January 14, 2022

Using PilotAware ATOM Stations for Situational Awareness at Airfields.

The CAA has released Supplementary Amendment CAP 670. Does this affect how you can use the ATOM GRID at your airfield?

The UK CAA has released Supplementary Amendment CAP670 Air Traffic Services Safety Requirements Effective 15th December 2021, which outlines what surveillance equipment can be used to assist in the provision of a Flight Information Service (FIS).

This PilotAware paper outlines what the ATOM ground station can be used for and the suitability of the VRS display used.

In January 2022 there were over 240 PilotAware ATOM stations installed in the UK, growing at a rate of 2 a week. About 120 of these are at Airstrips, Airfields, and Gliding sites. An ATOM station uses very low-cost, innovative, cooperative and disruptive technology using COTS equipment.

Using the open-sourced VRS system, PilotAware ATOM GRID displays more aircraft EC types than any other system. These include;

·       ICAO approved ADSB from transponders (DF17),

·       Mode-S transponders (using multilateration),

·       none certificated electronic conspicuity EC devices CAP1391 (DF18)

·       EC sources in the non-aviation-pro 860MHz band

o   Flarm,

o   PilotAware,

o   Fanet+.

o   OGN trackers

The CAA rules on the use of none approved systems such as PilotAware ATOM stations have been clarified in the release of Supplementary amendment CAP 670.

Supplementary amendment CAP 670. Number: 2021/02 Issued: 15 December 2021 Version: 1 Effective: 15 December 2021.  

The purpose of Supplementary Amendment CAP 670;

The purpose of the Supplementary Amendment to the Air Traffic Services Safety Requirements (CAP 670) is

(i) the provision of the minimum technical specification for a Cooperative Surveillance system for use by an Air Navigation Service Provider (ANSP) to support Flight Information Service (FIS) provision;

(ii) a description of the functionalities of a Flight Information Display (FID) which may be used as part of a Cooperative Surveillance system in support of FIS.

Is it, therefore, possible to use a PilotAware ATOM GRID system to provide a Flight Information Service at your airstrip or airfield?

A flight information service (FIS) is an air traffic service (ATS) that is provided:

·       to all flights with an air traffic control service in controlled airspace;

·       to participating VFR flights in class E airspace;

·       to participating IFR and VFR flights in uncontrolled, class G, airspace

·       by air traffic controllers (ATCOs) or flight information service officers (FISOs).

The conclusion in the Supplementary Amendment CAP 670 is that a Flight Information Service CANNOT be provided using the PilotAware ATOM GRID system. However, it CAN be used to provide improved Situational Awareness.

The exact words are ‘Note: EC sources in a non-aviation protected band can be used to supplement the above sources for situation awareness only and must not be used to supplement the provision of Flight Information Services’.

Electronic Conspicuity Systems

The Supplementary Amendment CAP670 categorises current Electronic Conspicuity Systems as

(i)              Approved including ICAO-approved ADSB from a 1090 transponder (DF17) and Mode-S transmissions identified through SSR and MLAT.

(ii)            Non-certificated electronic conspicuity (EC) devices.  CAP670 defines these as compliant to ‘Industry standards for these EC devices have been published in CAP1391, Electronic Conspicuity Devices’. The only CAP1391 devices currently in the UK are therefore SkyEcho 1 or 2.

(iii)          EC sources in a non-aviation-protected band. This is what EASA calls the 860 Band and includes  Flarm, PilotAware, Fanet+ OGN Trackers

From this, the implication is that all the traffic displayed by the PilotAware ATOM GRID system can be used for situational awareness of all traffic received by the ATOM Station. But at the moment this information must not be used for Flight Information Services either by air traffic controllers (ATCOs) or flight information service officers (FISOs).

This is also the position taken by PilotAware in the release of ATOM GRID software which is currently provided solely for situational awareness and must not be used to supplement the provision of Flight Information Services.

Further information to help ATOM GRID users get the most out of their installations;

The Supplementary Amendment CAP 670 provides information on how surveillance systems should be used.  This is very useful even when the ATOM GRID is only used for situational awareness. Therefore, some of the information is reproduced below to show what is thought to be best practice by the CAA. This includes information on displays, maps, testing, MLAT, power etc.  It is hoped that this will be useful to get the best out of the ATOM GRID system at your site.

Display requirements;

PilotAware ATOM software uses the open-sourced VRS, which is also used in the trials initiated by the CAA at Goodwood and Barton. The VRS is an excellent display and the comments below are relative to the use of VRS for situational awareness only. The VRS is NOT an approved Flight Information Display (FID).

For best practice, the display screen should be located such that it is readable in all ambient light conditions over a range of viewing angles, both vertically and horizontally.

The following bullets are in response to the requirements of Supplementary Amendment CAP 670.

·       Altitude information from MLAT is based on barometric pressure.

Surveillance data from multiple surveillance sources are available for presentation on the VRS.  The individual sources are selectable.

·       Messages for all types of EC devices display the most recent positional data.

·      Multiple target reports from a single aircraft from different EC sources are concatenated together against the unique ICAO code and the most recent report is used.

·      Flight Identification, position and height information is regularly updated on the VRS.

·       Aircraft positions are not to be corrected (or extrapolated) to allow for movement between the time of receipt or applicability of data and the time of display.

·       Prediction data is not displayed.

·      The VRS has the following operator functions,

·       Labels are used to provide, in alphanumeric form: Aircraft identity (e.g., Flight identification, SSR code and Aircraft Registration).

·       A solid line is provided to illustrate the past position of a selected aircraft.

·       Aircraft position targets are removed after a period of no data received.

·       If required the VRS will automatically move labels to prevent label overlapping and provides leader lines.

Display map requirements;

Display maps on VRS are based on general ground-feature maps, airspace maps based on controlled or uncontrolled sources, and custom-made maps or map features.

Testing and Verification of Required Performance Requirements;

Supplementary Amendment CAP 670 makes recommendations that the provider of Flight Information Services checks the performance of the equipment that they are using. This is also recommended by PilotAware when the ATOM GRID system is used for situational awareness as the performance of the system will vary from site to site due to site topology.

Supplementary Amendment CAP 670 states

‘The performance requirements for Accuracy and Target Latency of ADS-B are outside the control of the ANSP, and the CAA will commission ongoing testing to demonstrate continued compliance of CAP1391 devices with the national requirement. However, the ANSP shall carry out its own testing to provide assurance that the display system can meet all the Required Performance Parameters’.

·       CAP 1391 and other carry-on devices exhibit significant airframe blocking when positioned incorrectly particularly when remote antennas can’t be used to alleviate the problem. This will manifest itself in the targets’ detection and presentation on the VRS being spasmodic. The ATOM GRID minimises the problem by joining together detections from multiple ground stations and sharing the information with neighbouring ATOM stations. However, if the information is only picked up from a single ground station the trace will be poor and appear only occasionally. Please be aware that this is the fault of the transmission of the carry-on device not the reception by the ATOM GRID.

·       It is a good idea to check the positional accuracy and latency for each airborne target type (e.g., ADS-B, Flarm PilotAware Mode-S/3D or MLAT) by checks on circuit/runway position of aircraft or by checking pilot position reports against visual reference point (VRP) markers although the latter pilot position reports can be notoriously inaccurate.

·       For Mode-S transmitting targets detected using MLAT, each location will have a height at which the aircraft will not be interrogated by the ANSP terrestrial interrogators. The consequence of this is that the target will disappear from the VRS at a certain height. This can be as low as 300 ft at some airfields. Get to know what this is at your location. Note that high-flying commercial aircraft using TCAS will provide interrogation so it is possible to multilaterate down to ground level if there are enough receivers in range, but having enough receivers in your area cannot be guaranteed.

·       It will be beneficial to know of any local zone where situational awareness is difficult or not achieved. You will get to know your area and if experience shows that multilateration is not as good in a certain area, this may be overcome by the installation of a 360 RADAR 1090MHz receiver to overcome the deficiency.

·       The same exercise goes for different types of aircraft and EC types. Let your colleges know if their EC is spasmodic. Use PilotAware VECTOR to get them to improve their EC signature.


·       Multilateration (MLAT) is a form of independent cooperative surveillance. MLAT systems use the time difference of arrival (TDOA) of the existing 1090 MHz transmissions from aircraft, between several ground receivers to determine the position of the aircraft

·       PilotAware uses data from 1300+ 360 RADAR locations in the UK. This data is collected from Mode-S transponders that have been interrogated by third-party interrogators only. Neither 360 RADAR nor PilotAware employs interrogators, therefore this is a best-effort service.

·       The PilotAware system uses over 1300, 1090MHz receivers located throughout the UK to provide accurate MLAT data. All ATOM sites contribute to and are supplemented by all of the 360 RADAR sites. These sites have not been positioned for optimal performance at any location however, the performance provided is good due to the sheer volume of sites available. The performance will be optimised at any single site by the addition of strategically placed, low-cost, 360 RADAR receivers.

·       The available quantity of receivers in the PilotAware network and 360 RADAR networks is such that the loss of data from any single receiver or interrogator does not cause a loss of coverage.

·      The national broadband data network is used to connect the 1300+ sensors and the central servers of the PilotAware and 360 RADAR systems. There is in-built redundancy due to the size of the network and the links to the ATOM stations are encrypted. This is eminently suitable for a situational awareness solution and will be tested to see if it is suitable for future enhanced services.

Redundant Sensor Configuration;

The PilotAware system mitigates the loss of individual sensors by oversubscribing the number of sensors required throughout the UK. However, MLAT fidelity could be improved by the installation of more sensors in specific locations. If it is noticed that MLAT is less reliable in a certain area around your site then the installation of an additional 360 RADAR receiver may help.

Sensor Siting Requirements;

Supplementary Amendment CAP 670 recommends,

The structure upon which the receivers/transmitters, and antennas are mounted shall be of sufficient stability to withstand all expected weather conditions in the operational environment, especially with respect to maximum wind speed and icing. Where possible, the maximum wind speed, temperature and humidity conditions expected in the operational environment should be identified.

This is very good advice when a proper job is to be done installing the ATOM antennas.  

Power Supply;

Supplementary Amendment CAP 670 States

‘The stability of the power supply to the system shall be consistent with the availability and continuity of service requirements’.

This is true for the installation of PilotAware ATOM stations as it is for the installation of in-flight EC systems. Power over Ethernet has proven to be very reliable.

Low-Level Coverage;

Supplementary Amendment CAP 670 States

‘The coverage and the probability of detection in the low levels of altitude shall meet the performance requirements necessary for the intended application in the lower levels of the area of interest. Note: MLAT system coverage and the probability of detection can significantly vary across vertical levels.’

This is a very important principle not only for MLAT-derived targets but for any EC including ADSB, PilotAware Flarm and CAP1391, all of which are detected through line-of-sight UHF signals. Line of sight to any EC target, from a ground station at, very low levels is very problematic and is proportional to the number of ground stations available in any location. This is exacerbated in mountainous areas due to topographical obscuration.

With the PilotAware system, this is minimised through the use of PilotAware SkyGRID which uses in-flight PilotAware Rosetta data sensors to look down, collect and relay information to in-range neighbouring stations and hence to the national servers. This increases the probability that an individual target is not lost through topographical or airframe obscuration  This latest technology is not available to ANSP-centralised systems that are constrained to use aviation frequencies.  


Supplementary Amendment CAP 670 is a welcome document that recognises that technology and innovation move faster than regulation, none more so than in the development of Electronic Conspicuity. It acknowledges that many (over 120) airfields have installed high functionality yet extremely low-cost technology that arguably, provides situational awareness, of more aircraft than the traditional centralised systems in use by their larger commercial neighbours.

The document concludes that systems such as PilotAware ATOM GRID and SkyGRID have merit but are not fully acceptable for the provision of flight information services (FIS).  Conversely, they have sufficient maturity to allow them to be used to increase local situational awareness to improve aviation safety, as they have done on many documented occasions.

However, the new technology used has not yet been subject to the rigour of the intensive testing that the existing systems have undertaken over the past 80 years.  In this respect neither had the internet when it was introduced in the early 1990s but its disruptive technology has proven to be indispensable.

Consequently, companies such as PilotAware will continue to develop, without the constraint of developing in a regulated or monopolistic environment, to further the art of the possible using all technology available.

Time will be the arbiter of best practices in the development of interoperable electronic conspicuity.