October 11, 2020

Why doesn't PilotAware do ADS-B out? [Updated]

At PilotAware we have been asked why we don’t develop PilotAware to transmit ADSB-out at 1090MHz as this is the frequency recommended by the CAA for products compliant with the UK  publication CAP 1391.

Initial Short Response (1minute 20 seconds read time)

The answer is that PilotAware has been consistent in its response that it will not make the considerable investment to produce such a UK only device until it has been shown that the 1090MHz Aviation Band, has the physical capacity to cater for all GA and CAT aircraft let alone drones transmitting on this frequency. PilotAware, like ICAO, the FAA in the United States and EASA in Europe, think that this is unlikely. Aviation transponders such as Mode-A, Mode-C, Mode-S and ADSB-out, all use the same 1090MHz band with its inherently limited capacity, modulation technique, pulse position modulation (PPM).

Consequently in line with current EASA policy PilotAware puts all of its effort into increasing interoperability between the current systems that are voluntarily used by the various aviation communities. This provides all users with a choice in using which EC suits them best, as no one size fits all.

To be clear this does not mean that PilotAware is in any way against ADSB-out. In fact, we have provided software on all PilotAware units that will output GPS messages through a simple inexpensive cable connection to an existing transponder (equipped with Extended squitter) which will, in turn, make it transmit ADSB out with a SIL=0 showing that it is using an unapproved GPS source. We recommend that everyone who can do this does so for greater situational awareness and flight safety.

The More Detailed Response. (9 minutes read time)

The acronym ADSB is used often. However, unless it is understood in context then its use can be misunderstood unless a few very basic principles are understood. These are explained below.

ADSB stands for Automatic Dependent Surveillance Broadcast. This is a generic statement which describes equipment that is  

  1. Automatic. It doesn’t require an external stimulus such as secondary surveillance radar to prompt a transmission.
  1. Dependent. It is dependent on a GPS input to transmit its location.
  1. Surveillance. It is used to show its position in space and time.  
  1. It is a Broadcast signal for all to pick up on the frequency on which it is transmitted.

This is a generic statement and can be applied to other modern systems such as FLARM, Fanet, PilotAware etc.

In the context of transmissions on the regulated aviation frequencies, ADSB is associated with 2 frequencies.

1090MHz using Pulse Position Modulation (PPM). This frequency was first chosen for identify friend or foe (IFF) by the military in the 1940s and is a simple technique which whilst it is very effective, it is very inefficient in its use of the allocated spectrum. 1090MHz has also been used by all other technologies that followed IFF, such as Modes A, C and S.  

The International Standard

The international standard specification for ADSB at 1090MHz can be found at.  

RTCA DO-260 minimum operational performance standards for 1090 MHZ extended squitter automatic dependent surveillance - broadcast (ADS-B) and traffic information services - broadcast (TIS-B)

This specification describes the requirements of ADSB transponders with a minimum transmission power of 70Watts and is used globally. ADSB transponders transmit a code called DF17 to show they are compliant with the RTCA DO-260 standard.


This is a National standard introduced by the United Kingdom Civil Aviation Authority that describes the specification for a low powered transceiver using ADSB on 1090MHZ using PPM. It has not been harmonised by ICAO, FAA or EASA and is regarded as an ICAO member State standard, not an international one. CAP1391, has however also been adopted by Australia as a national standard. EASA has not harmonised CAP1391 and therefore devices using this specification cannot be currently used in any European State other than as an ADSB receiver. The manufacturer of CAP 1391 devices has therefore correctly stated that the transmitting function must be turned off when leaving UK airspace. ADSB transceivers transmit a code called DF18 to show they are compliant with the CAP1391 standard.

978MHz (UAT) This is a second international aviation frequency that uses a more modern and much more efficient modulation technique called FSK (Frequency Shift Key). Within the United States, the  978MHz UAT system can provide traffic- and government-generated graphical weather information at no cost through TIS-B and FIS-B application. TIS-B and FIS-B are funded by the US Federal Government.  The service is not universal elsewhere in the world due to the high cost required by the national governments to set up the ground infrastructure.

Therefore when discussing ADSB one should be clear on which version of ADSB is being cited.

ADSB as a generic technology

ADSB 1090MHz PPM compliant with international standard RTCA DO-26 used by transponders at >/=70Watts and transmitting DF17

ADSB 1090MHz PPM compliant with the UK CAP1391standard used by low powered transceivers (No Mode A) and transmitting DF18

ADSB 978MHz (UAT) FSK used for TIS-B and FIS-B mainly in the US.  

Using ADSB-out (109MHz PPM) either RTCA DO-26 or CAP 1391 on Drones?

In the United States

In the United States, the FAA has prohibited the use of ADSB-out on drones post January 1st 2022.  

The proposed rule prohibits the use of ADS-B Out and transponders for UAS operations under 14 CFR part107 and part 91 unless otherwise authorized by the FAA. The FAA is concerned that the potential proliferation of ADS-B Out transmitters on UAS may negatively affect the safe operation of manned aircraft in the airspace of the United States. The projected numbers of UAS operations have the potential to saturate available ADS-B frequencies, affecting ADS-B capabilities for manned aircraft and potentially blinding ADS-B ground receivers.

For completeness, context and reference, follow the link below.


In the United Kingdom

In November 2020 the United Kingdom Civil Aviation Authority responded to the ICAO prohibition of ADSB 1090MHz on drones below 500 ft by making the following statement.  

“Under existing arrangements, ADS-B devices exchange information at 1090 MHZ. However, this could lead to spectrum congestion in low level airspace. ICAO has issued a letter to States prohibiting the use of 1090 MHz below 500 feet. The UK is currently exploring the use of 978 MHz for UAS to mitigate the risk of spectrum overloading at 1090 MHz”  Reference CAP 722 Edition 8 section .


In Europe

Europe is made up of 27 member states who agree to harmonise regulations amongst themselves. If a member state does have a  different standard, as did the CAA with CAP1391 before Brexit then this is possible. However, because one member state has such a ruling it is not automatically ratified across other member states. EASA has not adopted CAP1391 and it is not recognised in any European Union single state.  

Regarding ADSB designed to the international standard RTCA DO-2 the following statement was issued in 2019 in Amsterdam.

‘AOPA-Netherlands has attempted to set up an experiment (similar to those in the UK and Denmark) to test ADSB equipment but the Dutch Ministry of Transportation wants to wait until EASA makes up their mind.

At this moment it is not at all clear what course EASA will take. An attempt has already been made years ago to implement ADS-B for all aircraft, also below 5700kg, but they had to realize that it can´t work based on Mode S [109MHz] technology for all.

In the European Core Area (within the polygon Paris → London → Hamburg → Vienna → Zurich → Paris) the Mode S saturation is already so high, that equipping all remaining GA aircraft and air-sports-vehicles like sailplanes and microlights with Mode S based ADS-B would lead to overstressing the Mode S system. For instance in Germany on a nice summer-weekend when 25% of the GA fleet would become airborne at the same time, ATC would already have a massive problem. Targets could become invisible for a couple of seconds, and also the ACAS system on board of the airliners would only work with decreased range.

As a consequence EASA is considering other ADS-B enablers with higher capacity, one of them would be UAT, but they are also discussing about the integration of FLARM and other sources via their ground-system Open Glider Network.’ Ends.

For completeness and reference, the actual document can be found using the link below.


So why not use 978MHZ (UAT)

This solution is available in the US where the Federal Government funded the necessary ground station infrastructure that integrates ADSB 1090MHz and ADSB 978MHZ together. Other National Governments or ICAO Member States are not so wealthy or inclined so to do. Therefore this has not been yet adopted at scale outside the US.

As described in the OFCOM UK Frequency Allocation Table (UKFAT) the 978MHZ frequency is part of the radio spectrum used for;  

DME Equipment  

Radio Astronomy services

Services for Programme Making and Special Events (PMSE) assignments and authorised by OFCOM  

MoD communications systems operate with the agreement of CAA.

Currently, 978MHZ is not universally available in the UK and Europe for the introduction of a UAT service. However, as mentioned earlier in CAP 772 the UK CAA are investigating the possible use of 978MHz which will no doubt include negotiations with OFCOM. 978MHz has therefore the potential for a standard frequency for EC in the UK if the spectral and financial considerations can be overcome. PilotAware is watching this possibility closely for future product development if financially viable.



As supporters of PilotAware know, we are champions of interoperability between existing systems and are against a mandate of any single technology.

To assist in providing as much interoperability as possible we have invested significant amounts of time money and energy in developing the PilotAware ATOM GRID Network to achieve this for the benefit of all. Read all about this and how you can help here.

PilotAware is not against any technology so long as it is safe, inexpensive, adds to the safely of General Aviation and maintains the integrity of all existing uncontrolled airspace. In our world, all forms of aviation are equal in importance but are different in operation and so no one type of EC suits all. If the UK is to become a world leader in aviation then sports aviation must be encouraged to flourish from the grassroots upwards and not limited to the few.  

It is inevitable that the rapid advances in technology will produce new innovative and disruptive services for GA, foot-launched aircraft, drones and other aircraft. These technologies will include traditional radio, 4G and 5G mobile, free space lasers and other innovations.  

Innovation does not flourish in a regulated environment. If 978MHz does become an international standard for Electronic conspicuity this will be a good step forward but other technologies will still be required to provide services not available on the regulated frequency. Watch this space it’s all very exciting.  

Fly safely.