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
Basic ADS
Ground vector
Air vector
Flight ID
Aicraft ID
Projected profile
Meteological
Information
Short-term Intent
Intermediate
intent
Data items
Latitude,Longitude, Altitude, Time, FOM
Track, Ground speed, Rate of climb or descent
Heading, Mach or IAS, Rate of climb or descent
Flight ID
24bit ICAO address
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
Wind speed, Wind direction, Temperature, Turbulence
Latitude at projected intent point, Longitude at projected
intent point, Altitude at projected intent point,
Time of projection
Distance from current position to change point,
Track from current position to change point, Altitude
at change point, Predicted time to change point
1
Satellite
t
es
AD
S
ntr
Co
t
ac
qu
Re
AD
rt
S
po
Re
AES
GES
ATC Center
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
ATC
Center
GES
AES
Contract Reque
st
Access Request
Reservation
ADS Report
t1
t2
t3
Access Request
Reservation
ADS Report
●
●
●
Access Request
Reservation
ADS Report
Contract Cance
l
Fig. 2
●
●
●
Request
ADS report transmission sequence for T-channel
2
Time　（s） 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
GPS Processing
Data Transfer
CMU Processing
SDU Processing
Mod. & Demo.
Waiting Time
Propagation
Mod. & Demo.
GES Processing
t1
t3
t2
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
Transmission Delay (s)
60
50
40
30
20
10
0
0
100
200
300
400
500
Data Length (octet)
Fig.4
T channel transmission delay with data length for 600 bit/s
3
600
Transmission Delay　（s）
300
Mean
95%
250
200
150
100
50
0
0
20
40
60
80
100
Channel Load　（％）
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
Transmission Delay (s)
60
600bit/s
1200bit/s
10500bit/s
50
40
30
20
10
0
0
100
200
300
400
500
600
Data Length (octet)
Fig.6 T-channel transmission delay with data length for various channel rates
4
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.
4.1 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
Satellite
1
N
2
rt
Re
po
…
AES 2
D
S
AES 1
A
1
2
N
A
…
D
S
Re
po
r
tR
eq
u
es
t
AES N
ATC Center
Spot Beams
GES
Fig.7 A new concept of ADS
5
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.
4.2 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.
(bit)
8 7
6
5
4
3
2
1
(octet)
Sequece Ind.
SU Type
1
Q number
D/R Reference No. 2
3
AES ID
4
5
GES ID
6
7
8
9
10
11
User Data
12
13
14
15
16
17
18
CRC
19
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
4.3 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
6
ATC
Center
AES AES
1
2
GES
AES
N
ADS Report R
equest
ADS Report R
ADS Report
equest
ADS Report
●
●
●
●
●
●
ADS Report R
●●●
equest
ADS Report
ADS Report R
equest
ADS Report
Fig.9 Transmission sequence of ADS report using polling scheme
TAES
TADS TG
GES
TX
AES
RX
AES
TX
GES
RX
REQ 1
・ ・ ・
REQ 2
REQ 1
ADS Report
REQ 2
AES 1
REQ N
・ ・ ・
REQ N
・ ・ ・
AES 2
AES 1
AES 2
ADS Report
Request
REQ 1
REQ 1
AES N
・ ・ ・
AES 1
AES 4
AES 4
Transmission
Delay
Fig.10 Time chart for ADS report transmission using polling scheme
7
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 T AES (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
1200
2400
4800
10500
0.96
0.46
0.21
0.1269
0.0846
0.08
0.08
0.08
0.08
0.08
9
18
34
48
60
Capacity(Number of AES)
1200
1 channel
2 channels
1000
800
600
400
200
0
0
1
2
3
4
5
6
7
8
9
Channel Rate　(kbit/s）
Fig.11 Number of AES to be handled for six spot beams
8
10
11
4.4 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
GES e.i.r.p.(dBW)
Propagation Loss(dB)
Rain Fade(dB)
Satellite G/T(dBK)
Return Link
63.8 AES e.i.r.p.(dBW)
206.6 Propagation Loss(dB)
10.5
189
11.8
-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)
Rain Fade and Additive
noise(dB)
-26 GES G/T(dBK)
205.2
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
AES G/T(dBK)
12
39
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)
4.5 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
9
Time　（s） 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
GPS Processing
Data Transfer
CMU Processing
SDU Processing
&Mod.
Propagation
Demo. &
GES Processing
Transmission
Delay
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.
10
Table 6 Comparison of ADS Systems using ICAO AMSS and Polling protocols
ICAO AMSS
Transmission delay
Transmission interval
or Update rate
Report content
Polling
- 26 seconds (600 bit/s),
6 seconds (10500 bit/s)
- less than
2 seconds
- Increase by channel congestion
- constant
- multiple of 8 seconds, 16 seconds (minimum)
- 10 seconds
- non-uniform interval
- almost uniform
- Basic ADS block
- Basic ADS block
- Extended ADS blocks
11