def get_near_airport(features, csv_path=''):
    """
    Filter trips data to being just trips whose first call originates from a
    tower near the airport.

    Args:
        features (Pandas.DataFrame): a set of user trips with features for
            trip number and when the call was placed in the timeline of the trip
        csv_path (string): file path for where to save CSV data to

    Returns:
        Pandas.DataFrame: The filtered subset of trip features for just trips
            whose first call is in the airport.
    """
    at_airport_data = features.loc[features['trip'].isin(['first', 'trip']) &
                                   features['calls_near_airport'] > 0]

    data = at_airport_data.groupby('date_').nunique('cust_id')

    if csv_path:
        data.to_csv(csv_path)

    return data