Swarm GVO datafiles:

SW_OPER_VOBS_1M_2__20131215T000000_20220415T000000_0101.cdf
SW_OPER_VOBS_4M_2__20140301T000000_20220301T000000_0101.cdf
SW_OPER_VOBS_12M_2__20140701T000000_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.
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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
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The GVO processing using the 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 
    -GVO altitudes are 490km during Swarm period
    -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
%==========================================================================
The GVO Swarm cdf files have a format in accordance with GVO Swarm cdf files described in Ref 2).

1) 1-month GVOs
    Data used:
    - Swarm Level 1b data, SW_OPER_MAGA_LR_1B, versions 0505/0506
    - Down-sampled to 15 sec data sampling rate
    - No data selection criteria applied
	
    B_OB: observed field time series
    - covers period: 2014-2022

    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. 

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

2) 4-month GVOs
    Data used
    - Swarm Level 1b data, SW_OPER_MAGA_LR_1B, versions 0505/0506
    - Down-sampled to 15 sec data sampling rate
    - 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-7.10 magnestospheric and induced fields removed
	
    B_OB: observed field time series
    - covers period: 2014-2022
	
    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
    - Swarm Level 1b data, SW_OPER_MAGA_LR_1B, versions 0505/0506
    - Down-sampled to 15 sec data sampling rate
    - 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 magnestospheric and induced fields removed
	
    B_OB: observed field time series
    - covers period: 2014-2021

    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
%==========================================================================

