Figure 11-137.  A cockpit display of ADS-B generated targets (left) and an ADS-B airborne receiver with antenna (right). surface features and bounces back toward the aircraft where  a second antenna receives the return signal. The transceiver  processes the signal by measuring the elapsed time the signal  traveled and the frequency modulation that occurred. The  display indicates height above the terrain also known as  above ground level (AGL).  [Figure 11-140] ADS-B IN offers features not available in TCAS. Equipped  aircraft are able to receive abundant data to enhance  situational awareness. Traffic information services-broadcast  (TIS-B) supply traffic information from non-ADS-B aircraft  and ADS-B aircraft on a different frequency. Ground radar  monitoring of surface targets, and any traffic data in the  linked network of ground stations is sent via ADS-B IN  to the flight deck. This provides a more complete picture  than air-to-air only collision avoidance. Flight information  services-broadcast (FIS-B) are also received by ADS-B  IN. Weather text and graphics, ATIS information, and  NOTAMS are able to be received in aircraft that have 987  UAT capability.  [Figure 11-138] A radar altimeter is more accurate and responsive than an air  pressure altimeter for AGL information at low altitudes. The  transceiver is usually located remotely from the indicator.  Multifunctional and glass cockpit displays typically integrate  decision height awareness from the radar altimeter as a digital  number displayed on the screen with a bug, light, or color  change used to indicate when that altitude is reached. Large  aircraft may incorporate radio altimeter information into a  ground proximity warning system (GPWS) which aurally  alerts the crew of potentially dangerous proximity to the  terrain below the aircraft. A decision height window (DH)  displays the radar altitude on the EADI in  Figure 11-141 .  ADS-B test units are available for trained maintenance  personnel to verify proper operation of ADS-B equipment.  This is critical since close tolerance of air traffic separation  depends on accurate data from each aircraft and throughout  all components of the ADS-B system.  [Figure 11-139] There are three common types of weather aids used in an  aircraft flight deck that are often referred to as weather radar:  A radio altimeter, or radar altimeter, is used to measure the  distance from the aircraft to the terrain directly beneath it. It  is used primarily during instrument approach and low level  or night flight below 2500 feet. The radio altimeter supplies  the primary altitude information for landing decision height.  It incorporates an adjustable altitude bug that creates a  visual or aural warning to the pilot when the aircraft reaches  that altitude. Typically, the pilot will abort a landing if the  decision height is reached and the runway is not visible. On-board weather radar systems can be found in aircraft  of all sizes. They function similar to ATC primary radar  except the radio waves bounce off of precipitation instead  of aircraft. Dense precipitation creates a stronger return than  light precipitation. The on-board weather radar receiver  Using a transceiver and a directional antenna, a radio  altimeter broadcasts a carrier wave at 4.3 GHz from the  aircraft directly toward the ground. The wave is frequency  modulated at 50 MHz and travels at a known speed. It strikes  11-62 3.  Satellite or other source weather radar information that  is uploaded to the aircraft from an outside source. 2.  Lightning detectors; and 1.  Actual on-board radar for detecting and displaying  weather activity; Weather Radar Radio Altimeter