UAT or 1090ES?

If you are considering ADS/B, there is a choice to make. Do you install a Universal Access Transceiver (UAT) or the Mode S transponder that has an extended squitter (1090-ES)? It all depends...

What country are you in? If you aren't in the USA, then the choice is pretty much made. The USA offers the option of a UAT. The rest of the world needs Mode S transponders for ADS/B installations.

If you are in the USA, and you mostly fly above FL180, then the choice is pretty much made again. The FAA doesn't allow aircraft flying above 18,000ft to use the UAT. It just makes sense to get the 1090-ES transponder that will do Mode S if you want take advantage of ADS/B and fly about FL180.

The UAT transmits and receives on 978MHz, the 1090-ES transmits and receives on 1090MHz. The ADS/B system will allow all participating aircraft to see each other. If the two devices work on different frequencies, how does a 1090MHz transceiver see a 978MHz transceiver? The ground stations will repeat the 978MHz messages on 1090MHz, as well as repeat the 1090MHz message on 978MHz. The ground station will also show both messages on the "RADAR" scope, so the air traffic controller knows where everyone is.

The FAA separated the two systems for a couple reasons. The 978MHz devices can handle more data (has more bandwidth), so more aircraft in a concentrated area will work without overloading ground stations or other aircraft. The 1090 Mode S transponders are already on the larger faster aircraft that are flying higher, so the expense should be minimized (I am repeating the FAA here, in reality, most operators will need to replace the transponders they have to get the extended squitter feature).

The UAT's are even more useful, since the FAA will broadcast extra information. The two extra messages that the FAA is broadcasting are the TIS/B and FIS/B. The 1090-ES system will get TIS/B, but not FIS/B.

TIS/B is Traffic Information Service-Broadcast, where non-ADS/B equipped aircraft will show up on the aircraft display, similar to ADS/B equipped aircraft. The ground station will broadcast the position of aircraft that are only visible on RADAR. As a pilot, you will be able to see more of what the controller sees.

FIS/B is Flight Information Service-Broadcast. Flight information includes weather, and aeronautical products. While XM provides some weather, that you must subscribe to, the FIS/B is free to everyone. The XM product may have additional information, or be more timely. The FIS/B data is what the FAA will be looking at, including potentially air traffic control. The aeronautical products appear to be weather like items, such as NOTAMs and SUA status.

Exactly what device to get will depend on the capability of the chosen display. Many of the MFD manufacturers will take either device for input, the displayed information may help make the choice. Some will show the weather RADAR information in great detail, others will show it blocky or not at all. Over the next couple years, the MFDs are sure to get better.

Should you wait, or should you buy today? Today the ADS/B MFD technology is being developed. Over the next 5 years, the technology will surely mature. Having ADS/B in on a tablet computer will allow a pilot to get their feet wet, sooner. By 2020, most aircraft will be required to have ADS/B out, which probably means, unless someone builds an under $1000 solution to ADS/B out only, most aircraft will be equipped with ADS/B in and out.

Can you get rid of your transponder once you have ADS/B? No, the Mode/C component will still be needed for RADAR service and TCAS for non-ADS/B equipped aircraft.

It'll be an interesting couple years going forward. What do you think?