Activities for 1030/1090 MHz Spectrum Saturation Alleviation

The paper describes the danger of continuing inevitable growing of transmissions on 1030/1090 MHz RF band and actual efforts for alleviation of RF pollution in the light of development of new technologies. Keywords-component; SSR, Mode S, ACAS, 1090 ES, Extended Squitter, Hybride Surveillance, Phase Modulation replies


INTRODUCTION
1030 and 1090 MHz remain the worldwide RF bands used to support cooperative aeronautical surveillance systems.These bands are shared by different applications such as SSR Mode A/C, SSR Mode S, MLAT (Multilateration Systems), ACAS, ADS-B 1090ES (1090 MHz Extended Squitter), including military applications.The global use of these RF bands allows a cost effective interoperable system [8].
On the other hand sharing this one spectrum among all above mentioned systems take the RF band very congested in high density airspace.The predicted growth in air traffic and an increased use of the aeronautical surveillance spectrum by both the civil and military surveillance communities creates the possibility that sometimes in the future, the spectrum may become congested to a point that existing and planned aeronautical surveillance and collision avoidance systems may not meet their desired performance requirements [1].A Mode S transponder shall be able to generate short Mode S replies at the rate of 50 Mode S replies in any one second interval and in addition to any downlink (a transponder level 2, 3 and 4), shall be able to generate at least 16 long replies of 50 Mode S replies in any one second interval.
We should note that the presented sample represent peak transmission load but on the other hand such results are nothing exceptional in European high traffic density areas.Moreover due to continual traffic growth it could be hardly expected that mode S load will be decreasing during a foreseeable future.

III. HYBRID SURVEILLANCE
Since a significant level of interrogations and replies fall to ACAS operation as you can see on Fig. 2, thus methods for reducing ACAS interrogation rates would offer great benefit to usage of the spectrum.Over time, ACAS standards have been enhanced to provide mechanisms to reduce interrogation rates.Last activities in this area were focused on technique known as Hybrid Surveillance.
Hybrid surveillance takes advantage of 1090 MHz extended squitter to reduce ACAS interrogations.For aircraft at surveillance range, whose announced position pass through the validation test, the position is further tracked by mean of passive acquisition of extended squitter.The validation test mentioned above includes comparison of relative range, relative bearing and barometric altitude determined by the active ACAS interrogations with derived relative range, bearing and altitude from information reported in squitter.If this test is satisfied then an active interrogation to validate the passive position occurs once every 60 seconds.
When the intruder is a near threat in altitude or range, but not both, it is tracked with passive surveillance, and that passive position is actively interrogated once every 10 seconds for validation.Exact conditions are specified below [3]: Since the length of a validation reply is twice the length of standard ACAS reply, a target that is tracked passively and validated once every 10 seconds doesn't provide reduction in 1090 MHz channel utilization relative to a target that is tracked actively once every 5 seconds [7].
The considered solution is simply use of interrogations which elicit short replies for the effect of subsequent more difficult computational process.

Additional considered improvement could be achieved by means of exclusive use of 1090 ES for surveillance of aircraft meeting certain conditions. This technique is sometimes called as Extended Hybrid Surveillance
The targets that meet the following conditions could be surveyed without any active interrogations [1] Many other ACAS improvements are suggested as far as the 1090 MHz channel load reduction is concerned.For example, significant restriction of ACAS interrogating (reduce interrogating power by 10 dB) when is aircraft operating on the airport surface.Or modify validating rate and prevent early transition to active surveillance because of a validating reply is not received.

IV. PHASE ENCOCED PULSES IN 1090 MHZ REPLIES
The present specification in ICAO Annex 10 determines that the reply data block shall consist of 56 or 112 data bits formed by binary pulse position modulation encoding of the reply data.Besides that a new standard is developed by ICAO based on phase modulation type reply.This technique is primary intended for 1090 MHz Extended Squitter messages where it could offer increased data capacity.
The technique consists of phase encoded pulses within a single standard pulse associated with a single phase state for each PPM data bit and in each of the four preamble pulses.For phase modulated signals, the PPM modulated reply data shall additionally contain 8PSK modulated data within the bit interval containing the pulse, either the first half or the second half of the interval.(i.e. 3 additional bits within one standard pulse are encoded by means 8PSK modulation).This encoding affords three additional 112-bit message segments within the standard 1090ES transmission.The phase overlay has the potential to increase data throughput, while maintaining backward compatibility to existing 1090 ES transmissions, without the need to increase the Extended Squitter rate.

V. CONCLUSION
Although the EUROCONTROL measurements were performed in high penetrated Mode S area, Mode A/C reply rate still remains relatively high.TCAS activities (especially DF=0) and All Call replies are the two main sources of Mode S RF pollution on 1090 in Europe.It is need to carefully predict future use of this link and development of new technologies and applications based on usage 1030/1090 RF band.For example, this is very important from the point of view the TIS-B (Traffic Information Service -Broadcast) implementation.

Figure 1 .
Figure 1.Mean number of replies per second on 1090 MHz frequency band [2].

Figure 2 .
Figure 2. Mean number of Mode S replies per second on 1090 MHz frequency band [2].

Figure 3 .
Figure 3. Mean duration of replies per second on 1090 MHz frequency band [2].
Where: a = intruder altitude separation in ft ȧ = altitude rate in ft/s r = intruder slant range in NM ṙ = range rate in NM/s When the intruder is a near threat in altitude and range, it is tracked with active surveillance at the standard once per second interrogation rate.If the following conditions are met, full active surveillance occurs [3]: This basic concept of utilizing passive positions information transmitting within extended squitter (1090ES) provides a means of reducing 1030/1090 MHz activities.Nevertheless researches have identified inefficiencies within the algorithm presented above as far as RF band consumption is concerned.ACAS without hybrid surveillance interrogates non thread target once every 5 seconds which elicit a short reply and potential threat target once every second by means of short interrogations and replies.Whereas validation interrogations (transmitted once every 10 s) during hybrid surveillance elicit long reply messages which includes extended squitter position register.This model was designed by reason of minimize processing effort for real time interpolation to be done by ACAS unit.The acquirement of position based on active measurement and position announced within 1090ES in the same time enable simpler comparison tasks.