def valid_svid(gdf):
    """
    removed invalid satellites from geodataframe

    Parameters
    ----------
    gdf : geodataframe
        geodataframe of gnss data which has column of svid
        
    Returns
    -------
    geodataframe:
        geodataframe of gnss data and the satellites are gps,glonass,beidou and galileo
    """
    #define all valid satellites
    svid = ('G01', 'G02', 'G03', 'G04', 'G05', 'G06', 'G07', 'G08', 'G09', 'G10', 'G11', 'G12', 'G13', 'G14', 'G15', 'G16', 'G17', 'G18', 'G19', 'G20', 'G21', 'G22', 'G23', 'G24', 'G25', 'G26', 'G27', 'G28', 'G29', 'G30', 'G31', 'G32',
'R01', 'R02', 'R03', 'R04', 'R05', 'R06', 'R07', 'R08', 'R09', 'R10', 'R11', 'R12', 'R13', 'R14', 'R15', 'R16', 'R17', 'R18', 'R19', 'R20', 'R21', 'R22', 'R23', 'R24',
'C01', 'C02', 'C03', 'C04', 'C05', 'C06', 'C07', 'C08', 'C09', 'C10', 'C11', 'C12', 'C13', 'C14', 'C15', 'C16', 'C17', 'C18', 'C19', 'C20', 'C21', 'C22', 'C23', 'C24', 'C25', 'C26', 'C27', 'C28', 'C29', 'C30', 'C31', 'C32', 'C33', 'C34', 'C35', 'C36', 'C37',
'E01', 'E02', 'E03', 'E04', 'E05', 'E06', 'E07', 'E08', 'E09', 'E10', 'E11', 'E12', 'E13', 'E14', 'E15', 'E16', 'E17', 'E18', 'E19', 'E20', 'E21', 'E22', 'E23', 'E24', 'E25', 'E26', 'E27', 'E28', 'E29', 'E30', 'E31', 'E32', 'E33', 'E34', 'E35', 'E36')
    
    gdf = gdf.loc[gdf['svid'].isin(svid),['svid','time','Cn0DbHz','geometry']]
    
    return gdf