Transponders Explained
Understand the difference between Mode A, Mode C, Mode S, and ADS-B. Learn how transponders work and why Stratux can receive but not replace them.
Transponder Modes Comparison
| Mode | Frequency | Altitude | Position | Typical Use |
|---|---|---|---|---|
| Mode A | 1030/1090 MHz | No | No | Legacy aircraft, rarely used alone |
| Mode C | 1030/1090 MHz | Yes (100ft increments) | No (radar-derived) | VFR in most controlled airspace |
| Mode S | 1030/1090 MHz | Yes (25ft capable) | No (radar-derived) | Standard for IFR, required in most EU airspace |
| ADS-B Out (ES) | 1090 MHz (broadcast only) | Yes (GPS + pressure) | Yes (GPS-derived) | Required for IFR worldwide, some VFR airspace |
How Transponders Work
Secondary Surveillance Radar (SSR)
Mode S Extended Squitter
ADS-B Out Transmission
Stratux as ADS-B In Receiver
MLAT - Multilateration
Multilateration (MLAT) calculates aircraft positions by measuring the time difference of arrival (TDOA) of transponder signals at multiple ground stations. This allows tracking of Mode A/C aircraft that don't broadcast GPS position.
Networks like Flightradar24 and OGN use MLAT to display non-ADS-B traffic. Stratux can contribute to MLAT networks when configured with OGN transmission-your device becomes a ground station that helps track nearby aircraft.
Transponder Requirements by Region
European Union
Mode S mandatory in most controlled airspace. ADS-B Out required for IFR above FL100 (expanding).
United States
ADS-B Out required in Class A, B, C airspace, and above 10,000ft MSL. Mode C acceptable in some Class E.
Australia
ADS-B Out required above FL290 and in designated airspace. Expanding to more regions.
VFR/Uncontrolled
Requirements vary widely. Check local regulations. Many VFR aircraft fly without transponders in Class G.
Important: Stratux Cannot Replace Your Transponder
Stratux is a receive-only (ADS-B In) device that enhances your situational awareness. It shows traffic from ADS-B, FLARM, and OGN-equipped aircraft on your EFB. For regulatory compliance and visibility to ATC, you must use certified transponder equipment. Stratux with OGN transmission makes you visible on the Open Glider Network, but this is not a replacement for certified ADS-B Out or Mode S transponder.
Frequently Asked Questions
Related Technology Guides
Receive Transponder Traffic with Stratux
All Stratux devices receive Mode S and ADS-B traffic on 1090 MHz. See IFR and commercial traffic alongside FLARM-equipped gliders.
Disclaimer
Stratux is not certified aviation equipment. It does not replace a transponder or relieve the pilot of traffic awareness responsibilities. Use as supplementary situational awareness only. Dross:Aviation assumes no responsibility or liability for the use of Stratux devices, firmware flashing, or hardware modifications. All DIY guides are provided without warranty-use at your own risk.