National Radio Astronomy Observatory
Site Studies

Radiosonde Flight Notes

Radiotheodolite setup

  1. The radiotheodolite manual has detailed instructions.
  2. When repacking, leave the antenna arms attached to the pedestal.
  3. Orient the pedestal and initialize the azimuth and elevation as described in the manual.
  4. The decoder box is newer than the radiotheodolite, so the intercabling is undocumented. The radiotheodolite I/O jack (J2) and the decoder box data I/O jack are connected with a multiconductor cable (SA-106336). There's also an extension cable for this connection. The decoder box RT data and PTU data jacks are connected to the com ports on the Austin laptop computer. It doesn't matter which com port is connected to which data jack.
  5. The main radiotheodolite cable (black Y-cable) connecting the pedestal to the panel is only long enough for one 360 degree azimuth wrap. Take the upper atmosphere wind direction into account when setting up the radiotheodolite!
  6. Connect an external monitor to the computer. This is optional, but makes it much easier to see the displays.
  7. The equipment has been verified with 110 VAC, 60 Hz electrical power. Take care before using any other source.

Balloon inflation

  1. Install regulator on helium tank and open tank valve.
  2. Open inflation (tilt) valve and purge supply hose.
  3. The valve is weighted to balance a net lift of 300 g, which should give an ascent rate of about 5 m s-1 (1000 ft min-1; see below).
  4. Insert the inflation valve into the balloon neck and fill the balloon to neutral buoyancy when weighted by the valve assembly and about 60 cm of hose. Caution: when the balloon is flaccid, the canopy can be punctured by the gas flow from the inflation valve.
  5. Tie off balloon neck with string.
  6. Close tank valve.

Startup, calibration, and flight

  1. Boot the computer and choose option 8 (old version of MS-DOS) from the start up menu. The radiosonde software will start automatically. Turn on radiotheodolite and decoder box.
  2. From the main menu, select run flight program. The program automatically selects and sets up the communications ports.
  3. In the station and flight data initialization screen, enter station 2, altitude 5000 m. Leave other parameters to defaults. Press F10 to continue to the next screen.

    4. When the sensor verification screen is displayed, unpack a sonde, deploy the antenna and sensors, and turn it on by installing the jumper on the bottom. Place it outside near the radiotheodolite. If necessary, adjust the radiotheodolite frequency with the hand paddle to lock onto the signal (see manual). The sonde frequency can be adjusted with a small screwdriver through a hole on the bottom. Turn the sonde off and on again as necessary to record the calibration data. The sonde uses three standard 9 V alkaline batteries, should it ever be necessary to replace them.
  4. After those data are recorded, press F10 to continue to the next screen. Don't enter surface values. Make sure the radiotheodolite has autotrack and AFC enabled on the hand paddle. Attach the sonde to the balloon with 2 m of string.
  5. From the prelaunch sensor data screen, the program will automatically detect the launch or you can initiate data acquisition by pressing F10.
  6. Make sure the radiotheodolite is tracking the sonde and release the balloon about 30 m away from the radiotheodolite..
  7. Some time after launch, the program will display data in the text screen. Pull down the graphics menu for more useful displays. Ignore the repeated requests for surface data.
  8. The radiotheodolite should track the balloon to burst at about 20 km altitude. In good weather, the balloon can be seen in the boresight telescope until burst. The program may automatically detect balloon burst. Then it may be possible (and interesting) to track the descent. The humidity sensor has, however, a substantial time constant. Use the quit menu to terminate data acquisition and then quit from the main menu to DOS.
  9. Reboot the computer to Win95 (option 1, default) to examine data, etc.

Balloon ascent rate

The balance between the balloon's buoyant lift and its aerodynamic drag and the balloon and package weight determines the ascent rate,
L = D + W ,
(4p/3) r3 (rair - rHe) g = Cd p r2 rair v2/2 + (Msonde + Mballoon) g .
Here r is the balloon radius, the sea level densities of air and helium, rair = 1.225 kg m-3 and rHe = 0.169 kg m-3, the gravitational acceleration g = 9.8 m s-2, v is the ascent rate, the sonde mass Msonde = 250 g, and the balloon mass Mballoon = 100 g. For a flow of 5 m s-1 and a characteristic length of 1 m, the Reynolds number R = vl/n » 3 × 105, where the kinematic viscosity of air n = 1.46 × 105 m2 s-1. This is just the critical Reynolds number for the onset of a turbulent boundary layer when the drag coefficient Cd drops abruptly from » 0.4 to » 0.1. Hence the ascent rate cannot be calculated precisely, but Cd = 0.2 is adequate for estimation. Inasmuch as air and helium are perfect gases, the buoyant lift of a sealed balloon is independent of altitude. The drag is a weakly decreasing function of altitude, so the ascent rate slowly increases with altitude. At sea level, a net lift of 300 g inflates a balloon to about 1 m diameter and the approximate initial ascent rate is 5.2 m s-1 (1030 ft min-1). At 5000 m altitude, the same lift will inflate the balloon to 1.26 m diameter and the ascent rate will be about 5.7 m s-1.

Reference: Daily, J. W., & Harleman, D. R. F., 1966, Fluid Dynamics (Addison-Wesley)

Hardware weights

sonde
250 g
balloon
100g
tilt valve with pipe fitting
69 g
small hose clamp
5.5 g
larger hose clamp
6.5 g
3/8" clear vinyl tube
0.75 g cm-1
3/8" reinforced vinyl tube
1.65 g cm-1
3/4"-10 steel nut
50 g
3/4" flat washer
13.5 g

Software note

If ever necessary, reinstall the software from the AIR diskette following the directions in the manual. Reboot and choose option 8 (old version of MS-DOS) from the start up menu before doing this. Choose the color EGA/VGA CRT video drivers, not the monochrome LCD.
Last modified 1998 September 28
Simon Radford