17 Sept 2008
- Integration of PTH Sondes into the NexSTAR UA
- Flight test of PTH Sondes
Two NCAR supplied radio sondes were integrated into the wings of a Hobbico NexSTAR aircraft that has been modified for autonomous flight and participation in an ad-hoc network. This deployment is a proof-of-concept system to demonstrate the feasibility of the use of UA as in-situ atmospheric sensing platforms.
The flight on 09-17-2008 was conducted at the Table Mountain radio quiet site north of Boulder, CO. The data from the flight was somewhat incomplete due to issues with the service discovery mechanism, but still provided some valuable information.
One of the sonde's GPS unit was identified to be non-functioning during integration (interface over the serial port was possible, but the device never tracked any satellites). To correct this error, the most recent data from the other sonde was assumed for its GPS information. Given the position solution was calculated at 2Hz, the error created by this method should be a spacial difference of at most 4.5m, but is likely to be less. The same sonde's humidity sensors didn't seem to be as well calibrated as the other sonde.
NexSTAR-1 Flight 1
Flight Time: 12.75 mins
Autonomous Flight Time: 10.63 mins
The experimental data from the pressure, temperature, and humidity are shown in the graphs (click for a higher resolution version).
One disturbing result from the analysis of the data is the time it takes for the sensors to become stable after takeoff. Notice how the internal board temperature takes nearly half the flight to reach a steady state and this seems to have had a significant affect on the pressure data. The humidity measurement might have been affected by this as well. The sensors are located in a position so as to be minimally affected by the board temperature, but it is likely that the conditioning performed by the onboard microprocessor was affected by the temperature and pressure readings.
Electronic integration required a 5V supply, TTL to RS232 level conversion, and the addition of serial ports to the Soekris net4526 board. Adding serial ports was done by adding a Mini PCI card from Aaeon, the PER-C40C, and compiling in the linux kernel module to support the card. Furthermore, the number of allowed serial devices by the kernel had to be upped to 7.
The TTL level output from the sonde also had to be converted to RS232 levels. This was performed using a module from Acronname, the S13-SERIAL-INT_CONN. The db9 connectors both on the level converter and the cables from the Mini PCI card were quite heavy, so a quick modification was made by cutting the connectors off either side and soldering the ribbon cables directly to the level converter pads. For further power savings, the two LEDs on the level converter boards can be unsoldered. The extender cables were attached to the level converter boards and they were encased in heat shrink.
To provide power and allow for connection to the wings, a simple board was constructed that contained a 5V regulator and wiring with a dean's connection for input power. The extender cables were soldered to the board and connected to power and ground. Another connector was created using a 10-pin Molex MicroFit 3.0 mm clip connector and the wires were connected to the four TTL UART inputs from the extender cables, and to power and ground.
On the wing side the other half of the Molex connector was wired to ribbon cables that were run through the wings for connection to the two sondes.
Using the covers on the wings provided for aerodynamic stability, we were able to easily create a hatch for containing a sonde in each of the wingtips.
The GPS antenna are attached to the underside of the top surface of the wing. The probe has been stabilized using a small strip of balsa.