NMEA specification
Overview
Reach RX MFi supports streaming of the following NMEA messages:
| Message ID | Description |
|---|---|
| GGA | Time, position and fix type data |
| GSA | GPS receiver operating mode, satellites used in the position solution, and DOP values |
| GST | Position error statistics |
| GSV | Number of satellites in view, ID numbers, elevation, azimuth, & SNR values |
| RMC | Position, velocity, and time |
| VTG | Course and speed information relative to the ground |
| ZDA | UTC day, month, and year, and local time zone offset |
Reach RX outputs all the messages with the GN talker ID at the following frequences:
| Message ID | Talker ID | Default frequency |
|---|---|---|
| GGA | GN | 5 Hz |
| GSA | G* | 1 Hz |
| GST | GN | 1 Hz |
| GSV | G* | 1 Hz |
| RMC | GN | 1 Hz |
| VTG | GN | 1 Hz |
| ZDA | GN | 1 Hz |
*Reach RX outputs GSA and GSV messages with the successive talker IDs-GP, GL, GA, GB.
Protocol
iOS device accessories shall declare at least one “SupportedExternalAccessoryProtocol” parameter in their identification information during authentication. This protocol is not standardized and allows for the exchange of any data between the device and its proprietary application on the iPhone.
For example, we use this protocol to send a standard set of NMEA messages to ArcGIS Field Maps. Reach RX creates special External Accessory profiles: com.emlid.nmea for sending NMEA messages. ArcGIS FieldMaps uses this profile for reading NMEA stream.
Exchange process is the following:
-
iPhone sends StartExternalAccessoryProtocolSession (0xEA00).
-
The device starts sending data.
-
iPhone sends StopExternalAccessoryProtocolSession (0xEA01).