Working Party 8D Draft Report: Impact of emissions of the ...



|[pic] |INTERNATIONAL TELECOMMUNICATION UNION |AMCP WF7/WP14 |

| |RADIOCOMMUNICATION |Document 8D/TEMP/144-E |

| |STUDY GROUPS |31 October 2001 |

| | |English only |

Source: Document 8D/127

Working Party 8D

(SWG 8D5)

Report

IMPACT OF EMISSIONS OF THE DISTANCE MEASURING EQUIPMENT AND TACTICAL AIR NAVIGATION SYSTEM (DME/TACAN) OPERATING IN

THE AERONAUTICAL RADIONAVIGATION SERVICE IN THE BAND

1 164-1 215 MHZ ON THE RADIONAVIGATION-SATELLITE SERVICE

ON-BOARD RECEIVERS

1 Interference scenario

In this annex, we consider only the interference from ground-based DME/TACAN as the main source of interference to proposed future RNSS systems. Interference to RNSS receivers due to out-of-band emissions from other equipment on-board the aircraft has not been taken into account.

DME and TACAN are pulse-ranging navigation systems that operate in the 960-1 215 MHz frequency band. DME systems provide distance measurement for aircraft, TACAN, a military navigation system, provides both azimuth and distance information. DMEs and TACAN operate in four modes (X, Y, W, Z) as shown in Figure 1. Only those X mode replies in the 1 151-1 213 MHz frequency band are predicted to potentially interfere with the operation of proposed future RNSS systems. The DME/TACAN navigation system consists of an airborne interrogator and ground-based transponder. An aircraft's interrogator transmits pulse pairs on one of 126 frequencies with 1 MHz spacing. The X-mode ground transponder (beacon) transmits pulses in pairs with a pulse interval of 12 microsec. The assumption of 2 700 pulses pairs per second (ppps) for DME and 3 600 ppps for TACAN is well accepted as a worst-case scenario to be used for simulation.

The regions of the world having the densest DME/TACAN concentrations include Europe and the United States. The following sections in this annex provide a methodology for assessing the impact of emissions of DME/TACAN on RNSS systems followed by simulation results for Europe and the United States.

[pic]

Figure 1

Standard DME/TACAN channel plan

2 Simulation modelling

2.1 DME/TACAN modelling

In first approximation, the instantaneous received power of a DME/TACAN X-mode pulse pair separated by 12 microseconds can be written as:

[pic] (A2-1)

where:

p = is the received radiated power of the undesired signal from one DME/TACAN pulse (Watts)

( = 4.5 ( 1011 (seconds–2)

A typical ground-based DME/TACAN transponder antenna gain pattern is shown in Figure 2. An envelope useful for simulation is also shown in Figure 2. Most (around 87.5%) of the DME/TACAN radiated power is contained within a 0.5 MHz bandwidth centred on the 1 MHz channels.

[pic]

figure 2

Typical DME/TACAN antenna gain vs. elevation angle

2.2 RNSS receiver modelling

Two types of RNSS systems are considered in this annex. Type 1 systems include only wideband signals and associated receivers are assumed to have the filtering response shown in Figure 3 (5.5 dB/MHz). Type 2 systems include both wideband and narrow-band signals. Type 2 receivers are assumed to employ two filtering chains - one for the wideband signal with the response shown in Figure 3 and one for the narrow-band signal with response shown in Figure 4.

[pic]

figure 3

Assumed RNSS wideband receiver filter response

[pic]

figure 4

Assumed RNSS narrow-band receiver filter response

The minimum RNSS receiver antenna gain in any satellite direction is (4.5 dB, which corresponds to a 5 degree elevation angle. For simulation purposes, only that value is considered.

The RNSS receiver is assumed to employ pulse blanking. When the instantaneous received power is above the thresholds below, it is assumed that the receiver zeroes out the received signal:

• narrow-band signal: (126.5 dBW;

• wideband signal: (116.5 dBW.

A 1 microsecond period is assumed for the blanking reaction time.

2.3 Criteria of interference

The methodology for determining compatibility consists of determining the degradation to the RNSS receiver's post-correlation signal-to-noise power density ratio (S/N0) due to the presence of DME/TACAN signals. An acceptable level of degradation is defined to be one that yields the S/N0 attained at L1 for SBAS (see Table). From the table, it can be seen that acquisition is the worst-case in terms of RNSS susceptibility to interference.

| | |Narrow-band (2 MHz) |Wideband (20 MHz) |Wideband (20 MHz) | |

| |L1 SBAS |or wideband (20 |(157 dBW |(152 dBW | |

| | |MHz), | | | |

| | |(154 dBW | | | |

|Received signal power |(161 |(154 |(157 |(152 |dBW |

|Implementation losses |(2 |(2 |(2 |(2 |dB |

|Minimum antenna gain |(4.50 |(4.50 |(4.5 |(4.5 |dBi |

|Sky temperature |100 |100 |100 |100 |K |

|Receiver excess temp |412.90 |626 |626 |626 |K |

|(Need to be confirm) | | | | | |

|Noise floor |(201.50 |(199.99 |(199.99 |(199.99 |dB(W/Hz) |

|Minimum S/N |34 |39.5 |36.5 |41.5 |dB-Hz |

|Track mode | | | | | |

|S/N no margin |30.50 |30.50 |30.5 |30.5 |dB-Hz |

|S/N with safety margin |32.8 |32.8 |32.8 |32.8 |dB-Hz |

|Io max tolerable, no margin |(200.5 |(191.5 |(195.2 |(189.3 |dB(W/Hz) |

| |(110.5 |(101.5 |(105.2 |(99.3 |dBm/MHz |

|Io max tolerable, with safety margin|(116.5 |(104.4 |(108.7 |(101.9 |dBm/MHz |

|Maximum S/N degradation, no margin |3.5 |9 |6 |11 |dB |

|Maximum S/N degradation, with safety|1.2 |6.7 |3.7 |8.7 |dB |

|margin | | | | | |

|Acquisition mode | | | | | |

|S/N no margin |32.80 |32.80 |32.80 |32.8 |dB-Hz |

|S/N with safety margin |33.7 |33.7 |33.7 |33.7 |dB-Hz |

|Io max tolerable, no margin |(206.5 |(194.4 |(198.7 |(191.9 |dB(W/Hz) |

| |(116.5 |(104.4 |(108.7 |(101.9 |dBm/MHz |

|Io max tolerable, with safety margin|(122.5 |(105.5 |(110.4 |(103.0 |dBm/MHz |

|Maximum S/N degradation, no margin |1.2 |6.7 |3.7 |8.7 |dB |

|Maximum S/N degradation, with safety|0.3 |5.8 |2.8 |7.8 |dB |

|margin | | | | | |

An equation to determine the S/N degradation for an RNSS receiver due to DME/TACAN is:

[pic]

[pic]

(A2-2)

where:

n = is the number of DME/TACAN in visibility (distance between DME/TACAN and RNSS receiver < horizon)

= is one of the DME among the n in visibility

To = is the Gaussian pulse pair period

PDCB = is the percentage of time when the RNSS receiver is blocked

Bd = the receiver bandwidth: 20 MHz for wideband RNSS signal and 2 MHz for narrow-band signal.

p(t) = has been defined above, with p as follows:

p = DME/TACAN radiated power + DME/TACAN antenna gain (() + RNSS antenna gain (ϕ) - free space loss (f, d) + RNSS filter (f) - atmospheric attenuation and the diffraction attenuation.

Use of the equation to predict an RNSS receiver's performance in the presence of many undesired pulsed signals is only valid when the pulses are very short relative to the minimum predetection integration time used by the receiver (1-10 ms, depending on the mode of operation). This constraint is satisfied for DME/TACAN.

3 European results

3.1 Simulation

The methodology described in Section 2 was employed in this study with some specific assumptions:

• A value of 2 700 pps was used for both DME and TACAN (this value is well accepted as a worst-case average value for systems operated in Europe).

• The RNSS receiver antenna gain in the direction of DME/TACAN is taken into consideration as follows. The assumption used when the aircraft is flying in a horizontal position is –10 dB for both LCP and RCP (antenna gain in lower hemisphere) (Figure 5). The RNSS receiver antenna radiated pattern used for simulations with an aircraft having a roll angle of 33° is:

G(Φ) = 7 ( 1.957 ( 10(3 Φ2.01 for 0 < Φ < 80° (A2-3)

G(Φ) = 21.59 ( 0.3882 Φ + 5.455 ( 10(4 Φ2 for Φ > 80°

where:

Φ = 0 is the vertical upwards axis of the aircraft.

[pic]

Figure 5

RNSS receiver gain assumption (aircraft flying in a horizontal position)

• The gaseous attenuation and the diffraction attenuation are taken into account.

• All DME/TACAN ground transmitters are considered, up to the electromagnetic horizon (4 130(h with h in m).

3.1.1 Simulation steps

In order to evaluate the interference from ground-based DME/TACAN to RNSS receivers, a simulation tool was created, taking into account the characteristics mentioned above. A simulation round consists of:

– simulating the S/N and blanking percentage of an airborne receiver at a specified altitude, in each position over Europe sampled every 0.5 degrees of latitude and 0.5 degrees of longitude. The aircraft is placed at the horizontal position. The tool provides the worst figure found: worst position for the aircraft and the corresponding S/N degradation;

– the aircraft is placed at the worst position and the simulation at this position is made 10 000 times varying the initial pulse positions of the DME/TACANs in order to obtain a good statistic of the events;

– the cumulative distribution function (CDF) which gives the probability to have an S/N degradation below a certain value is calculated at the worst aircraft position over Europe.

3.1.2 Results

3.1.2.1 Altitude impact

The altitude impacts the S/N degradation, and the worst case found corresponds to the highest altitude (40 000 feet) due to the high number of DME/TACAN in visibility. Therefore all the simulations presented in this paper are with an altitude of 40 000 feet.

3.1.2.2 Aircraft inclination

When an aircraft banks from a horizontal flight, the RNSS antenna attenuation for DME signal varies. An inclination up to 33° has been considered here.

The simulation has been performed with an aircraft at 40 000 feet having a roll angle of 33° towards eight different directions (N, NE, E, SE, S, SW, W, NW). Compared to horizontal flight, the simulation with banking showed an increase of 0.5 dB of the worst-case S/N degradation and an increase of 11% of the worst-case percentage of blanking time for all locations and azimuth.

3.1.2.3 First scenario

For the first scenario, a Type 2 RNSS (see Section 2.2) was used, comprising a wideband signal at

–157 dBW and a narrow-band signal at –154 dBW with a frequency centre at 1 202.025 MHz.

The following figures (Figures 6-7) show the cumulative distributions functions CDF for an aircraft at the horizontal position, at 40 000 feet and at the worst-case position over Europe. This is also valid for Section 3.4.

The probability of having a S/N degradation above the maximum tolerable S/N degradation in acquisition mode and with the worst condition in term of position (altitude, longitude and latitude), is 100% with the wideband receiver and 0% with the narrow-band receiver.

[pic]

FIGURE 6 ((157 dBW)

CDF for the wideband receiver

[pic]

FIGURE 7 ((154 dBW)

CDF for the narrow-band receiver

3.1.2.4 Second scenario

For the second scenario, a Type 2 RNSS (see Section 2.2) was used, comprising a wideband signal at –157 dBW and a narrow-band signal at –154 dBW with a frequency centre at 1 176.45 MHz.

The probability of having a S/N degradation above the maximum tolerable S/N degradation in acquisition mode and with the worst condition in term of position (altitude, longitude and latitude), is as the previous case 100% with the wideband receiver and 0% with the narrow-band receiver.

Thus, the choice of frequency has no major impact.

3.1.2.5 Third scenario

For the third scenario, a Type 1 RNSS (see Section 2.2) was used, comprising a wideband signal at

–152 dBW with a frequency centre at 1 202.025 MHz.

The probability of having a S/N degradation above the maximum tolerable S/N degradation in acquisition mode and with the worst condition in term of position (altitude, longitude and latitude), is 0%.

[pic]

FIGURE 8

CDF for the wideband receiver ((152 dBW)

3.1.2.6 Possible solutions to enhance the wideband receiver performance

Figures 6 shows that receivers will have difficulty in the acquisition of the low-power ((157 dBW) wideband RNSS signal at high altitudes over regions with a large number of DME/TACAN.

Some mitigation techniques could be investigated including:

( The use of frequency adaptive filters in order to minimize the impact of DME in addition to time blanking technique.

( Possible simultaneous use of both the wide and narrow-band RNSS signals.

3.1.2.7 Consideration on the impact of the introduction of potential new ARNS system in the band 1 164-1 215 MHz on RNSS systems

Concerning the RNSS receiver interfering environment, current ARNS systems characteristics were used (DME/TACAN) in the studies[1].

3.1.3 Theoretical simulation conclusions

The results of this simulation with the assumptions presented in this annex, show that we can be optimistic regarding the use of RNSS narrow-band receivers until the altitude of 40 000 feet without harmful interference from DME/TACAN. Regarding wideband receivers operating with –157 dBW of wanted signal, in certain geographical areas of the world, due to a higher number of DME/TACAN emitting in the receiver band and the lower wanted wideband signal power, the acquisition of an RNSS satellite could be more difficult. However, in the case of wideband receivers using (152 dBW of wanted signal, we can be optimistic regarding the capacity for RNSS wideband receivers to operate with DME/TACAN environment. This needs to be confirmed and

ARNS band environment measurements could be useful. We must also take into account some solutions as presented in Section 3.1.2.6 which can improve the wideband receivers satellite acquisition and then to ensure their use at altitudes up to 40 000 feet.

2 Environment measurement of the band 1 164-1 215 MHz

Two independent measurements campaign were performed.

3.2.1 First measurement campaign

The last simulation provides theoretical results. The next step consists to validate the simulation tool comparing them to flights measurements. That is why the ARNS band environmental measurement was performed using an aircraft as shown in Figure 9.

3.2.1.1 Measurements description

3.2.1.1.1 Antenna measurement position on the aircraft

One antenna was used for the measurement: a GPS L2 antenna. The following figure represents its position on the aircraft.

[pic]

FIGURE 9

Aircraft

3.2.1.1.2 Flight route over France

Two days of measurements have been performed over France: (Toulouse to Lille and Lille to Lanion).

Each route is described in the following figures:

[pic]

FIGURE 10

Aircraft altitude:

[pic]

FIGURE 11

[pic]

FIGURE 12

3.2.1.1.3 Measurement line

The following figure describes the measurement line used when the measurements are performed using the GPS L2 antenna.

[pic]

FIGURE 13

Line transmission measurement:

[pic]

FIGURE 14

Antenna characteristics

GPS L2 antenna radiated pattern:

[pic]

FIGURE 15

Antenna adaptation measurement (in the band 1 164-1 215 MHz)

[pic]

FIGURE 16

Cable characteristics

Cable 1 loss: 0.5 dB

Cable 2 loss: 1.6 dB

RF Filter characteristics

Pass band (1 164-1 300 MHz)

Insertion loss 2 dB with the objective of ................
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