co-located stations are known as VORTACs. This is a station composed of a VOR for civil bearing information and a TACAN for military bearing information and military/civil distance measuring information. The TACAN transponder performs the function of a DME without the need for a separate co-located DME. Because the rotation of the antenna creates a large portion of the azimuth (bearing) signal, if the antenna fails, the azimuth component is no longer available and the TACAN downgrades to a DME only mode. Accuracy Theoretically a TACAN should provide a 9-fold increase in accuracy compared to a VOR, but operational use has shown only an approximate 3-fold increase. Accuracy of the 135 Hz azimuth component is ±1° or ±63 m at 3.75 km. Accuracy of the DME portion must be or 3 percent of slant range distance, whichever is greater, per FAA 9840.1 1982. and FAA N8200.121 TACAN stations can provide distance up to 390 nautical miles. Modern TACANs are much more accurate. The requirement now is to have portable TACAN that is IFR certifiable, both station and portable systems. The latest modern version of TACAN has been tested and could be a feasible back-up to future Air traffic control systems and may even be integrated into systems for a seamless back up. Past TACANs have relied on high output power (up to 10,000 watts) to ensure good signal in space to overcome nulls present in antenna design and to provide their required 200 mile range. With the advancement of technology, antenna design has improved with higher gain antennas, much shallower nulls, and lighter construction. Now it's feasible to have a 200 nmi range with a 400 watt TACAN DME transmitter, making the TACAN package much smaller, more portable and more reliable (a decrease in power also reduces heat, which lengthens the life of electronics). On the first Space Shuttle flight, Capcom Joseph P. Allen reported up to the crew that their TACANs had locked onto the Channel 111 signals at St. Petersburg, FL at a range of 250 miles. TACAN is getting smaller: full TACAN coverage can now be provided in a system that can be carried on a single trailer weighing less than 4000 lbs, and set up by two people in less than an hour. TACAN Transceivers can now be as small as lunch boxes (with full coverage and range) and the antennas can be reduced from 800