## 3 Current Measurement Techniques
### Other Techniques Applicable to Both Soil Moisture and Snow Property Determination
#### 3.3.3 Satellite Products
They are both, however, still subject to large, scenario dependent measurement uncertainties, which are particularly applicable in alpine terrain. Active microwave sensing is a relatively new method of directly measuring SWE through interferometric methods using a Synthetic Aperture Radar (SAR) [73]. This is similar to GPR, and indeed, inherits the same limitations, but adds the limitations of measuring from a satellite. The first, and perhaps most important of these limitations, is that the pass frequency of these satellites is also only a maximum of twice per month, making it of little use for the scenarios under discussion. The geometry of the terrain is also a significant factor. Active microwave measurements use the phase of the reflected and subsequently refracted wave emerging from the snowpack, hence corrections must be made such that it appears that the signal is orthogonally incident on the snowpack [74]; although active microwave satellite resolution is extremely good [66], the resolution of the measurement is also degraded in mountainous terrain due to the additional length component added when a horizontal measurement is converted to a slope parallel measurement. Active microwave measurements are also subject to errors caused by inhomogeneous snowpacks (caused by snow metamorphosis) and by vegetation. More information on errors caused by vegetation is provided under the passive microwave measurement discussion. The latest developments in active SAR, entails the use of Ku-band and X-band radiation, combined. This provides a combination of surface scattering and volume scattering effects [66], which could possibly improve the errors caused by reflections within an inhomogeneous snowpack. As with GPR, the size of the errors above depends very much on the scenario and area of application. There is little quantitative literature for alpine areas and the authors do not know of any validated datasets for the Alps. Passive Microwave Sensing  
Passive microwave SWE measurement products have been produced since 1978 [66]. In passive microwave measurements, the naturally emitted microwave radiation from the soil is measured and SWE is inferred from the attenuation of the values measured with no snow present. Passive microwave measurements use a 25 km wide swath measurement [66, 67], which immediately provides a significant limitation for the scenarios of interest, as information can only be interpreted at regional to hemispheric scales [66]. This resolution is again, further degraded when translated to the slope-parallel distances of a mountainous environment. Passive and active microwave measurements are also subject to issues of snow metamorphosis and liquid water content, as with GPR, but in the case of passive detection, this is for different reasons detailed below. The passive microwave--SWE conversion widely used [75] assumes constant snow density and constant grain size [66, 76], hence snow grains, enlarged through snow metamorphosis, result in increased calculated SWE, and increased density (and in-particular the presence of liquid water) results in decreased calculated SWE [77]. These two parameters are particularly important in alpine and sub-alpine regions, where snow is often around its melting point and temperature gradients within the snowpack may be large. These parameters also change on a spatial scale of tens of meters [25] and throughout the snowpack respectively, as well as temporally [76, 78, 79] on a scale of hours. As HS and HNS are calculated from SWE, these are also affected by the constant assumptions. As mitigation, the presence of liquid water can be detected by the method proposed by [80] and grain size can be measured through hyperspectral remote sensing [66]. The same attenuating process used for SWE measurement, also means that passive microwave measurements may only be used for SWE of \(<\)150 mm or 10-100 times the microwave length [77, 81, 82].