How to read a constant stream of NMEA http data using ruby - ruby

Question

I have the iPhone app called 'gps2ip', which launches a web server you can visit to get streaming NMEA data.
You can directly connect to this stream using qgis to get an updated location position on your map. I'd like to access this stream programmatically.
If I type in this into my browser url window: http://192.168.1.116:11123 where 192.168.1.116 is the ip of my smartphone as indicated by the gps2ip app
I get a constant stream of newline separated NMEA strings on my safari/chrome/mozilla browser screen, constantly being updated at the bottom with constantly new lines of data.
GPS 2 IP Server started. "exit" to finish.
$GPGGA,005730,3403.415,N,07914.488,W,1,8,0.9,13.6,M,46.9,M,0,2*56
$GPRMC,005730,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*66
$GPGGA,005730,3403.415,N,07914.488,W,1,8,0.9,13.6,M,46.9,M,0,2*56
$GPRMC,005730,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*66
$GPGGA,005731,3403.415,N,07914.488,W,1,8,0.9,13.7,M,46.9,M,0,2*56
$GPRMC,005731,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*67
$GPGGA,005731,3403.415,N,07914.488,W,1,8,0.9,13.7,M,46.9,M,0,2*56
$GPRMC,005731,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*67
$GPGGA,005732,3403.415,N,07914.488,W,1,8,0.9,13.6,M,46.9,M,0,2*54
$GPRMC,005732,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*64
$GPGGA,005732,3403.415,N,07914.488,W,1,8,0.9,13.6,M,46.9,M,0,2*54
$GPRMC,005732,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*64
$GPGGA,005733,3403.415,N,07914.488,W,1,8,0.9,13.5,M,46.9,M,0,2*56
$GPRMC,005733,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*65
$GPGGA,005733,3403.415,N,07914.488,W,1,8,0.9,13.5,M,46.9,M,0,2*56
$GPRMC,005733,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*65
$GPGGA,005734,3403.415,N,07914.488,W,1,8,0.9,13.4,M,46.9,M,0,2*50
$GPRMC,005734,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*62
$GPGGA,005734,3403.415,N,07914.488,W,1,8,0.9,13.4,M,46.9,M,0,2*50
$GPRMC,005734,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*62
$GPGGA,005735,3403.415,N,07914.488,W,1,8,0.9,13.3,M,46.9,M,0,2*56
$GPRMC,005735,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*63
$GPGGA,005735,3403.415,N,07914.488,W,1,8,0.9,13.3,M,46.9,M,0,2*56
$GPRMC,005735,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*63
$GPGGA,005736,3403.415,N,07914.488,W,1,8,0.9,13.2,M,46.9,M,0,2*54
$GPRMC,005736,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*60
$GPGGA,005736,3403.415,N,07914.488,W,1,8,0.9,13.2,M,46.9,M,0,2*54
$GPRMC,005736,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*60
$GPGGA,005737,3403.415,N,07914.488,W,1,8,0.9,13.4,M,46.9,M,0,2*53
$GPRMC,005737,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*61
$GPGGA,005737,3403.415,N,07914.488,W,1,8,0.9,13.4,M,46.9,M,0,2*53
$GPRMC,005737,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*61
$GPGGA,005738,3403.415,N,07914.488,W,1,8,0.9,13.4,M,46.9,M,0,2*5C
$GPRMC,005738,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*6E
$GPGGA,005738,3403.415,N,07914.488,W,1,8,0.9,13.4,M,46.9,M,0,2*5C
$GPRMC,005738,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*6E
$GPGGA,005739,3403.415,N,07914.488,W,1,8,0.9,13.4,M,46.9,M,0,2*5D
$GPRMC,005739,A,3403.415,N,07914.488,W,0.00,,120115,003.1,W*6F
$GPGGA,005739,3403.415,N,07914.488,W,1,8,0.9,13.4,M,46.9,M,0,2*5D
I know how to parse these lines of NMEA code into latitude/longitude pairs, I just need to be able to access "the last one" easily inside a ruby environment.
I want to be able to pluck the last one or two lines and parse the NMEA strings manually, but I haven't figured out a way to "sip from the firehose" of data without generating an error message.
When I try this:
require 'open-uri'
open("http://192.168.1.116:11123")
I get this error:
Net::HTTPBadResponse: wrong status line: "GPS 2 IP Server started. \"exit\" to finish."
Where "GPS 2 IP Server Started. 'exit' to finish." is of course the first line of the response.
What ruby gem should I use to sip from this firehose of data? Apparently open-uri wants html headers and my stream has none of that. I just need to stream pure text apparently.

Answer 1

Since this isn't the HTTP protocol, you'll need the more generic Socket. You can do something like this:
require 'socket'
s = TCPSocket.new '192.168.1.116', 11123
while line = s.gets
puts line
end
s.close
Depending on how fast the data arrives and how long it takes to process each line, you may need to investigate putting each line into a queue such as Sidekiq so that multiple workers can process lines simultaneously.

Answer 2

Since what you have is a never-ending stream of data, you can't just grab the last message. You must consume the stream, and decide when you've received enough to start your processing. You could probably take advantage of the GPRMC message to do that.
In nmea_plus (full disclosure, I wrote it), you could do it this way:
require 'socket'
require 'nmea_plus'
allowable_delay = 3 # threshold to consider the stream as fresh
caught_up = false # flag to say whether we've crossed the threshold
io_source = TCPSocket.new('192.168.1.116', 11123)
source_decoder = NMEAPlus::SourceDecoder.new(io_source)
source_decoder.each_message do |msg|
case msg.data_type
when 'GPRMC'
if Time.now - msg.utc_time < allowable_delay
caught_up = true
end
when 'GPGLL'
if caught_up
puts "Fix: #{msg.latitude}, #{msg.longitude}"
end
end
end
By default, the decoder ignores any lines that don't parse as NMEA.