nmea

Author:: Max Lapshin (mailto:[e-mail]) Copyright:: Copyright (c) 2007 Max Lapshin, Getalime License:: Distributes under the same terms as Ruby

To use NMEA parser, you should somehow get data stream from Yor gps device. For example, You can use ruby-serialport (which seems to be dead, but working). The example on Mac with installed USB-Serial driver:

require 'serialport'
require 'nmea'
@sp = SerialPort.open("/dev/tty.usbserial", 4800, 8, 1, SerialPort::NONE)
@handler = NMEAHandler.new
while(@sentence = @sp.gets) do
  NMEA.scan(@sentence, @handler)
end

NMEAHandler is a user class, that implements following interface:

class NMEAHandler
  def rmc(time, latitude, longitude, speed, course, magnetic_variation)
    # read further about types of latitude and longitude
  end
  def gsv(flag, satellites)
  end
  def gsa(mode_state, mode, satellites, pdop, hdop, vdop)
  end
  def gga(time, latitude, longitude, gps_quality, active_satellite_count, gsa_hdop, altitude, geoidal_height, dgps_data_age, dgps_station_id)
  end
end

As you can see, the following NMEA sentences are supported currently: $GPRMC, $GPGSV, $GPGSA, $GPGGA. If you need more, please contact me and provide me with several examples. I will add support for them. NMEA::scan will not try to call unexistent method on your handler, thus you can implement only those methods in your handler, you need to.

GSV handler has parameter flag. This flag can take one of the following values: :start, :continue and :finish. GSV messages appears in packs of several sentences, but NMEA::scan is stateless, thus your NMEAHandler should keep accumulate these messages.

It is important to mention, that NMEA::scan assumes, that You have classes GPS::Latitude and GPS::Longitude, which initializer, takes two arguments: degrees and minutes. You can use this example:

module GPS
  AngleValue = Struct.new :degrees, :minutes

  class AngleValue
    def to_s
      "%d %.4f%s" % [degrees.abs, minutes, symbol]
    end
  end

  class Latitude < AngleValue
    def symbol
      degrees >= 0 ? "N" : "S"
    end
  end
  class Longitude < AngleValue
    def symbol
      degrees >= 0 ? "E" : "W"
    end
  end
end

Supported devices.

Ruby NMEA has full support of Globalsat BU-353 device (http://www.usglobalsat.com/item.asp?itemid=60&catid=17).

NMEA syntax is assumed to be as in document nmea.html, taken from http://www.werple.net.au/~gnb/gps/nmea.html