# NMEA Module¶

This module implements a Zerynth driver and parser for a generic NMEA GNSS receiver.

The following functionalities are implemented:

• retrieve the current location fix if present
• retrieve the current UTC time

The driver supports reading NMEA sentences from a serial port only.

Location fixes are obtained by parsing NMEA sentences of type RMC and GGA, and optionally GSA. Obtaining a fix or UTC time are thread safe operations.

readline(serial, buffer, timeout=5000)

Wait for a full NMEA sentence from the specified serial interface and copy it to the specified buffer (bytearray), with optional timeout (in milliseconds).

Returns the length of the NMEA sentence or a negative error code:

• -1, if the line does not start with NMEA header (missing '\$')
• -2, if the line has incomplete NMEA sentence (missing '*')
• -3, if the line has invalid or mismatching NMEA checksum
parseline(buffer, length, tm, fix)

Parse the content of the specified line buffer (bytearray) up to length bytes and fill the two sequences tm and fix with date/time and fix data if available.

Returned value is 0 if the line does not have valid data, or a combination (sum) of:

• 4, if the tm sequence (7 items) has been filled (from RMC sentence)
• 1, 2 or 3, if the fix sequence (9 items) has been filled (from RMC, GGA or GSA respectively)
class NMEA_Receiver

This class is meant to be used as a base class to provide a uniform interface for GNSS receivers.

Instances of this class are fed with NMEA sentences using the parse method and can be queried for UTC time and location data.

fix()

Return the current fix or None if no fix is available. A fix is a tuple with the following elements:

• latitude in decimal format (-89.9999 - 89.9999)
• longitude in decimal format (-179.9999 - 179.9999)
• altitude in meters
• speed in Km/h
• course over ground as degrees from true north
• number of satellites for this fix
• horizontal dilution of precision (0.5 - 99.9)
• vertical dilution of precision (0.5 - 99.9)
• positional dilution of precision (0.5 - 99.9)
• UTC time as a tuple (yyyy,MM,dd,hh,mm,ss,microseconds)
has_fix()

Return True if a fix is available

utc()

Return the current UTC time or None if no UTC time is available. A UTC time is a tuple of (yyyy,MM,dd,hh,mm,ss,microseconds).

UTC time can be wrong if no fix has ever been obtained.

has_utc()

Return True if a UTC time is available

parse(buffer, count)

Parse count bytes from the specified buffer (bytearray) and updates internal state from valid NMEA sentences found.