Uploading APRS/CWOP weather data

Radio Amateurs built up a network called Automatic Position Reporting System (APRS), where they show positions of stationary, mobile and portable radio stations on a map. The station reports are normally transmitted by radio transmitters, but they can also be transmitted over the internet. These reports can also contain additional data like weather reports or telemetry data. All details about APRS can be found in the APRS Protocol Reference.

Normally, it is only allowed for licensed radio amateurs to transmit APRS data that will show up on the APRS map. But there is also a public-private partnership called Citizen Weather Observer Program (CWOP), which stations are also shown on the APRS map.

If you sign up for CWOP, you will get a station ID and you will be able to upload your weather data, so that your weather station will show up on aprs.fi, together with your station ID. On the Raspberry Pi, it offers itself to us a Python script for uploading APRS/CWOP data. There are a few little pitfalls within the APRS protocol which I want to mention. First, United States customary units are used which can be a bit confusing, if you are not familiar with it.

The other thing is the format of the coordinates. Although it contains a dot, the values are supposed to be in degrees, minutes and seconds. If you keep this in mind, should should be able to use the script below to successfully transmit APRS/CWOP data over the internet. Only thing you have to do is to add your station ID, your location and your weather data.

If you want to use the script as a licensed radio amateur, you just have to change the name and the port of the server (to one of the APRS network ones) and to use your call sign instead of the station ID. And of course, you have to provide your APRS password (instead of -1 in the script).

#!/usr/bin/env python

import sys, os, time
import rrdtool
from datetime import datetime, timedelta
from socket import *

serverHost = 'cwop.aprs.net'
serverPort = 14580
address = 'YOURSTATIOIDHERE>APRS,TCPIP*:'
# Attention, format of the location is bit special. Although there is a dot, the values are in degrees, minutes and seconds!
position = 'DDMM.SSN/DDDMM.SSE_'

def send_packet():
	# Get the values from the database
	weather = rrdtool.fetch("./../weather_info/weather.rrd", 'AVERAGE', '-r 720', '-s -720')
	press = rrdtool.fetch("./../pressure_info/pressure.rrd", 'AVERAGE', '-r 600', '-s -600')
	# Attention, temperature in Fahrenheit!
	fahrenheit = 9.0/5.0 * weather[2][0][0] + 32
	humidity = weather[2][0][1]
	# Attention, barometric pressure in tenths of millibars/tenths of hPascal!
	pressure = press[2][0][1] * 10

	# If you have wind and rain data, get it here. Be aware that values are required in mph and in hundredths of an inch!
	wind_degrees = ...
	wind_mph = ...
	wind_gust_mph = ...
	precip_1hr_in = ...
	precip_today_in = ...

	# Prepare the data, which will be sent
	wx_data = make_aprs_wx(int(float(wind_degrees)),float(wind_mph),float(wind_gust_mph),fahrenheit,f_precip_1hr_in,None,(float(precip_today_in)*100),humidity,pressure)
	# Use UTC
	utc_datetime = datetime.now()
	# Create socket and connect to server
	sSock = socket(AF_INET, SOCK_STREAM)
	sSock.connect((serverHost, serverPort))
	# Log on
	sSock.send('user YOURSTATIOIDHERE pass -1 vers Python\n')
	# Send packet
	sSock.send(address + '@' + utc_datetime.strftime("%d%H%M") + 'z' + position + wx_data + 'Raspberry Pi\n')
	# Close socket, must be closed to avoid buffer overflow
	sSock.shutdown(0)
	sSock.close()

def make_aprs_wx(wind_dir=None, wind_speed=None, wind_gust=None, temperature=None, rain_last_hr=None, rain_last_24_hrs=None, rain_since_midnight=None, humidity=None, pressure=None):
	# Assemble the weather data of the APRS packet
	def str_or_dots(number, length):
		# If parameter is None, fill with dots, otherwise pad with zero
		if number is None:
			return '.'*length
		else:
			format_type = {
				'int': 'd',
				'float': '.0f',
			}[type(number).__name__]
			return ''.join(('%0',str(length),format_type)) % number
	return '%s/%sg%st%sr%sp%sP%sh%sb%s' % (
		str_or_dots(wind_dir, 3),
		str_or_dots(wind_speed, 3),
		str_or_dots(wind_gust, 3),
		str_or_dots(temperature, 3),
		str_or_dots(rain_last_hr, 3),
		str_or_dots(rain_last_24_hrs, 3),
		str_or_dots(rain_since_midnight, 3),
		str_or_dots(humidity, 2),
		str_or_dots(pressure, 5),
	)

try:
	send_packet()
except:
	sys.exit(-1)