I n s t r u m e n t s

a n d

M e t h o d s

Fig. 2. Basic setup of the snow stratigraphy profiler.

passing throughpin holes in thefoil of the Polariod sensor.
Vapor that condenses may freeze the foil to the backplane
of the sensor.

Snow Profiler

TheFrequencyModulatedContinuousWaveprofiling
Radar(FMCW)showninFig.2measuresaspectrumof
distances from the ground surface to reflecting layer inter-
faces and the snow surface. Specular reflection of micro-
waves dependson the difference of the index of refraction
between consecutive layers and layerthickness.
Specialsoftware at thebase station (e.g. at theavalanche
forecaster's office) permits on-line graphicpresentation of
time-series of electromagnetic profiles of the snow cover.
The electromagnetic profile looksvery similarto a tradi-
tional layerprofile.With additional software depth and
settlingofnew snowlayers can bedetermined withfair
accuracy (5%).Settling speed relates directly tothe rate
of increase of strength and viscosity but also to the state of
metamorphism(depthhoarinhibitssettling).Other
importantparametersthat canbedeterminedare:snow
depth, water equivalence of a dry snow cover,significant
layer interfaces, avalanchereleases,releaseheight,dammed
melt water at layer interfaces(in spring occasionallyacting
as weak layers),and surface wetness.Because wet snow
significantly attenuatesmicrowavesat frequenciesused for
theprofiler,installationsarelimitedtohighmountain
release zones where the snow coverstays essentially dry
duringthewinter season. Themain advantageof thesystem
is the possibility toget most important information from
withinorclosetopotentialr eleasezonesoflarge
avalanches.In large bowls several radar may be installed
atdifferentrepresentativelocationstoallowforthe
monitoring of aspectdependentloading and partial relea-
ses.

132

Specifications:

*Frequency modulated continuous C-band radar

*Triangular modulation 4 to 8Ghz at 40Hz

*Approx.conversion rate in air2kHz/m.

*Approx.conversion rate in dry snow2.2 to 2.8 kHz/m
depending on snow density.

*Maximum range in dry snow forsnow densities below
400kg/m³is 8m.

The accuracyof the measurementsdependson the qual-
ity of the microwave reflection at the layer interfaces and
ofthesnowsurface,and forthemeasurementswithout
reference reflector,onthe accuracyofthesnowdensity
estimates (a change of densityby 100kg/m³corresponds
to a 7% changeof optical distance). The absolute error for
measurementsofthewaterequivalenceofdrysnow
amounts to about 5kg/m ²or5mm of waterequivalence.
The systemconsistsof twoseparate parts:the radar is
mounted in a water protected cylindrical container with a
max. outer diameter of 68cm and a heightof approx. 80cm.
Infieldinstallationsthiscylindricalradar-containeris
installed in a concrete tube that isburied in the ground.
The upperendof thetube is closedby a mechanicallystable
radome that supports the weight of the snow.
The radar connects with a coaxial RG/U 58 cable (up to
700m) tothe field data controllerthat controls the radar,
recharges batteries,initiates measurements and performs
thenecessarycomputing(FastFourierTransform).The
radarinterface controllerconnects tothe CR10Xbythe
SDM-bus.Theradarcontroller (CR10X) transmits frequency
spectra(256 points per measurements)to the base station.
The base station software allows individual analysesof the
spectra andconversion to a bitmap presentation as shown
in Fig. 4/5.