def get_geo_delay_horizontal(a0, a1, src):
    """
    The delay is negative if a1 is closer to the source than a0

    t_a1 - t_a0 = (t_a1 - t_origin) - (t_a0 - t_origin)
    """
    d0 = a0.get_geo_delay_horizontal(src)
    d1 = a1.get_geo_delay_horizontal(src)
    return d1 - d0
