# VanderSat Frequently Asked Questions (FAQ)¶

### What type of satellites are used in your algorithms?¶

We use passive microwave satellites which measure microwave signals that are naturally radiating from the Earth’s surface. This enables observations to be acquired during cloudy conditions, because of the physical properties of waves transmitted in this spectrum’s range. Examples of the satellites/sensors we use are:

### When is the data observed?¶

Currently, VanderSat uses the night time observations during the descending orbits. Each satellite has it’s own time of observing an area which is roughly the same time every day. For the satellites we use, these overpasses are either around 02:00 solar time, or 06:00 solar time. The data acquired at that time can be regarded as a snapshot of the soil conditions at that time. Therefore, the measurements we provide are representative for the time of overpass.

### What is the depth of the measurements?¶

The sensors we use, measure signal originating from the top layer of the soil. Typically, this is up to 10 cm deep, though the strongest contribution is received from the most upper layer. Commonly assumed is a depth of roughly 5 centimeter, though in reality the depth of this measurement varies slightly with moisture content. If the soil is drier the sensor can see deeper into the soil.

### Can you measure anything deeper than the top-surface?¶

Not directly, a direct measurement is only possible on the soil surface. However, there are ways of translating this surface measurement into something representative for deeper layers as well. This always requires some sort of model (which is inherently limited to its assumptions). A very simple, yet powerful way to do this through the Derived Root Zone Soil Moisture calculation that VanderSat delivers. Through this calculation, one can approximate the water content for the rootzone, up to 50 cm deep.

### What does the unit $$m^3 m^{-3}$$ represent?¶

The unit $$m^3 m^{-3}$$ of the VanderSat data products is Volumetric Soil Moisture which indicates the volume fraction of water in a volume of soil. A value of 0.4 is equivalent to 400 liters of water in one $$m^3$$ of soil.

### What do you measure in cities?¶

The Soil Moisture values that we measure in urban areas are simply the measured dielectic constant values of the city’s surface converted into soil moisture. Although we do show soil moisture values over urban areas, these values don’t actually represent soil moisture.

### When can I expect my data when having a Near-Real-Time service?¶

As soon as the satellite data has been transferred from the satellite to the ground segments, VanderSat needs about an hour before this data is transformed into soil moisture data. Generally this is within 6 hours after the satellite has observed an area, given that the satellite system is fully operational.

### When is my data available after the start of the contract?¶

Generally, this depends on whether this region has been processed before or not. Processing a region requires a great deal of processing, which means that only those parts of the world are updated where clients have requested data. If a new area needs to be processed, especially when the full archive is requested, the data needs to be checked for error sources. After this, an account is created and set-up for use (or the data is added to the Kisters platform). At the moment we strive to deliver an archive within two weeks. If one only needs a Near-Real-Time service, the required time can be reduced significantly.

### Do you need in-situ data in order to start processing a region anywhere in the world?¶

No, the Land Parameter Retrieval Model relies on physical quantities that can directly be inferred from satellite measurements.

### What is the coverage of the data in terms of observations?¶

Each satellite observes the globe each day, as their orbit is synchronized with the sun moving from east to west as it orbits from pole to pole in about 90 minutes (sun-synchronous orbit). Nevertheless, not every single area is observed daily since the satellite’s field of view is not wide enough. This results in the least amount of measurements around the equator (where the Earth is ‘widest’ around its longitude belt) and most for high (in absolute value) latitudes where the longitude belts are less wide. One can expect between 200 and 330 measurements per satellite, per year, depending on geographical location.

### Are there any weather conditions for which no valid measurements can be derived?¶

Most conditions are suitable for deriving a measurement, especially cloudy conditions are generally not a problem. There are however exceptions, especially when ice is involved. When the soil is frozen, the signal that the satellite observes is completely different from unfrozen soil and cannot be transformed into soil moisture. This also applies for heavy thunderstorms when the top of the clouds are frozen. In general: when frozen conditions apply.

### Are there any other conditions for which no valid measurements can be derived?¶

This can happen when there are radio sources transmitting on the exact frequency on which the satellite measures are directed at the satellite. Because of this, the measurement can be disturbed causing the soil to look more dry than the actual case. This disturbance is called Radio Frequency Interference (RFI). We have a filter in place that checks for these disturbances to filter them out in an early stage of the processing chain. However, when areas become too obscured by RFI, the soil moisture retrieval will eventually fail.