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mDOS NMEA description

Some sensor data collected by the Medusa gamma-ray spectrometers is broadcast over a serial line in NMEA sentences, similar to the NMEA-0183 protocol. The typical baud rate used for sending these NMEA messages is 115.200 baud. Note that the NMEA output is a subset of the data that is available in JSON format. In the web interface of the mDOS system, a selection can be made from the sentences that are available.

Raw spectrum

The raw spectrum contains integer counts for each measurement channel. The number of channels (resolution) depends on the hardware configuration but is available as a field in the output. Note that this NMEA sentence may be longer than the 82 bytes specified by the NMEA-0183 protocol. The mDOS web interface contains a setting to combine multiple channels in the NMEA output. This allows reducing the number of bytes that have to be transmitted per spectrum. The cost is a reduced resolution in the spectrum. For example, if the spectrometers collects spectra of 512 channels, and 8 channels are combined in the NMEA output, the NMEA spectra will contain 512 / 8 = 64 channels each. This has no effect on the resolution of the spectra that are being stored on-board. Note that the bandwidth of serial communication is limited. In order to get more channels per spectrum, you could increase the baud rate of the serial communication, or set the spectrometer to record spectra at a lower frequency. Note that these settings are considered to be advanced user settings, there is no check if the available bandwidth is sufficient.

Meusa NMEA string example

CODE
$MSSPE,1634718239893,1.001,0.982,64,0,113,574,450,369,259,186,155,83,84,69,70,71,44,41,35,33,47,52,31,23,24,22,14,23,10,23,56,44,13,5,5,10,9,6,4,1,0,1,5,4,3,3,2,3,1,2,1,2,8,8,4,5,2,0,0,0,0,0,0,0,0,0,1*53



#ExampleFormatDescription
0$MSSPE%sSentence label, always $MSSPE
11634718239893%dA unix system timestamp, the number of milliseconds since Jan 1st 1970
21.001%.3fReal time in seconds
30.982%.3fLive time in seconds
464%dResolution of the spectrum (the number of channels)
50%dNumber of counts in the first channel
6 .. 4 + resolution113 .. 1%dNumber of counts in the other channels


Activity concentrations

The mDOS system will analyse the recorded data and returns activity concentrations. Note that energy stabilization is required before activity concentrations can be calculated. Depending on the size of the scintillation crystal, this may take up to a few minutes. After a stabilization has been determined, it is continuously updated until the system is powered down.


Activity concentrations

CODE
$MSACT,1634738741388,1.002,0.983,0.682,1365.484,136.370,136.429,14.500,86.641,19.458*58



#ExampleFormatDescription
0$MSACT%sSentence label, always $MSACT
11634738741388%dA unix system timestamp, the number of milliseconds since Jan 1st 1970
21.002%.3fReal time in seconds
30.983%.3fLive time in seconds
40.682%.3fStabilization parameter. This number is used to translate a raw spectrum into an energy stabilized spectrum.
51365.484%.3fActivity concentration for 40K in Bq/kg
6136.370%3fUncertainty in the activity concentration for 40K in Bq/kg
7136.429%.3fActivity concentration for 238U in Bq/kg
814500%3fUncertainty in the activity concentration for 238U in Bq/kg
986.641%.3fActivity concentration for 232Th in Bq/kg
1019.458%3fUncertainty in the activity concentration for 232Th in Bq/kg


PTH sensor

A PTH sensor is used to monitor the environmental properties air pressure, temperature and humidity.

PTH sensor

CODE
$MSPTH,1634738741389,1007.000,30.000,112.000*47



#ExampleFormatDescription
0$MSPTH%sSentence label, always $MSPTH
11634738741389%dA unix system timestamp, the number of milliseconds since Jan 1st 1970
21007.000%.3fReal time in seconds
330.000%.3fLive time in seconds
4112.000%.3fStabilization parameter. This number is used to translate a raw spectrum into an energy stabilized spectrum.


GPS

The spectrometer can process GPS data from the on-board GPS, or from an external source. The position data is available as an NMEA sentence.

GPS data

CODE
$MSGPS,1634738741389,53.211048,6.612187,2.857,1634636299006*6E



#ExampleFormatDescription
0$MSPTH%sSentence label, always $MSPTH
11634738741389%dA unix system timestamp, the number of milliseconds since Jan 1st 1970
253.211048%.3fGPS latitude
36.612187%.3fGPS longitude
42.857%.3fGPS altitude in meters
51634636299006%dGPS timestamp


Stabilized counts

The number of counts is calculated in certain windows of the stabilized (energy) spectrum. This allows for a temperature independent measurement. To convert counts to count rate, divide the number by the live time.

Stabilized counts

CODE
$MSSC,1634738741389,1.000,672,524,318*0B
#ExampleFormatDescription
0$MSSC%sSentence label, always $MSSC
11634738741389%dA unix system timestamp, the number of milliseconds since Jan 1st 1970
21.000%.3fLive time in seconds
3672%dNumber of counts in the region >= 200 keV
4524%dNumber of counts in the region >= 300 keV
5318%dNumber of counts in the region >= 400 keV


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