VDL-ACARS Adjacent Channel Interference Test Results

VDL-ACARS adjacent channel interference test results.

Purpose of tests

The tests were conducted to measure the effect an impact of operating a VDL Mode-2 channel adjacent (25 kHz apart) to an ACARS MSK channel. These tests considered the following 4 cases of interference:

1. reception of POA ACARS traffic by an aircraft in the presence of VDL interference
2. reception of POA ACARS traffic by a ground station in the presence of VDL interference
3. reception of VDL Mode-2 AOA traffic by an aircraft in the presence of ACARS interference
4. reception of VDL Mode-2 AOA traffic by a ground station in the presence of ACARS interference

This report shows the results for the first test case scenario since this one is deemed to be the most critical at this time due to:

1. majority of datalink equipped aircraft during the intial rollout of VDL will still be using ACARS and VDL will be perceived as the ‘intruding’ traffic
2. effect of VDL interfence on the aircraft are more difficult to mitigate than at the ground station where several measures can be taken, such as spacial separation, transmitter-receiver isolation etc.

Test environment

The tests were conducted at the SITA Aircraft Services testbed in Montreal using the test setup in place to perform VAQ ACARS avionics qualification. In this setup, shown below, the ACARS environment is simulated using production ground station and avionics equipment as well as RF combiners/splitters and attenuators to simulate the RF channel.

The testing was performed as follows:

1. the ACARS avionics and the ground system were allowed to establish an ACARS link (using Q0 and other messages) and the level of the ACARS RF signal at the aircraft VHF transceiver was measured. The frequency at which the ACARS system was operating was 131.550 MHz.
2. the VDL XCVR was then commanded to transmit continuously VDL bursts to simulate a busy VDL channel at a frequency of 131.525 MHz. The RF signal level of the VDL interference was then adjusted to the desired level.
3. A script was started at the uplink message generator to transmit a 120 byte long (approximately 450 msec long) ACARS message a 100 times and measure the number of acknowledgements received from the avionics.
4. After each 100 message run the ACARS and VDL interference levels were adjusted to measure the effect in different conditions. The results were entered into the table shown in section 3.

Characteristics of the interfering VDL signal

The duration and duty cycle of the interfering VDL signal is shown below. This represents a considerably loaded channel.

Figure1. The timing characteristics of the VDL interference signal.

Figure 2. The spectral characteristics of the interfering signal.

The characteristics of this spectrum are:

-center frequency = 131.525 MHz

-span = 50 kHz

-Resolution BW = 1 kHz

-Video BW = 1 Khz

-Reference level = 0 dBm

-Scale = 10 dB/division

Test results

The results of the tests are shown in the table below:

Note 1: tests in these conditions were not performed because they represent unrealistic conditions. In order to achieve a condition where the ACARS signal received at the aircraft would be between –40 dBm and
–100 dBm the aicraft would have to be in flight, relatively far from the ground station, yet the interfering signal would be quite strong, meaning that it would have to originate from another aircraft. In this case, the interfering aircraft would be most likely too close to the ACARS aircraft from safety and separation point of view and in addition would at most generate very infrequent interference transmissions.

Note 2: under these conditions, ie.: VDL weaker than ACARS, it is certain that ACARS receptions will not be affected.

Note 3: tests under these conditions were not conducted due to limitations (temporary) of the test environment. However, they represent a minority of the possible test conditions and its impact is not judged to be important.

Note 4: The test environment was slightly changed in order to be able to achieve the necessary differences in signal levels. As a result, the actual ACARS signal level was –84 dBm and the VDL interference was at –27 dBm. The characteristics of the interfering signal, however, remained as they are shown in Figure 1.

Note 5: The timing characteristics of the interfering signal were modified for this test. The RF signal levels remained the same but the duty cycle of the interference was lowered to 10%.

Note 6: The timing characteristics of the interfering signal were modified for this test. The RF signal levels remained the same but the duty cycle of the interference was lowered to 5%, which is the lowest that the interference generator allowed.

Conclusions

Based on the results of the tests conducted so far it appears that there is sufficient confidence that the simultaneous operation of VDL and ACARS on adjacent channels in the VHF frequency band is possible with no apparent impact on the incumbent ACARS traffic. These tests, however, were conducted in an enclosed, electromagnetically ‘quiet’ environment and not in the ‘noisy’ RF channel that is currently encountered by aircraft. For example, this testing did not produce any 1-st and 2-nd order intermodulation products which are known to affect VHF transceivers.

The limiting factor for operating ACARS and VDL on adjacent channels appears to be not only the relative difference in signal levels but also the timing characteristics of the interfering signal. Even when the VDL interference signal strength is 57 dB stronger than ACARS, it has negligible impact at very low channel occupancy level, or duty cycle ie.: at 5% or below.

Further tests will be conducted to determine the impact of adjacent ACARS traffic on VDL Mode 2 operations.

Prepared by:

Zbig Jasiukajc and Giuseppe Capobianco

SITA Aircraft Services

System Integration and Avionics Qualification

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