Mode S

[G-V]

I've been told that the current heading (set in the heading window if operating in the heading mode) is the part of the information package transmitted by the Mode S transponder to the ground station (to the ATC).

Can anyone comment and point me in the right direction to verify this info please?

Thanks

[DaveReidUK]

I've been told that the current heading (set in the heading window if operating in the heading mode) is the part of the information package transmitted by the Mode S transponder to the ground station (to the ATC).

Yes, provided the aircraft is compliant with Mode S EHS (Enhanced Surveillance), which most are.

SKYbrary - Mode S

[Cough]

Always thought it was actual heading, not selected heading...

[Scuzi]

Originally Posted by Cough

Always thought it was actual heading, not selected heading...

You are correct, it's the actual magnetic heading which is displayed.

[DaveReidUK]

Always thought it was actual heading, not selected heading...

Yes, I'd assumed by the OP's use of the word "current" that they meant the direction the pointy end was facing at any given instant.

[G-V]

Many thanks for the info.

It must be a ADS-B equipt Aircraft correct?

[ATCast]

It's not required to transmit ADS-B in order to provide magnetic heading on Mode S. But most aircraft that have the capability to send magnetic heading are also ADS-B equipped. And the other way around, when an aircraft is ADS-B equipped it is no guarantee that it will be able to provide magnetic heading on Mode S. Typically aircraft do not transmit magnetic heading on ADS-B but send out a GPS based ground speed vector with respect to the true north.

Mode S Magnetic Heading is located in register 60, together with indicated airspeed, mach and vertical rate (baro & inertial). A Mode S radar can extract the information from the transponder by uplinking the appropriate Reply Request Code. It is not transmitted in every standard interrogation.

[DaveReidUK]

Typically aircraft do not transmit magnetic heading on ADS-B but send out a GPS based ground speed vector with respect to the true north.

Yes, though a surprising number of aircraft in this day and age still have ADS-B that isn't based on GPS.

[ATCast]

a surprising number of aircraft in this day and age still have ADS-B that isn't based on GPS.

About 90% of the ADS-B transmitting aircraft have GNSS based data, approximately 10% uses INS data. The 10% will be phased out or fitted with GPS in the next 5 years.
ADS-B that is not based on GNSS is pretty much useless.

[10W]

Where an aircraft is required to have Enhanced Mode S in Europe, the magnetic heading is a mandatory part of the data set which is downlinked.

ADS-B is of course not mandatory in Europe and nor are all aircraft covered by the Enhanced Mode S mandate, so some aircraft will indeed not transmit their magnetic heading. For those who must have it, then there is no choice but to have the capability to transmit magnetic heading.

[ATCast]

ADS-B is of course not mandatory in Europe and nor are all aircraft covered by the Enhanced Mode S mandate, so some aircraft will indeed not transmit their magnetic heading. For those who must have it, then there is no choice but to have the capability to transmit magnetic heading.

Indeed, ADS-B is not mandatory yet and not all aircraft are subject to the Mode S mandate. But there are some things changing in the next 4 years.
From December 2017 all general air transport aircraft with an MTOM > 5700 kg or a maximum cruising speed > 250 KTAS flying in Europe must have the following:

Mode S ELS (Elementary Surveillance)

• 24 bit address
• Mode A code
• pressure altitude
• ground/airborne indication
• aircraft identification (flight id/call sign)
• special position indicator ("ident")
• emergency status
• TCAS RA activity (if TCAS II equipped)

EHS (Enhanced Surveillance)(for fixed wing aircraft):

• selected altitude
• roll angle
• true track angle
• ground speed
• magnetic heading
• indicated airspeed
• vertical rate (baro or baro-inertial)
• barometric pressure setting
• track angle rate (or true airspeed if track angle rate is not available)

ADS-B (Automatic Dependent Surveillance Broadcast)

• 24 bit address
• aircraft identification
• Mode A code
• Special Position Indicator
• Emergency Status
• ADS-B Emitter Category
• Horizontal position (WGS 84)
• Pressure altitude
• Geometric altitude
• Ground speed vector
• Quality indicators for position, geometric altitude and velocity
• vertical rate (baro, baro inertial or GNSS)
• selected altitude
• barometric pressure setting
• TCAS RA activity (if TCAS II equipped)
• ADS-B version 2

[FlightPathOBN]

It appears the thread is blending Mode S and ADSB.

Mode S transponder can be stand alone, or have a combined Mode S/ADSB

In answer to the OP, the answer is no, depending on the Countries requirements for Mode S, it does not necessarily include current heading.

Currently, Mode S E is the basic mode that is required in most States for registered aircraft. Aircraft registered after March 2007, must have Modes S EHS, but this is only required in UK, France, Germany, and AUS and Alaska.

Mode S E provides the following:

• Aircraft ID
• Altitude (every 25 feet if able)
• Integrity (basic ability for ground to determine reflectivity ie false signal)
• Flight Status (On ground/In air)

Mode S ELS provides:

• Aircraft ID
• Enhanced integrity
• Improved ground tracking
• Compressed SSR coding to help alleviate congestion/interference
• Altitude (mandates every 25 feet) to decrease alerts by ground SSR system

Mode S EHS Enhanced Surveillance:

• Select altitude (manually entered by pilot)
• Roll Angle, True Track Angle and Track Angle Rate
• Ground Speed
• Magnetic Heading
• Indicated airspeed
• Vertical rate (barometric rate of climb / descent)
• TCAS downlinked resolution advisories.

Transponders may or may not be connected to the ADSB and/or GPS system

.........

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[FlightPathOBN]

In regards to GPS location, keep in mind that GPS lets the aircraft know where it was, not where it is.
The estimate of the current location of the aircraft is from the Kalman filter.

As noted, the minimum requirement for GPS system update is 2 seconds, and the typical GPS system on a commercial aircraft is about 0.6 seconds.

What this means is the location of the aircraft can be 2 seconds off, which at 500 kts, is a good bit.

The Kalman filter is the predictive part of the GPS system, which indicates to the FMS where the aircraft is.

Final approach maneuvers, such as turns on final, are difficult to predict, hence the reported position accuracy will go down...

[G-V]

Many thanks to all who conrtibuted.

[FlightPathOBN]

speaking of heading...can the display show grid instead of MAG?