Alan Nelson's Daily Commentary for 27 August 1999

Kwajalein (Day 9), Republic of Marshall Islands

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I had promised you a better picture of the DC-8 airplane. Here it is. The little hill behind the airplane, where the antenna is sitting, is "Mt. Olympus" the highest point on the island. Doesn't amount to much as hills go, but it would rival the highest point in the Marshall Islands which is only 34 feet above sea level!

I had some more training on radiosondes today. I didn't really learn very much new, but today was my day to DO things rather than to watch or be helped. I am ready to head out to the island of Meck tomorrow (boat leaves at 6:05 AM). It is a slightly different setup at Meck than here at Kwajalein so any more practice here at Kwaj would hardly be worth it.

I thought I would show you some pictures of these sondes. Here is a picture of a sonde as it comes out of the sealed package. The battery and humidity sensor are packed in airtight containers. The temperature sensor is packed in it's own little box because it is extremely fragile. Two wires of different metals are welded together to form a "thermocouple". The two different metals generate a tiny amount of electric current when welded together. Slightly more current when warm than when cold. Those properties are sufficiently well known that the amount of current generated can be used to determine the temperature of the bimetal weld. These wires are barely thicker than a human hair!

The humidity sensor changes resistance as the humidity changes. For this type of sensor, a known voltage is passed through the sensor. If the resistance is higher, less current flows through the sensor. Lower resistance means more current flows. By measuring the amount of current, we can calculate the amount of humidity.

Every sonde with its sensors is unique and has been calibrated at the factory under a variety of conditions. All the sensors are either generating current (like the temperature sensor) or changing resistance (like the humidity sensor). The equations that need to be used to calculate temperature or humidity are also unique to each sensor. So all that information about the specific sensors for this sonde are contained on that blue calibration tape. By running the tape through a tape reader, all the equations are fed into the computer receiving the information from the sonde.

These particular sondes are almost considered primitive by today's standards. I have used computers almost from their earliest days, but this is the first time that I have used paper tape. It turns out that all the antennas around Kwajalein would interfere with most modern sondes and vice versa (the sondes would interfere with the Army's antennas). These old sondes are using communication frequencies that don't interfere. In fact, we need to tune each one down to a frequency that won't interfere.

In addition to my first use of paper tape, I also had my first work experience where playing Nintendo games helped! I have mentioned before that the sonde is attached to a helium balloon and then released to rise through the atmosphere. The sonde sends back the information that the sensors are measuring during its ascent.

The antenna on the ground has to be pointed at the sonde in order to get a good clear signal. The antenna automatically follows the sonde once they get "locked on" or pointed right at the sonde. This tracking by the antenna also tells us where the balloon is located. That information is stored every two seconds as the balloon ascends. The wind speed and direction are calculated by comparing each current position with the previous position. All is great once you get locked on. But for the first few seconds after release, the antenna needs some help. You need to point the antenna right at the sonde using the joystick at the tip of the arrow in this picture. The antenna behaves just like the cars in Mario Kart! If you turn it right it keeps going for a fraction of a second after you quit turning it. My sons can stomp me at Mario Kart (I've only played it once). Today I really wanted to turn the tracking of the balloon over to them. Eventually I got the hang of it. The experienced radiosonde people are much better at it than I am.

Our sonde today reached over 100,000 feet in the atmosphere. At that height the air pressure was only 12 millibars. A bar is essentially the average air pressure at sea level. One bar is also equivalent to 1,000 millibars. So that poor helium balloon started out today on the ground at +28 degrees Celsius. It then climbed to 100,000 feet where the temperature was -70 degrees Celsius and the pressure was barely 1% of that at the ground. The balloon starts out at the ground about four feet in diameter. By the time it gets to 100,000 feet it is about 30 to 40 feet in diameter! So it remains flexible after being stretched that much and being chilled down to -70 degrees. I'm impressed!

The receiving station is still experiencing some problems. I will be going to Meck tomorrow in the hopes that we can get some sonde data. Keep your fingers crossed!

Alan Nelson