HSDI L1B File Structure

24SEP22 Add notes
Contents

Overview

Level 1B (L1B) files are netCDF format files containing calibrated measurement data (filter transmittances in the case of HSDI). Within the Cubemap End-to-End (E2E) simulator this can be generated either by the Level 1 processing module or by the Instrument Bypass module.

Each file contains the measurements from a single occultation event.

Example: l1b_hsdi.nc, which can be read with the IDL function read_l1b_hsdi.pro.

The file was written with IDL procedure write_l1b_hsdi.pro

File Contents

Global Attributes
Name Type Value
Title String 'HSDI L1B Transmittances'
Created String eg 'Sun Jan 16 14:38:46 2022'
Source String eg 'Created by make_l1b_hsdi.pro v12JAN22'
Dimensions
Name Size Definition
NImg eg 60 Number of images [1]
NChn up to 16 expected Number of spectral channels
NMos eg 9 Number of different mosaics defined for file [2]
NDat ≤ (NMos × NImg ) Total number of mosaic data points in file [2]
Variables
Name Type Definition Units Range
Satellite String Satellite ID eg 'Cubemap 1'
Instrument String Instrument ID eg 'HSDI'
Orbit Long Orbit number > 0
Sunrise Byte 1=Sunrise, 0=Sunset 0 or 1
Mos_X Int(NMos) x-coordinate of left of Mosaic within Image (1=left column) [3] pix ≥ 1
Mos_Y Int(NMos) y-coordinate of bottom of Mosaic within Image (1=bottom row) [3] pix ≥ 1
Mos_Alt Float(NMos) Tangent altitude displacement of Mosaic relative to Image reference [3] [4] km −20 : +20 expected
Chn_Lab String(NChn) Code used to identify each Channel [5] eg 'HSDI_01'
Chn_X Int(NChn) Column of Channel within Mosaic (1=left colummn) [2] [5] pix ≥ 1
Chn_Y Int(NChn) Row of Channel within Mosaic (1=bottom row) [2] [5] pix ≥ 1
Chn_Alt Float(NChn) Tangent altitude displacement of Channel wrt Mosaic reference. [4] [5] km −1 : +1 expected
Julian_Day Long(NImg) Day since 1st Jan 2000 (=Day 0) > 0
Milliseconds Long(NImg) Milliseconds since midnight UT ms 0 – 864000000
Altitude Float(NImg) Geometric Tangent Altitude of Image reference [4] km 0 – 120 expected
Latitude Float(NImg) Geometric Tangent Latitude of Image reference deg N −90 : +90
Longitude Float(NImg) Geometric Tangent Longitude of Image reference deg E −180 : +180
Rad_Curve Float(NImg) Earth radius of curvature in LOS plane at Image reference km 6300 – 6400 expected
NUse Int(NImg) No. Mosaics used for each Image [2] 0 ≤ NUse ≤ NMos
Idx_Mos Int(NDat) Index of Mosaic used for each data point [3] 0 : NMos-1
Quality Long(NChn,NDat) Quality Control flags 0=OK, other values TBD
Noise Float(NChn,NDat) Noise in each transmittance value [6] > 0
Transmittance Float(NChn,NDat) Transmittance measurements [6] 0 – 1

Notes

  1. An 'image' refers to the complete set of measurements taken at a particular time/tangent altitude.

  2. A complete HSDI image is divided into a number of rectangular Mosaics of (eg) 4×4 pixels, with a different spectral filter assigned to each pixel within the Mosaic, hence (eg) 16 spectral channels. Not all mosaics from the image are expected to be included in the L1B file since some will never image the sun hence there will be no associated transmittance measurement. Also different subsets of mosaics will probably be used from each image as the location of the sun on the detector array varies with tracking and refraction distortion.

  3. Separate from the Image data, the L1B file also contains a list of NMos mosaics, together with associated information. All the Image datasets within the file use a (different) subset of this list, identified by the index Idx_Mos in range 0, 1, .... NMos-1. Mosaic coordinates Mos_X, Mos_Y are not actually required for L2 but just used to identify each mosaic by its location within the full array. The coordinates represent the location of the bottom left pixel of the Mosaic, so the bottom left Mosaic in the full image will always have coordinates (1,1) and, in the case of mosaics of 4×4 pixels, the adjacent Mosaics will be (5,1) and (1,5).

  4. The Geometric Tangent Altitude of the centre of each measurement (iImg, iMos, iChn) is constructed as

    z = Altitude(iImg) + Mos_Alt(iMos) + Chn_Alt(iChn)

    where Altitude is the absolute measurement and Mos_Alt and Chn_Alt are relative measurements. Consequently the definition of reference points within the Image and Mosaic need only satisfy this overall requirement, but the centroids are suggested for simplicity. The Mos_Alt values for mosaics within the same row across the image would be expected to show some variation due to the curvature of the Earth.

  5. The ordering (and labelling) of the channels is arbitray

  6. Transmittance is dimensionless quantity nominally in the range 0–1 and Noise is the 1 σ uncertainty in Transmittance. However, it is possible that noise will lead to reported transmittance measurements outside the range 0–1 and that a particularly noisy channel might have a noise value > 1.

Document Changes

24SEP22 Add Notes.
16JAN22 Add Noise and Quality