Surveillance sources, correlation and meaningful limitations
A controller display combines observations, processing and labels. Begin by asking where each displayed quantity originates and what must remain serviceable for it to be available. Position, identity, pressure altitude and a predicted trajectory do not necessarily come from one independent source. A clear label on a screen is not proof that every component of the underlying picture is valid.
Different surveillance mechanisms
Primary radar detects reflected energy and therefore does not require the same cooperative transponder response as secondary surveillance radar. A primary return does not itself contain an aircraft's coded identity. Secondary surveillance uses aircraft replies, with modes carrying different information. An aircraft that does not reply cannot be assumed absent simply because it is missing from a secondary-only picture.
ADS-B involves broadcasts of information derived from onboard systems rather than the same interrogation-and-reply process as classical SSR. Multilateration determines position using timing differences at suitable receiving stations; it is not simply reading a broadcast latitude and longitude. Each technique has dependencies and limitations. Two symbols using the same navigation input do not provide wholly independent confirmation of that input.
Source, reference and quality
Mode C pressure altitude and geometric height are different quantities. A label's altitude reference must be understood before comparing it with an obstacle, a pilot report or another sensor. Likewise, radar range, DME slant range and horizontal chart distance are different measurements. Directly above a station, nonzero vertical separation can still produce a nonzero slant distance.
Coverage depends on geometry, terrain, equipment, interference and the approved service, not merely a theoretical horizon. A line-of-sight calculation can illustrate a relationship without proving that a real aircraft is identified or that a specified surveillance minimum is authorized. The chosen system must provide the integrity and performance needed for the particular use.
Alerts and continued monitoring
An alert supports attention; it does not replace the controller's understanding of what is happening. A missing alert does not establish that no conflict exists, and a predicted alert must be interpreted with the actual track, altitude data, settings and available information. Changes in sensor serviceability can require a different operating method and coordination under the unit's procedures.
Study which observations are independent, which can be stale and which depend on cooperative aircraft equipment. Explain the mechanism before choosing a response. Actual identification methods, tolerances, fallback procedures and operating minima are established through current applicable rules and unit instructions. This chapter supplies neither an invented universal numeric tolerance nor permission to continue a degraded service without those checks.