GEO-REFERENCING RADAR PLOT DATA FOR THE TRAFFIC INFORMATION SERVICE BROADCAST Jeffrey D. Giovino, The MITRE Corporation’s Center for Advanced Aviation Systems Development (CAASD), McLean, Virginia Abstract Background The Federal Aviation Administration (FAA) has gained some significant early operational ADS-B and TIS-B experience with the Traffic Information Service The FAA is enhancing the legacy radar Broadcast (TIS-B), both on the East Coast of the surveillance system with ADS-B. ADS-B provides United States, and in the Anchorage, Alaska area. Global Positioning System (GPS) positions of TIS-B, for the first time, puts geo-referenced radar aircraft for separation by controllers. It also data directly in the cockpit where, for a number of provides the unique capability of displaying reasons, the customer can be more sensitive to surveillance data received air-to-air on a cockpit inaccuracy and other anomalies than the traditional display for use by the pilot. During the initial period user (i.e., the air traffic controller). of ADS-B equipage, the FAA plans to supplement The Broadcast of Automatic Dependent this cockpit display capability with TIS-B. TIS-B is Surveillance (ADS-B) is the air-to-air transmission the ground-to-air uplink of primarily radar derived of aircraft position and velocity information. In surveillance data. Supplementing ADS-B with TIS- order for ground surveillance radar data and ADS-B B will allow the ADS-B equipped pilot to see the data to integrate seamlessly in the cockpit, a “truly remaining aircraft that have not yet equipped with geo-referenced” radar alignment technique is ADS-B. In short, the cockpit display will depict needed for radar to ADS-B correlation. Successful both ADS-B air-to-air and TIS-B (derived from synergy between ADS-B and radar requires accrued RADAR) from the ground uplink. Other sources of radar registration. Incorrect geo-referencing can TIS-B exist, such as the rebroadcast of ADS-B and result in a number of anomalies that can be difficult multi-lateration. These non-radar sources of TIS-B for both designers and pilots to deal with are beyond the scope of this paper. appropriately. This paper describes the process of geo- Need for Geo-Referencing Radar Data referencing radar data, describes some real world Radar measurements consist of range and limitations of the radar sensors, the anomalies that azimuth (ρ, θ ) plots. Range, or rho, is the distance can be encountered and their cause, techniques for from the radar antenna which is derived from mitigating these anomalies, and finally, this paper measuring the time between the transmitted and discusses one of the radar alignment techniques received radio frequency (RF) pulses. Azimuth, or used for the system providing TIS-B in Anchorage theta, generally, is the direction the antenna sail is for the FAA’s Capstone program. facing. Pressure altitude may also be available in This paper will attempt to bridge the gap the Mode C reply to a radar interrogation. Rho, between the ground systems and airborne systems. theta, pressure altitude (ρ, θ, h ) coordinates, Given the insight into the processing performed on relative to a radar, are very different from the ADS- the ground, the avionics developers may adapt or B GPS coordinates depicted in latitude, longitude, create new ways to overcome these issues. and both geometric and pressure altitudes. Simple geometry is used to convert from the radar centric polar coordinate system to the WGS- 84 coordinate system used by ADS-B. However, many subtleties exist in both physical and temporal dimensions. These subtleties are overcome through With the advent of ADS-B, two fundamental massaging the conversion algorithm with learned changes will take place: values in order to get the radar plot data as close to truth as possible. These variables are defined in the 1) The plot data will now needs to be truly “radar alignment technique” section of this paper. geo-referenced for the first time, matching the capability inherent in ADS-B as defined by WGS- Radar Alignment is the process of obtaining 84. the radar specific geographic attributes used to geo- reference the radar plot data. From an avionics and 2) Discontinuities at ARTCC boundaries pilots perspective, radar alignment is the process cannot be tolerated. All surveillance radars must used to determine the parameters necessary for be truly geo-referenced across ARTCC bounds. converting radar plot data to the same coordinates as used by ADS-B. From a ground infrastructure and Air Traffic Control (ATC) perspective, radar Limitations and Anomalies alignment is what enables adjacent radar plots to be TIS-B is used primarily for an aid to visual correlated as well as correlating the plot data with acquisition. TIS-B will give a pilot access to a ADS-B. This correlation process is what allows an wealth of powerful information. This means that ATC tracker to maintain a single track from pilots will want to use this information as part of multiple surveillance sources, such as, adjacent their decision making processes, even though, they overlapping surveillance radars and ADS-B sensors make these decisions without TIS-B today. This can be beneficial during instances, such as maintaining Radar Processing for ATC Use traffic awareness in an airport traffic pattern. It can also be a detriment. If this advisory information was Historically, each Air Route Traffic Control misleading, it might lead a pilot to deviate course Center (ARTCC) receives radar plots from the inappropriately to avoid traffic that is not really available radars in their respective air space. This there. ATC controllers are trained to identify such data is then converted to the local system plane for anomalies when they occur. They also have the tracking, flight plan association, and controller benefit of large scale displays. For the pilot display. With this technique, the radar alignment however, this is new territory. Both avionics parameters would only be adjusted enough for designers and pilots may encounter several smooth transition between the adjacent radars that anomalies, including: provide coverage for one ARTCC. The radars are aligned to each other and not to truth. The ATC display context is a little more forgiving than what Shadows is needed for TIS-B on a cockpit display. A pilot A “shadow” is the term used when a TIS-B using the cockpit display to visually acquire report is up-linked representing a target that has crossing traffic 1 mile ahead using a 5 mile range already transmitted its ADS-B position. The scale will generally be more sensitive to latency and avionics display depicts two targets flying in close positional errors than an air traffic controller proximity. The aircraft generating the ADS-B separating traffic at 3 miles using a 30 mile (or position receives its own TIS-B report which might larger) range scale. Furthermore, due to the be interpreted as an immediate danger. coarseness of the surveillance radar plot resolution, The TIS-B service will not contain messages the alignment process does not need to be that represent a target that has already transmitted completely thorough. It only needs to be good its ADS-B position. If implemented perfectly, this enough to fall within the radar error in will eliminate the existence of shadows. It will also measurement. This is completely acceptable, since ease the transition from primarily radar surveillance all of the data is still accurate relative to adjacent to ADS-B surveillance. As aircraft equipage of radar plots. If one is off, relative to the ATC ADS-B reaches completion, there is significantly automation system plane, they all are off by the less need for the TIS-B service. Therefore as time same amount and there is no separation concern. progresses, the load on the data-link channel will smoothly migrate from a majority of TIS-B messages to a majority of ADS-B messages. This perform noise filtering and position estimation on transition will keep the data-link channel utilization raw data. For trackers that do not have access to consistent in time. Doppler velocity, such as ATC trackers, this filtering process can not keep up with aircraft accelerations. Even a simple turn is acceleration in Pop ups one dimension. Current ATC Tracker filters treat A “pop up” occurs when a non ADS-B target acceleration as noise. This anomaly manifests itself climbs into coverage of a ground RADAR. The as a lag in the velocity vector and the reported track ground tracker will initiate its first track and start position is in error tangential to the turn. The figure transmitting. To the avionics this target will appear below shows an exaggeration of this condition. out of the blue. It is possible for this target to annoyingly appear and disappear while on the fringes of radar coverage RUNWAY Non-Transponder Equipped Aircraft Actual aircraft flight path At least in the near-term, TIS-B service will TIS-B report position and vector not include radar returns without mode-C (altitude) or radar returns only from primary, skin paint, radar. This means the pilot may find themselves near another aircraft that does not appear on the Figure 1. Tracker positional lag in a turn avionics display. In extreme cases this anomaly can result in If primary only, or non Mode C, returns are substantial position errors that could be seen by a included in the TIS-B service, the accuracy of the pilot. The example above is a tight turn in the position will be severely reduced. This means that, pattern on approach. The avionics display will show at close range, the target might be incorrectly the aircraft arcing well beyond the approach depicted on the display.