def findPassingDistances ( distances, threshold ):
    """
    This function takes a list of distances and a threshold and returns all indices of
    distances which pass the threshold, or -1 if no distances is below the threshold.

    Parameters
    ----------
    list : distances
        The list of the distances to be searched.
        
    float : threshold
        The threshold below which any passing distance needs to be.

    Returns
    -------
    list : indices
        The indices of all passing distances or -1 if no distance is below the threshold.

    """
    ### Initialise variables
    minInd                                            = []
    indCtr                                            = 0
    
    ### Find all passing distances
    for val in distances:
        if val[0] < threshold:
            minInd.append                             ( indCtr )
        indCtr                                       += 1
        
    ### Check for at least one
    if len( minInd ) == 0:
        minInd                                        = -1
    
    ### Return the index
    return                                            ( minInd )