Compare IGRFv13 vs v12 vs AIPS #1
Comments
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I've written some fitting software that I use to extract the ionospheric behaviour from the EVPA on the limb of a disk --- very similar to software Bryan / Eric worked on for AIPS workflow. This is here: https://github.com/bennahugo/lunaticpolarimetry/. We can use this to compare with results from ALBUS with different earth magnetic models. Re the AIPS model: SUBROUTINE MAGDIP (GLAT, GLONG, RADIUS, H)
C-----------------------------------------------------------------------
C! Calculate Earth's magnetic field components
C# Util
C-----------------------------------------------------------------------
C; Copyright (C) 1996
C; Associated Universities, Inc. Washington DC, USA.
C;
C; This program is free software; you can redistribute it and/or
C; modify it under the terms of the GNU General Public License as
C; published by the Free Software Foundation; either version 2 of
C; the License, or (at your option) any later version.
C;
C; This program is distributed in the hope that it will be useful,
C; but WITHOUT ANY WARRANTY; without even the implied warranty of
C; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
C; GNU General Public License for more details.
C;
C; You should have received a copy of the GNU General Public
C; License along with this program; if not, write to the Free
C; Software Foundation, Inc., 675 Massachusetts Ave, Cambridge,
C; MA 02139, USA.
C;
C; Correspondence concerning AIPS should be addressed as follows:
C; Internet email: aipsmail@nrao.edu.
C; Postal address: AIPS Project Office
C; National Radio Astronomy Observatory
C; 520 Edgemont Road
C; Charlottesville, VA 22903-2475 USA
C-----------------------------------------------------------------------
C-----------------------------------------------------------------------
C Routine to compute the earth's magnetic field strength from an
C offset dipole model. Adapted from Handbook of Geophysics and Space
C Envirnoments (circa 1965) S. L. Valley ed. Air Force Cambridge
C Research Laboratories and Chapman and Bartels, 1940, GEOPHYSICS,
C Oxford)
C NOTE: The Gaussian coefficients from Chapman and Bartels give
C a slightly better representation of the field than Valley so these
C values are used here.
C Values of H returned are probably good to better than 20%.
C At the VLA the model is 6% low in total intensity and 11 deg W in
C magnetic declination.
C Inputs:
C GLAT R Geocentric latitude (radians)
C GLONG R Geocentric EAST longitude (radians)
C RADIUS R Distance from the center of the earth (m)
C Output:
C H(3) R Magnetic field vector (gauss),
C (1) = positive away from earth center,
C (2) = positive east,
C (3) = positive north.
C-----------------------------------------------------------------------
REAL GLAT, GLONG, RADIUS, H(3),
* RE, FACT, GLATMP, GLONMP, PI,
* H02, L0, L1, L2, E, SQRT3,
* G10, G11, G20, G21, G22, H11, H21, H22, X0, Y0, Z0
PARAMETER (PI = 3.14159265)
C Geographic coordinates of
C North magnetic pole.
PARAMETER (GLATMP = 78.63 * PI / 180)
PARAMETER (GLONMP = 289.85 * PI / 180)
C Gaussian coefficients(gauss):
C From Handbook of Geophysics...
C Epoch 1960.
C Modified??????
C PARAMETER (G10 = -0.30509)
C PARAMETER (G11 = -0.02181/2.0)
C PARAMETER (G20 = -0.02196/2.0)
C PARAMETER (G21 = 0.05145/3.0)
C PARAMETER (G22 = 0.01448/4.0)
C PARAMETER (H11 = 0.05841/2.0)
C PARAMETER (H21 = -0.03443/3.0)
C PARAMETER (H22 = 0.00172/4.0)
C Chapman values Epoch 1922
PARAMETER (G10 = -.3095)
PARAMETER (G11 = -.0226)
PARAMETER (G20 = -.0067)
PARAMETER (G21 = 0.0292)
PARAMETER (G22 = 0.0143)
PARAMETER (H11 = 0.0592)
PARAMETER (H21 = -.0122)
PARAMETER (H22 = 0.0113)
C SQRT3 = sqrt (3.0)
PARAMETER (SQRT3 = 1.732050808)
C Compute dipole center in units
C of earth radius.
PARAMETER (H02 = G10*G10 + G11*G11 + H11*H11)
PARAMETER (L0 = 2.0*G10*G20 + (G11*G21 + H11*H21) * SQRT3)
PARAMETER (L1 = -G11*G20 + (G10*G21+G11*G22+H11*H22) * SQRT3)
PARAMETER (L2 = -H11*G20 + (G10*H21-H11*G22+G11*H22) * SQRT3)
PARAMETER (E = (L0*G10 + L1*G11 + L2*H11) / (4.0*H02))
PARAMETER (X0 = (L1 - G11*E) / (3.0*H02))
PARAMETER (Y0 = (L2 - H11*E) / (3.0*H02))
PARAMETER (Z0 = (L0 - G10*E) / (3.0*H02))
REAL X0M, Y0M, Z0M, HMAG, HD(3), CLA, SLA, CLO, SLO
DOUBLE PRECISION POS0(3), POS1(3), POSTMP(3), POST2(3), RADDIP,
* COLAT,
* LONDIP, CA, SA, CB, SB
C RE = Radius of earth (avg polar
C and equitorial)
DATA RE /6367650.0/
C-----------------------------------------------------------------------
C Center of dipole
X0M = X0 * RE
Y0M = Y0 * RE
Z0M = Z0 * RE
C Convert to earth center x,y,z
C Here y=> 90 e long.
POS0(1) = RADIUS * COS (GLAT) * COS (GLONG)
POS0(2) = RADIUS * COS (GLAT) * SIN (GLONG)
POS0(3) = RADIUS * SIN (GLAT)
C Translate
POSTMP(1) = POS0(1) - X0M
POSTMP(2) = POS0(2) - Y0M
POSTMP(3) = POS0(3) - Z0M
C Rotate to dipole coord.
CA = COS (GLONMP)
SA = SIN (GLONMP)
CB = SIN (GLATMP)
SB = -COS (GLATMP)
POST2(1) = (POSTMP(1)*CA + POSTMP(2)*SA) * CB +
* POSTMP(3) * SB
POST2(2) = POSTMP(2) * CA - POSTMP(1) * SA
POST2(3) = POSTMP(3) * CB - SB * (POSTMP(1)*CA + POSTMP(2)*SA)
C Polar coordinates in dipole.
RADDIP = SQRT (POST2(1)*POST2(1) + POST2(2)*POST2(2) +
* POST2(3)*POST2(3))
COLAT = ACOS (POST2(3) / RADDIP)
LONDIP = ATAN2 (POST2(2), POST2(1))
CLA = SIN (COLAT)
SLA = COS (COLAT)
CLO = COS (LONDIP)
SLO = SIN (LONDIP)
C Terms of dipole, local
FACT = SQRT (H02) * ((RE / RADDIP) ** 3)
H(1) = -2.0 * FACT * COS (COLAT)
H(2) = 0.0
H(3) = FACT * SIN (COLAT)
C Rotate to dipole centered
HD(1) = (H(1)*CLA - H(3)*SLA) * CLO - H(2) * SLO
HD(2) = H(2) * CLO + (H(1)*CLA - H(3)*SLA) * SLO
HD(3) = H(3) * CLA + H(1) * SLA
C Modulus of HD
HMAG = SQRT (HD(1)*HD(1) + HD(2)*HD(2) + HD(3)*HD(3))
C Find position 1 km from
C position in the direction of HD.
POST2(1) = POST2(1) + 1000.0 * HD(1) / HMAG
POST2(2) = POST2(2) + 1000.0 * HD(2) / HMAG
POST2(3) = POST2(3) + 1000.0 * HD(3) / HMAG
C Rotate new position to earth
C system.
POSTMP(1) = (POST2(1)*CB - POST2(3)*SB) * CA - POST2(2) * SA
POSTMP(2) = POST2(2) * CA + (POST2(1)*CB - POST2(3)*SB) * SA
POSTMP(3) = POST2(3) * CB + POST2(1) * SB
C Translate to earth center
POS1(1) = POSTMP(1) + X0M
POS1(2) = POSTMP(2) + Y0M
POS1(3) = POSTMP(3) + Z0M
C Earth centered field
HD(1) = (POS1(1) - POS0(1)) * 0.001 * HMAG
HD(2) = (POS1(2) - POS0(2)) * 0.001 * HMAG
HD(3) = (POS1(3) - POS0(3)) * 0.001 * HMAG
C Earth local field
CLA = COS (GLAT)
SLA = SIN (GLAT)
CLO = COS (GLONG)
SLO = SIN (GLONG)
H(1) = (HD(1)*CLO + HD(2)*SLO) * CLA + HD(3) * SLA
H(2) = HD(2) * CLO - HD(1) * SLO
H(3) = HD(3) * CLA - (HD(1)*CLO + HD(2)*SLO) * SLA
C
999 RETURN
END |
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It will be useful to translate this into / wrap it with some python so we can get apples to apples with what AIPS assumes, CASA assumes, RMextract assumes and ALBUS assumes. |
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You can use RMextract to compare the EMM magnetic field model vs the WMM model. The comments in the code above indicate the AIPS code dates from 1965 - we're still using that - really? The IGRF model gets updated about every 5 years or so. and WMM/EMM are I believe, at least as recent as 2015. |
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Some progress on this front - for the same time ranges (kludging the date in igrfv11 to accept 2021): AIPS uses IONEX and ALBUS of course uses RINEX from stations close to site, so the difference is mainly in the measured TEC to source. I will try to next get the fortran MAGDIP.f bound into ALBUS to see what kind of order of effect this has on the resultant RM to quantify the errors |
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Just a note that there are now Python interfaces to the IGRF magnetic field model. Would be great to get away from having to shoehorn the IGRF Fortran into ALBUS |
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Yup we should - including f2c code is a bit ugly. There are also python-based RINEX conversion routines that I want to include instread of the proprietary binaries that we currently have to volume mount in. |
Making a note to link this to an investigation of upgrading the current IGRFv12 inside casacore as mentioned on casacore/casacore#1114
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