A New Concept of Satellite-Based Automatic …



AMCP/WG-M6/WP/11

12 December 2002

Aeronautical Mobile Communication Panel (AMCP)

Working Group-M Meeting

Sixth Meeting

Montreal, Canada, 12 to 18 December 2002

Agenda Item: 4

A new concept of satellite-based Automatic Dependent Surveillance

INFORMATION PAPER

Presented by S. Takahashi

(Prepared by A. Ishide, M. Fujita)

|SUMMARY |

|We have evaluated the ADS transmission characteristics using a T-channel protocol that meets ICAOAMSS SARPs. The results |

|showed that it takes about 26 seconds for transmitting a basic ADS block at a channel rate of 600 bit/s in Aeronautical |

|Telecommunication Network (ATN). Although the transmission delay reduces to about 6 seconds for a channel rate of 10500 |

|bit/s, channel congestion increases it. Since ADS is a surveillance system like a Secondary Surveillance Radar (SSR), it is|

|undesirable that the transmission delay is affected by communication traffic conditions. |

|This paper first describes the measured transmission delays in ADS report transmissions using T channel protocol. Then it|

|describes a new concept of ADS system in which a transmission delay is almost constant and no traffic congestion occurs. It|

|uses a polling scheme in which each AES in a spot-beam coverage area responds an interrogation from a ground system and |

|sends back an ADS report. It was first validated in the Engineering Test Satellite V (ETS-V) experiment. The transmission |

|delay is estimated to be less than 3 seconds using a channel rate of 4800 bit/s. The update rate for each AES is 10 |

|seconds. |

|Such an ADS system can be regarded as one of the candidates for ADS in a next generation AMSS system. |

| |

1. Introduction

We have evaluated the transmission characteristics of ADS report using T-channel protocols that meets ICAOAMSS SARPs[1]. The results showed that it takes about 26 seconds for transmitting a basic ADS block at a channel rate of 600 bit/s in Aeronautical Telecommunication Network (ATN). Although the transmission delay reduces to about 6 seconds for a channel rate of 10500 bit/s, channel congestion increases it. Since ADS is a surveillance system like a Secondary Surveillance Radar (SSR), it is undesirable that the transmission delay is affected by communication traffic conditions.

This paper first describes the measured transmission delays in ADS report transmissions using T channel protocol. Then it describes a new concept of ADS system in which a transmission delay is almost constant and no traffic congestion occurs. It uses a polling scheme in which each AES in a spot-beam coverage area responds an interrogation from a ground system and sends back ADS report. This concept was first validated in the Engineering Test Satellite V (ETS-V) experiment[2]. The transmission delay is estimated to be less than 2 seconds using a channel rate of 4800 bit/s. The update rate for each AES is 10 seconds.

2. ADS Summary

Automatic Dependent Surveillance (ADS) is defined as a function by which data, derived from on-board navigation system, are transmitted automatically to the ground so that air traffic controllers can monitor aircraft positions on a display almost in real-time. Table 1 lists the items to be transmitted in ADS report. Basic ADS block must be included in every ADS report, but other blocks are included in ADS report when air traffic controllers request them. There are three types of ADS report: periodic contract, demand contract and event contract. In the rest of this paper, we only deal with periodic contract.

Table 1 Content of ADS report

|Block |Data items |

|Basic ADS |Latitude,Longitude, Altitude, Time, FOM |

|Ground vector |Track, Ground speed, Rate of climb or descent |

|Air vector |Heading, Mach or IAS, Rate of climb or descent |

|Flight ID |Flight ID |

|Aicraft ID |24bit ICAO address |

|Projected profile |Next waypoint, Estimated altitude at next waypoint, |

| |Estimated time at next waypoint, (Next+1)waypoint, |

| |Estimated altitude at (next+1) waypoint, Estimated |

| | time at (next+1) waypoint |

|Meteological Information |Wind speed, Wind direction, Temperature, Turbulence |

|Short-term Intent |Latitude at projected intent point, Longitude at projected |

| | intent point, Altitude at projected intent point, |

| |Time of projection |

|Intermediate intent |Distance from current position to change point, |

| |Track from current position to change point, Altitude |

| |at change point, Predicted time to change point |

Fig. 1 Concept of ADS

3. T-channel Protocol

Figure 1 shows the ADS report transmission sequence in the T-channel protocol. When the length of user data is more than 33 octet, T-channel is used for transmission of data. If the content of ADS report is Basic ADS block, the length of the user data is 11 octet. But, if it is transmitted in Aeronautical Telecommunication Network (ATN), the length of the user data increases to about 230 octet because of upper layer headers

(more than 150 octet) attached to the original ADS information (11 octet). Therefore, any ADS report is transmitted on T-channel.

Figure 2 shows the transmission sequence for ADS report using the T-channel protocol. In the T-channel protocol, an ADS reporting procedure is initiated by an ATC center. First, an ATC center sends an ADS contract request to an aircraft earth station (AES). The ADS contract request specifies the content and transmission interval of

Fig. 2 ADS report transmission sequence for T-channel

Fig. 3 Time chart for ADS report transmission on T channel

ADS report. Upon receipt of the request, the AES makes an ADS report and stores it in a transmission buffer, and sends an access request to a ground earth station (GES) for reservation of T-channel transmission slots. Then, the GES checks a reservation table and sends back reservation information to the AES. The AES transmits the buffered ADS report to the GES. Thereafter, the AES repeats the procedure from access request transmission to ADS report transmission at the specified time interval.

Figure 3 shows the time chart for ADS report transmission on 600 bit/s T-channel. In this case, the data length is about 100 octet. Assuming that t1 is the time required for making ADS report, that t2 is the time from access request transmission to data transmission, and that t3 is the time required for data transmission, the transmission delay is defined as t= t1 + t2+ t3. This suggests that the time required for reservation of transmission time slots (t2) in T-channel protocol is fairly large.

Figure 4 shows the transmission delays measured for various data lengths on 600 bit/s T channel. The transmission delay is about 26 seconds for the data length of 230

Fig.4 T channel transmission delay with data length for 600 bit/s

Fig.5 Transmission delay vs. channel load for 600 bit/s T channel

octet.

Figure 5 shows the transit delay (average transmission delay) and transfer delay (95 percentile transmission delay) measured for random transmissions of user data (230 octet) on 600 bit/s T-channel. The distribution of transmission interval is exponential. The abscissa represents the channel load. It is found that the transit and transfer delays increase with channel load.

Figure 6 shows the transmission delay measured for various channel rates on T-channel. In this measurement, the channel rate of P and R channels is 600 bit/s. The result shows that the transmission delay for the data length of 230 octet is about 26 seconds for 600 bit/s, about 18 seconds for 1200 bit/s and about 11 seconds. If the

Fig.6 T-channel transmission delay with data length for various channel rates

channel rate of P and R channels is 10500 bit/s, the transmission delay comes to about 6 seconds for the channel rate of 10500 bit/s on T channel.

This result indicates that the transmission delay decreases as the channel rate increases. It is true as long as there is no congestion on the link. But, if the channel load increases, the transmission delay increases. In the global beam coverage, the AES must be equipped with a high-gain antenna to communicate at a channel rate of 10500bit/s. Although it is possible that airliners can have a high-gain antenna, small airplanes can’t.

These results suggest that the followings are the problems to be resolved for ADS transmissions on T-channel.

1) Even if original ADS data is short, the data length in the datalink layer increases due to addition of headers in the upper layers of OSI, which necessitates the use of T-channel. But, the reservation procedure of transmission slots increases the transmission delay in T-channel protocol.

2) In the T-channel protocol, priority is given to a reliable data transfer between air and ground. When data is not transmitted to the recipient successfully, the data is retransmitted until successful data transfer. This may occur when the link is congested. It results in the increase of transmission delay.

3) The procedures described in (1) and (2) also cause non-uniform receipt interval of ADS report.

4. New ADS Protocol

This section describes a new ADS concept that resolves the problems described in the previous section. In the concept, we assume the use of spot beams so that small airplanes with a small antenna may also be able to use the system.

1. Concept

Figure 7 shows a new ADS concept. In this concept, we adopt a polling scheme as a multiple access. An ATC center sends an ADS report request to each AES in the coverage of a spot beam sequentially. Then, each AES sends back an ADS report to the ground in response to the request. The use of spot beam reduces the required gain of AES antenna. To minimize the data size, the ADS report only includes a basic ADS block in Table 1. The interval of ADS report for each AES is 10 seconds, the same

Fig.7 A new concept of ADS

value as Second Surveillance Radar (SSR). When CRC indicates errors in a received ADS report, it is abandoned. Such protocol minimizes the transmission delay and avoids its increase, and keeps the receipt interval almost constant.

2. Channel Rate and ADS Report Length

As described in 4.1, the content of ADS report is Basic ADS block. Since the most important items required for surveillance are included in Basic ADS block, it is an appropriate choice. The data size of Basic ADS block is 11 octet and the size of user data field in a R-channel Signal Unit (SU) is 11 octet as shown in Fig.8. Although we can use any data format for transmitting ADS report, we assume to use the data format and burst structure of R-channel for convenience. Table 2 lists the relation between the channel rate and the burst length.

Fig.8 Data format of R-channel SU

Table 2 Burst length and channel rate

|Channel Rate（bit/s） |Burst Length（s） |

|600 |0.96 |

|1200 |0.46 |

|2400 |0.21 |

|4800 |0.1269 |

|10500 |0.0846 |

3. Access Scheme and Capacity

Figure 9 shows the transmission sequence of ADS report using a polling protocol. An ATC center sends an ADS report request to each AES sequentially at a constant interval, and the AES that received the request addressed to it sends back an ADS report to the ground. Since the number of ADS report handled at a certain time is one on a channel and no retransmission exists, no congestion occurs in this protocol. It keeps the

Fig.9 Transmission sequence of ADS report using polling scheme

Fig.10 Time chart for ADS report transmission using polling scheme

transmission delay almost constant, and it also keeps the receipt interval of ADS report at the ground.

Now, let consider how many AES can be handled using this protocol. We assume to use one forward link and one return link for one spot beam. If TAES (s) is the transmission interval for a certain AES and TCH (s) the minimum interval of successive bursts on a channel, the number of AES that can be handled per channel, N, is obtained by

N=[TAES/TCH]

where [A] denotes the integer part of A. If TADS (s) is the burst length of ADS report and TG (s) the guard time between successive bursts, then, TCH =TADS+ TG as shown in Fig.10. The guard time is required to avoid the collision of burst signals from different geographical areas due to the difference of propagation distances. Table 3 lists the number of AES that can be handled for one spot beam with one forward channel and one return channel calculated by the equation above for different channel rates. We assumed TADS is 10 seconds and TG is 0.08 seconds. Figure 11 shows the total number of AES that can be handled in the coverage of 6 spot beams.

Table 3 Number of AES that can be handled per channel

|Channel Rate |Burst Length |Guard Time |Number of AES |

|（bit/s） |TADS（s） |TG　（s） |per channel |

|600 |0.96 |0.08 |9 |

|1200 |0.46 |0.08 |18 |

|2400 |0.21 |0.08 |34 |

|4800 |0.1269 |0.08 |48 |

|10500 |0.0846 |0.08 |60 |

Fig.11 Number of AES to be handled for six spot beams

4. Link Budget

In this section, we study the link budget for the case of MTSAT as an example. The satellite G/T at L band is -9 dBK for global beam and -2 dBK for spot beam. This implies that the C/N0 for the link from AES to Satellite increases 7 dB using a spot beam instead of global beam. Since 1200 bit/s data can be transferred from AES to GES using a low gain AES antenna in the coverage of global beam, it is conceivable that the transfer of 6000 bit/s data (5 times of 1200 bit/s) may be possible in the coverage of the spot beam.

Table 4 lists an example of link budget for 4800 bit/s data transmission. The parameters are determined on a basis of the link budget for MTSAT. The required C/N0 is the value for 4800 bit/s AQPSK quoted from the table of SDM (Table 8.1.1.2). The BER of less than 10-5 is obtained at the C/N0 of more than the required C/N0. This link budget shows the transfer of 4800 bit/s data for AES with a low gain antenna using a spot-beam antenna on a satellite.

If an intermediate gain antenna is used for AES, the channel rate can be increased to 10500 bit/s.

Table 4 Example of link budget

|Forward Link | Return Link　 |

|GES e.i.r.p.(dBW) |63.8 |AES e.i.r.p.(dBW) |10.5 |

|Propagation Loss(dB) |206.6 |Propagation Loss(dB) |189 |

|Rain Fade(dB) |11.8 |　 |　 |

|Satellite G/T(dBK) |-1 |Satellite G/T(dBK) |-2 |

|Up-link C/N0(dBHz) |71.9 |Up-link C/N0(dBHz) |48.1 |

|Satellite e.i.r.p.(dBW) |31.3 |Satellite e.i.r.p.(dBW) |-1.5 |

|Propagation Loss(dB) |188.5 |Propagation Loss(dB) |205.2 |

|　 |　 |Rain Fade and Additive noise(dB) |12 |

|AES G/T(dBK) |-26 |GES G/T(dBK) |39 |

|Down-link C/N0(dBHz) |45.4 |Down-link C/N0(dBHz) |48.9 |

|Total C/N0(dBHz) |44.3 |Total C/N0(dBHz) |44.8 |

|Required C/N0(dBHz) |42.2 |Required C/N0(dBHz) |42.2 |

|Frequency Band : Ku band(GES-Satellite), L band(Satellite-AES) | |

|Required C/N0: 4.8 kbit/s AQPSK(SDM) | |

|Satellite C/IM: 51 dBHz (Forward), 53 dBHz (Return) | |

5. Transmission Delay

In this section, we estimate possible transmission delay for the polling system. Figure 12 shows the time chart for ADS report transmission on 600 bit/s R channel. The times for GPS processing, data transfer and CMU (Communication Management Unit) processing on AES are obtained from measurement using experimental AES equipment. The time from SDU processing on the AES to GES processing is obtained by using the calculated times of data transfer and propagation and the actual processing time for modulation and demodulation for the experimental equipment. The resultant value of transmission delay comes to about 2.9 seconds for the channel rate of 600 bit/s.

We can estimate the transmission delay for other channel rate. Table 5 lists the

Fig.12 Time chart for ADS report transmission using polling scheme

(channel rate : 600 bit/s)

Table 5 Estmated transmission delays

|Channel rate |Estimated transmission |

|(bit/s) |delay (s) |

|600 |2.9 |

|1200 |1.9 |

|4800 |1.3 |

|10500 |1.1 |

estimated transmission delay for various channel rates. This table shows that the transmission delay for the channel rate of 4800 bit/s is about 1.3 second. It should be noted that the values in the table don’t include the transmission delay for terrestrial networks. If the transmission delay for terrestrial networks is assumed to be 0.5 seconds (This value was typical when the DDX-P was used in the satellite datalink experiment we conducted in 1992-1994.), the total transmission delay will be about 2 seconds.

5. Conclusion

Table 6 lists the differences of the ADS systems using ICAO AMSS protocol and the proposed polling protocol. The concept assumes the use of a low-gain AES antenna in the coverage of spot beams. The polling protocol gives a constant transmission delay of less than 2 seconds and a short update rate of about 10 seconds at a channel rate of 4800 bit/s.

A more efficient error correcting code and/or a larger spot beam gain increases the channel rate further and it leads to less transmission delays.

Such improved ADS can be regarded as one of the candidates for a next generation AMSS system.

References

[1] T. Nakata:”Comparison of Transmission Delay for Experiment and Simulation(II)”, AMCP/WG-A, Sept. 1999.

[2] A.Ishide:”ATC Demonstration Experiment Using ETS-V”, AMSSP/WG, Sept. 1989.

Table 6 Comparison of ADS Systems using ICAO AMSS and Polling protocols

| |ICAO AMSS |Polling |

|Transmission delay |- 26 seconds (600 bit/s), |- less than |

| |6 seconds (10500 bit/s) |2 seconds |

| |- Increase by channel congestion |- constant |

|Transmission interval |- multiple of 8 seconds, 16 seconds (minimum) |- 10 seconds |

|or Update rate | | |

| |- non-uniform interval |- almost uniform |

|Report content |- Basic ADS block |- Basic ADS block |

| |- Extended ADS blocks | |

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