Ranges, Coords and Projs
pyDARN uses several different measurement and plotting systems to easily allow the user to customise their plots, this page aims to describe their uses: - Range Estimation: the estimate of how far the target (echo) is from the radar - Coordinate systems: determines the unique position using a set of points - primarily we use geographic and magnetic coordinate systems for Earth - Projections: used primarily on spatial plots (FOV, Fan, Grid...) projection choices allow the user to choose what type of projection the plot appears in
RangeEstimation.RANGE_GATE a rectangle section determined by beam width and set distance for each range (nominally 45 km). RAWACF and FITACF data give their parameter values with respect to range gates. Range gates are a unit-less measure of estimating distance.
RangeEstimation.SLANT_RANGE is a conversion from range gates to km units. Slant range estimates the distance of ionospheric echos from the radar, using the time it takes for the radio wave to travel to the ionosphere and return, assuming the radio wave is travelling at the speed of light. Measured in km.
RangeEstimation.HALF_SLANT is slant range divided by two, measured in km.
Ground Scatter Mapped Range:
RangeEstimation.GSMR uses echos from ground scatter to adjust slant range coordinates to be more accurate based on Dr. Bill Bristow's paper. Implemented by Dr. Nathaniel Frissell and Francis Tholley from University of Scranton. Measured in km.
Slant range is calculated from the value of
frang, the distance to the first range gate. In pyDARN, we assume
that this value is the distance to the inner edge of the first range gate. We are aware that not all radars use this
exact definition, this is outside the remit of pyDARN and should be addressed at a higher level.
rxrise is also used in the definition of slant range. This is the receiver rise time of the radar, however,
due to discussion outside of pyDARN's remit the value of
rxrise is adjusted in FITACF files and may not match
the value given in hardware files. Currently, pyDARN has decided to use the values for
rxrise given in the
hardware files. We will amend or reconsider this approach as and when a solution to the differing values is found.
In some plots, the user can change the
frang value to fit their needs.
Used to determine the position of data in spatial plots: fan, grid and convection map plots
Range time plots now allow for
Coords use. The y-axis can be converted to latitude, longitude or MLT using a the
lat_or_lon='lon' in the method call will convert the chosen range estimate (see above) into Geographic Longitudes.
Coords.GEOGRAPHIC is the standard geographical coordinate system for latitude and longitude (degrees)
Coords.AACGM is Altitude Adjusted Corrected Geogmagnetic Coordinates developed by Dr. Simon Shepherd are an extension of corrected geomagnetic (CGM) coordinates that more accurately represent the actual magnetic field. In AACGM coordinates points along a given magnetic field line are given the same coordinates and thus are a better reflection of magnetic conjugacy. pyDARN uses AACGM-V2 from the aacgmv2 python library.
Coords.AACGM with the geomagnetic longitude converted to magnetic local time.
RangeEstimation methods can be used with a
Coords calculation. For example, using
RangeEstimation.GSMR together, will give a plot of ionospheric echoes at a distance from the radar calculated in ground scatter mapped range, in geographic coordinates.
Spatial plots have two options for projections.
Projs.POLAR sets up the axis of the spatial plot in polar coordinates common in studies that show data over the poles.
Projs.GEO sets up the axes of the spatial plots in geographic coordinates using Cartopy.
The 'look-direction' of the two projections are different for the Southern hemisphere. Polar projections show a view of the south pole as if looking down through the planet from above the north pole, geographic projections show a view of the south pole as if looking from above the south pole.
Some combinations of Projs/Coords/RangeEstimates are not designed to work. For example, you cannot plot a fan plot using range gates, spatial plots require a value in kilometers. At the moment, AACGM Coordinates do not plot on Geographic projections as it has not been developed yet. Convection maps only support polar projections due to lack of interest in requiring geographic projections.