The AOPA Air Safety Foundation has just released GPS/Loran -A Guide to Modern Navigation. The new guide provides concise and easy-to-understand descriptions of how GPS and Loran work. The guide briefly explains the ins-and-outs of certifying GPS and Loran units for I FR. It also details why existing hand-held units cannot be certified and should not be used for IFR navigation. The guide provides an overview on how GPS approaches are flown and the distinction between “overlay” and “stand-alone” approaches. It also discusses why GPS-generated altitude will probably differ from altimeter indications. For a free copy, write: GPS/Loran -A Guide to Modern Navigation, AOPA Air Safety Foundation, 421 Aviation Way, Frederick MD 21701. Enclose a large (6″x9″) self-addressed envelope with 55 cents postage affixed.