ADS-B Explained: How It Works, the Performance Rules, ADS-B In Weather and Traffic, and NextGen
- Nathan Hodell
- Dec 17, 2025
- 13 min read
Updated: Aug 26
ADS-B has become the cornerstone of modern air traffic surveillance — mandated by the FAA as part of the NextGen system, it tracks aircraft more accurately than radar ever could, feeds traffic and weather directly to the cockpit, and reshaped how the airspace is monitored. Most pilots know they need "ADS-B Out" in certain airspace, but fewer understand how the technology actually works, why it's a leap beyond radar, what performance standards make equipment compliant (or not), and how much genuinely useful traffic and weather ADS-B In delivers. Understanding ADS-B as a system, not just a mandate, makes you a more informed and capable pilot.
This post covers ADS-B in practical depth: what the name actually means and how the technology works, why it's better than radar, the airspace where ADS-B Out is required, the performance requirements that determine compliance, the 1090ES versus UAT choice, ADS-B In traffic and weather in detail, anonymous mode and privacy, and how to verify your installation is compliant.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What the Name Actually Means
ADS-B stands for Automatic Dependent Surveillance–Broadcast, and unpacking each word explains how it works.
Automatic:
It transmits continuously and automatically
No pilot action or ATC interrogation required
Once installed and on, it just broadcasts
Unlike a transponder (which replies to interrogation), ADS-B broadcasts on its own
Dependent:
It DEPENDS on the aircraft's own navigation system (GPS) for position
The aircraft determines its own position and broadcasts it
"Dependent" because it relies on the onboard GPS (not ground-based measurement)
This is a key distinction from radar
Surveillance:
It's a surveillance technology (tracking aircraft)
Provides position information for ATC and other aircraft
The function is knowing where aircraft are
The purpose of the system
Broadcast:
It BROADCASTS the information (one-to-many)
Any receiver in range (ATC ground stations, other aircraft) can receive it
Not a directed transmission — a broadcast
Enables both ATC surveillance and aircraft-to-aircraft awareness
Putting it together:
Automatic: transmits on its own
Dependent: uses onboard GPS for position
Surveillance: for tracking aircraft
Broadcast: to anyone in range
That's how ADS-B works in four words
How ADS-B Works
The actual mechanism, which clarifies why it's different from radar.
The process:
The aircraft's GPS receiver determines its precise position
The ADS-B Out equipment packages this with altitude, velocity, and identification
It broadcasts this data (on 1090 MHz or 978 MHz)
Ground stations receive it and relay it to ATC
Other ADS-B In-equipped aircraft can also receive it directly
What ADS-B Out broadcasts:
GPS position (latitude/longitude)
Pressure altitude
Ground speed and track (velocity)
Aircraft identification (call sign or ICAO address)
Additional status data
The ground infrastructure:
A network of ADS-B ground stations across the country
They receive the broadcasts and feed ATC
They also transmit TIS-B and FIS-B (for ADS-B In)
The ground network is the backbone
The update rate:
ADS-B updates roughly once per SECOND
Radar updates every several seconds (as the antenna sweeps)
The near-real-time updates are far more current
This is a major advantage
The satellite element:
Space-based ADS-B (via satellite constellations) now extends coverage over oceans and remote areas
Where ground stations can't reach
Global surveillance
An expanding capability
Why ADS-B Is Better Than Radar
Understanding the advantages explains why the FAA mandated the transition.
Accuracy:
ADS-B uses GPS position (very precise)
Radar has inherent inaccuracies (especially at range, where the beam widens)
ADS-B is accurate to meters; radar to fractions of a mile
Far more precise
Update rate:
ADS-B: about once per second
Radar: every 5-12 seconds (antenna rotation)
The faster updates track aircraft more currently
Better for closely spaced traffic
Coverage:
Radar requires line-of-sight from ground antennas (gaps in mountains, low altitude, remote areas)
ADS-B ground stations (and satellites) fill many gaps
Coverage at lower altitudes and in remote areas
Fewer blind spots
Cost:
Radar installations are enormously expensive to build and maintain
ADS-B ground stations are far cheaper
The FAA can provide broader coverage for less
An economic driver
Capabilities:
ADS-B enables cockpit traffic and weather (ADS-B In)
Radar provides none of that to the cockpit
ADS-B is a platform for additional services
More than just surveillance
The NextGen context:
ADS-B is a foundation of NextGen (the FAA's air traffic modernization)
Moving from ground-based radar to satellite-based (GPS) surveillance
More efficient routing, better capacity, enhanced safety
The future of air traffic management
ADS-B Out vs. ADS-B In
The two components, which serve different functions.
ADS-B Out (required):
Broadcasts YOUR aircraft's data (position, altitude, velocity, ID)
One-way (out from the aircraft)
Required by regulation in specified airspace (91.225)
Makes your aircraft visible to ATC and other aircraft
ADS-B In (optional):
RECEIVES data (traffic and weather) into the cockpit
Requires a receiver and a display
NOT required by the FAA
Provides valuable situational awareness
The relationship:
Out: you broadcast (mandatory in rule airspace)
In: you receive (optional bonus)
Many systems do both
Out is the mandate; In is the added value
Why Out is required but In isn't:
Out serves the SYSTEM (ATC surveillance, others seeing you) — mandated for safety/surveillance
In serves the individual pilot (your awareness) — beneficial but optional
The mandate is about being seen; receiving is your choice
Both are valuable, but only Out is required
Where ADS-B Out Is Required
The airspace requirements (91.225), which largely mirror the transponder airspace.
The rule airspace:
Class A: 18,000 MSL to FL600 (IFR only)
Class B: all of it, plus the Mode C veil (30 NM around the primary airport, surface to 10,000 MSL)
Class C: all of it (and above, to 10,000 MSL)
At or above 10,000 MSL: except at and below 2,500 AGL
Gulf of Mexico: Class E airspace at and above 3,000 MSL, within 12 NM of the coast
The "essentially the same as transponder airspace" summary:
If a Mode C transponder is required, ADS-B Out is generally required too
The main addition is the Gulf of Mexico provision (3,000 MSL)
The airspace overlaps heavily
Both required in the busy airspace
Where ADS-B Out is NOT required:
Most Class D airspace
Class E below 10,000 MSL (outside the veil, away from Class B/C)
Class G airspace (unless otherwise specified)
But transponder requirements may still apply where relevant
The Gulf of Mexico distinction:
The one place ADS-B airspace differs notably from transponder airspace
Class E over the Gulf, at/above 3,000 MSL, within 12 NM of the coast
For offshore helicopter and other operations
A specific addition
(For the full airspace breakdown and the Mode C veil versus Class B shelves distinction, see our dedicated post on transponder-required airspace — the ADS-B airspace tracks it closely.)
The Performance Requirements: What Actually Makes Equipment Compliant
A crucial topic the basic treatment doesn't cover: ADS-B Out has performance standards, and failing them means non-compliance even with equipment installed.
The regulation (91.227):
91.225 says WHERE ADS-B Out is required
91.227 defines the PERFORMANCE the equipment must meet
Meeting the performance standards is required for compliance
Installed equipment that doesn't meet the standards isn't compliant
The position source requirement:
ADS-B Out requires a qualified GPS position source
Must meet specific accuracy and integrity standards
A WAAS-capable GPS typically meets the requirement
The position source quality is critical
The key performance parameters:
NACp (Navigation Accuracy Category for position): how accurate the position is
NIC (Navigation Integrity Category): the integrity of the position
SIL (Source Integrity Level): the probability the position exceeds the integrity bound
SDA (System Design Assurance): the design assurance level of the system
These must meet minimum values
Why these matter:
The system broadcasts these quality indicators
ATC uses them to trust (or not trust) the position
Below the required values, the ADS-B Out is non-compliant
The FAA monitors these
The "NIC/NACp too low" problem:
A common compliance issue is a position source that doesn't meet the accuracy/integrity standards
The equipment transmits, but the quality indicators are too low
This shows up as non-compliance
Requires a qualified position source
Why you can't just install any GPS:
The GPS position source must meet the standards (typically a certified WAAS GPS)
A basic GPS may not qualify
The position source is as important as the ADS-B transmitter
Both must meet the requirements
1090ES vs. UAT: The Two Technologies
The two approved ADS-B Out link types and how to choose.
1090ES (1090 MHz Extended Squitter):
Uses the Mode S transponder at 1090 MHz
An "extended squitter" is the ADS-B message on the transponder frequency
REQUIRED for aircraft operating at or above 18,000 feet (Class A)
Required for international operations
Integrated with the Mode S transponder
UAT (978 MHz Universal Access Transceiver):
A separate frequency (978 MHz) and separate equipment
Only usable BELOW 18,000 feet (U.S. only)
Often more affordable for GA
Supports FIS-B weather (a bonus)
The choice:
Fly above 18,000 or internationally: you need 1090ES
Stay below 18,000 in the U.S.: UAT is an option (often cheaper, includes FIS-B)
Many GA aircraft below 18,000 use UAT
High-performance/turbine aircraft use 1090ES
The FIS-B advantage of UAT:
FIS-B (weather) is broadcast on the UAT frequency (978 MHz)
UAT-In receivers get FIS-B directly
1090ES doesn't carry FIS-B (need a separate UAT receiver for weather)
A reason some choose UAT (for the weather)
The dual-frequency reality:
ADS-B In from BOTH frequencies requires dual-band capability (or the ground stations rebroadcast)
TIS-B rebroadcasts traffic so you see aircraft on the other frequency
The system handles the frequency mix
Dual-band receivers see everything
ADS-B In: Traffic (TIS-B) in Detail
The traffic component of ADS-B In, which has genuine value and some limitations.
What TIS-B is:
Traffic Information Service–Broadcast
Ground stations broadcast traffic information to your aircraft
Includes traffic detected by radar and other ADS-B aircraft
Displayed on your cockpit device
What you see:
Nearby aircraft positions
Their altitude (relative to you)
Their track and speed
A traffic picture on your display
The value:
Enhanced situational awareness
See traffic you might miss visually
Especially valuable in busy areas
A significant safety enhancement
The limitations (important):
TIS-B requires YOU to have ADS-B Out to trigger the service (in many cases the ground station provides TIS-B based on your ADS-B Out)
Traffic without ADS-B Out (and not seen by radar) may not appear
The picture may be incomplete
Don't rely on it as the sole traffic awareness — keep looking outside
The ADS-B In "client" requirement:
To get TIS-B, your aircraft generally needs ADS-B Out (to be a "client" the ground station serves)
ADS-B In alone (without Out) gets a limited picture
Out + In together gives the full service
The system is designed around participating aircraft
The see-and-avoid caveat:
TIS-B supplements but doesn't replace visual scanning
Not all traffic appears
Use it as an aid
Keep your eyes outside
ADS-B In: Weather (FIS-B) in Detail
The weather component, which delivers remarkably useful (and free) weather to the cockpit.
What FIS-B is:
Flight Information Service–Broadcast
Ground stations broadcast weather and aeronautical information
Free (no subscription)
Received via UAT (978 MHz) ADS-B In
The FIS-B products:
NEXRAD radar (regional and national precipitation)
METARs and TAFs (airport weather)
AIRMETs and SIGMETs (hazard advisories)
PIREPs (pilot reports)
NOTAMs
TFRs (temporary flight restrictions)
Winds and temperatures aloft
Special Use Airspace status
The value:
In-cockpit weather without a subscription (unlike XM/satellite weather)
Regional weather picture
Helps strategic weather decisions
A major safety benefit
The NEXRAD latency caveat (critical):
FIS-B NEXRAD is NOT real-time
It can be several minutes old (the mosaic has inherent age)
The timestamp may not reflect the actual age of the data
NEVER use FIS-B NEXRAD for tactical thunderstorm avoidance (penetrating close to cells)
Use it strategically (big-picture), not tactically (dodging individual cells)
Why the latency matters:
Storms move and develop fast
Minutes-old NEXRAD can show a cell in the wrong place (or not show new growth)
Pilots have flown into storms trusting old NEXRAD
Strategic use only — give storms wide berth
The frequency note:
FIS-B is on 978 MHz (UAT)
1090ES-only aircraft need a UAT receiver for FIS-B
This is a reason UAT is attractive for GA (built-in weather)
Know which frequency provides FIS-B
Anonymous Mode and Privacy
A real pilot concern the basic treatment doesn't address.
The privacy issue:
ADS-B Out broadcasts your identification and position
Anyone with a receiver (including public flight-tracking websites) can see it
Some pilots have privacy concerns (being tracked)
A legitimate consideration
Anonymous mode (UAT only):
UAT equipment can support an anonymous mode
It broadcasts a temporary, randomized identifier instead of the tail number
Only available when NOT receiving ATC services (VFR, not on a discrete code)
Provides some anonymity
The limitations:
Anonymous mode is only for UAT (not 1090ES)
Only when squawking 1200 (VFR, not receiving services)
You still broadcast position (just not your identity)
Limited privacy, not invisibility
The FAA Privacy ICAO Address (PIA) program:
A program allowing eligible aircraft to use an alternate ICAO address (not tied to the registration)
Reduces public tracking
For 1090ES aircraft with privacy concerns
An option for those who qualify
The LADD program:
Limiting Aircraft Data Displayed (LADD)
Requests that your aircraft not be displayed on public tracking sites
Doesn't stop the broadcast, but limits public display
Another privacy option
The practical reality:
ADS-B Out means you're broadcasting
Some privacy options exist (anonymous mode, PIA, LADD)
Complete anonymity isn't possible while compliant
Options for the privacy-conscious
Verifying Your ADS-B Out Is Compliant
A practical topic: how to confirm your installation actually works and meets the standards.
The ADS-B Out Performance Report (PAPR):
The FAA offers a Public ADS-B Performance Report (PAPR)
Request it after a flight in ADS-B-monitored airspace
It analyzes your ADS-B Out performance
Confirms compliance (or identifies problems)
How to get it:
Fly in airspace with ADS-B ground station coverage
Request the report from the FAA (online)
Receive an analysis of your transmission
Free
What it checks:
Whether your ADS-B Out meets the performance requirements
The position source quality (NIC, NACp, SIL, SDA)
Any failures or anomalies
Confirms your equipment is compliant
Why verify:
Installed equipment doesn't guarantee compliance (performance standards)
The PAPR confirms it's working correctly
Catch problems before they're a violation
Recommended after installation
The compliance confirmation:
After installing ADS-B Out, get a PAPR
Confirm it's compliant
Fix any issues
Peace of mind
Operating Without ADS-B Out
The deviation provision, when you don't have compliant ADS-B Out.
The authorization:
Aircraft without ADS-B Out may operate in ADS-B-required airspace only with ATC authorization
Request in advance (typically at least 1 hour before)
Via the ADAPT tool (the FAA's ADS-B Deviation Authorization Preflight Tool) for some requests
Authorization is limited and not guaranteed
The ADAPT tool:
The FAA's online tool for requesting ADS-B deviation authorizations
For flights in ADS-B airspace without compliant equipment
Submit the request in advance
Get approval (or not)
Common situations:
Aircraft not yet equipped
ADS-B Out equipment failure
Ferry flights
Aircraft without electrical systems (exceptions apply)
The not-guaranteed reality:
Authorization depends on the airspace and traffic
Not always granted
Plan ahead
Have a backup
Common Pilot Mistakes
Assuming ADS-B is optional everywhere:
ADS-B Out is REQUIRED in the rule airspace (91.225)
Not optional in Class A/B/C, the veil, above 10,000
Know where it's required
Mandatory, not optional
Confusing transponder and ADS-B rules:
Separate regulations (91.215 vs. 91.225)
Both may apply
A Mode S/1090ES unit can cover both
Know both
Flying into a Mode C veil without ADS-B Out:
The veil requires ADS-B Out (like the transponder)
Even under the Class B shelves
Within 30 NM of the primary airport
A common violation
Incorrect aircraft identification programming:
The ADS-B Out must broadcast the correct identification
Misprogrammed ID (wrong call sign, wrong ICAO address) is a compliance issue
Verify the programming
Shows up on the PAPR
Trusting FIS-B NEXRAD tactically:
FIS-B weather is delayed (minutes old)
Don't use it to dodge individual cells
Strategic use only
A dangerous mistake
Not verifying compliance:
Installed equipment might not meet the standards
Get a PAPR to confirm
Don't assume
Verify
Final Thoughts
ADS-B represents a major advancement in air traffic surveillance — more accurate than radar, updated every second, and the foundation of NextGen. Understanding it as a system clarifies the whole picture: ADS-B Out broadcasts your GPS-derived position automatically (required in Class A/B/C, the Mode C veil, above 10,000, and over the Gulf), while ADS-B In brings traffic (TIS-B) and free weather (FIS-B) into the cockpit. The performance standards in 91.227 mean the position source matters as much as the transmitter, and a PAPR confirms your installation actually complies.
Knowing where ADS-B Out is required, ensuring your equipment meets the performance standards, using ADS-B In wisely (strategic weather, supplemental traffic), and understanding the privacy options makes you a more informed pilot. As the airspace continues to evolve, ADS-B remains the foundational technology shaping aviation safety and efficiency.
On the Written Test and Checkride
ADS-B appears on tests and checkride orals. The most commonly tested topics:
What ADS-B is (Automatic Dependent Surveillance–Broadcast)
ADS-B Out (required) vs. ADS-B In (optional)
Where ADS-B Out is required (91.225 airspace)
1090ES vs. UAT (and the 18,000-foot distinction)
FIS-B weather and the NEXRAD latency caveat
The position source/performance requirement
Quick Reference
ADS-B = Automatic Dependent Surveillance–Broadcast:
Automatic: transmits on its own
Dependent: uses onboard GPS for position
Surveillance: tracks aircraft
Broadcast: to anyone in range
How It Works:
GPS determines position → broadcast (1090 or 978 MHz) → ground stations relay to ATC + other aircraft receive
Updates ~once per second (radar: every 5-12 seconds)
Better Than Radar:
More accurate (GPS)
Faster updates
Better coverage (fewer gaps)
Cheaper
Enables cockpit traffic/weather
ADS-B Out (REQUIRED) vs. In (OPTIONAL):
Out: broadcasts your data (mandatory in rule airspace)
In: receives traffic (TIS-B) and weather (FIS-B) (optional)
Where ADS-B Out Is Required (91.225):
Class A, B (+ Mode C veil), C
At/above 10,000 MSL (except at/below 2,500 AGL)
Gulf of Mexico E airspace at/above 3,000 MSL (within 12 NM)
≈ same as transponder airspace + Gulf
Performance Requirements (91.227):
Qualified GPS position source (typically WAAS)
NACp, NIC, SIL, SDA must meet minimums
Low position quality = non-compliant
The position source matters as much as the transmitter
1090ES vs. UAT:
1090ES | UAT (978) | |
Altitude | Any (required ≥18,000) | Below 18,000 only |
Basis | Mode S transponder | Separate equipment |
International | Yes | No (US only) |
FIS-B weather | No (need UAT-In) | Yes |
ADS-B In Traffic (TIS-B):
Nearby traffic on your display
Generally requires your ADS-B Out to get full service
Incomplete (not all traffic appears)
Supplements, doesn't replace, visual scanning
ADS-B In Weather (FIS-B):
Free weather (NEXRAD, METARs, TAFs, AIRMETs/SIGMETs, PIREPs, NOTAMs, TFRs, winds aloft)
On 978 MHz (UAT)
NEXRAD is DELAYED (minutes old) — strategic use only, NOT tactical cell avoidance
Privacy Options:
Anonymous mode (UAT, VFR/1200 only)
Privacy ICAO Address (PIA) program
LADD (limits public display)
Complete anonymity not possible while compliant
Verifying Compliance:
Request a PAPR (Public ADS-B Performance Report) from the FAA
Confirms your Out meets the standards
Free, after flying in monitored airspace
Without ADS-B Out:
ATC authorization required (advance, via ADAPT tool)
Not guaranteed
Key Principle:
ADS-B is automatic, GPS-dependent, broadcast surveillance — more accurate and current than radar. ADS-B Out (required in Class A/B/C, the Mode C veil, above 10,000, and the Gulf) broadcasts your position; ADS-B In brings traffic and free weather to the cockpit. The position source must meet performance standards (91.227) to comply — verify with a PAPR. And FIS-B NEXRAD is delayed: use it strategically, never to dodge individual storms.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.