Co-site Interference Testing
of
VDL Mode 4 and AM-DSB Voice

Kim O'Neil
Advanced Aviation Technology Ltd.

Abstract

The introduction of new datalink services into civil aviation is seen as one of the essential enabling technologies in the ICAO CNS/ATM concept. Datalinks are required for new services such as Automatic Dependent Surveillance - Broadcast (ADS-B) and Controller Pilot Data-Link Communications (CPDLC), as well as improved airline operational communications (AOC). These technologies will enable improvements in safety, capacity and operational efficiency. Despite the obvious desirability of these services, there is considerable debate about the merits of the various technologies on offer. This paper is provided as information to airlines considering installing ADS-B and datalink services based on ICAO's VDL mode 4 datalink.

1. Background

The datalink debate covers issues such as functionality, performance, spectrum efficiency and aircraft integration and has proven to be very contentious. Consequently, practical demonstrations of datalink characteristics, have become a necessary requirement. VDL mode 4 has already been subject to widespread trials across a very wide range of operational applications. These trials have been extremely successful, and have demonstrated the potential benefits of ADS-B, CPDLC and other CNS/ATM applications. VDL mode 4 trials have involved many aircraft types, clocking up more than a hundred and fifty thousand flight hours.

The aircraft trials and testing described here, were carried out on a Boeing 737-600 to investigate any possible adjacent or co-channel interference effects between VDLmode 4 and DSB-AM Voice in a realistic operating environment, with radios operating in a typical configuration. A realistic appraisal of radio quality was made in addition to practical measurements. The trial was carried out by CNS Systems AB on an SAS B 737-600 test aircraft and were supervised by the Swedish CAA. Practical demonstrations were made to an invited party of experts including ICAO AMCP members and Eurocontrol participants. The full results and voice channel recordings are available in a CD-ROM for comparison.

The tests demonstrate compliance to European Radio Standards and successfully refute the somewhat surprising claims made by proponents of other systems.

2. Available Datalink Technologies

Datalink technologies under consideration today include: several VHF DataLinks (VDL), Satcom, Mode S and its variants and the Universal Access Transceiver (UAT). For comparison purposes, these are summarised:

2.1 VDL Mode 2

VDL mode 2 is an air/ground D8PSK datalink, nominally operating at 31.5kbps, but with a net capacity estimated at around 3 to 4kbps. VDL2 is a Carrier Sense Multiple Access (CSMA) link, consequently the link is not available all the time - severely reducing its capacity. Significant frequency planning is required to protect its channels - due to low frequency re-use rates caused by high CCI (Co-Channel Interference) values of 22 - 26 dB. Although nominally a 25kHz service, VDL2 requires adjacent channels to be sterile. VDL 2 does not support tactical and time critical ATC datalink requirements.

2.2 VDL Mode 3

VDL3 is a Time Division Multiple Access (TDMA) D8PSK 25kHz service, operating at 31.5 kbps, presumably synchronised to GPS time and providing 4 channels per 25 KHz frequency (nominally 3 voice and 1 data). It has the potential to provide wide area datalink services with data rates up to 12.4 kbps. Intended as a digital replacement for analogue VHF voice, it is a key element of the FAA's planned ATC Nexcom upgrade.

2.3 VDL Mode 4

VDL4 is a Time Division Multiple Access (TDMA) GFSK 25 kHz service operating at 19.2 kbps with a net link capacity of 14kbps. VDL4 was specifically designed to be independent of ground infrastructure. It permits any number of Users to simultaneously share the link, enabling both air/air and air/ground communications. The FM modulation scheme also permits frequency re-use, due to a low CCI value of 10dB - meaning the loudest (and nearest) transmissions are always heard - and is consequently very spectrum efficient. VDL4 has good range and capacity and is seen as a candidate for all long-term applications. It has the greatest demonstrated capability in a wide range of intensive studies across many aircraft types and ground ATM platforms.

2.3 Mode S Enhanced Surveillance

Mode S Enhanced Surveillance (EHS) consists of Elementary Surveillance (ELS) with the addition of various Downlink Airborne Parameters (DAPs). Although there are some overlaps, this technology does not support air-to-air applications.

2.4 1090 Extended Squitter (ES)

1090ES is a derivative of Mode S technology, with messages being squittered at a high rate. The channel is shared with SSR Mode A/C, SSR mode S, Mode S EHS, TCAS/ACAS and military datalinks. The air-to-air range of 1090ES is relatively poor, making it unsuitable for long-term purposes and it suffers from multi-path (e.g. reflections from buildings, aircraft etc.), also making it unsuitable for airport applications. This is well documented in independent simulations and trials. 1090ES will saturate in Europe around 2010. There are also concerns about overloading 1090MHz (also occupied by TCAS) with yet another application.

2.5 Universal Access Transceiver (UAT)

UAT was developed by Mitre Corporation, and is a random squitter broadcast technology operating at around 960MHz with a bandwidth of 2-3MHz. Two message types are supported: air/air and air/ground ADS-B messages and ground-station uplink messages (weather, Flight Information Services - Broadcast FIS-B). This technology is a direct competitor to 1090 ES, but has better range and capacity. Work on ICAO UAT SARPs will start in the near future. Finding and protecting spectrum for UAT may be difficult.

Table 1: Comparison of Datalink Functionality 3. VDL Mode 4 to AM-DSB Voice Test

Objectives

The purpose of the test was to investigate the potential for co-site interference between a VHF Datalink Mode 4 radio operating (i.e. transmitting ADS-B messages) simultaneously with a AM-DSB voice radio onboard the same airframe. The tests investigate possible co-channel and adjacent channel interference of VDL 4 datalink to VHF Voice. Necessarily, protection of voice channels is an absolute primary safety concern.

4. Test set-up

The operational test was carried out at Linkoping Airport in Sweden in May 2003, using an SAS Boeing 737-600 aircraft. The VDL mode 4 Transponder Unit was manufactured by CNS Systems. The test was supervised by the Swedish Civil Aviation Administration (LFV) and carried out by CNS Systems AB, with the cooperation of SAS.

5. Hardware

Table 2 summarises the hardware configuration employed in the test, whilst Table 3 details the specific frequency test set up.

Table 2: Hardware Configuration

6. Setup

Table 3: Test Set up

Note that frequencies normally interfering with the regular air traffic were not used. The interfering transmitter was the co-site VDL Mode 4 transponder transmitting ADS-B data at the specified frequency. The load was 13.3 ms bursts at 1 Hz repetition rate.

PICTURE HERE Figure 1: Illustration of Test Scenario.

7. Test Results

A qualitative analysis of interference from VDL Mode 4 transmitter to VHF radio 1 victim during voice communication, gives the following results:

Table 4: Analysis of Impact of VDL 4 on VHF Voice

Recordings and analysis of the voice channel supports the results of Table 4. Recordings of the voice channels are available on a CD-ROM for independent analysis.

8. Analysis of Potential Interference

Analysis of potential interference from VDL Mode 4 transmitter to AM-DSB victim during voice communication is recorded in Table 5:

Table 5: Analysis of VDL4 interference into AM-DSB Voice VDL Mode 4

Table 6: VDL Mode 4 TX interference to AM-DSB victim on idle voice channel.

The AM-DSB receiver must be saturated at this high input level. However, the squelch did not open in any case.

9. Results

The results of the live test are consistent with the calculations indicated above. Audible ADS-B transmissions were only present in the co-channel case when the S/P value was much worse than the required 12 dB. Even with -20 dB S/P (as calculated above) the audible clicks from ADS-B transmissions with 1 Hz rate were quite acceptable and did not adversely affect voice communications. There was no audible interference on the first adjacent channel or at further channel separations. This is expected, since the radio should have ACR 60 dB @ +/- 17 kHz according to the ARINC 716-10 standard, and even better ACR at 25 kHz and larger offsets.

The test set-up and original results are documented on video, pictures and in audio files. A CD with the complete material can be obtained from kim.oneil@aatl.net or fredrik.lindblom@lfv.se

10. Conclusions

There was no audible interference on any of the adjacent channels - whether or not the squelch was open or closed. At the co-channel with the squelch closed, no ADS-B transmissions could be heard. Significantly, the squelch was not opened by the ADS-B transmissions. With the squelch open, the ADS-B transmissions could be heard as very short clicking sounds. The voice test message being read on the same channel was still heard loud and clear. Any reasonable aircraft installation should provide excellent and well-behaved VDL mode 4 datalink performance with no adverse impact on voice communications on any of the voice channels. This well-behaved performance is consistent with the practical experience gained in the many VDL mode 4 trials carried out on many different aircraft platforms.

Advanced Aviation Technology Ltd.
The Old Post Office,
The Street, Compton,
Surrey GU3 1ED. ENGLAND.
Tel. +44 1483 811 311.
Email: kim.oneil@aatl.net