Dependent surveillance is aircraft-supplied data — position, altitude, and identity — shared through 1090 MHz Mode S extended squitter, 978 MHz UAT, and interrogated Mode S replies, with ground services rebroadcasting coverage to fill gaps. Master each link in that chain and practice tracing signals end to end rather than memorizing isolated definitions.
What 'Dependent' Means and Why ADS-B Is Not Radar
Dependent surveillance relies on the aircraft to determine and transmit its own position, altitude, and identity, usually derived from GNSS. Independent surveillance, such as primary radar, detects reflected energy without any cooperation from equipment on the aircraft.
In a dependent system, the quality of the output is inherited from the aircraft's own sensors. If the position source degrades, the broadcast degrades with it, because the receiving network simply forwards what the aircraft reports. Primary radar works the opposite way: the ground measures range and bearing from reflected energy, so a target with no working equipment can still be painted, but with no altitude or identity attached. Keeping those two error models separate — sensor-originated versus measurement-originated — makes the rest of the syllabus easier to place.
A second distinction is cooperative versus non-cooperative. A transponder-equipped aircraft is cooperative: it can answer interrogations or broadcast its state. ADS-B is both cooperative and autonomous, transmitting without being asked; secondary radar and TCAS are cooperative but reactive, transmitting only in reply to an interrogation. Build a two-axis mental model — dependent versus independent on one axis, cooperative versus non-cooperative on the other — and place every system in the topic list into one quadrant before moving on. That single exercise clarifies labels that otherwise blur together.
1090ES vs UAT: Which ADS-B Datalink the Scenario Assumes
ADS-B Out can be transmitted as 1090 MHz Mode S extended squitter or, where rules permit, on a 978 MHz universal access transceiver. The datalink choice changes the hardware, the message formats, and which services the aircraft can receive.
1090ES appends extended squitter messages to the aircraft's Mode S transponder, so identity, position, and velocity ride the same equipment that ATC transponders already use. UAT is a separate 978 MHz link with greater uplink capacity, which is why flight-information services are associated with it in the United States service architecture. The two are not interchangeable: message formats differ, and acceptance for particular airspace or services depends on the rules of the operating region. A scenario that names the datalink is therefore telling you which equipment set and capabilities to reason about.
Worked scenario: a shop completes an ADS-B Out installation on a light aircraft, and a ground check shows no position in the network's track picture. The tempting move is to condemn the transmitter. The better decision is to first verify that the fitted datalink actually meets the requirement for the aircraft's intended airspace, then confirm the position source is supplying valid data. A correctly functioning transmitter cannot broadcast a position it never received, and airspace acceptance often hinges on which link is installed, so this order protects both the diagnosis and compliance.
Mode S Enhanced Surveillance: From Interrogated Replies to Broadcasts
Classic Mode A/C transponders reply only when interrogated, and Mode S adds selective addressing with a unique 24-bit code. Enhanced surveillance extends Mode S with spontaneous broadcasts of position, velocity, and identity — the basis of 1090ES ADS-B.
Understanding ES requires knowing what it extends. Mode A/C replies carry identity or altitude only, and multiple replies to one interrogation can overlap and garble, a known limitation of older secondary surveillance. Mode S assigns each aircraft a unique address, letting a ground station or a TCAS unit interrogate one specific aircraft and exchange richer data. Extended squitter then uses that datalink to append longer messages that are broadcast without prompting. Every DST topic touching 1090 MHz inherits from this lineage, so learn the lineage once and reuse it everywhere.
Worked scenario: a ground network displays an aircraft's callsign but no position, and a technician replaces the transponder with no effect. The better decision is to trace forward from the GNSS position source and check whether valid position data, together with its quality indication, actually reaches the transponder. ES position messages originate in the aircraft's navigation equipment, not in the transponder itself, so an invalid position source can produce identity-only broadcasts. The fault-isolation order — position source, interconnect, then transponder — reflects how the system genuinely passes data.
TCAS and ACAS: Independent of Ground, Dependent on Target Transponders
TCAS/ACAS operates without ground infrastructure: it interrogates nearby transponders at 1030 MHz and computes traffic and resolution advisories from the replies. That makes it independent of surveillance networks while remaining dependent on each target's transponder performance.
The dependence is easy to miss because the system sits entirely in the cockpit. A Mode A target yields bearing and range but no altitude, so it cannot support altitude-based advisory logic; a Mode C or Mode S target supplies the pressure altitude that the collision-avoidance logic needs. Mode S equipage additionally allows coordination between two TCAS-equipped aircraft so their advisories do not conflict. When evaluating any TCAS scenario, inventory what each target's transponder can actually report before judging the behavior of the TCAS unit itself.
Worked scenario: a pilot reports an intruder displayed without altitude, and maintenance suspects the TCAS computer. The better decision is to read the report closely — a target lacking altitude indicates something about the other aircraft's transponder reply, because range and bearing were evidently obtained successfully. Display behavior is a direct readout of what the target transmitted. Recognizing this prevents removal of serviceable cockpit equipment and explains why identical TCAS units can behave differently against different targets on the same flight.
TIS-B and FIS-B: Matching Each Ground Uplink Service to Its Link
TIS-B rebroadcasts ground-surveillance-derived traffic, including targets without ADS-B, to suitably equipped aircraft; FIS-B delivers flight information such as weather. The services ride specific uplinks — a relationship worth memorizing rather than deriving.
TIS-B exists because not every tracked aircraft broadcasts ADS-B: a ground network can fuse radar and ADS-B tracks and uplink a merged traffic picture, which fills gaps at the edge of air-to-air ADS-B reception. FIS-B is an information service, not a surveillance service, and in the United States service architecture it is carried on the uplink capacity that UAT provides. Both services exist only within ground-station coverage, so their absence over remote terrain is a coverage fact, not an aircraft fault.
Apply this when interpreting a cockpit traffic display. A target that appears and disappears as the aircraft crosses coverage boundaries is consistent with a rebroadcast service, while a target present continuously at the same range is consistent with direct air-to-air ADS-B. In practice a display blends several sources, which is exactly why strong practice scenarios force you to attribute observed behavior to one specific link. Labeling each displayed target with its probable source is a reliable rehearsal technique for this skill.
Worked scenario: during a test flight, a technician observes a traffic target vanish while the aircraft crosses a ridge and then reappear minutes later. The tempting conclusion is an intermittent aircraft antenna fault. The better decision is to check the ground-station coverage picture first: a target that tracks coverage boundaries points to a rebroadcast service rather than to intermittent air-to-air reception, and confirming the source before touching hardware saves a needless inspection.
| System | Link or band | Depends on | Ground station needed | Primary role |
|---|---|---|---|---|
| ADS-B Out (1090ES) | 1090 MHz Mode S extended squitter | Aircraft GNSS position source and Mode S transponder | No for broadcast; network for ATC use | Aircraft self-reports position, velocity, identity |
| ADS-B Out (UAT) | 978 MHz UAT | Aircraft GNSS position source and UAT transmitter | No for broadcast; network for ATC use | Same data on the alternate datalink where permitted |
| Mode S (basic and ES) | 1030 MHz interrogations; 1090 MHz replies and squitter | Unique 24-bit address; ES adds GNSS-derived state | Interrogator or receiving ground station | Selective addressing, datalink, enhanced surveillance |
| TCAS/ACAS | 1030/1090 MHz | Each target aircraft's Mode C or Mode S replies | No | Cockpit traffic and resolution advisories |
| TIS-B | Uplinks on 1090ES and UAT (architecture varies) | Ground surveillance network and station coverage | Yes | Rebroadcasts tracked traffic, including non-ADS-B targets |
| FIS-B | 978 MHz UAT uplink (in the U.S. architecture) | Ground station coverage | Yes | Flight information such as weather, not surveillance |
Maintenance Practices: Testing the Chain, Not Just the Box
DST maintenance verifies an end-to-end chain — position source, interconnects, transponder or UAT, antennas, and displayed output — against approved maintenance data, using appropriate test equipment rather than conclusions drawn from a unit that simply powers up.
A transmitter that passes a bench check can still fail a system check: a corroded antenna connector, an encoder supplying incorrect altitude data, or a position source with degraded accuracy will each produce symptoms that no bench test of the unit alone reproduces. Sound practice isolates the chain in data order, from the sensor that originates the information to the point where it is transmitted or displayed, and documents each verification. The approved maintenance data for the specific installation governs methods, tolerances, and return-to-service steps.
Regulatory context matters but varies by region and equipment class, so treat the applicable rules as part of the topic to study rather than assumptions to import from elsewhere. Confirm broadcast content, not merely broadcast presence: correct identity, correct altitude encoding, and position consistent with the aircraft's actual location are separate checks that catch separate faults. Keep records of transmissions observed during testing, because intermittent faults are diagnosed from patterns across repeated observations rather than from a single passing run.
A Four-Week DST Sequence with a Scoring Rubric and Readiness Checks
Study DST as one signal chain over four weeks: fundamentals and datalinks, then Mode S and ES, then TCAS with ground uplink services, then maintenance and review. Score yourself weekly with the rubric below as a learning milestone, not a pass prediction.
Weeks one and two: build the dependent-versus-independent and cooperative-versus-non-cooperative model, the 1090ES/UAT comparison, and the Mode S-to-ES lineage, ending each week by tracing one full signal path aloud. Week three: add TCAS dependence on target transponders and the TIS-B/FIS-B service split, again tracing paths end to end. Week four: work maintenance fault-isolation scenarios and re-trace every path from memory. Administrative details such as scheduling and credential requirements belong to NCATT itself, so a short note: check ncatt.org for anything procedural rather than attempting to study it here.
The core exercise is signal-path tracing. On paper, draw a 1090ES transmission from the GNSS position source through the transponder to a ground station, then draw a UAT path including one uplink service, annotating what each element contributes and what fails if it is removed. Expected observations: you can name the originating sensor for every data item, identify one failure consequence per block, and mark where ground infrastructure is or is not required. Repeat until you produce both paths from memory in under ten minutes.
- Score each item 0 (cannot recall) to 3 (can explain and apply): place ADS-B, Mode S, TCAS, TIS-B, and primary radar in the correct dependent/independent and cooperative/non-cooperative quadrants.
- Trace the 1090ES path and the UAT path, including one uplink service, with no gaps in data origin.
- State which cockpit behaviors point to a target transponder problem rather than a TCAS fault.
- Order a fault isolation for 'position not reported' from the position source forward, with a reason recorded for each step.
- A consistent 3 on every item after week four, plus two clean blind tracings, is a reasonable readiness check for this material.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.