Remote Data Transmission (Proceedings of the Vancouver Workshop, August 1987). IAHS Publ. no. 178, 1989.
Use of data collection platforms for snow water equivalent measurement
and other parameters
HEINZ G. MARTENS L.
Universidad de Chile
Centro de Estudios Espaciales
Division NASA
Arturo Prat 1171, Casilla 411-3
Santiago 6981703 NASA-Colina
Chile
ABSTRACT
This paper contains a brief description of the snow water equivalent and
hydrometeorological parameter measurement in the Chilean Andes using Landsat
and GOES data collection platforms DCP's.
After a short introduction of the history of the development of the DCS
system in Chile, some remote stations as well as the receive sites are
described. Finally, the results obtained over 5 years of operation and the
main failures experienced are presented.
INTRODUCTION
The use of satellites to obtain hydrometeorological data in Chile began at the
end of 1977. At that time the Direocion General de Aguas (a governmental
organization responsible for managing all water resources in Chile), Endesa
(electrical power generating enterprise) and the University of Chile's Center
for Space Studies, agreed to a joint venture to determine the feasibility of
collecting snow water equivalent and meteorological data in the Chilean Andes
and to transmit them to a receiving station using a satellite as a relay.
Landsat was used during this experimental phase to relay the data to a
tracking station operated by the University of Chile for NASA, which is
located h0 km north of Santiago. Landsat, a polar-orbiting satellite, is in
view of the receiving antennas two times each day at 12 hour intervals for 8
to 12 minutes during each orbit, depending on its inclination.
NASA permitted the University to receive and process the data on a noninterference basis to NASA projects. The system was mainly composed of NASA
equipment used in DCS activities.
The Data Collection Platforms (DCP's) were two loaned by the USGS, which
were first installed close to meteorological stations in order to obtain data
comparison. Depending upon the results, they will be relocated.
This test period lasted two years.
It demonstrated successfully that
sensors and electronics survived snow, ice and temperature cycles experienced
in the Andes. Next, the DCP's were relocated close to snow routes in order
to evaluate if they could be replaced by DCP's.
Two aspects were considered: Reliability of the system and accuracy of the
measurements. The advantage of this method would be continuous measurement
of the parameters, almost real-time data, both of prime importance during long
lasting snow storms and the melting season.
The present method consists in a monthly in-situ measurement of the snow
route.
Although all transmission were received correctly, the system did have
severe limitations:
45
Heinz G. Martens L.
46
(a) Part of the equipment involved was not exclusively dedicated to
receive and process data from remote sites.
(b) Data could only be recovered twice a day.
(c) The amount of data, (measurement interval) was limited by the time of
mutual visibility of transmitting and receiving antenna, (DCPsatellite-receiving station).
The principal objective of its test demonstrated that air temperature,
relative humidity, solar radiation, wind velocity and snow water equivalent
can be collected by remote telemetry. Experience was also gained regarding
the behavior of sensors and electronics under the changing conditions
prevailing in the Andes.
Five data collection platforms were deployed based on the considerations
indicated above and next to the existing snow routes. Two more were installed
several years later for measuring meteorological data: one in the north,
close to the Bolivian border at 5,600 m above sea level and one in the south,
next to Lake Dickson.
THE PRESENT SYSTEM
The next step was the evaluation of the GOES satellite as an active repeater
using a 15 ft antenna available at the receive site. Using GOES would not
only increase the amount of data available but would also make the data
available in almost real-time.
Once it was determined that the receiving capability operated correctly,
a microprocessor to decode and store the data was designed and built. The
processing system is capable of converting the data to engineering units under
operator control or transmitting the engineering units over a microwave link
and telephone line to the user's office.
A system like the one described makes a continuous measurement of
hydrometeorological parameters possible. This cannot be done by conventional
methods as the areas of interest are uninhabited, mainly during winter.
To increase the probability of receiving the data correctly, the same set
of values is transmitted 3 times (triple redundancy), thus an average of 95?
of all transmissions are received during the evaluation period.
Receive Site
The signals are received by a 15 ft dish which has 35 db gain, amplified by
low-noise preamplifiers with 28 db gain and 1.2 db NF. Downeonversion,
demodulation and synchronizations of the data pulses are conventional. At this
point the signal (data and clock) is fed to the microprocessor input port to
be recorded on a floppy disk and is simultaneously recorded on magnetic tape
as a back-up. Further processing is done once a week under operator control
as both users, Endesa and DGA require the data for statistic and runoff
forecasts. Reception processing and transmissions over a microwave link and
telephone line is also possible.
Remote Stations
The 5 hydrometeorological remote stations which measure mainly snow water
equivalent are all deployed in the Andes between 30 dg. SL and 36 dg. SL
(figure 1). Four of them are at 3>500 m above sea level and the fifth one at
2,200 m.
Use of data collection platforms for snow water equivalent measurement
47
1
DCP
CERRO OLIVARES
DCP
CERRO VEGA NEGRA
Pv> LAS RAMADAS
DCP ELSOLDADO
DCP
GLACIAR
ECHAURREN
PV MAITENES
PV CIPRESES
DCP
LO AGUIRRE
—36'
MIT
FIG. 1
Reference Map
All of the stations are next or close to existing snow routes, since these
areas are representative for the basin. In-situ snow measurements render the
data required to evaluate the automatic measurement.
One site evaluated in the study is Lo Aguirre. Its layout is shown in
figure 2. At this location the normal snow accumulation during the winter
season in 6 m to 8 m. Some years it is even more.
There is a high probability that ice layers will "bridge" the snow pillows
and thus prevent the pressure of the water in the fresh snow from being
sensed. To minimize the probability of this bridging effect to occur, four
snow pillows were connected in parallel as shown in figure 3- No negative
effects have been detected.
Heinz G. Martens L.
48
LO AGUIRRE T Y P I C A L SNOW
PILLOW
SCHEMATIC
FIG. 2 Typical snow pillow i n s t a l l a t i o n a t Lo Aguirre
All but two remote s t a t i o n s have sensors to measure:
(a) snow water equivalent
(b) solar radiation
(c) relative humidity
(d) air temperature
(e) wind speed
(f) wind direction
Snow water equivalent is measured using 1.20 m X 1.50 m X 15 mm stainless
steel snow pillow filled with a mixture of 50? alcohol and 50? water. The
pillow is connected to a differential pressure transducer. Its measuring
range is 0-3 or 0-6 lb/in2 depending on the amount of snow expected.
In order to minimize the bridging effect, up to four pillows are connected
in parallel.
To insure that the pressure of the snow is evenly distributed over the
whole area of the pillow, the surface on which the pillow rests was leveled
and covered with sand or similar material available.
The rest of the sensors, the solar panel, and the antenna are attached to
a 2" pipe used as a tower.
Guy wires are used at two levels to avoid
collapsing of the pipe due to snow weight (figure 4 ) .
The height at which all elements are mounted has to be sufficient to
prevent their being covered by the snow. At 3 installations they are at 3.5
Use of data collection platforms for snow water equivalent measurement
49
\
f wee
S now - p i 1 t o w
T
-M-
-wr-txf-
-t<r-
J
pressure
transducer
w e a t he r p r o o f
Û
V .
SNOW
FIG. 3
PILLOW
INSTALLATION
enclosure
/~
SIGNAL TO DCP
y>
SCHEMATIC
Snow pillow installation schematic
m. At two other sites, they were attached to a pipe mounted on top of the
shelter.
Interface, DCP and battery are installed in two different forms: at sites
where a shelter already existed they were installed inside. Where no shelter
was available, the electronics and battery were put in a waterproof container
and buried (figure 5 ) . Two other DCP's were installed north of Chile at a
site called Pampa Lirima, to measure rainfall. The other one was deployed in
the far south to measure level fluctuation of Lake Dickson.
Heinz G. Martens L.
50
4.V -
-k
1
N
* • * * #
FIG. 4
'
Picture of Pampa Lirima DCP
RESULTS OBTAINED
Regular operation of the remote stations started in 1981. Although data was
gathered for several parameters, only the snow water equivalent data can be
evaluated as there is only data available for this one against which to
compare.
The snow pillows were installed next to existing snow routes in order to
take snow samples next to the pillow, while taking samples along the route.
Snow pillow data was also compared to accumulated precipitation measured at
the same basin but at a lower altitude. The evaluation of the snow pillow
behavior covers 3 DCP's during 1982, 1983, and 1984.
(a) Cerro Vega Negra, installed 400 km north of Santiago at 3,600 m above
sea level,
(b) Glacier Echaurren next to Santiago at 3,800 m above sea level,
(c) Lo Aguirre, 280 km south of Santiago, at 2,200 m above sea level.
Figure 6a, b, and c, represent the snow water equivalent as measured by the
pillow and in-situ measurements in 1982, 1983, and 1984 at one of the
stations, Cerro Vega Negra.
Their graphs are representative of the
correlation between the snow pillow measurements and the in-situ measurements
at the other stations.
Use of data collection platforms for snow water equivalent measurement
FIG. 5
Picture of Yelcho (Antarctic) installation
Figure 7 represents the accuracy of the snow water equivalent data.
Deviation of the straight line indicated errors of the data collected by the
DCP as compared to data obtained by traditional methods.
Good correlation exists in all cases between pillow data and accumulated
precipitation up to the point of maximum snow water equivalent.
On the pillow installed at Glacier Echaurren near Santiago there is
precipitation even during the melting season.
This is indicated by an
increase of the accumulated precipitation and a decrease of the snow water
equivalent. The foregoing relation indicates further that more of the snow
remains during the whole winter and that a direct relation exists between the
snow water and the precipitation accumulated during the same period.
It can be considered that the data obtained using a snow pillow and
pressure transducer satisfies hydrological requirements.
When operating data collection platforms in the Andes the following aspects
have to be considered: A high probability of data recovery is only assured
when
data is transmitted in three consecutive transmissions (triple
redundancy). Lightening rods and an adequate grounding system has to be
installed to avoid failures of the electronics caused by lightening, which is
very frequent in the Andes. When no shelter is available, the DCP interface
and battery can be buried. Before each snow period the DCP has to be
overhauled and the sensor data checked against data obtained by instruments
considered as acceptable standards.
A continuous record of snow data along with meteorological parameters, such
as solar radiation, temperature, relative humidity, wind, etc., helps to
better understand how snow accumulation and melting occurs, yielding a more
precise run off forecast.
The results obtained so far indicate that snow-routes can be replaced by
snow pillow considering that data is continuous and reliable. In several
cases, the frequency of in-situ measurements was decreased, especially at
Heinz G. Martens L.
FIG. 6 DCP Cerro Vega Negra, snow water equivalent 1982:
(a) 1982, (t>) 1983
52
Use of data collection platforms for snow water equivalent measurement
53
m m
1500-
(c)
A
f
SNOW P I L L O W
0
I
A
1000-
500-
PREC.
L A S RAMADAS
1
o SP s a m p l i n g
A snpw
i
0
may.
/-—U
ju n
,~J,
1
Jul.
1
aug
1
sep
roule
1
oct.
]
nov.
MIT
FIG. 6(c)DCP Cerro Vega Negra snow water equivalent 1984
places difficult to reach and where the probability for an accident to occur
is high.
Calibration of Sensors
All instruments are calibrated in the laboratory. However, after installation
is completed, the values obtained by the sensors are compared against readings
of standard instruments.
There are always small discrepancies between
readings, discrepancies which are not the same over the range of measurement
of the sensor.
The range of measurement of all parameters that generate a voltage is from
0 to 5 volts.
Since the sensors are linear, all values measured will fall on a straight
line when plotted on an x/y coordinate system. X represents the values
measured with the instrument considered standard (or correct value) and x_the
voltage obtained by the sensors of the remote station.
This line can be represented in the equation:
(1) y = bx + a. The
constants "a" and "b" can be calculated by the least square method after
obtaining an adequate set of values.
Heinz G. Martens L.
54
SP s a m p l i n g
2500'
•
CERRO VEGA
NEGRA
& GLACIAR ECHAURREN
a L0
SNOW
WATER
EQUIVALENT
FIG. 7
DC P
IN
SITU
AGUIRRE
MEASUREMENTS
Snow water equivalent
DCP in-situ measurements
When applying this method to calibrate the snow pillow pressure transducer,
the first step is to determine the linearity of the transducer. This is done
at the laboratory. A transparent hose is connected to the transducer; then
it is attached to a vertically mounted pole. Next, regulated voltage is
applied to the transducer, the hose filled up to 1.50 m or 1.80 m with water
and the output voltage of the transducer measured. (The water in the hose
replaces the water in the snow.) The water is then released in 10 cm steps
and the corresponding output voltage measured. Since the pressure range of
the transducer used allows for over 4 m of water, which is impractical to set
up in the laboratory, only a height of a 1.5 m to 1.8 m is reproduced.
Once the transducer is installed in the mountains, this procedure is
repeated. We have then a set of x/y values and are able to determine both
coefficients of the equation y = bx + a.
The snow pillows are then
reconnected and the output voltage measured. Since there is no snow over the
pillows, the value measured is due to the pressure of the pillow which was
installed approximately 20 cm higher than the transducer. This pressure is
a second constant which has to be considered.
55
Use of data collection platforms for snowwater equivalent measurement
Data Collection Platforms Installed for Other Purposes
(a) Automatic remote stations are being used for gathering meteorological
data at the Antarctic. The stations are still in their experimental phase;
one of them has not operated during the whole year.
(b) The Center for Space Studies is measuring the amount of water
obtained from fog condensation. The idea of obtaining water from fog is not
new and several projects were carried out over the last years, but DCP's made
it possible to measure the amount of water obtained from different collectors.
Installing several, varying in shape and size, permits to determine the best
collector. The water is measured using modified tipping bucket type, rain
gages; also meteorological parameters are measured at the same time.
(c) Snow avalanche is a hazard which causes loss of life and interruption
of mining activity. The problem is of prime importance having as much as 150
avalanche routes on a 10 km long road. Unfortunately not all the parameters
which cause a snow avalanche can be measured. Nevertheless one mining company
is testing the system by measuring wind speed and direction at the top of the
avalanche road. The data obtained is transmitted every hour to the avalanche
control office of the mine in the mountains.
(d) A flood early-warning system was developed. During snow melting
season in 1982/1983 a DCP and water level gage was installed at two rivers
next to Santiago. Collected data were transmitted almost real time over a
microwave link and telephone line to flood control office. The results were
excellent.
Evaluation of the System
An average of 78.75? of all possible transmissions was received. This figure
includes stations deployed in the mountains during 5 years of operation (Jan.
1982-1986), one station deployed near Lake Dickson (50 dg. 50* SL; Vi dg 50'
WL) for 2 years and shows the efficiency of the system for the indicated
period.
The results are acceptable but the system is not sufficiently
reliable.
The low efficiency of Limari and Lirima in 1982; Elqui in 1983 and 1985;
Choapa, Echaurren, Limari and Dickson in 1986, (about 60$) are due to extended
outages, caused by several reasons.
A principal reason is that it is
impossible to reach the remote site during winter time. Helicopter flight is
expensive (US $465.00/hour).
It should be noted that these figures also include instrument failures.
Data was also lost during eclipse periods of the satellite. Table 1 lists the
percentage of data recovered from the remote stations during 1982-86.
As said at the beginning, the first DCP was installed during 1978.
Sensors, interfaces, solar panels, DCP etc. are still operating. They have
failed many times, were repaired and continued measuring and transmitting.
Many failures can be attributed to the fact that the DCP's were the first
or almost the first to be on the market; other troubles to an inadequate
installation.
Following is a list of the main failures experienced:
- Transmission on a different time slot, caused by lightning or by low battery
voltage.
- Drift of the transmission time, causing interference with the DCP
transmitting on the next time slot.
Heinz G. Martens L.
56
TABLE 1 Data recovered
SITE
YEARS
1982
1983
198U
1985
1986
ECHAURREH
93. 31*
98.25
98.25
16.9k
27.2
78.8?
ELQUI
88.50
6U.ll
97.56
6k.kl
81+.3
79-78?
CHOAPA
78. lk
9^-77
98.25
90.27
50.1+
82.1+9?
LIMARI
63 A l
90.59
97-56
73.56
k.9
66.0?
MAULE
8U.32
82.23
95. Vf
9^.3
91.1+2
89.69?
LIRIMA
62.81
97-1
90.28
92.51
93.1
87.16?
*75-93
**58.69
67.31?
81.2k
58.57
78.75?
DICKSON
SYSTEM EFFICIENCY
67.30
75.29
82. U8
AVERAGE
NOTES :
* Started transmitting Jan.25, 1985
** Battery failure in MayFigures include: DCP failures, Instrument failures, Receive site failures
and Satellite eclipse.
- Solar panel:
Failure of the zener diode, which caused the voltage to
increase and the fuse on the DCP to blow.
- The installed DCP's use at the sensor input circuit an analog to digital
converter (CD 4051 AE) which is sensitive to negative voltage inputs. A
negative voltage at the input of one channel affects data at all the other
channels, making it useless. The negative voltage can be generated due to a
failure of a component of the interface, or when the instrument is not
calibrated correctly, or due to loss of calibration.
- The snow pillows have failed twice due to' mechanical problems.
- Anemometer and vane failed several times, a couple of weeks after
installation, also due to mechanical problems.
- Calibration of the humidity sensor changed during operation after a couple
of weeks of installation.
- A problem which is common to all installed DCP's is the continuous radio
frequency drift.
Unfortunately, while the frequency increases on several
DCP's, it decreases on others. The drift does not follow any pattern and
57
Use of data collection platforms for snow water equivalent measurement
appears not to depend on temperature. The behavior of a DCP is similar
whether it is installed in the Antarctic or in the mountains, buried or
located in a shelter.
The erratic behavior may cause that the frequency distribution of all
DCP's will sometimes exceed the bandwidth permitted by the satellite and earth
stations.
To avoid the loss of data, the transmissions frequency of all DCP's is
periodically monitored and one of the local oscillators (a HP 5100 A
Synthesizer) retuned so that all DCP's or at least as many as possible fall
within the demodulators bandwidth.
It has to be noted that our receive site is "home made," built using
existing equipment, designing or modifying other pieces and buying only the
demodulator. The Hewlett Packard frequency synthesizer model 5105 A and 5100
A generate the appropriate frequencies for signal downconversion.
Reliability of the system will increase when replacing DCP1s which are in
the field for almost 8 years by new equipment, which uses up-to-date
technology.
The main difficulty in expanding the network is its installation cost as
compared with the traditional data collecting methods.