INITIAL RESULTS ON NARROWBAND AIR-GROUND PROPAGATION CHANNEL MODELING USING OPPORTUNISTIC ADS-B MEASUREMENT

FOR COVERAGE DESIGNJ. Naganawa, H. Miyazaki, T. Otsuyama, J. HondaElectronic Navigation Research Institute (ENRI),

National Institute of Maritime, Port and Aviation Technology, Tokyo, Japan

AbstractFor making deployment of aeronautical

surveillance systems successful, an appropriate propagation channel model should be selected in the coverage design. The selection of the channel models is particularly important for Automatic Dependent Surveillance – Broadcast (ADS-B) because it suffers from 1090 MHz co-channel interferences. Meanwhile, many aircraft have already started to transmit ADS-B signals. Therefore, as a new means of air-ground propagation study, ADS-B signals of target-of-opportunity can be measured. The resulting data covers various route, attitude, weather, aircraft models. Then, such large-scale data can be analyzed to verify the existing channel models and to derive empirical models and parameters. In this paper, discussions and a measurement are made to investigate the limitations and measurable propagation characteristics by the proposed approach.

IntroductionTo increase safety, efficiency, and capacity of

Air Traffic Control, deployment of Automatic Dependent Surveillance – Broadcast (ADS-B) have been started/planned worldwide as a high performance surveillance system [1-6]. However, weakness of ADS-B is that its performance is more sensitive to the received signal strength and co-channel interference than the conventional secondary surveillance radars because of the following reasons. First, the ADS-B antennas have a wide beamwidth or are omnidirectional in

horizontal plane, which results in lower gain and more interfering signals. Second, ADS-B is passive unlike SSR that can repeat interrogations.

To obtain the stable ADS-B performance, coverage design plays an important role [7,8]. Coverage design is a process to decide places to install ground stations. When the ground station are installed densely, good performance is expected, but the cost for installation and operation increases. Therefore, coverage design is important to achieve a required performance while minimizing the cost.

Successful coverage design needs appropriate channel models. In [7,8], CRC-Predict Model [9] was used for designing the station layout in U.S. after verifying it with IF-77 Model [10]. Propagation curves in an ITU-R commendation are also available [11]. An extensive literature review on the air-ground channel models is available in [12], and recent flight experiments and modeling were reported in [13-19].

As seen in [13-19], a traditional and reliable approach of radio propagation study is flight experiments. However, generally speaking, flight experiments are limited in the variety of measurement conditions. As a complementary approach, measurement of ADS-B signals broadcasted by targets-of-opportunity is promising. Although exact channel transfer functions cannot be measured, the received signal can be used for validating existing models or empirical modeling. Such approach have been suggested in [20-21], where a simple log-distance path loss model was obtained. In addition, a system architecture to

analyze massive surveillance data obtained from the ADS-B signal is proposed in [22].

Although applying ADS-B signals to propagation measurements sounds straightforward and easy, there are several issues to be addressed. For example, the effect of the antenna diversity is so large to extract the contribution of the propagation channel. This issue has been addressed by an estimation technique [23], and then the validation against the free space model has become possible.

In this paper, an additional effort for enabling the ADS-B-based propagation analysis is provided. The contribution of this paper is three-fold as follows.

1. By discussing the limitation and measurable propagation characteristics, this paper identifies signal strength variations (path loss, fading, and shadowing) are a promising characteristic to be measured and analyzed.

2. This paper further identifies the two issues in the measurement and analysis of the signal strength variation. First is a measurement accuracy issue; a recommended decoding technique generates a “reference level” but this may be different from what the propagation researchers expect. Second, analysis of path loss, fading, and shadowing are limited by transponder power deviation, ADS-B transmission rate, and antenna diversity, respectively.

3. To examine the issue of measurement accuracy, this paper further conducts a measurement-based verification. In particular, a practical ADS-B receiver and a software defined radio (SDR) receiver are compared. This comparison is also aimed at verifying the long-term measurement data stored by the authors’ experimental system.

This paper is organized as follows. In Section 2, discussions on the expected usage, advantage, limitation, and measurable propagation

characteristics are made. In addition, issues related to the signal strength variation are identified. In Section 3, measurement-based verification is conducted, where the measurement results by a SDR receiver and a practical receiver are compared. In Section 4, this paper is concluded.

ADS-B Approach

Expected UsageAircraft that supports ADS-B out broadcasts

signals called “squitter” to disseminate information about itself necessary for air traffic control or collision avoidance [24-26]. Three major types of squitters, i.e., airborne position, airborne velocity, and identification, are transmitted periodically. By measuring and analyzing them, following two approaches of radio propagation study will be possible.

1. Verification of the existing models

By comparing the measured propagation characteristic and prediction by the existing models, verification can be made. For example, received signal strength can be compared with the models for coverage prediction.

2. Derivation of empirical models

By applying the statistical analysis to the measurement data, empirical models can be derived. For example, measured signal strength variations can be modeled by a known probability distribution.

AdvantageThe opportunistic approach has several

advantages. First, various measurement conditions can be covered in terms of flight path, attitude, aircraft models, and weather. Second, measurement is conducted in passive manner so that a transmitter is not necessary, which significantly reduces the measurement cost. Moreover, since the ADS-B signal is not encrypted and simple, either off-the-

shelf receiver, SDR receiver, or operational equipment can be used. This widens the opportunity for many researchers to start air-ground propagation study. Third, information contained in the ADS-B signals are helpful for analysis. For example, the airborne position squitter provides the aircraft latitude, longitude, and barometric altitude, which can be used as a transmitter position. In addition, the aircraft Mode S address allows to identify which aircraft have transmitted the signal.

LimitationOn the other hand, the proposed approach is

suffered from some limitations, which are clarified in this subsection. Here, factors that are unique to the ADS-B signals are mainly discussed. Considerations common in propagation measurement can be found in the literature, e.g., [27].

1. Exact transmission signal unknown

Since the measurement is passive, the exact transmit waveform is not available. The provision [24-26] is not such strict to satisfy the accurate propagation measurement. For example, the carrier frequency is allowed to be plus or minus 1 MHz from 1090 MHz [24]. The provision on the transponder power specifies a relatively wide range, e.g., 21 to 27 dBW [24].

2. Successful decoding necessary

The 1090-MHz band is shared by all the aircraft. To identify the originating aircraft of the received signal, it is necessary to read out the aircraft address. This means the signal must be successfully decoded for being used in the propagation analysis. For example, when the signal is significantly distorted by multipath, such signal cannot be used for analysis.

3. Co-channel interference

Another constraint due to the 1090-MHz band sharing is co-channel interference. This problem is known as False Replies Unsynchronized to Interrogator Transmission (FRUIT) or garble. The collided signal cannot be used for analysis.

4. Transmission scheme

The airborne position squitter and airborne velocity squitter are broadcasted at a random interval uniformly distributed from 0.4 s to 0.6 s [24], which limits the sampling rate. In addition, the transmission is made alternately from the top and bottom antennas [24], which causes significant variation in the received signal strength. This effect is so large that the contribution of the propagation channel is hidden, but this issue has been addressed by a technique to estimate the transmit antenna [23].

Measurable Propagation CharacteristicsBased on the discussion in the previous

subsection, measurable propagation characteristics are discussed. Among the different characteristics, signal strength variations are found promising.

1. Signal Strength Variation

Received signal strength variations are easily achieved just by measuring the amplitude of detected pulses. The variations include the effect of path loss, fading, and shadowing. However, the decoding process may degrade the accuracy of the measured signal strength (limitation 2). Moreover, the transmit interval and transponder power deviation may constrain the analyzable propagation mechanisms (limitation 1 and 4). These issues will be discussed in more detail in the following subsection.

2. Delay

Two reasons make the time delay measurement challenging. First, the exact transmit signal is

not known (limitation 1), which further makes it difficult to estimate exact channel transfer functions. Second, multipath with a large delay spread may distort the signal undecodable. As limitation 2 says, a signal must be decodable to be used in analysis. Thus, signals that have experienced a large delay spread may be discarded. Only the signals that have experienced a small delay spread constitute the available measurement data.

3. Doppler

As limitation 1 says, the relatively loose provision on the transponder frequency allows a deviation from 1090 MHz. Consequently, it is difficult to associate measured frequency shift with the Doppler or the transponder characteristic.

4. Angular

Array signal processing allows to decompose the overlapped signals. When the same ADS-B signal appears at different angle of arrival, this implies the effect of multipath. Thus, the angular characteristic is measurable in principle and suggested as future work.

Issues in Signal Strength MeasurementAs discussed in the previous section, variations

of received signal strength are promising to be measured and analyzed. However, the measured data is still suffered from the limitation of the ADS-B approach, which will be discussed here in more detail.

Issue 1: Measurement AccuracyAs limitation 2 stated, the measured signal

strength is given after successful decoding. When the decoding technique generates a kind of received signal strength indicator as its function, it is natural to employ this value as a measurement result. However, this result may be different from what the propagation researchers expect because of decoding process. In particular, there is a recommended

decoding technique in Do-260B [32] which is referred to the enhanced reception technique. This technique generates a “reference level” which can be considered as a received signal level. The process to generate the reference level is summarized as follows, where the samples are in dB scale and the sampling frequency is typically 10 MHz.

1. The preamble, which consists of four pulses, is detected by a “valid pulse position” (consecutive samples above the threshold) and a leading edge (slope). Further, preamble pulses that will be used in the reference level generation are selected based on its timing.

2. For each selected preamble pulse, 3 samples after the leading edge sample are selected, meaning that only the samples in the middle of a pulse are used.

3. For each sample, the number of other samples that are within 2 dB is computed.

4. If the maximum of these counts is unique, then the sample that forms the maximum is taken as

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Figure 1. Comparison between the “accurate signal strength” and the reference level

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the reference sample. This process can be understood as that the sample close to other samples is selected as a representative.

5. Otherwise, if the maximum count is not unique, any samples whose counts are less than the maximum are discarded. Furthermore, the minimum power of the remaining samples is computed, and then any samples that are more than 2 dB stronger than the minimum are discarded. These processes are for discarding outliers. Finally, the average of the remaining samples is taken as the reference level.

As seen above, the reference level generation is a complex process. On the other hand, the average of all the pulse samples is a more appropriate measure in terms of a propagation measurement. Considering that the received signal is a convolution of the impulse response and the transmit signal, discarding some samples means discarding some delay taps. Furthermore, to increase the estimation accuracy, not only the preamble pulse but also the data pulses should be used. In the rest of this paper, this method, which computes the average of all the pulse samples, and the resulting value are referred as the “accurate method” and the “accurate signal strength”, respectively. Figure 1 illustrates the difference between the accurate signal strength and the reference level.

Difference between the reference level and the accurate signal strength should not be significant as long as the channel is narrowband and SNR is sufficient. However, when the channel is wideband or SNR is not sufficient, the two values may be different. When the channel is narrowband, the pulses are flat and all the preamble samples are within 2 dB. Then, in step 5 of the reference level generation, the average of all the preamble samples is taken. Thus, the reference level is identical to the accurate signal strength. On the other hand, when the channel is wideband, the signal is distorted. Then, in step 4 or 5, some of the samples may be

discarded. The resulting reference level is different from the accurate signal strength.

Note that the difference between the reference level and the accurate signal level appears when SNR is not sufficient as well. A signal of low SNR has fluctuated sample levels which may result in discarding some samples.

Issue 2: Analysis of Propagation MechanismThe signal strength variation is caused by the

combination of multiple propagation mechanisms: namely path loss, fading, and shadowing. The limitation of the proposed approach have different effects depending the type of the propagation mechanisms as follows.

By observing the trend of signal strength over large distances, path loss can be analyzed. However, it should be reminded that the received signal strength may contain a bias of the transponder power which is unique to individual aircraft (limitation 1). This result in either overestimated or underestimated path loss. On the other hand, from the operational point of view, the distribution of the transponder power is also another important information. Therefore, an ideal analysis is such that separately estimating the transponder power distribution and path loss.

The analysis of fading is limited by the data acquisition interval or the ADS-B transmission rate (limitation 4). For example, the aggregated transmission rate of the airborne position, airborne velocity, and identification squitters is 4.2 Hz in average. Assuming that the aircraft speed is 900 km/h, the resulting acquisition interval is 59.5 m. Only fading which varies slower than this value can be analyzed.

Here, shadowing refers to one by the airframe. Aircraft shadowing is affected by the alternate transmission from the top and bottom antenna (limitation 4). Actually, aircraft shadowing occurs at the most of time; the top antenna is usually behind the airframe seen from the ground. Taking this as an assumption, shadowing can be analyzed.

After separating the received signals to those of the bottom antenna and those of the top antenna, taking their difference in the signal strength derives the effect of aircraft shadowing [28]. However, the problem is when the aircraft makes a deep turn. Which antenna is in LOS depends on each case. If the top antenna is in LOS, the signal strength for the top antenna becomes stronger than that for bottom antenna, which makes the analysis difficult [23]. Thus, analysis of the shadowing has to overcome the alternate transmission.

Measurement-Based VerificationAs discussed in the previous section, signal

strength measured by the proposed approach may be affected by the decoding technique in multipath environment. To examine this, measurement-based verification is conducted. Furthermore, this process is also aimed at verifying the long-term measurement data stored by the authors’ experimental system.

Measurement SystemA SDR receiver, National Instruments

Universal Software Radio Peripheral (USRP) 2900, and a prototype receiver from the authors’ experimental wide-area-multilateration (WAM)/ADS-B system were used. The accurate signal level and reference level were measured by USRP, while the prototype ADS-B/WAM receiver measured only the reference level. These three values were compared.

The SDR receiver generated complex baseband signals of a 10 MHz bandwidth at 1090 MHz. The generated baseband signal was transferred to a host computer. The host computer then applied a threshold for initial preamble detection. When a possible preamble was detected, 1220 samples were stored such that the detected preamble and following data pulses were included. The stored samples were decoded offline by the enhanced reception technique to extract the ADS-B signal. Due to a limitation in implementation time, only the brute-force error correction was implemented, and the conservative error correction was not implemented.

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Figure 2 shows the WAM/ADS-B receiver. Having been manufactured by NEC as a prototype of operational en-route surveillance systems, the receiver can represent practical systems. The enhanced reception technique was embedded, and the prototype type receiver generated the decoded messages. The detail of the receiver is available at [28-31]. Because the receivers have been continuously operated for several years and large-scale data have been obtained, verification of the receiver will justify the analysis result of the stored data. Note that only the conservative error correction was used, and the brute-force error correction was not used due to the computation power. However, the difference in the error correction method should not affect the value of the reference level.

CalibrationThe ADS-B/WAM receiver was calibrated

such that its reference level was the same as the accurate signal strength measured by USRP in the laboratory environment, i.e., a sufficient SNR and no multipath. An ADS-B transmitter was used. The calibration was performed in the following steps, see Figure 3.

1. USRP was calibrated with a continuous wave (CW) supplied by a signal generator. The power of the CW was measured by a spectrum analyzer in advance. By comparing the output of the USRP and the spectrum analyzer, a coefficient that converts the samples measured by the USRP to dBm scale was obtained.

2. The accurate signal strength of the ADS-B transmitter was measured by the USRP.

Figure 3. Procedure of the calibration

Figure 2. Photo of the ADS-B/WAM receiver

3. The ADS-B transmitter was connected to the ADS-B/WAM receiver. The ADS-B/WAM receiver decoded the signal and generates a reference level. By comparing the reference level and the signal strength measured in step 2, the receiver was calibrated such that its output in dBm agrees with the USRP.

In the calibration, non-linearity in the reference level of the ADS-B/WAM receiver was observed. The receiver outputted the reference level as an integer called a log video amplitude. Then, Figure 4 takes the output level of the receiver in the horizontal axis and the input signal level as the vertical axis. The measurement result showed a non-linearity when the input signal level is small. The reason has not been identified yet, but to consider this effect, two linear equations were fitted to convert the log video amplitude to a dBm value.

In addition, it was observed that the ADS-B/WAM receiver showed some distribution of the reference level despite of the ideal condition. Figure 5 shows an example when the input level is -77.6 dB, where the standard deviation is 0.40 dB. Such a distribution was observed even with a higher input level. A possible reason is receiver noises.

MeasurementFigure 6 shows the measurement setup. A

sector antenna was connected to the USRP and ADS-B receiver via a splitter. The output of them are compared. The measured airspace covers mainly departure from and arrival to Tokyo International Airport and Narita International Airport. Figure 7 shows the trajectory of the target.

Figure 8 shows the variations of the received signal strength of an aircraft, where three values are plotted: the accurate signal strength measured by USRP, Pr , USRP, the reference level measured by USRP, P 'r , USRP, and the reference level measured by the ADS-B/WAM receiver, P 'r , ADS−B. The three values follow the same trend, and a rough agreement is observed among them. Note that the

separated series observed in the figure are due to the transmissions from the top and bottom antennas. Further, following three errors are computed by

Δsig=P'r ,USRP−Pr ,USRP [dB ]

Δrx=P 'r , ADS−B−P'

r ,USRP [ dB ]Δsig+rx=P' r , ADS−B−P r ,USRP [ dB ] .

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Figure 6. Measurement setup for comparing USRP and ADS-B/WAM receiver

Figure 7. Measured and analyzed target

Δsig shows the effect of the decoding technique, Δrx shows the effect of hardware difference, and Δsig+rx shows the combined effect. Figure 10 shows the distribution of Δsig, Δrx and Δsig+rx. The statistics are summarized in Table 1. The means are less than 1 dB, and this suggests that a relatively long-term trend such as path loss can be estimated from the reference level measured by a SDR or the practical receiver by this accuracy. However, the standard deviations is beyond 0.5 dB. If the distribution is approximated by a normal distribution, the range for 99% samples is ± 1.5 dB or more. Fading and shadowing under the analysis are required to show a larger variation than this value, otherwise, these mechanisms are unobservable. Moreover,

comparing the calibration (Figure 5) and the measurement, increase in the standard deviation was not significant. This means the standard deviation is mainly due to the characteristic unique to the ADS-B/WAM receiver itself such as noise.

Summary and Future WorkIn this paper, the limitation and measurable

propagation characteristics are discussed. As a result, signal strength variations (path loss, f adding, and shadowing) were identified as a promising characteristic to be measured and analyzed. However, our detail discussion revealed two issues in the measurement and analysis of the signal strength variations. First issue is that the

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Figure 9. Measurement result: distribution of the measurement errors

measurement accuracy might be degraded by the decoding technique because the reference level generation executes a complex process against samples distorted by multipath. Second issue is that the analysis of path loss, fading, and shadowing may be limited by transponder power deviation, ADS-B transmission rate (acquisition rate in measurement), and antenna diversity.

Among the two issues, the accuracy issue was further examined by a measurement-based verification. A practical ADS-B/WAM receiver and USRP was compared. This comparison is also aimed at verifying the long-term data measured by the authors’ experimental WAM system. As a result, differences due to the decoding process and the receiver were obtained. It was found that a relatively long-term trend such as path loss can be estimated from the reference level measured. On the other hand, the standard deviation is not small, and fading and shadowing under the analysis are required to show a larger variation, otherwise, they are unobservable.

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AcknowledgmentThis work is supported by JSPS KAKENHI

grant (No. 17K14688).

Email AddressesJunichi Naganawa

naganawa@ mpat .go.jp

2018 Integrated Communications Navigation and Surveillance (ICNS) Conference

April 10-12, 2018