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ADS-B out is available on the 1090 (Mode S) band, and can even be implemented as a firmware update to some Mode S transponders.

In the case of 978 ADS-B Out, I believe that it is still necessary to keep operating a Mode C transponder for operations where Mode C is currently required, as noted in This video at 16:25, in order to work with TCAS systems. Continuing on to 41:24, there is a recommendation stating that if a Mode C transponder fails, it should be replaced with an ADS-B "out" transponder. Here are my questions:

  1. Is there such thing as a 978 transponder which will meet ads-b out requirements and eliminate the need for Mode C, or does this recommendation essentially mean that you move to a 1090ES transponder?
  2. Perhaps this recommendation is assuming that there was no ads-b out at the time. Given an aircraft with a failed mode-c transponder, what equipment options which meet the mandates are worth considering for GA aircraft? Apparently John Zimmerman does not recommend Mode C + 978 out. It appears he recommends 1090ES. Either one of these would seem to benefit from having 978 in. I can see no advantage to 1090ES in, as it appears to offer only a subset of the 978 services, unless it was desired to be able to receive 1090ES air-to-air or our operate outside the US.
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  • $\begingroup$ Even with ADS-B, the rules for entering class B and C airspace don't change, you still need a working Mode-C transmitter. $\endgroup$ – Ron Beyer Dec 31 '15 at 21:18
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For Class A,B and C, you'll need a Mode C transponder as per the present regulations (unless cleared by ATC). From 14 CFR §91.215 ATC transponder and altitude reporting equipment and use:

§91.215 ATC transponder and altitude reporting equipment and use.

(b) All airspace. Unless otherwise authorized or directed by ATC, no person may operate an aircraft in the airspace described in paragraphs (b)(1) through (b)(5) of this section, unless that aircraft is equipped with an operable coded radar beacon transponder ... and that aircraft is equipped with automatic pressure altitude reporting equipment having a Mode C capability that automatically replies to Mode C interrogations by transmitting pressure altitude information in 100-foot increments. This requirement applies—

(1) All aircraft. In Class A, Class B, and Class C airspace areas;

FAA also addresses this in its FAQs regarding ADS-B:

Will Mode C transponders be required indefinitely?

The FAA is considering additional changes in the national airspace system, such as for TCAS. These changes, may at some future date, reduce or eliminate the need for transponder equipage.

So, at present, you'll need Mode C anyway.

Also, you'll need 1090ES in Class A. From FAA FAQs:

Aircraft that fly internationally and aircraft that require TCAS (Traffic Collision Avoidance System) are already operating with Mode S transponders and many have older versions of ADS-B Out on 1090ES MHz. Aircraft flying in Class A airspace must operate on the 1090ES MHz frequency. General aviation users who choose the UAT (978 MHz) link may take advantage of the ADS-B traffic, weather, and aeronautical services that are transmitted on the UAT frequency at no charge but must retain their current ATCRBS transponder. Mode S, 1090-ES users can operate in all airspace, but cannot receive FIS-B services.

I agree with you that there are a number of services (through FIS-B) for 978 UAT. In that case, (if you are not going above 18,000 ft) I suggest you check the availability of UAT receivers before taking a decision.

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1.) Not at this time, but someday maybe. It is reasonable that transponders will someday become obsolete with the advent of NextGEN infrastructure (including ADS-B) being implemented worldwide.

The transponder lends itself to a radar based ATC system for determining 'who' and 'where' aircraft are located. Although the radar can determine a 'where', the transponder fills in the 'who'.

NextGEN is planning to do away with radars (for the most part). ADB-S (a major component of NextGen) can provide ATC both the 'who' and the 'where' (and then some) on its own (no radar needed).

It could be years or decades before transponder requirements are relaxed however.

2.) '978 in' or 'ADS-B in' is a compelling reason for installing a UAT. I'd wager that even aircraft meeting the 2020 mandate with a 1090ES mode S transponder will eventually install UAT receivers. One advantage, however, to 1090ES is it is the only recognized ADB-S equipage anywhere outside of the USA at this time.

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