First, a traditional anemometer is used to measure wind
speed. This unit consists of three cups
mounted on a rotating shaft. The more
wind, the faster the shaft rotates. A
magnet inside the shaft produces electrical impulses, which are measured by the
data logger.
A second type of wind speed sensor is also used. This is an ultrasonic type, model WS-425
from Vaisala. Ultrasonic wind sensors
contain several microphones and speakers which are in the path of the
wind. The speakers produce an
ultrasonic tone, which is carried by the wind to the microphone. If the tone received at the microphone is
higher than the one sent from the speaker, then the wind is toward the
microphone. The technical term for this
phenomenon is called “Doppler Shift”.
We like to use this type of sensor because there are no moving parts to
break in the harsh conditions.
There are two temperature sensors on the weather station,
one close to the ground, and another near the top of the structure.
Data Storage
The sensors are measured every 10 minutes, and the data
are stored in a Campbell Scientific CR10X data logger. Once an hour, the data logger sends the
sensor data to the telemetry unit. The
unit is powered by a bank of lithium batteries. Everything is housed in a rugged, watertight enclosure.
Data Logger Unit
Telemetry
The telemetry unit consists of a Polartronix PX-503
modulator, and a simple VHF transmitter.
Transmitter Enclosure (Receiver shown for scale)
The
radio transmitter is an ordinary voice grade VHF unit, which operates at about
150MHz. The transmitter power is about
1/10 of a watt. (less than an ordinary
cell phone) The receiver for the
weather station is about 100 miles away.
This is quite a long distance to send radio signals of such low
power! To do this reliably requires
special modulation techniques. This is
where the PX-503 comes in. Tones are
sent over the radio to represent data.
For example 1060 Hz is a “1” and 860 Hz is a “0”. By changing these tones, the entire data
word can be sent over the link. This is
generally known as FSK or “Frequency Shift Keying”. In this case the data are sent very slowly, about 10 baud. Another technique that is used to send the
data is something called forward error correction. Essentially, all the data are sent twice, so that any bad or
missing data can be reconstructed at the receiver. These techniques allow us keep the power consumption of the
transmitter to a minimum, while still allowing reliable communications. Even with all this, the batteries are still
the largest part of the system, as you can see from the picture.
PX-503
The
Polartronix PX-503 radio modulator represents the “brains” of the telemetry
system. The PX-503 uses a basic stamp
micro controller to communicate with the data logger and to generate the tones
for the radio. The unit has a solid
state relay to turn on the power for the VHF radio. The Radio is off most of the time, and is only turned on for
several seconds each hour to send the data.
One problem that is encountered at high elevations is the increased
amount of cosmic radiation. (About 40X normal)
These high energy particles can wreak havoc when they pass through the
circuits of the computer chips.
Sometimes when this happens, the computer needs to be power cycled
(turned off then on again). The problem
with this is that there is usually no one on the mountain who can do this! To solve this problem, we have a second
computer chip, which turns on a mechanical relay to cycle the power. The reason for the mechanical relay is that
it is spring loaded, and will supply power to the main processor even if the
secondary processor were to fail during reset.
The PX-503 also contains several other features so that it can be used
as a stand alone instrument package.
For example there is the ability to read analog voltages, as well as
digital inputs. There is also a built
in temperature sensor and battery voltage monitor.
PX-503 Radio Modulator
PX-503
Specs
Components of the receiver system
Building and
Installing the weather station
Although I have been involved in the design and
construction of the Denali weather station for several years, there are many
others who also play an important role.
Working with the folks at Climatec in Japan was really great. They are both knowledgeable and easy to get
along with. In fact, Climatec have
extensive experience in Antarctica so their expertise is very useful for this
project. This picture shows me with Mr.
Kobayashi and Mr. Sasaki at the Climatec facility in Japan.
Denali is a large mountain. From base to the summit is 18,000 ft, which is larger than Everest from
the base to the summit.
The weather station climbing party use the "West Butress" route
In
2004, I climbed Denali with the Japan Alpine club to assist in the
installation. Actually, I made it only
to 15,600 ft. However I was able to
provide assistance with the setup of the weather station at 14,000 ft. Here, team members inspect the equipment
before heading the rest of the way up the mountain for final installation.
From left: Team Leader Yoshitomi Okura, Tohru Saito and
Sosyaku Kanamori
