT) effort seeks to develop navigation service to continue efficient, high-density operations even if Global Navigation Satellite System (GNSS) service is degraded. One technology being examined to support APNT is Automatic Dependent Surveillance Broadcast (ADS-B). ADS-B is being currently being implemented to provide improved surveillance capabilities. An ADS-B equipped aircraft broadcasts its precise positions derived from GNSS to provide high accuracy surveillance for air traffic control and traffic awareness to nearby aircraft. Additionally, ADS-B ground stations, sometimes called ground base transceivers (GBT), re-transmit ADS-B information as well as provide traffic and weather information (TIS-B and FIS-B, respectively). This work investigates the performance of ADS-B signals to provide APNT capabilities.
 
ADS-B and APNT can complement each other. Currently ADS-B relies on GNSS for its position reports. APNT can provide a high accuracy navigation source that is robust to GNSS degradation for ADS-B. This improves ADS-B availability and robustness. The ADS-B system offers multiple possible means to provide an APNT capability. One means is to use ADS-B and its related signals to provide pseudo ranging. In the United States, the ADS-B transmission uses either 1090 MHz Mode S extended squitter (Mode S ES) or 978 MHz Universal Access Transceiver (UAT). UAT has pseudo ranging features built into its transmissions from an ADS-B ground station. Another possibility is to use the ground based multilateration (MLAT) that is being developed and implemented as part of ADS-B to provide verification of navigation data. MLAT derived position, when sent back to the aircraft, may serve as a source of navigation information. Hence, two aspects of the work looks at the ranging capabilities of the ADS-B signals and the positioning performance with ADS-B based MLAT.
 
The first step is studying the performance ADS-B signals when used for ranging. ADS-B transmissions were collected and analyzed to study their nominal range precision. Signals from multiple ADS-B ground stations throughout the country were collected and assessed. A concern with using ADS-B signals for ranging is multipath as the transmission will either be transmitted from or received by the ground. An analysis of the multipath performance of the ADS-B signals is performed and presented.
 
The paper then discusses the multilateration testbed being developed using relatively low cost commercial off the shelf components. Both the hardware and algorithm processing are described. A data collection system is built based on the universal software receiver peripheral (USRP) to collect raw ADS-B transmissions on the ground. As MLAT requires multiple geographically separated but synchronized measurements of the same signal, a means of synchronizing the data from each USRP is developed. The method developed synchronizes of each measured ADS-B signal sample to a GPS sample for each data collection unit. This allows for synchronization of the samples between geographically separated data collection units and allows for accurate calculation of position. Another factor that needs to be accounted for is altitude. Analysis will show the effect of altitude error on position error.
 
This presentation is valuable for showing the performance of ADS-B signals for ranging and discussing what is needed to allow ADS-B signals to provide pseudo ranging. It also provides details on how to build a simple low cost MLAT testbed which is important for researchers evaluating and developing its capabilities as current MLAT installations use proprietary systems and processing.