Article: 1005 of rec.aviation.ifr Path: newshost.ncd.com!ncd.com!gossip.pyramid.com!decwrl!decwrl!uunet!spool.mu.edu!uwm.edu!rutgers!uhog.mit.edu!xn.ll.mit.edu!xn.ll.mit.edu!stevebu From: stevebu@bullwinkle.atc.ll.mit.edu (Steve Bussolari x5956 ) Newsgroups: rec.aviation.ifr Subject: Data Link for DGPS Message-ID: Date: 2 Sep 93 23:52:05 GMT Sender: usenet@xn.ll.mit.edu Reply-To: bussolari@ll.mit.edu Distribution: rec.aviation.ifr Organization: M.I.T. Lincoln Lab - Group 41 Lines: 55 Our newsgroup posting system has been broken for a couple of weeks and some of my posts regarding data link/GPS have not reached all of you. I'm repeating a summary of them here. Please forgive the long post. As many of you know, the RTCA Task Force 2 ("Transition to Digital Communications") has been discussing the use of data link by both transport and general aviation. Many organizations, FAA, AOPA, airlines, manufacturers, etc. have been involved in these discussions, which have focused recently upon the appropriate data link to use for transmission of local area DGPS corrections to permit Cat 1 approaches. Of the many candidates, two were selected for special consideration: the upper end of the VOR frequencies (VHF) and the Mode S frequencies (uplink 1030 Mhz downlink 1090 Mhz). Ground stations would be located near airports, each serving about a 20nm radius. For a while, it appeared that not only the differential corrections for each satellite in view, but the key geographic coordinates of each precision approach would be required to be broadcast. This requirement stemmed from the concern that the on-board databases could not be maintained with sufficient reliability and would need to be checked against the broadcast. This requirement would have likely eliminated the VOR option because of bandwidth limitations, particularly in congested areas where there are many runway ends. However, this requirement appears to have been dropped. From the user standpoint, the installation in the aircraft would be as follows: 1) VOR option. You probably would be able to use your existing VOR receiver by hooking up the audio to your GPS set. Within the GPS set would be an audio demodulator that would decode the corrections. You would, of course, be required to tune your VOR receiver to the appropriate frequency. The VOR receiver would not be available for VOR navigation while performing the DGPS duty. An alternative would be to incorporate a scanning VOR receiver in the GPS unit that would acquire the frequency automatically based on position and do the whole thing without manual intervention. This would be more costly than the above implementation. 2) Mode S option. A 1030 Mhz receiver would be incorporated into the GPS receiver. Alternatively, a data link capable Mode S transponder could be used to send the data to the GPS receiver. The cost of the VOR audio demodulator or the 1030 receiver as an add on to a GPS receiver are expected to be the same (about $300). Because of bandwidth, frequency availability and protection, and other reasons, it appears that RTCA is going to adopt the 1030 broadcast as their primary recommendation. This may be a very good thing for GA since it opens up an avionics path to many data link services over Mode S (now euphemistically called 1030/1090 to avoid the stigma of the name "Mode S" resulting from FAA's ill-fated effort to legislate equipage). Steve Bussolari