up the transponder and initiates tracking as soon as it detects  the assigned code. For this reason, the transponder should  remain on standby until takeoff clearance has been received. The aircraft appears on the controller’s radar display as a  target with an associated data block that moves as the aircraft  moves through the airspace. The data block includes aircraft  identification, aircraft type, altitude, and airspeed. A TRACON controller uses Airport Surveillance Radar  (ASR) to detect primary targets and Automated Radar  Terminal Systems (ARTS) to receive transponder signals;  the two are combined on the controller’s scope.  [Figure 2-9] At facilities with ASR-3 equipment, radar returns from  precipitation are not displayed as varying levels of intensity,  and controllers must rely on pilot reports and experience  to provide weather avoidance information. With ASR-9  equipment, the controller can select up to six levels of  intensity. Light precipitation does not require avoidance  tactics but precipitation levels of moderate, heavy, or  extreme should cause pilots to plan accordingly. Along  with precipitation, the pilot must additionally consider the  temperature, which if between –20° and +5 °C causes icing  even during light precipitation. The returns from higher levels  of intensity may obscure aircraft data blocks, and controllers  may select the higher levels only on pilot request. When  uncertainty exists about the weather ahead, ask the controller  if the facility can display intensity levels—pilots of small  aircraft should avoid intensity levels 3 or higher. At many locations, instrument flights can be conducted  entirely in terminal airspace. These tower en route control  (TEC) routes are generally for aircraft operating below  10,000 feet, and they can be found in the A/FD. Pilots desiring  to use TEC should include that designation in the remarks  section of the flight plan. Pilots are not limited to the major airports at the city pairs  listed in the A/FD. For example, a tower en route flight from  an airport in New York (NYC) airspace could terminate  at any airport within approximately 30 miles of Bradley  International (BDL) airspace, such as Hartford (HFD).  [Figure 2-10] A valuable service provided by the automated radar  equipment at terminal radar facilities is the Minimum Safe  Altitude Warnings (MSAW). This equipment predicts an  aircraft’s position in 2 minutes based on present path of  flight—the controller issues a safety alert if the projected  path encounters terrain or an obstruction. An unusually  rapid descent rate on a nonprecision approach can trigger  such an alert. ARTCC facilities are responsible for maintaining separation  between IFR flights in the en route structure. Center radars  (Air Route Surveillance Radar (ARSR)) acquire and track  transponder returns using the same basic technology as  terminal radars.  [Figure 2-11] Earlier Center radars display weather as an area of slashes  (light precipitation) and Hs (moderate rainfall), as illustrated  in  Figure 2-12 . Because the controller cannot detect higher  levels of precipitation, pilots should be wary of areas  showing moderate rainfall. Newer radar displays show  weather as three levels of blue. Controllers can select the  level of weather to be displayed. Weather displays of higher  levels of intensity can make it difficult for controllers to  see aircraft data blocks, so pilots should not expect ATC  to keep weather displayed continuously. Center airspace is divided into sectors in the same manner  as terminal airspace; additionally, most Center airspace is  divided by altitudes into high and low sectors. Each sector  has a dedicated team of controllers and a selection of radio  frequencies because each Center has a network of remote  transmitter/receiver sites. All Center frequencies can be found  in the back of the A/FD in the format shown in  Figure 2-13; they are also found on en route charts. Each ARTCC’s area of responsibility covers several states;  when flying from the vicinity of one remote communication  site toward another, expect to hear the same controller on  different frequencies. The majority of airports with instrument approaches do not  lie within terminal radar airspace and, when operating to or  from these airports, pilots communicate directly with the  Center controller. Departing from a tower-controlled airport,  the tower controller provides instructions for contacting the  appropriate Center controller. When departing an airport  without an operating control tower, the clearance includes  instructions such as “Upon entering controlled airspace,  contact Houston Center on 126.5.” Pilots are responsible  for terrain clearance until reaching the controller’s MVA.  Simply hearing “Radar contact” does not relieve a pilot of  this responsibility. If obstacles in the departure path require a steeper- than-standard climb gradient (200 feet per nautical mile  (FPNM)), then the controller advises the pilot. However,  it is the pilot’s responsibility to check the departure airport  listing in the A/FD to determine if there are trees or wires  in the departure path. When in doubt, ask the controller for  the required climb gradient. Air Route Traffic Control Center (ARTCC) Tower En Route Control (TEC) Center Approach/Departure Control 2-7