LUA Export NMEA AHRS & GPS over TCP/IP

Background

• It is possible to export ownship data from DCS via LUA
  
• There are a number of Glass Cockpit apps for iOS and Android that accept NMEA data over TCP/IP
  

Goal

Create code for export.lua that extracts relevant ownship data, formats it as NMEA sentences, and sends it over TCP/IP to a Glass Cockpit app on a mobile device

Requirements

• Exctract ownship data from DCS
  
• Format ownship data as NMEA sentences for AHRS and GPS
  
• Transmit data over TCP/IP to a mobile device running client software
  
• Alternatively/additionally: transmit data via UDP
  

Resources

1. AHRS Devices

Levil have AHRS devices that sends data to iOS and Android devices via TCP/IP over WLAN:

Wireless Transmission

 

Some mobile devices that do not have a USB or serial port, require wireless communication to

the AHRS (i.e iPad/iPhones). The iLevil and AHRS-G mini series have an embedded wireless

transmitter that works as the router at home and creates a WiFi access point to which your tablet will link to. Once connected, navigation programs can then telnet to the transmitter using either

TCP or UDP transmission:

 

- TCP/IP connection is very robust and requires the remote device to confirm receipt of data

every time the AHRS sends a package. Although TCP connection guarantees no data is lost on

the way, it may cause some latency on the transmission if either the sender or the receiver is not

properly responding in a timely manner. For example, iPad devices have shown latency when

you move the device around because it is trying to figure out the screen rotation. You can also

expect some latency if you are inside a building with multiple Wi-Fi access points or if you are

accidentally blocking the iPad transmitter with your hand. The best way to test the AHRS

transmission is to fly with it. TCP can only be used by one application at a time.

 

- UDP transmission is not as robust as TCP, but allows multiple devices to access the same data

simultaneously. If you’d like to use two iPads, for example, you might want to use the device in

UDP mode. UDP is the default mode and is recommended for in-flight use.

InertialLabs make and AHRS that does the same, and we can see via their AHRS documentation (pp. 21-22) that NMEA has a proprietry sentence for AHRS data ($PAHR):

At the “NMEA Output” the AHRS output data are transmitted in the form of sentences with printable ASCII characters like the NMEA 0183 format. Each sentence starts with a "$" sign and ends with <CR><LF> (carriage return 0xD and line feed 0xA symbols). All data fields are separated by commas.

 

The general form of the “NMEA Output” sentence is the text

PAHR,RRRR.rr,PPP.pp,HHH.hh,TTT.t,V.vv,SSSS*CC<CR><LF>

where PAHR is identifier and other fields are listed in the Table 6.7.

 

Тable 6.7.The AHRS message in NMEA format

(at NMEAcont or NMEAreq command)

Field RRRR.rr PPP.pp HHH.hh TTT.t V.vv SSSS CC

Parameter Roll Pitch Heading Temperature Vdd USW Checksum

Note deg deg deg ºC VDC Hex*

 

*Hex written in ASCII; AHRS state information (Ready/Sleeping, Errors, BIT, etc)

 

Notes:

1. USW is unit status word (see section 6.5 pp. 43-44 for details).

2. Temperature is averaged value for 3 accelerometers.

3. Vdd is input voltage of the AHRS.

4. Check sum consists of a "*" and two hex digits representing XOR of all characters

between, but not including "$" and "*".

Important note: The AHRS maximum data rate is limited to 50 Hz in the NMEA output

format

2. NMEA Sentences

Although the NMEA standard costs $$$ to obtain, there are a number of resources that explain some of the sentences

For example: http://aprs.gids.nl/nmea/

   $GPBOD - Bearing, origin to destination
  $GPBWC - Bearing and distance to waypoint, great circle
  $GPGGA - Global Positioning System Fix Data
  $GPGLL - Geographic position, latitude / longitude
  $GPGSA - GPS DOP and active satellites 
  $GPGSV - GPS Satellites in view
  $GPHDT - Heading, True
  $GPR00 - List of waypoints in currently active route
  $GPRMA - Recommended minimum specific Loran-C data
  $GPRMB - Recommended minimum navigation info
  $GPRMC - Recommended minimum specific GPS/Transit data
  $GPRTE - Routes
  $GPTRF - Transit Fix Data
  $GPSTN - Multiple Data ID
  $GPVBW - Dual Ground / Water Speed
  $GPVTG - Track made good and ground speed
  $GPWPL - Waypoint location
  $GPXTE - Cross-track error, Measured
  $GPZDA - Date & Time

3. Client Applications

Air Navigation Pro

Air Navigation is a flight planning and real-time aircraft navigation application with 2D moving map and 3D synthetic vision for iPhone, iPad and Android devices. A Desktop version for Mac OS X is also available. Air Navigation Apps support free charts of almost the entire world as well as commercial aviation VFR charts for many countries. The application costs a fraction of the price of a dedicated portable aviation GPS and will help you to plan your flight, saving you time and money.

 

Air Navigation can be connected to the X-Plane or FSX flight simulators by installing a plugin on your favorite simulator. This is a great feature for training at home when bad weather prevents real flying! Also, simulator enthusiasts can add a full moving map system to their Sim setup. Currently we have an X-Plane 9/10 plugin for Windows and Mac OS X and and FSX plugin for Windows.

List of Other Client Apps: http://aviation.levil.com/compatible-apps.html

4. DCS export.lua Variables

Get the relevant paramaters:

LoGetAltitudeAboveSeaLevel()
LoGetADIPitchBankYaw()
LoGetVerticalVelocity()
LoGetTrueAirSpeed()
MyPlane.LatLongAlt.Lat
MyPlane.LatLongAlt.Long

Format them for NMEA:

local t = LoGetModelTime()
local altBar = LoGetAltitudeAboveSeaLevel()
local pitch, bank, yaw = LoGetADIPitchBankYaw()
local vertvel = LoGetVerticalVelocity()
local trueairspeed = LoGetTrueAirSpeed()
local LatPos = MyPlane.LatLongAlt.Lat
local LongPos = MyPlane.LatLongAlt.Long
local av = LoGetAngularVelocity()

Send as NMEA sentences:

$PAHR - AHRS info
$GPGLL - Geographic position, latitude / longitude

I have also found this script that is used to export UH-1 telemetry to an Android app. It looks like I could use this code as the bones of an NMEA export script over UDP:

---------------------------------------------------------------------------------------------------
-- Export UH-1H instruments
---------------------------------------------------------------------------------------------------

f_uh1h =
{
Start=function(self) 
package.path = package.path..";.\\LuaSocket\\?.lua"
package.cpath = package.cpath..";.\\LuaSocket\\?.dll"
socket = require("socket")

my_init = socket.protect(function()	
	-- export telemetry to instrumeny panel on android
	host2, port2 = "10.0.0.3", 6000 -- replace IP with android device ip 
	udp = socket.try(socket.udp())	
end)
my_init()	
end,

AfterNextFrame=function(self)
   --local player = LoGetObjectById(LoGetPlayerPlaneId())
--print(player.Name)
--if (player and player.Name == "UH-1H") then
	-- read from UH-1H main panel instruments
	local MainPanel = GetDevice(0)
	local AirspeedNeedle = MainPanel:get_argument_value(117)*150
	local Altimeter_10000_footPtr = MainPanel:get_argument_value(178)*100000
	local Altimeter_1000_footPtr = MainPanel:get_argument_value(179)*10000
	local Altimeter_100_footPtr = MainPanel:get_argument_value(180)*1000
	local Variometer = MainPanel:get_argument_value(134)*4000
	local TurnNeedle = MainPanel:get_argument_value(132)*math.rad(1.5)
	local Slipball = MainPanel:get_argument_value(133)
	local CoursePointer1 = MainPanel:get_argument_value(159) * 2.0 * math.pi
	local CoursePointer2 = MainPanel:get_argument_value(160) * 2.0 * math.pi
	local CompassHeading = MainPanel:get_argument_value(165) * 2.0 * math.pi
	local Torque = MainPanel:get_argument_value(124) * 103 + -3
	local Engine_RPM = MainPanel:get_argument_value(122)*7200
	local AHorizon_Pitch = MainPanel:get_argument_value(143) * math.pi / 2.0
	local AHorizon_Bank = MainPanel:get_argument_value(142) * math.pi
	local AHorizon_PitchShift = 0
	local GyroHeading = MainPanel:get_argument_value(165) * 2.0 * math.pi
	local Oil_Temperature = MainPanel:get_argument_value(114) * 150
	local Oil_Pressure = MainPanel:get_argument_value(113) * 100
	local Fuel_Pressure = MainPanel:get_argument_value(126) * 50
	local Fuel_Tank = MainPanel:get_argument_value(239) * 1580.0 / 100.0
	local VerticalBar = MainPanel:get_argument_value(151) * -2.0
	local HorisontalBar = MainPanel:get_argument_value(152) * -2.0
	local ToMarker = MainPanel:get_argument_value(153)
	local FromMarker = MainPanel:get_argument_value(154)		
	local RotCourseCard = MainPanel:get_argument_value(156) * 2.0 * math.pi
	
	my_send = socket.protect(function()
		local json = string.format("{ 'AirspeedNeedle':%.2f, 'Altimeter_10000_footPtr':%.2f, 'Altimeter_1000_footPtr':%.2f, 'Altimeter_100_footPtr':%.2f, 'Variometer':%.2f, 'TurnNeedle':%.2f, 'Slipball':%.2f, 'CoursePointer1':%.2f, 'CoursePointer2':%.2f, 'CompassHeading':%.2f, 'Torque':%.2f, 'Engine_RPM':%.2f, 'AHorizon_Pitch':%.2f, 'AHorizon_Bank':%.2f, 'GyroHeading':%.2f, 'Oil_Temperature':%.2f, 'Oil_Pressure':%.2f, 'Fuel_Pressure':%.2f, 'Fuel_Tank':%.2f,'VerticalBar':%.2f,'HorisontalBar':%.2f,'ToMarker':%.2f,'FromMarker':%.2f,'RotCourseCard':%.2f}\n", 
			AirspeedNeedle, 
			Altimeter_10000_footPtr, 
			Altimeter_1000_footPtr, 
			Altimeter_100_footPtr, 
			Variometer, 
			TurnNeedle, 
			Slipball, 
			CoursePointer1, 
			CoursePointer2, 
			CompassHeading, 
			Torque, 
			Engine_RPM, 
			AHorizon_Pitch, 
			AHorizon_Bank, 
			GyroHeading, 
			Oil_Temperature, 
			Oil_Pressure, 
			Fuel_Pressure, 
			Fuel_Tank,
			VerticalBar,
			HorisontalBar,
			ToMarker,
			FromMarker,
			RotCourseCard
		)
		--print(json)	
		socket.try(udp:sendto(json, host2, port2))
	end) -- my_send
	my_send()
--end -- if UH-1H	
end,


Stop=function(self)
my_close = socket.protect(function()
	socket.try(udp:close())
end)
my_close()
end
}

-- =============
-- Overload
-- =============
do
local PrevLuaExportStart=LuaExportStart
LuaExportStart=function()
	f_uh1h:Start()
	if PrevLuaExportStart then
		PrevLuaExportStart()
	end
end
end

do
local PrevLuaExportAfterNextFrame=LuaExportAfterNextFrame
LuaExportAfterNextFrame=function()
	f_uh1h:AfterNextFrame()
	if PrevLuaExportAfterNextFrame then
		PrevLuaExportAfterNextFrame()
	end
end
end

do
local PrevLuaExportStop=LuaExportStop
LuaExportStop=function()
	f_uh1h:Stop()
	if PrevLuaExportStop then
		PrevLuaExportStop()
	end
end
end