A D S MODE-S CHANNEL PERFORMANCE FOR AIRCRAFT SURVEILLANCE USING GPS POSITIONING

Michael S. K. Sushko Kensington & Icknield

5775 Wayzata Blvd, Suite 700 Minneapolis, Minnesota 55416

ABSTRACT

The current Mode S squitter technology is being considered for the basis of a Automatic Dependent Surveillance (ADS) system. Under this proposal, aircraft position would be determined by GPS equipped aircrafi and the Mode S data link would be used to transmit the positional information, along with current Mode S usage.

This paper studies the downlink data channel efficiently of the Mode S data link. The downlink channel is modeled as a random access ALOHA channel in both a fading and non- fading signal environment. The paper presents results for both unslotted (pure ALOHA) and slotted channel performance for Mode S GPS data traffic.

I. INTRODUCTION

The FAA and other related industry groups are studying the Mode S squitter technology currently in use for aircraft iden~cation for the basis of a Automatic Dependent Surveillance (ADS) system. The usage of the Mode S data link for GPS position reports would be in addition to the current Mode S usage for aircrafi identification. The FAA is interested in a integrated surveillance system that provides both identification and position.

The Mode S squitter is currently used on most U.S. transport aircraft. It is the basis of the current Traffic Collision Avoidance System (TCAS). The ADS Mode S system being proposed would involve the use of the Mode S data squitter message format. The Mode S downlmk channel would be used for GPS position information while sharing the channel with existing Mode S Secondary Surveillance Radar (SSR) that currently interrogate aircraft for identification. The uplink channel would be used for differential GNSS (DGNSS) corrections. It would be shared with ATC alerts and other information data traffic.

The Mode S aircraft transponder system for SSR returns, TCAS and data link support operate over two different

communication channels. The upllnk data channel operates at 1030 MHZ with a raw 4 Mbps data rate and the downlink channel operates at 1090 MHZ at 1 Mbps.

There is currently two basic squitter message formats: a basic squitter format with a length of 56 bits and the extended squitter format with a length of 112 bits. The current use of the extended Mode S format is for ADS operation. It consists of the same format as the basic squitter except that it contains an additional 56 bit ADS message field deiked for ADS support. For TCAS operation, aircraft. Mode S transponders continuous broadcast their Mode S address contained in the squitter message every second. The actual start of each squitter message is randomized to prevent transponder interference with other aircraft squitter broadcasts are monitored by TCAS equipped aircraft.

Aircraft surveillance is based on the concept of using a Mode S broadcast (“squitter”) containing the aircraft’s GPS position. It was originally proposed and studied by MIT Lincoln Laboratory under FAA sponsorship[ 1][2]. Under their proposal, a Mode S squitter containing the aircrafl’s GPS position would be a candidate for the FAA’s ADS program. The Mode S beacon system would be extended to include new squitter message types. The extended squitter message formats would include the aircraft’s position determined from a onboard GPS receiver.

The ADS Mode S proposal would use the existing Mode S transponder equipment with minor modifications. The transponder would continue to support SSR interrogations while providmg ADS reports. It would also provide support for real-time data link for ATC alerts, GPS differential corrections and other traffic information.

11. SQUITTER DATALINK TRAFFIC

Downlink data tr&c under the ADS Mode S system would consist of a Mode S transponder address transmission every second. The ATCRBS SSR beacon interrogation replies would continue to be a part of the downlink Mode S data

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traffic. The number of ATCRBS replies depend on the aircratl density and could range up to 60 replies per aircraft per second. The GPS squitter extended Mode S messages (‘‘Airborne Position” and ‘‘Stdace Position”) for supporting ADS would broadcast every !4 second. The ICAO flight ID squitter would be provided every 5 seconds[ 1][2].

decrease of squitter traffic.

The attempted traffic G, is continuously spread over the terminal area. The normalized packet traffic per unit area at distance r is G(r). The offered channel traffic G (in packets per time slot) has quasi-dorm spatial distribution [3][4]

Other data llnk traffic includes Mode A and Mode C ground interrogation replies. Mode C replies can also result from interrogations fiom TCAS equipped aircraft. The

measured at various airports and are known currently to range fiom 75 to 85 per second[l][2]. Transmission of Mode A, Mode C, and Mode S SSR replies along with aircraft Mode S and GPS squitter data traflk including proposed downlink data Iink traffic would all occur on the 1090 MHZ RF channel. Squitter interference has been a known problem [1][2].

(1) G

G(r) = L e v ( -F4) Mode A and Mode C interrogation rates have been 7c

Both the test and interfierence signal mean powers decreases with increased distance according to fixed propagation loss exponent p. The local mean power p, for a test terminal j out to a normalized radius distance r (0 < 5 I; 1) from the base station is taken as

111. TRAFFIC CHANNEL MODEL

Antenna position for all aircraft Mode S transreceivers in this paper are located on the bottom of the aircraft. When aircraft are landing, taking off or taxing on the ground, a bottom mounted Mode S antenna position results in indirect line-of-sight signal components and shadowing. For airbome aircraft higher than the height of the Mode S ground station antenna, a direct line of sight signal power exists. A single Mode S ground base station is located on the airport field. The base station antenna is omni-directional and is effected by local signal reflections from ground-base aircraft, hangers, building structures and passenger terminals.

All data packets have equal length of 1 12 bits (“single ADS GPS packet”). Therefore, each data packet duration is equal in length. The attempted or offered traffic G, is the average number of attempted transmissions by all Mode S transmissions in the channel expressed as “packets per unit of time” (ppt). The number of participating aircraft (Mode S terminals) is N even through probability modeling requires the total population to be infinite (N- m)[4] [5] [6]. This provides a bounded total channel traffic for G,.

The Mode S data channel traffic is modeled as a stationary Poisson process. For packets of unit duration, the arrival rate for the channel traffic is G, = h where A. = ,A (Mode S squitters) + h, (Mode A squitter replies) +3h (Mode C squitter replies) + A, ( SSR squitter replies). All generated traftic comes from continuously transmitting Mode S equipped aircraft. Squitter packets destroyed in collisions with other squitters are lost - there is no squitter packet retransmission. Aircrd: entering a terminal area cause a increase in squitter traffic where aircraft leaving result in a

- 1 p .=-

where the propagation loss exponent p is determined by the model being used.

IV. TERMINAL-AREA MODEL

The Terminal-Area Model covers the various phases of flight where there is a direct line of sight and the only signal attenuation is due to the path loss statistical model p j = 1/r; for fiee space lost. Any Rayleigh fading or shadowing effects are not part of the model. The model is based on the “vulnerability circle” concept [5] [4] where capture occurs for a test packet at radius 5 if no other transmission occurs within a circle of radius CA where the parameter q is a system constant. This implies that the area mean power p,of the test signal must exceed the largest of any of the interfering area mean powers p, by a threshold factor or capture ratio z = .’, [4]. For a test packet at distance r, from the base station center, the probability of no overlapping packets is Pr = exp(-2GJ where the total traffi~ G generated withm a circle of radius 5 is [5]

(3)

Using the total generated traffic (3) G, in the expression for probability of no packets overlapping, this gives the probability of no interfering arrivals in the interval 0 < r < CA for test packet at distance 5.

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Similarity, probability of successful transmission Q(r) for an slotted channel is

&[no interfering pkts]=e

Q(r,) =exp( -zNA r 2)exp

By definition for unconditional probability of succesdul reception, this becomes the probability of successful

unslotted channel [5] [4] transmission Q(r) for a test packet at distance 5 in a (10)

An important measure of performance is the total channel throughput S, The total channel throughput is defined as the average number of packets per unit time that are received correctly. Therefore, the total channel throughput S, for a non-slotted channel is

Q(r> =e*[ : p ( r ) 4 . - ) (5)

In this model, additive noise with power PN = No B, is st= present. The No term is the one-sided AWGN spectral power density at the receiver’s input and B, is the RF- bandwidth. Then, the probability that the wanted signal is above the receiver noise floor is [4] (1 1)

Similarity, the total channel throughput S, for a slotted pNd = exp(-zNd rP) (6) channelis

where f3 = 2 for thls model. When the additive noise expression (6) is included, the probability of successful transmission Q(d is

(12)

Q(rJ) =exp( -zNA r 2)exp - 4xrG(r)dr (7) V. GROUND/ENROUTE-AREA MODEL

The GroundEnroute Area model covers the phases of aircraft movement on the ground and in-flight where there is no direct line of sight signal. In this case, the signal is Rayleigh faded and shadowing is not considered. Under this model [4], capture is defined to occur only if the test packet’s power exceeds the joint interference power by the capture ratio z over the duration of test packet’s transmission. In addition, transmission of the test packet must complete before the start of the next fade. When the joint interference signal is approximated by a Nakagami faded signal, then the probability of successfbl reception can be determined for a nonfade duration that lasts longer than T, [4]. It is

[ c r )

[ c: 1 When the quasi-uniform spatial distribution (1 ) G(r) is inserted into expression (7), then Q(r) becomes

e@,) =exp( -zNAr 2)exp - 1 4rGtexp( - 3 4)dr (8)

From the defmtion of the error function erf, then probability of successful transmission (8) Q(r) for non-slotted channel

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When including the noise power NA [4] as part of the total interference, the probability of capture during a nonfade period becomes

For a slotted channel, all packet transmissions must begin on a slot boundary. Probability of successful access Q(r) [4] is

The pdfof the total joint intdaence power [3][4] is (19)

The total channel throughput St for a non-slotted channel

- (2Gt)" m +g ~eXP(-2Gt)*S2nsexp2ns.exp The probability of capture q,,(q for a wanted Rayleigh-faded 0 signal over a non-fading interval with Nakagami-faded interference with mean powerp, [4] is ( -zr$ ++N,)-En 4 2 s 4 - r - f , T s , / w ) dsdr

St= 2rGte --%4-Gt-NAr4-r2 I 4 4

Substituting terms (1 4) and (1 5) into ( 1 6), the probability of capture after substituting the expressionp,=l/s' is-

m - (G)"

on-i n!

m

+/E &xp( -Gt)./2nrexp w 0 ( -zr;'(s -4 +NA) - 2 n 2s4

qn(rj) =/2ns exp(-z(s -4 +NA)r;' 4 (17)

0

For a non-slotted channel, the "vulnerability" interval is two packet lengths. Given the stated conditions on test and interference power, then probability of successful access is

VII. CONCLUSION

There was two general random access models developed to examine Mode S ADS channel performance in fading and non-fading environment. Expressions for the probability of

developed using the Vulnerability Circle model. Probability 0 of capture, successful transmission and total system

Mode-S operates as a non-slotted system and the slotted system was used for comparison. Slotted system performed better than the non-slotted system.The results show a slotted Mode-S channel performs better under high traffic loads.

Q(r) =e*( -2G$h.(~,lp;=r~-~,F~ =0) m successful transmission and total system throughput were

throughput for the Non-Fade Interval model. The current

+E -e*( (2Gt)" -2Gt)-/2nsexp n-1 n!

( -zr;(s -4 +NA) - 3 4 ds

(1 8)

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&anrflAxem t I I I I I I I I I

Na =NONE

SUCCESSFUL A CCESS

NONFADE INTBBVAL MODXL

R 4 D I O C I U ” X L - 1090MHa MTA PACKUTSIZH = 113 EITS

M T A RAT8 - 1 Mbpr

J!ECIUVllR THRESHOLD - 6 dB MTA TElFPIC Qt - I

AIRCRAFTSPEED = S. l IM/S (10 KNOTS)

W R O U N D OPERATION”

0 &I A4 &U &a I 1.1 1.4 1.6 1.1 a-, . 0 U &4 &# &I 1 1.1 1.1 1.6 1.) 1

a-.

Figure 1 : Terminal Area Performance Figure 3 : Ground/Enroute Area Performance

REFERENCES

0.4s TOTAL THROWORPUT

VULNEMBILITYCIBCL3 MODEL WLNXMBILITYPARMElZR Cz - 3 UICXIYHB THRESHOID = 6 dB

THROUQHPVTQRAPHRANQE: 0 - 50%

0 0.1 I.6 2.1 J.1 4 U LI 61 7.1 I

Y I R I w m 6 0

Figure 2: Terminal Area Performance

[l] Bayliss, E., R. Boisvert, M. Burrows, and W. Harman, “Aircraft Surveillance Based on GPS Position Broadcasts from Mode S Beacon Transponders”, Proceedings of the ION, GPS-94, Salt Lake City, Ut Sept 1994.

[2] Bayliss, E., R. Boisvert, and G.Kmtte1, “Demonstration of GPS Automatic Dependent Surveillance of Aircraft Using Spontaneous Mode S Beacon Reports”, Proceedings of the ION-94,5Oth Annual Meeting, Colorado Springs, CO June 1994.

[3] Widipangestu, I, A J ‘T Jong and R. Prasad, “Capture Probability and Throughput Analysis of Slotted ALOHA and Unslotted np-ISMA in a RicianRayleigh Environment”, IEEE Trans. Veh. Technol., Vol43, No. 3, Aug 1994.

[4] Lkar tz , J P, “Narrowband Land-Mobile Radio Networks”, Artech House, 1993

[5] Abramson, N., “The Throughput of Packet Broadcasting Channels”, IEEE Trans Commun.,Vol COM-25, No 1, Jan 1977

[6] Kleinrock, L and R A Tobagi, “Packet Switching in Radio Channels: Part I - Carrier Sense Multiple-Access Mcdes and Their Throughput-Delay Characteristics”, IEEE Trans. Commun., Vol COM-23, No.12, Dec 1975.

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