ATM Modernization, Business & GA

Non-certified GPS Found to Be Sufficient for ADS-B, NATS Says

By Woodrow Bellamy III  | March 14, 2016
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[Avionics Today 03-14-2016] U.K. Air Navigation Service Provider (ANSP) NATS has released a report on the results of a six-month trial analyzing the functionality of Mode S Extended Squitter Automatic Dependent Surveillance – Broadcast (ADS-B) transponders when connected to non-certified GPS position sources. Data recorded over the six-month trial saw General Aviation (GA) pilots transmitting sufficiently accurate Automatic Dependent Surveillance-Broadcast (ADS-B) position information when their transponders were connected to non-certified GPS sources.
Computer generated concept of ADS-B flight operations. Photo: NATS UK.
NATS’ primary goal in performing the trial was to assess the quality of ADS-B data generated by GA aircraft equipped with Mode S transponders using broadcast position information generated by non certified GPS sources. According to the NATS’ report on the trial results, the British ANSP sees controlled airspace infringements by non-transponding aircraft as a risk to flight operations in NATS-controlled airspace. This is especially a risk within the London Terminal Maneuvering Area (LTMA), according to the report. 
The trial was also backed by the U.K.’s aviation safety regulatory body, the Civil Aviation Authority (CAA), which has not permitted the pairing of non-certified GPS receivers with Mode S transponders that are capable of broadcasting ADS-B. CAA has historically not permitted this type of equipage pairing because non-certified GPS position sources typically lack certain features required by the Minimum Operational Performance Standards (MOPs) and Minimum Aviation System Performance Standards (MASPs) that are applicable to certified units, specifically GPS integrity and interference protection mechanisms for the 1090 MHz band.
During the six-month trial, the ADS-B data collected from a total of nine GA aircraft equipped with Mode S transponders using non-certified GPS position sources was assessed against aircraft equipped with certified GPS sources (DO-260B/ED-102A compliant). For the trial results, NATS officials overseeing the trial assumed that GPS satellites were in a fault free condition. 
“The accuracy of the non-certified GPS position reports was found to be very similar to that of the certified aircraft; however the trial did record several extremely large horizontal position errors. Investigation of these errors determined they were broadcast by one particular airframe and were caused when the reported longitude position swapped from negative to positive for an update. It is not expected that a single large error would lead to significant issues to ATC as a single report would be considered as an outlier by a surveillance tracker and not lead to credible corruption,” NATS stated in its report analyzing the trial results. 
Trig Avionics and Funke Avionics provided technical support for the trial, including information for connecting and configuring their respective transponders. Several manufacturers were reluctant to allow users of their device to participate in the trial, as they expressed concern with the impact on certification of the transponder used in the trial. 
The NATS trial was also conducted as part of a wider Single European Sky ATM Research Joint Undertaking (SESAR JU) project called Electronic Visibility via ADS-B. SESAR JU can use the results of the NATS trial to validate its belief that ADS-B equipage allows for improved surveillance of general aviation aircraft flying in controlled and uncontrolled airspace. 
As part of the EVA project, NATS has been working with Funke Avionics to develop a new Low Power ADS-B Transceiver that would provide the GA pilots with the minimum functionality needed to make their aircraft visible to other airspace users. 
“This trial has successfully shown we can use non-certified GPS sources with capable transponders to provide ADS-B and therefore increase visibility and situational awareness of GA pilots,” said Mark Watson, head of research and development at NATS. 

NATS produced several recommendations regarding ADS-B flight operations as a result of the trial as well. One of those recommendations is to implement a continual independent re-verification of ADS-B derived information, to ensure ADS-B solutions with poor performance are quickly identified for rectification. The FAA’s current method for addressing this issue in the United States is to receive emailed aircraft N number, ADS-B transmitter and GPS make/model numbers to its ADS-B office where the equipment can be verified as properly functioning.

See the report by NATS on the full results of the trial here

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