def get_drifter_ids_from_ascii(filename):
    """
    Returns a list of all drifter ids that are available in the GLAD
    dataset

    Parameters
    ----------
    filename : string
        Path to GLAD ascii data file.

    Returns
    -------
    ids : list
        A list of integers that represent all drifter ids available in 
        the GLAD data file.
    """
    data = [line.rstrip() for line in open(filename,'r').readlines()]

    ids = []
    for line in data[5::100]:
        id = int(line[7:10])
        if not id in ids:
            ids.append(id)

    return ids