Weather Data

Spacetime uses weather data when estimating link attenuation between any candidate pair of compatible transceivers as part of its continuous forward analysis of candidate links and their utility. The weather data is input to several ITU-R link attenuation models, each of which has specific requirements for the types of data necessary to perform its attenuation calculations.

Sources

Spacetime pulls weather data on-demand, caching for subsequent access to the same meteorological variables. It can pull from any service that generates GRIB2 files (and indexes thereof), including NOAA (GFS) and ECMWF.

Weather-based Attenuation Models

Several ITU-R attenuation models are available, and when paired with live or forecast weather data are used in estimating link attenuation. A survey of attenuation models and input data they require follows.

ITU-R P.676

This “Attenuation by atmospheric gases and related effects” model needs at least the following data types:

Pressure (Pascals)
Temperature (Kelvin)
Water vapor pressure (Pascals)

at several heights above local orography, distributed across all latitudes and longitudes through which a link may pass.

ITU-R P.838

The “Specific attenuation model for rain for use in prediction methods” model needs at least the following data types:

Rain height (meters)
Rain rate (meters/second or equivalent like mm/hour)

The local rain rate is the key datum, but a surface level rain rate can be combined with an approximate rain height to estimate the volume of space impacted. Data for all latitudes and longitudes through which a link may pass should be supplied.

ITU-R P.840

The “Attenuation due to clouds and fog” model needs at least the following data types:

Cloud ceiling (meters)
Cloud layer thickness (meters)
Cloud liquid water density (grams/meter³)
Cloud temperature (Kelvin)

distributed across all latitudes and longitudes through which a link may pass. This suffices for a single layer of clouds; multiple layers of clouds would require the same data for each layer, also distributed across all latitudes and longitudes of interest.

ITU-R P.618

The “Propagation data and prediction methods required for the design of Earth-space telecommunication systems” includes tropospheric scintillation modeling for satellite links. Spacetime implements the ITU-R P.618-12 scintillation fade depth calculations for frequencies between 4-50 GHz and elevation angles ≥ 5°.

Scintillation is caused by small-scale variations in atmospheric refractive index, particularly in the troposphere. The fade depth varies with:

Wet refractivity (N_wet): The wet term of surface refractivity, representing atmospheric water vapor contribution to the radio refractive index. This is automatically computed from ITU-R P.453-14 global maps based on ground terminal location and atmospheric probability level. Typical values range from 20-150 N-units, with higher values in humid equatorial regions and lower values in dry polar regions.
Frequency: Carrier frequency (4-50 GHz)
Elevation angle: Angle to satellite (≥ 5° in current implementation)
Antenna diameter: Physical aperture size (meters)
Antenna efficiency: Aperture efficiency (0-1, typically 0.5-0.7)
Availability: Target link availability percentage (e.g., 99.5%)

The wet refractivity parameter is critical for accurate scintillation modeling. Spacetime automatically determines N_wet values using:

ITU-R P.453

The “Radio refractive index: its formula and refractivity data” recommendation provides global maps of the wet term of surface refractivity. Spacetime uses ITU-R P.453-14 data derived from ECMWF ERA Interim reanalysis (1979-2014) on a 0.75° resolution grid with 18 probability of exceedance levels (0.1% to 99.0%).

For each ground terminal location, N_wet is interpolated from the global maps using:

Latitude and longitude: Ground terminal position
Probability of exceedance: Percentage of time the actual value exceeds the tabulated value (e.g., 50% for median conditions)

The interpolated N_wet value is then used in the ITU-R P.618-12 scintillation calculations to determine fade depth at the specified availability level. This location-based approach provides significantly more accurate scintillation predictions than using a global average value, particularly important for:

Equatorial regions with high atmospheric moisture (N_wet = 80-150 N-units)
Polar regions with dry conditions (N_wet = 20-40 N-units)
Links where ground terminal location changes as satellite moves across the sky

ITU-R P.1814

From “Prediction methods required for the design of terrestrial free-space optical links”, Spacetime estimates scintillation effects. Specifically, it uses the Hufnagel-Valley (H-V) model to estimate the refractive index structure parameter, derived from wind speed:

East and North components of wind (meters/second)

Weather data structure

Weather forecast products from ECMWF and NOAA have many data products useful for link attenuation modeling. Their delivery format may be broadly categorized as:

layered (3D, i.e. values vary by height) and
flat (values vary only by geographic location and do not vary with height)

ECMWF

A summary of ECMWF weather variables which can form a useful input data set:

Quantity	Layered / Flat	ITU-R Models	ECMWF Variable	Notes
pressure	layered	P.676	n/a	all layered data is keyed by pressure levels; no explicit variable required
height	layered	P.676, P.840	Z	requires additional conversion from geopotential to geometric height in meters above the WGS84 ellipsoid
temperature	layered	P.676, P.840	T	cloud temperature modeled as approximately equal to atmospheric temperature
water vapor pressure	layered	P.676	Q	converted via epsilon (gas constant for dry air / gas constant for water vapor)
rain height	flat	P.838	deg0l	modeled as equal to the height of the zero-degree isotherm
rain rate	flat	P.838	TP
cloud ceiling	flat	P.840	CC	source variable is layered; derived by walking from the ground layer up, inspecting the fractional cloud cover and noting when it crosses a density threshold (currently 50%)
cloud layer thickness	flat	P.840	CC	source variable is layered; delta between the bottom (ceiling, above) and the top of an assumed single layer of clouds (i.e. assuming no open air gaps between multiple layers of clouds). It is derived by walking down from the highest altitude, inspecting the fractional cloud cover (“CC”) and noting when it crosses a density threshold (currently 50%)
Cloud liquid water density	layered in principle; flat	P.840	TCLW	convert total cloud liquid water to density by dividing by cloud layer thickness (assuming one layer of clouds)
U component of wind	layered	P.1814	U	eastward component of the wind
V component of wind	layered	P.1814	V	northward component of the wind

NOAA

NOAA products have similar data in a variety of formats. Many products are limited to U.S. or North American territories.

Of interest for a future ITU-R P.531 implementation might be the Global Total Electron Content data product.

ITU-R P.453 Wet Refractivity Data

In addition to dynamic weather data from ECMWF/NOAA, Spacetime uses static global reference data for the wet term of surface refractivity:

Quantity	Data Source	ITU-R Models	Resolution	Probability Levels	Notes
wet refractivity (N_wet)	ITU-R P.453-14	P.618	0.75° global grid	18 levels (0.1% to 99.0%)	Derived from ECMWF ERA Interim reanalysis (1979-2014); used for tropospheric scintillation fade depth calculations

This data is bundled with the Spacetime software and requires no external weather service. The interpolator automatically determines location-specific N_wet values based on ground terminal latitude/longitude and the desired atmospheric probability of exceedance.