1.1 In recent years, the civil aviation community has recognized the safety and efficiency potential of broadcast data link services. One application in particular—known as Automatic Dependent Surveillance Broadcast (ADS-B)—is central to the community’s future vision of the “free flight”concept. ADS-B is the periodic broadcast of aircraft position, velocity and intent information for use by ground systems or proximate aircraft.
1.2 ADS-B will enable numerous new procedures (air-air, air-ground and on the airport surface) that hold the potential to significantly increase safety and efficiency of the FAA’s National Airspace System (NAS). Other related broadcast services such as uplink of traffic information from existing surveillance radars (Traffic Information System Broadcast, TIS-B) and uplink of ground-based weather radar imagery (Flight Information Service Broadcast, FIS-B) will increase pilot “situational awareness” thus also contributing to safety and efficiency improvements. There is currently activity in the RTCA to progress the definition and implementation of these various broadcast services. Significant progress has been made in defining ADS-B system level requirements. These are documented in the RTCA DO-242 MASPS.
2.1.1 The UAT was conceived in 1994 as a data link system designed specifically for support of multiple broadcast services including ADS-B while maintaining a simple, robust and high capacity architecture. Key in achieving this was freeing the system of constraints imposed by legacy systems—for example, the small payload available in a Mode S extended squitter, or the narrow 25 kHz channel spacing of the VHF band.
2.1.2 Initial UAT prototypes were developed and test flown as part of an independent research and development (IR&D) initiative at the MITRE Corporation’s Center for Advanced Aviation System Development. Subsequently the UAT was included in the FAA’s Safe Flight 21 (SF-21) program and Cargo Airline Association’s link evaluation efforts with a number of industry-produced units approved for installation on Cargo and other FAA-sponsored test aircraft. UAT is one of the three data link candidates being evaluated in the FAA/Eurocontrol Technical Link Assessment Team effort.
2.1.3 More recently, UAT has been selected by the FAA’s Alaska Region as part of their Capstone Program. Capstone is a safety improvement initiative, which initially will include the installation (at FAA expense) of production UAT radios and display systems in approximately 140 air taxi aircraft and establishment of UAT ground stations at approximately 12 sites. A key operational objective of Capstone is to use UAT/ADS-B in an area of Western Alaska--with little or no radar coverage--to provide “radar-like” ATC services. Currently about 60 aircraft installations have been completed and the end-end ADS-B system including the Anchorage Center automation system is being validated. The target date for initial ATC “radar-like” services is 1 Jan 2001. Upon successful completion of this initial phase of the program, plans are to develop infrastructure statewide in Alaska.
2.1.4 In response to significant vendor interest, the Program Management Committee of RTCA—on 13 September—approved the creation of a working group tasked with development of MOPS for UAT.
UAT Technical Overview
2.2.1 The UAT implementation for the FAA’s Safe Flight 21 and Capstone programs is mature and fully scaleable to a future high-density environment. The UAT design used for these programs forms a firm basis for the planned RTCA MOPS activity. The paragraphs below give an overview of the major design parameters and a rationale for their selection.
2.2.2 Channel Structure: UAT operates on a single common channel in all airspace. This allows for full seamless air-air ADS-B connectivity without the complication of multiple receivers or channel tuning procedures. In order to assure adequate update rates and surveillance capacity for high-density airspace, the channel bit rate is 1 Mbps.
2.2.3 Frequency Band: The band 960-1215 MHz was chosen for UAT due to its 1 MHz channelization and its designation as an Aeronautical Radio Navigation Service (ARNS ) band.
2.2.4 Waveform: Modulation is binary Continuous Phase Frequency Shift Keying (CPFSK), a form of Frequency Modulation (FM). The modulation was chosen to optimize power efficiency and performance in a self-interference environment.
2.2.5 Media Access: The UAT uses a one second frame period for media access. About 20% of the frame period is devoted to use by ground station transmitters through 32 defined time slots. The remainder of the frame time is devoted to use by aircraft for their ADS-B transmissions. Aircraft transmissions are randomized to prevent any two aircraft from repeatedly choosing the same transmission time. Each aircraft always makes exactly one transmission per second. Figure 1 shows the media access plan. This allows a single airborne receiver to support full air-air ADS-B and uplink broadcast products—without any tuning procedures. Media access is never dependent on sensing of the channel environment; a minimal installation could be a transmit-only implementation
Figure 1: UAT Media Access 2.2.6 Message Structure: ADS-B messages contain either 128 or 256 bits of “payload”—the portion conveying ADS-B information. This allows every transmitted message to include a full-uncompressed State Vector. The State Vector is the information related to position and velocity—the most dynamic of the ADS-B information. This allows every transmitted ADS-B message to “stand-alone” in that every received message can be used directly to update the traffic display without the need for tracking, message reassembly or ambiguity resolution of the compressed latitude and longitude as is required in other systems. Furthermore, each message includes adequate error checking power that the applications are not required to perform any integrity checks.
2.2.7 Message Time-of-Receipt: An important feature of the UAT is its ability to accurately measure message time-of-receipt (MTOR). The MTOR feature allows for validation of ADS-B messages by comparing the one-way propagation time to the range derived from the position information included in the ADS-B message. This allows the future ADS-B surveillance system to maintain an added level of integrity. Additionally, aircraft in view of 3 or more ground stations (with good geometry) can make a position determination using the MTOR feature and information in the Ground message header. This could allow for a future integrated satellite and ground-based navigator in the aircraft that provides a degree of diversity from GPS for dependent surveillance purposes.
3.1 The UAT has been designed from a “clean slate” specifically for ADS-B and other related broadcast data link applications. Basic to the design philosophy is simplicity, robustness and high capacity of the data link. The wideband channel and high reporting rate design support all RTCA DO-242 MASPS requirements in the highest density airspace while maintaining a consistent operation in all airspace. The simplicity of media access and channel structure minimizes any possible failure modes. Additionally, the partitioned bandwidth dedicated to uplink products (FIS-B) offers an important incentive to early equipage with ADS-B while sharing the same common receiver. Finally, the wide bandwidth available and the fact that this system should be limited primarily by self-interference makes it a very high capacity system.