Composite ATM and airspace surveillance system

15 February 2014
For several dozens of years, the major ATM surveillance asset has been secondary radars, while Mode S monopulse secondary radars have been de facto considered standard assets since the 2000s.
Recently the development of the Global Navigation Satellite System (GNSS), coupled with the desire of air navigation providers to reduce the price of the ground equipment and operating costs prompted another surveillance technology, namely the ADS-B.
The technology allows to reduce dramatically the surveillance equipment price, cut emission through assigning to the aircraft the most effective flight levels, potentially improve position fixing accuracy, as well as receive additional data via aircraft-ground links improving dramatically the surveillance quality and thus enhancing flight safety.
ADS-B makes the aircraft measure its own coordinates and other navigation parameters to transmit them along with the ID data to the ground. These are received by a ground station designed to receive messages from the aircraft, decode them, check their consistency and generate air situation reports in standard formats.
Such ADS-B ground stations are roughly 10 times cheaper than secondary radars in terms of equipment and operating cost due to simpler motionless antennas and lack of transmitters.
Difficulties of phasing in ADS-B constitute its disadvantages. The coordinates are received by the ground stations via communications lines, hence there is no way to check that what has been received is the actual aircraft coordinates. Moreover, one self-sustained ADS-B ground station cannot even check whether they were received from an aircraft at all and not from other source.

The reason is attributed to the vulnerability of GNSS signals caused by:
GNSS faults (failures or replacement of satellites), 
Deliberate limitation of the GNSS performance by the operator, 
Undetected faults of the on-board navigation system, 
Inadvertent jamming by signals of the same or adjacent frequency, 
Jamming or substitution of GNSS signals, 
Substitution of ADS-B signals.
For safety reasons the use of ADS-B data in the most critical surveillance areas should be augmented by a capability providing independent (from on-board navigation systems and GNSS) measurements of coordinates. 
This can be provided by secondary radars or series of multilateration stations.
The latter is based on extremely accurate, to the extent of nanoseconds, measurement of the time the signal is received at each station to be later used to calculate the coordinates at the MLAT server, for which the time difference is used. Additionally, the MLAT system can feature a secondary radar interrogator of its own to double as a secondary radar. This configuration will even be able to provide the range to the target and thus extend the surveillance envelope.
Once ADS-B and MLAT came into being the airspace surveillance started through another loop of its development. In the past 50 years it was provided by secondary radars backed-up by primary radars that had been relieved by the former.
As time goes on ADS-B will take over as the major surveillance system, backed up by secondary radars, where the field already exists, but the ultimate redundancy will be provided by MLAT, where radars are non-existent or have run out of their service life and thus are facing retirement.
Thus, ADS-B comprises a third surveillance layer after fully self-sustained systems (primary radars) and independent cooperative assets (secondary radars, MLAT). Each of the layers supported by appropriate technologies and types of equipment allows to perform its own set of tasks, either constituting new capabilities or providing redundancy and safety.
ADS-B communications lines
As of today standard ADS-B lines are as follows: 1090 ES, UAT, and VDL-4.
At the 11th ICAO International Conference decisions were made to use 1090ES based on Mode S (secondary radar) as primary in the world. Among the advantages listed were: the only communications line with a dedicated frequency band, availability of on-board antenna feed systems, no electromagnetic compatibility issues, appropriate for independent measurement of coordinates.
UAT was developed in the U.S. to unload the 1090MHz channel in some regions. It is used for surveillance of the lower airspace. The deployment of the second communications line came at a price. The ground infrastructure became much more expensive for it was necessary to equip ground stations to provide both 1090 ES and UAT modes and mutual signal repeating capability, let alone the requirement to install UAT transponders on the aircraft. 
Developed in Sweden in the 1980s, VDL-4 has never been introduced in any country ever since. Its problems range from electromagnetic incompatibility in the UHF band, in which it is used, and non-dedicated frequency band to being inappropriate for multilateration surveillance systems.
Many years that ADS-B 1090 ES has been operated in the U.S., Europe and Australia have resulted in terabytes of data. These were used to evaluate the advantages and shortcomings of the technology. In fact, making its way in the wake of the world experience with ADS-B saves Russia a fortune.
The major argument in favor of developing a second data transmission line is that 1090 MHz is overwhelmed by secondary radar signals, however the main sources of jamming for ADS-B are A/C radars (ATCRBS). One of the ways to reduce radiations in this band is to switch to Mode S, which is also favored by flight safety considerations. 
This will require all aircraft to be equipped with appropriate transponders. The task has already been accomplished in Europe. Surveillance of St.Petersburg’s airspace suggests that almost 100 percent of commercial aircraft already carry Mode S transponders, about 80 percent of this number are ADS-B 1090 ES capable.
Another argument supporting the single line solution for data transmission is the functionality margin of the 1090 ES signals that allows to boost the throughput several times over (efforts are already underway in the world), as well as the fact that several formats have been reserved for the military providing the capability to cipher data transmitted through 1090 ES.
All in all, if ADS-B 1090 ES is provided in Russia on compulsory basis, where the air traffic density is dozens of times lower than in the U.S., deployment of another data line particularly taking into consideration the need to set up and maintain double ground infrastructure, does not seem to make much sense economically.

Secondary radars for special purpose aviation
The special purpose aviation aircraft feature RBS 1090 MHz and ATC 740 MHz transponders. Although ATC is subject to conversion and retirement, still up to five aircraft are airborne at any given moment in the skies of St. Petersburg.
Apart from these systems required for ATM, the Russian AF aircraft are equipped with IFF transponders.

Composite surveillance concept
The Composite Surveillance System (CSS) provides surveillance by the secondary radar, ADS-B 1090 ES and IFF signals at all times at the required precision and update rate in various areas.
For CSS establishment, it is essential to use a combination of the following critical principles:
Gradual transition to new surveillance technologies (ADS-B, MLAT) with all current flight safety measures provided,
Exploitation of additional advantages provided by better performance of the new equipment, i.e. better accuracy, higher air situation update rate, etc. These may include surveillance in out-of-reach areas over cities, sea, and mountains or implementation of new applications, e.g. improved approach surveillance, surveillance on small airfields or at heliports,
No revolutionary changes on board of aircraft by virtue of using the current secondary radar and IFF radio channel system,
Prudent and sufficient changes of the ground surveillance infrastructure to the extent of deployment of compact and cheap ground stations, as well as improvement of communications channels,
Improvement of ground surveillance performance, including reliability and survivability provided by virtue of adopting a distributed arrangement for ground stations; operating cost reduction by virtue of deployment of smaller and less complex equipment and reduction of the power consumption.

As far as technical issues go, the CSS is based on technologies and capabilities as follows:
For the commercial aviation and ATM in general the current Mode S/1090 ES is used,
For the special purpose aviation, provided the appropriate on-board equipment is available, the current IFF system is used,
Employed are distributed receive and transmit ground stations, both secondary radars and IFF, with synchronized time scales,
The receive stations can receive both ADS-B signals and responses to the interrogation signals sent by their own interrogators or elsewhere: А/С/S and IFF,
The received signals are marked by a precise time tag and sent to the ADS-B/MLAT server to calculate the coordinates and generate aircraft tracks,
The server can transmit ADS-B and MLAT flight data to various users, either combined or independent, providing individual protocol for each user (user profile),
The ground stations can be fitted with various types of antennas depending on the area under surveillance and deployment place. They can be omnidirectional, sectoral, multi-sectoral or directional.