7.1. NMEA Protocol (RS-232)#
NMEA protocol is based on the standard protocol family widely used by the navigation equipment. NMEA protocol is sentence oriented, and is capable of sending multiple sentences with different information. The sentence content is designated by the starting keyword which is different for each sentence type. NMEA sentences are terminated with the checksum which makes this protocol extremely reliable. NMEA protocol is single-direction protocol: data is only transmitted from the AX500. At RS-232 interface the device periodically outputs following data sentences:
Air quality measurement report#
The following 3 sentences are transmitted every second:
$GXAMB,<temp °C>,<rh %>,<pressure kPa>,<aqi>,<average mV>,<min mV>,<max mV>
$GXGAS,<slot1 CO>,<slot2 SO2>,<slot3 NO>,<slot4 NO2>,<slot5 O3>
$GXPM,<pm less than 1 um>,<pm less than 2.5 um>,<pm less than 10 um>
$GXAMB: The keyword sent on the beginning of each report. This sentence contains information about the ambient conditions.
<temp °C>: Temperature (°C) – Internal air temperature measured in degrees Celsius.<rh %>: Relative Humidity (%) - Internal relative humidity expressed as a percentage. It represents the amount of water vapor in the air compared to the maximum amount the air can hold at the same temperature.<pressure kPa>: Atmospheric pressure measured in kilopascals. It indicates the pressure exerted by the surrounding air - Optional.<aqi>: Air Quality Index. A calculated indicator used to describe the overall air quality level based on measured pollutants. Higher values indicate poorer air quality.<average mV>: Average input supply voltage expressed in millivolts in the last 1 hour.<min mV>: Minimum measured input supply voltage expressed in millivolts since startup.<min mV>: Maximum measured input supply voltage expressed in millivolts since startup.
$GXGAS: The keyword sent on the beginning of each report. This sentence contains information about the measured ambient gas concentrations. The values represent the concentration of each gas pollutant in micrograms per cubic meter [µg/m³].
<slot1 CO>: CO [µg/m³] Carbon monoxide concentration in ambient air. CO is a colourless and odourless gas mainly produced by incomplete combustion processes, such as vehicle exhaust or fuel burning.<slot2 SO2>: SO₂ [µg/m³] Sulfur dioxide concentration in ambient air. SO₂ is a gas commonly associated with combustion of sulfur-containing fuels and industrial emissions.<slot3 NO>: NO [µg/m³] Nitric oxide concentration in ambient air. NO is a nitrogen oxide typically produced during high-temperature combustion processes, such as traffic and industrial combustion.<slot4 NO2>: NO₂ [µg/m³] Nitrogen dioxide concentration in ambient air. NO₂ is a reactive nitrogen oxide mainly emitted from combustion sources and is commonly used as an indicator of traffic-related air pollution.<slot5 O3>: O₃ [µg/m³] Ozone concentration in ambient air. Ground-level ozone is a secondary pollutant formed by photochemical reactions involving nitrogen oxides and volatile organic compounds under sunlight.
$GXPM: The keyword sent on the beginning of each report. This sentence contains information about the measured ambient particulate matter concentrations. The values represent the mass concentration of airborne particles in micrograms per cubic meter [µg/m³].
<pm less than 1 um>: PM1 [µg/m³] Concentration of particulate matter with an aerodynamic diameter smaller than 1 micrometre. These very fine particles can remain suspended in the air and may penetrate deep into the respiratory system.<pm less than 2.5 um>: PM2.5 [µg/m³] Concentration of particulate matter with an aerodynamic diameter smaller than 2.5 micrometres. PM2.5 is commonly used as an important indicator of fine particle pollution and air quality.<pm less than 10 um>: PM10 [µg/m³] Concentration of particulate matter with an aerodynamic diameter smaller than 10 micrometres. PM10 includes inhalable particles such as dust, pollen, smoke, and other airborne solid or liquid particles.