GDL 90 Extended Specification

ForeFlight offers the industry-standard GDL 90 Data Interface Specification defined below for third-party devices to transmit live inflight data to ForeFlight Mobile. Properly configured devices will be able to display ADS-B weather and traffic, AHRS, device name information, and GPS data in the ForeFlight Mobile app.

Please note: ForeFlight does not test or provide support for devices that use this specification. If you experience problems with a device that uses this specification, please contact the device manufacturer for assistance.

Connectivity

ForeFlight expects data sent using UDP to port 4000 on the iOS device. Data can be sent using broadcast or unicast packets, but implementers are advised to be aware of packet loss concerns when using broadcast packets.

iOS has an MTU of 1500 bytes. It is strongly recommended to avoid fragmentation and to keep all packets (including headers) smaller than 1500 bytes.

ForeFlight determines that a device is connected if data is regularly received. Devices should regularly send either or both of the Heartbeat or Ownship Report messages to ensure that the device is consistently reported as Connected.

Messages

The ForeFlight GDL90 Extension protocol defines messages based on the GDL90 protocol. Section 2.2 of the GDL90 specification describes the message structure and Section 3 outlines a set of standard messages. ForeFlight supports a subset of the standard messages and also extends the protocol with a pair of custom messages containing device ID and AHRS information.

Heartbeat Message

See GDL90 specification §3.1 for complete details. Only GPS validity bit is checked at this time.

Byte # Name Size Value
1 Message ID 1 010 = Heartbeat
2

Status Byte 1

Bit 7: GPS Pos Valid

1

1 = Position is available for ADS-B Tx

Other bits are ignored

3 Status Byte 2 1 All bits ignored
4-5 Time Stamp 2 Ignored
6-7 Message Counts 2 Ignored

 

UAT Uplink

See GDL90 specification §3.3 for complete details.

Byte # Name Size Value
1 Message ID 1 710 = Uplink Data
2-4

Time of Reception

3

24-bit binary fraction

Resolution = 80 nsec

5-436 Uplink Payload 432 UAT Uplink Packet. See §3.3.2 for details

 

Ownship Report

See GDL90 specification §3.4 for complete details. The position information in this message is used by ForeFlight to determine current position.

Byte # Name Size Value
1 Message ID 1 1010 = Ownship Report
2-28

Ownship Report

27

Defined in §3.5.1


Notes:
Accuracy information is encoded by setting the NACp value.
Altitude is defined as ownship pressure altitude (referenced to 29.92 inches Hg). For unpressurized aircraft a barometer in the cabin is close enough for practical purposes, but in pressurized aircraft, care must be taken to set this field to 0xFFF (Invalid or Unavailable) if the device does not have access to outside pressure. Setting ownship pressure altitude incorrectly will result in incorrect calculation of relative traffic altitude. 

Ownship Geometric Altitude

See GDL90 specification §3.8 for complete details. Note that the altitude may be interpreted as either relative to the WGS-84 ellipsoid as spec'ed, or to the WGS-84 geoid (MSL). The ID message described below defines how this altitude will be interpreted.

Byte # Name Size Value
1 Message ID 1

1110 = Ownship Geo Alt

2-3

Ownship Geo Altitude

2

Signed altitude in 5ft resolution.

Byte 2 is the Most Significant Byte

Altitude is interpreted as relative to the WGS84 ellipsoid unless Bit 0 of the ID Message Capabilities Mask is set, in which case it's treated as MSL. 

4-5 Vertical Metrics 2

Vertical Warning Indicator (MSB of Byte 4)

Vertical Figure of Merit (remaining 15 bits).

0x7FFF indicates VFOM not available

0x7EEE indicates VFOM is > 32766 meters

Byte 4 is the most significant byte.

 

Traffic Report

See GDL90 specification §3.5 for complete details.

Byte # Name Size Value
1 Message ID 1 2010 = Traffic Report
2-28

Traffic Report

27

Defined in §3.5.1

 

ID Message

For multibyte fields, the most significant byte should be sent first (Big Endian).

Byte # Name Size Value
1 ForeFlight Message ID 1 0x65
2

ForeFlight Message sub-ID

1

0

3 Version 1 Must be 1
4-11 Device serial number 8

0xFFFFFFFFFFFFFFFF for invalid

12-19 Device name 8

8B UTF8 string.

20-35 Device long name 16 16B UTF8 string. Can be the same as Device name. Used when there is sufficient space for a longer string.
36-39 Capabilities mask 4

Bit 0 (LSB): Geometric altitude datum used in the GDL90 Ownship Geometric Altitudes message.

0: WGS-84 ellipsoid (as the GDL90 spec states)

1: MSL

Bits 1-31: Reserved. Should be all 0's.

 

AHRS Message

For multibyte fields, the most significant byte should be sent first (Big Endian).
The AHRS message should be sent at 5Hz.

Byte # Name Size Value
1 ForeFlight Message ID 1 0x65
2

AHRS Sub-Message D

1

0x01

3-4 Roll 2

Roll in units of 1/10 degree

0x7fff for invalid.

Positive values indicate right wing down, negative values indicate right wing up.

The message will be rejected if roll is outside of the range [-1800, 1800]

5-6 Pitch 2

Pitch in units of 1/10 degree

0x7fff for invalid.

Positive values indicate nose up, negative values indicate nose down.

The message will be rejected if pitch is outside of the range [-1800, 1800]

7-8 Heading 2

Most significant bit (bit 15)
0: True Heading
1: Magnetic Heading

Bits 14-0: Heading in units of 1/10 degree

Track should NOT be used here.

0xffff for invalid.

The message will be rejected if heading is outside of the range [-3600,3600]

9-10 Indicated Airspeed 2

Value in Knots

0xffff for invalid.

11-12 True Airspeed 2

Value in Knots

0xffff for invalid.