Dank u zeer, Jesse – twee keer!
Becker
I don’t think Becker supports ADS-B to the point it would fulfill the US ADS-B mandate.
What you can do AFAIK is connect a NMEA GPS and then it would transmit the position with unknown containment radius. That’s at least what I got out of a telephone call to them.
Jesse, thanks much for the info. Obviously once you’re got the GPS based ADS-B / UAT system working and proven, there’s no particular ATC related reason for radar: the new system does the same thing without expensive radar equipment on the ground. I understand the original hope in the US was to phase out transponders immediately. Rightly or wrongly the perception of technical risk must’ve been too much to go without a period of overlap. I think I’ll go with UAT as opposed to Mode S if it ends up being cheap enough, and maintain the hope of throwing out the transponder some day. Buying a new transponder doesn’t make sense if the long term US intent is to replace ATC radar with an ADS-B GPS + transceiver based position reporting system.
There are a lot of misconceptions in the previous posts. There are at least three standards for ADS-B Out. The ones most discussed are based on the squitter capability of a mode S transponder. Squitter is a radar term that essentially means it is an unsolicited broadcast. A transponder generally operates on two frequencies, one used for interrogation of the transponder (1030 MHz receiver) and the other used for replies (1090 MHz transmitter). A mode S transponder differentiates itself from a mode A/C transponder in that it is addressable and has a greater data capacity for each reception and transmission, generally 56 bits, 24 of which are used for the address or parity/address. A squitter is used in mode S to alert other aircraft and radar systems that are not aware of the aircraft and don’t have its address. This enables the other aircraft or ground ATCRBS radar systems to individually address the mode S transponder and exchange information by transmitting on 1030 MHz and receiving on 1090 MHz. TCAS takes advantage of this to determine and exchange RA data. With ES (Extended Squitter) the message length is increased to 112 bits and can contain more data in each squitter.
ADS-B data is included in the 1090ES extended squitter. It is not generated in response to an inquiry on 1030 MHz. It is a broadcast on a timed basis or event driven. In other words it is not a reply to an interrogation. ADS-B In the case of a mode S transponder, the question regarding ADS-B eliminating the transponder is moot. In the case of the UAT 978 MHz technology, the transponder is still required for two main reasons. First, TCAS depends on it. Second, when GPS outages or failures occur in the ADS-B equipment, either on the aircraft or in the ATC system, ATCRBS radar is the backup system.
UAT is a MITRE designed system based on breaking the GPS second up into transmission opportunities. There are some 3500+ such opportunities per second. The individual message is much larger than that used for 1090ES and is 432 bytes long or 3456 bits in comparison to 112 bits. This enables the message structure to be simpler and complete with fewer messages whereas 1090ES needs about 6 messages per second to forward all the necessary data and data types.
An ADS-B Out device does not include the capability to receive ADS-B messages, just transmit them. To receive ADS-B messages, a separate receiver is normally required as the transponder only receives on 1030 MHz, not 1090 MHz. With UAT on 978 MHz, many are packaged with both an Out (transmit) and In (receive) capability.
A TCAS or TCAD system is the opposite of a transponder in that it receives on 1090 MHz and transmits on 1030 MHz. These devices are capable of being programmed to interpret ADS-B broadcasts. Avidyne, L3 and Bendix King have all indicated they intend to add this capability.
In the US, we have the ground stations (Ground Based Transceiver – GBT) that provide ADS-B Out aircraft with ground based services. The two services our GBT provide are TISB (Traffic Information Service Broadcast) and FISB (Flight Information Service Broadcast). TISB is provided on both frequencies 1090 MHz and 978 MHz and broadcasts radar detected mode A/C targets that are not ADS-B compliant. This fills in the traffic picture if one has ADS-B Out and In capability. Because there are two links, a GBT service called ADS-R is provided to Re-broadcast the ADS-B Out link information on the other link when a client indicates it only can receive on a single link. Garmin has set the defacto standard for ADS-B by including the ability to receive on both links with its ADS-B In products. FISB is the weather data and is only provided on the UAT 978 MHz link because of bandwidth limitations on 1090 MHz. My understanding is that outside of the US, no GBT services are provided other than receiving ADS-B broadcasts from aircraft and forwarding them to ATC.
ATC can’t use the position if the dynamic accuracy and integrity are unknown. It also uses the velocity and vertical velocity information and can cross check the GNSS altitude with the pressure altitude for reasonableness. In the US, the following data is required in the ADS-B broadcast, a total of 19 items, note that item 2 is the position and with most simple position sources this may be the only data complied with but without some of the other data, it can’t be used;
d) Minimum Broadcast Message Element Set for ADS-B Out. Each aircraft must broadcast the following information, as defined in TSO-C166b or TSO-C154c. The pilot must enter information for message elements listed in paragraphs (d)(7) through (d)(10) of this section during the appropriate phase of flight.
(1) The length and width of the aircraft;
(2) An indication of the aircraft’s latitude and longitude;
(3) An indication of the aircraft’s barometric pressure altitude;
(4) An indication of the aircraft’s velocity;
(5) An indication if TCAS II or ACAS is installed and operating in a mode that can generate resolution advisory alerts;
(6) If an operable TCAS II or ACAS is installed, an indication if a resolution advisory is in effect;
(7) An indication of the Mode 3/A transponder code specified by ATC;
(8) An indication of the aircraft’s call sign that is submitted on the flight plan, or the aircraft’s registration number, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address;
(9) An indication if the flightcrew has identified an emergency, radio communication failure, or unlawful interference;
(10) An indication of the aircraft’s “IDENT” to ATC;
(11) An indication of the aircraft assigned ICAO 24-bit address, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address;
(12) An indication of the aircraft’s emitter category;
(13) An indication of whether an ADS-B In capability is installed;
(14) An indication of the aircraft’s geometric altitude;
(15) An indication of the Navigation Accuracy Category for Position (NACP);
(16) An indication of the Navigation Accuracy Category for Velocity (NACV);
(17) An indication of the Navigation Integrity Category (NIC);
(18) An indication of the System Design Assurance (SDA); and
(19) An indication of the Source Integrity Level (SIL).
In the case of the UAT 978 MHz technology, the transponder is still required for two main reasons. First, TCAS depends on it. Second, when GPS outages or failures occur in the ADS-B equipment, either on the aircraft or in the ATC system, ATCRBS radar is the backup system.
FWIW I don’t see either the existing limitations of large aircraft TCAS technology (they need to keep up) or having a backup system (why?) as making the idea of eliminating transponders unfeasible for the light aircraft fleet, once the UAT based system is operating reliably. Its arguably the only benefit to ADS-B that most light aircraft owners would hope for. Without some kind of carrot that the average US aircraft owner cares about, owners are going to fight tooth and nail… as they did with Mode S which as a result died in the US, in any meaningful sense for light aircraft.
The problem with any plan to reduce radar capability is that somebody who does not want to be seen is not going to be co-operating by radiating his position 
So while ADS-B is a really slick surveillance solution, the moment the “national security” issue is raised, you find that radar capability cannot actually be reduced.
In the US, some redundant radars will be eliminated, but the bulk will not be removed. ADS-B does not eliminate radar as it is the backup and it is also part of the verification against spoofing. The US system will show the controller an integrated surveillance view with inputs from ADS-B, primary radar, secondary radar, and trilateration systems. Also unless the turbine fleet was mandated to upgrade its TCAS to include UAT as a required input, transponders will remain.
Another issue with ADS-B that will become a big issue by 2020 is installation compliance. Currently approximately 50% of the installations are not compliant and can’t be used for surveillance for one reason or another.
Jesse, thanks much for the info. Obviously once you’re got the GPS based ADS-B / UAT system working and proven, there’s no particular ATC related reason for radar: the new system does the same thing without expensive radar equipment on the ground. I understand the original hope in the US was to phase out transponders immediately. Rightly or wrongly the perception of technical risk must’ve been too much to go without a period of overlap. I think I’ll go with UAT as opposed to Mode S if it ends up being cheap enough, and maintain the hope of throwing out the transponder some day. Buying a new transponder doesn’t make sense if the long term US intent is to replace ATC radar with an ADS-B GPS + transceiver based position reporting system.
I agree with your decision to install a UAT system for flight in the US, but don’t expect the transponder requirement to go away for the reasons stated in my other posts. I see no benefit at all beyond compliance for installing a 1090ES transponder for ADS-B and that in some cases compliance is the least expensive route to go. In my case, it would have only cost me $1500 to have my existing GTX330 upgraded to ES and become compliant, but I chose to install the Garmin GDL88 UAT system at three times the price because it provided me with ADS-B traffic and weather, whereas the GTX330ES did not.
I am far from an expert on this matter, yet much curious. AFAIU, ADS-B means (in Europe):
-) transmitting on the same frequency as a transponder: 1090 MHz
True.
-) transmitting more or less the same data as a mode-S transponder
False, all of the Mode S data is included, but there are more than a dozen other data items included, also the mode S is a reply whereas the ES ADS-B is a broadcast. The reply only occurs if the transponder is interrogated by the ground or TCAS/TCAD. The ADS-B data is unsolicited.
-) in the same format as a mode-S transponder
Yes, but the format of the ES uses a different message type and its formatting is totally different than standard mode S. It uses a download code of 17 to distinguish it from other mode S reply or squitters.
-) without however requiring triggering from a ground station
True
If this is correct, implementing ADS-B is by far the easiest as a piggy-back function of a mode-S transponder. Those that exist (Trigg &c) seem to confirm this. So eliminating transponders would eliminate ADS-B, too.
This is true, but the mode S transponder is only part of the system and a certified position source with the correct data and performance are required. The position source can be the most expensive part of the equation. Shortly, I expect position sources to be available as an integrated option for $1000 to $1500 extra.
The problem with any plan to reduce radar capability is that somebody who does not want to be seen is not going to be co-operating by radiating his position So while ADS-B is a really slick surveillance solution, the moment the “national security” issue is raised, you find that radar capability cannot actually be reduced.
I’d agree that ground based radar is necessary for air defense, in fact I think I already said that 
However, I think the aim should be to eliminate transponders, which is a different thing. Having light aircraft buy, carry around and maintain two redundant position reporting systems for ATC strikes me as dumb.
