def clean(df, GPS = False, elevation = False, TEC = False, VTEC = False, locktime = False):
	"""Clean
	
	Removes erroneous values of VTEC
	
	Args:
		df (dataframe): Master dataframe containing TEC measurements
		GPS (bool): (default: False) If True, only GPS satellite data is included and vice-versa
		elevation (bool): (default: False) If True, only data for which elevation is greater than 30 degrees is included
		TEC (bool): (default: False) If True, only data with positive TEC values are included and vice-versa
		VTEC (bool): (default: False) If True, only data with positive VTEC values are included and vice-versa
		locktime (bool): (default: False) If True, only data with locktime greater than 3 minutes are included and vice-versa
	"""
	if elevation == True:
		df = df[df['elevation'] > 30]
	if TEC == True:
		df = df[df['TEC'] > 0]
	if VTEC == True:
		df = df[df['VTEC'] > 0]
	if locktime == True:
		df = df[df['locktime'] > 180]
	if GPS == True:
		df = df[(df['SVID'] >= 1) & (df['SVID'] <= 37)]
	return df