Module/Aeroprobe OTF

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Aeroprobe 3mm 5-hole probe
Aeroprobe On-The-Fly! air data computer
OTF! cable


The Aeroprobe On-The-Fly! air data computer measures air pressure from a 5-hole pitot tube and can write resulting data to an SD card or transmit it through an UART. It outputs speed, angle of attack, angle of sideslip and altitude.

Module name sensors/airspeed_otf
Sensor type airspeed, flow angle
Range (speed) 8m/s .. 63m/s
Range (angle) ±20°
Resolution (speed) 0.02m/s
Resolution (angle) 0.01°
Refresh rate 100Hz

User manual


The air data computer needs to be configured by the manufacturer to output data at the desired speed. The serial interface does have original RS-232 levels, that is not compatible with the autopilots 3.3V interface. You need a RS-232 level shifter.

Air data computer/pitot tube supplier: Aeroprobe

RS232 level shifter supplier: Sparkfun

OTF connector supplier: Farnell


Umarim UART1 pin Umarim UART1 Level shifter
3 +3.3V VCC
4 RX1 TX-O
5 TX1 RX-I

Level shifter Sub-D pin Level shifter Sub-D OTF OTF pin
2 RXD RX (data in) 4
3 TXD TX (data out) 3

BEC Power supply OTF OTF pin
+ +VCC 1
- GND 2

Ground usage

This is for the lab only, not for flight. The modem serial port of an Umarim will be occupied by the OTF! compter. Therefore the data is transferred through USB.

File: conf/airframes/myplane.xml
 <firmware name="fixedwing">
    <subsystem name="telemetry" type="transparent_usb"/>

To use it load the airspeed_otf module:

File: conf/airframes/myplane.xml
    <load name="airspeed_otf.xml"/>

The used UART has to be set globally:

File: conf/airframes/myplane.xml
   <target name="ap" board="<my_board_type>">
      <configure name="OTF_UART" value="UART1"/>

Result message

The output is airspeed (velocity), angle of attack (a_attack), angle of sideslip (a_sidesl) and altitude. The data is secured by a packet counter and a checksum.

File: conf/messages.xml
  <message name="WIND_AP_OTF" id="179">
     <field name="counter" type="uint32"/>
     <field name="velocity" type="int16" unit="cm/s" alt_unit="m/s"/>
     <field name="a_attack" type="int16" unit="centideg" alt_unit="deg"/>
     <field name="a_sidesl" type="int16" unit="centideg" alt_unit="deg"/>
     <field name="altitude" type="int32" unit="cm" alt_unit="m"/>
     <field name="checksum" type="uint8"/>

Sample log file lines

11.522 123 FLOW_AP_OTF 1076 1480 628 1636 -100 2
11.535 123 FLOW_AP_OTF 1077 1457 735 1707 900 29
11.548 123 FLOW_AP_OTF 1078 1451 661 1867 800 19
11.555 123 FLOW_AP_OTF 1079 1422 697 1685 -300 11