Composite GVO datafiles:

CO_OPER_VOBS_1M_2__19990415T000000_20220415T000000_0101.cdf
CO_OPER_VOBS_4M_2__19990301T000000_20220301T000000_0101.cdf
CO_OPER_VOBS_12M_2_19990701T000000_20210701T000000_0201.cdf

Variables:
Timestamp    - time in milliseconds since 01-Jan-0000 00:00:00.000 of B_OB and B_CF 
Latitude     - latitude in degrees of GVO
Longitude    - longitude in degrees of GVO
Radius       - radius in metres of GVO
B_OB         - observed field in nT
sigma_OB     - error estimate of observed field in nT
B_CF         - core field in nT
sigma_CF     - error estimate of core field in nT
Timestamp_SV - time in milliseconds since 01-Jan-0000 00:00:00.000 of B_SV
B_SV         - SV field in nT/yr
sigma_SV     - error estimate of SV field in nT/yr

Note: the version number for the 1 and 4 month CDF files has been changed from 0201 to 0101
in order to be consistent with the official ESA dataproduct release, where the end date is changing. 
For the 12 month CDF file was not compiled from the official ESA GVO dataproduct (although it is produced using the same processing scheme and code as the official dataproduct), and therefore retains the version 0201.
%==========================================================================
Documentation: 

Ref 1): Swarm Geomagnetic Virtual Observatories Description of the Processing Algorithm, Rev. 3A, SW-DS-DTU-GS-005_2_GVO_DPA

Ref 2): Swarm Geomagnetic Virtual Observatories Product Definition, Rev.2B, SW-DS-DTU-GS-004_2-1_GVO_PDD

%==========================================================================
The GVO processing using the Oersted, CHAMP, CryoSat-2 and Swarm data are in accordance with Ref 1).
- GVO model setup:
    -300 globally distributed GVOs using an equal area grid (Leopardi 2006)
    -GVO data search range is 700km around each VO location
    -GVO data are collected for 1 or 4 or 12 months ad a time 
    -Data along-track differences and sums are used
    -GVO fit using cubic potential description
    -Inversion limit = 30 data points
    -IGRF-13 used as main field model in processing
    
    -GVO altitudes are computed at their mean orbital altitude (GVO). Afterwards they have been relocated to a common altitude of 700km as follows:   
    ->GVO OBS correction del_OBS using internal SH degrees: B_COR = CHAOS(1-20)+LCS1(20-185) 
	del_OBS      = B_COR(700km) - B_COR(GVO)
	B_OBS(700km) = B_OBS(GVO) + del_OBS

    ->GVO CORE correction del_CORE using internal SH degress: B_COR = CHAOS(1-20)-LCS1(14-20) 
	del_CORE      = B_COR(700km) - B_COR(GVO)
	B_CORE(700km) = B_CORE(GVO) + del_CORE

    -For Oersted GVOs the error estimates are computed as: 
    ->sigma_CF error estimates of the GVO CORE field are computed based on the residuals towards internal CHAOS-7.8 field predictions (deg 1-20).
     The residuals are computed for two latitude bands: a) 50N-90N degrees together with 50S-90S degrees and b) 50S-50N deg.
     Using all residual within each band, the error estimates are computed as the square root of the residual robust mean squared plus the residual robust standard deviation squared.

    ->sigma_SV error estimates of the GVO SV field are computed based on the residuals towards internal CHAOS-7.8 field predictions (deg 1-20).
     The residuals are computed for two latitude bands: a) 50N-90N degrees together with 50S-90S degrees and b) 50S-50N deg.
     Using all residual within each band, the error estimates are computed as the square root of the residual robust mean squared plus the residual robust standard deviation squared.
%==========================================================================
The GVO composite cdf files have a format in accordance with GVO Swarm cdf files described in Ref 2).
The composite series have been compiled from version 0201 cdf files of Oersted, CHAMP, CryoSat-2 and version 0101 of Swarm.

1) 1-month GVOs
    Data used:
    - Oersted data (available from ftp://ftp.spacecenter.dk/data/magnetic-satellites/Oersted/), 15 sec data sampling rate, 1999-2004
    - CHAMP MAG-L3 data (available from https://isdc.gfz-potsdam.de/homepage/), 15 sec data sampling rate, 2000-2010
    - CRYOSAT-2 CHAOS calibrated data (Olsen et al., 2020), 15 sec data sampling rate, 2010-2018
    - Swarm Level 1b data, SW_OPER_MAGA_LR_1B version 0101, versions 0505/0506, 15 sec data sampling rate, 2014-2022
    - No data selection criteria applied
	
    B_OB: observed field time series
    - covers period: 1999-2022

    B_CF: core field time series
    - Estimates of LCS-1 crustal field for SH degree n=14-185 (static part) removed 
    - PCA denoising applied.
    - SHA denoising applied: external and toroidal terms estimated to SH degree 13 during CryoSat-2 and Swarm, and to degree 5 during CHAMP. 
    At times of insufficient data for SHA inversion, a linear interpolation was used. No SHA denoising applied to Oersted data.

    B_SV: core field time series
    - GVO secular variation computed as annual differences of GVO-CORE time series

2) 4-month GVOs
    Data used
    - Oersted data (available from ftp://ftp.spacecenter.dk/data/magnetic-satellites/Oersted/), 15 sec data sampling rate, 1999-2004
    - CHAMP MAG-L3 data (available from https://isdc.gfz-potsdam.de/homepage/), 15 sec data sampling rate, 2000-2010
    - CRYOSAT-2 CHAOS calibrated data (Olsen et al., 2020), 15 sec data sampling rate, 2010-2018
    - Swarm Level 1b data, SW_OPER_MAGA_LR_1B version 0101, versions 0505/0506, 15 sec data sampling rate, 2014-2022
    - Data selection criteria applied:
    	- The sun is at least 10deg below horizon
    	- Geomagnetically quiet conditions (kp<30,dDst<3,Em<0.8,Bz>0nT,abs(By)<10nT)
    	- Estimates of LCS-1 crustal field for SH degree n=14-185 (static part) removed 
    	- Estimates of CIY4 ionopsheric and induced fields removed
        - Estimates of CHAOS-7.8 (version 7.10 for Swarm data) magnestospheric and induced fields removed
	
    B_OB: observed field time series
    - covers period: 1999-2022

    B_CF: core field time series
    - SHA denoising applied: external and toroidal terms estimated to SH degree 13. At times of insufficient data for SHA inversion, a linear interpolation was used. 

    B_SV: core field time series
    - GVO secular variation computed as annual differences of GVO-CORE time series

3) 12-month GVOs
    Data used
    - Oersted data (available from ftp://ftp.spacecenter.dk/data/magnetic-satellites/Oersted/), 15 sec data sampling rate, 1999-2004
    - CHAMP MAG-L3 data (available from https://isdc.gfz-potsdam.de/homepage/), 15 sec data sampling rate, 2000-2010
    - CRYOSAT-2 CHAOS calibrated data (Olsen et al., 2020), 15 sec data sampling rate, 2010-2018
    - Swarm Level 1b data, SW_OPER_MAGA_LR_1B, versions 0505/0506, 15 sec data sampling rate, 2014-2021
    - Down-sampled to 15 sec data sampling rate
    - Data selection criteria applied:
    	- The sun is at least 10deg below horizont
    	- Geomagnetically quiet conditions (kp<30,dDst<3,Em<0.8,Bz>0nT,abs(By)<10nT)
	
    B_OB: observed field time series
    - covers period: 2000-2021

    B_CF: core field time series
    - Estimates of LCS-1 crustal field for SH degree n=14-185 (static part) removed 
    - Estimates of CIY4 ionopsheric and induced fields removed
    - Estimates of CHAOS-7.8 magnestospheric and induced fields removed
    - SHA denoising applied: external and toroidal terms estimated to SH degree 5 (degree 13 if 300 GVOs were available). At times of insufficient data for 	SHA inversion, a linear interpolation was used. 

    B_SV: core field time series
    - GVO secular variation computed as annual differences of GVO-CORE time series
%==========================================================================
*NOTE: High scatter level in the Oersted part of the Composite GVO time series are seen.  Likely due anisotropic errors, there can be large outliers.  Please beware if using Gaussian statistics.

