002 Aircraft Position Report Using DGPS Mode-S

EUROPEAN ORGANISATIONFOR THE SAFETY OF AIR NAVIGATION

EUROCONTROL EXPERIMENTAL CENTRE

AIRCRAFT POSITION REPORTUSING

DGPS & MODE-S

Subdivision B2.2. - Communications

EEC Task No. AT58EEC Note No. 01/95

Approved for publication bythe Head of Division B2

Issued : FEBRUARY 1995

The information contained in this document is the property of the EUROCONTROLAgency and no part should be reproduced in any form without the Agency's

permission.The views expressed herein do not necessarily reflect the official views or

policy of the Agency.

REPORT DOCUMENTATION PAGE

Reference :

EEC Note No. 01/95

Security Classification :

Unclassified

Originator Code :

EEC Division B2

Originator (Corporate Author) Name/Location :

EUROCONTROL Experimental CentreB. P. 15F - 91222 BRETIGNY SUR ORGE CedexTelephone 33 (1) 69 88 75 00

Sponsor Code :

EATCHIP DevelopmentDirectorate

Sponsor (Contract Authority) Name/Location :

EUROCONTROL AgencyRue de la Fusée, 96B - 1130 BRUSSELSTelephone 32 (2) 7299011

Title :

AIRCRAFT POSITION REPORT USING DGPS AND MODE-S

Author : Mr. P. HUNT

Mr. L. CROUZARD

Date

02/95

Pages

21

Figs

15

Refs

7

Annexes

Det. Task Specification AT 58

Period2nd Semester

1994

Task No.Sponsor

FCO.ET2.ST08

Task No.Originator AT 58

Distribution Statement :(a) Controlled by : Head of Division B2(b) Special limitations : None(c) Copy to NTIS : NO

Descriptors (keywords) : DGPS, Extended Squitter, Mode-S

Abstract :

This note describes the EUROCONTROL Experimental Centre contribution to the experimentset up by the French DGAC/STNA to assess the value of aircraft position reports usingdifferential GPS and Mode-S extended squitters.

The modifications made to the THOMSON-TRT transponder and the various formats used plusthe structure and method used in programming the PC based Data Link Processor aredescribed.

Aircraft Position Report using DGPS & Mode-S

EEC Task No. AT58EEC Note No. 01/95

Issued : February 1995

AIRCRAFT POSITION REPORTUSING

DGPS AND MODE-S

by

P. HUNT

L. CROUZARD

S U M M A R Y

This note describes the EUROCONTROL Experimental Centre contribution to the experiment set

up by the French DGAC/STNA to assess the value of aircraft position reports using differential

GPS and Mode-S extended squitters.

The modifications made to the THOMSON-TRT transponder and the various formats used plus

the structure and method used in programming the PC based Data Link Processor are described.


C O N T E N T S

1. GENERAL DESCRIPTION.....................................................................................................................................1

1.1. OVERALL ON-BOARD CONFIGURATION......................................................................................................................21.2. PART PROVIDED BY EUROCONTROL......................................................................................................................2

2. EXTENDED SQUITTER .........................................................................................................................................3

2.1. FORMAT TYPE CODES................................................................................................................................................32.2. AIRBORNE FORMAT CODING......................................................................................................................................4

2.2.1. Surveillance Status............................................................................................................................................52.2.2. Turn...................................................................................................................................................................52.2.3. Altitude..............................................................................................................................................................52.2.4. Time ..................................................................................................................................................................52.2.5. Lat/Lon..............................................................................................................................................................6

2.3. IDENTITY FORMAT CODING .......................................................................................................................................62.3.1. Type/Wake Field ...............................................................................................................................................62.3.2. ICAO Identifier Field........................................................................................................................................6

2.4. LATITUDE LONGITUDE CODING .................................................................................................................................62.4.1. CPR Algorithm Parameters and Internal Functions........................................................................................72.4.2. CPR Position Encoding Process.......................................................................................................................8

3. DESCRIPTION OF GPS UPLINK FORMATS ..................................................................................................10

3.1. RF FORMATS...........................................................................................................................................................103.2. MESSAGE BLOC FORMAT .........................................................................................................................................103.3. MESSAGE BLOCK HEADER.......................................................................................................................................103.4. MESSAGE DATA FORMAT.........................................................................................................................................103.5. CYCLIC REDUNDANCY CHECK.................................................................................................................................11

4. SPECIFICATIONS OF MODIFICATIONS TO TRANSPONDER SOFTWARE ..........................................12

5. SPECIFICATION OF THE DATA LINK PROCESSOR MODIFICATIONS ...............................................13

6. DLP SOFTWARE DESCRIPTION ......................................................................................................................14

6.1. IMPLEMENTATION PRESENTATION..........................................................................................................................146.2. HARDWARE SUPPORT...............................................................................................................................................146.3. SOFTWARE DESCRIPTION.........................................................................................................................................15

6.3.1. Uplink Chain...................................................................................................................................................156.3.2. Downlink Chain..............................................................................................................................................15

6.4. DETAILED DESCRIPTION...........................................................................................................................................166.4.1. Uplink Process (see Figure No. 10 Uplink Process).......................................................................................166.4.2. Downlink Process (see Figure No. 11 Downlink GPS)...................................................................................17

7. GLOSSARY.............................................................................................................................................................19

8. REFERENCES ........................................................................................................................................................20

9. FIGURES..................................................................................................................................................................21

1


1. GENERAL DESCRIPTION

Extended Squitter Experimentation with the TRT Mode-S Transponder

ICAO Annex 10 /Ref. 1/ specifies that Mode-S transponders send spontaneoustransmissions called « squitters » on a regular basis. LINCOLN Laboratory in the USAhas proposed to extend those messages to carry additional information such as presentposition. This initiative opens the way to very innovative applications such as passivesurveillance. See /Ref. 2/ for more information.

Recently, several experiments have been conducted or proposed using extendedsquitters.

In the USA, the FAA already conducted some tests using modified COLLINStransponders which squitter the aircraft GPS position. This enables a ground system totrack the aircraft with high precision whilst taxiing at the airport, hence allowing theground controller to monitor the position of the aircraft even in adverse weatherconditions.

In Europe, the French DGAC/STNA is conducting flight trials to evaluate the airborneposition reports received from an aircraft equipped with DGPS and Mode-S equipment. Due to its expertise in Mode S transponders and Data Link Processors, theEUROCONTROL Experimental Centre has been invited to contribute to these trials,which are relevant to the EATCHIP Future Concept Domain.

The ground equipment is provided by DASSAULT and THOMSON and is not describedhere.

The airborne equipment is provided by EUROCONTROL and STNA and is described inthis document.

A THOMSON-TRT Mode-S transponder modified to transmit Extended Squittermessages is provided by EUROCONTROL.

A special version Data Link Processor is also provided by EUROCONTROL. This DLPconsists of a ruggedised PC (provided by STNA) with ARINC 718/429 and RS 422interface boards to interface with the transponder and the GPS receiver respectively.

The airborne GPS receiver is provided by STNA (from SEXTANT).

The airborne equipment is mounted in a PILATUS aircraft to be flight-tested by STNA atToulouse-Blagnac.

2


1.1. Overall On-board Configuration

The overall on-board configuration is shown in Figure No. 1.

The MINILIR is an optical trajectography system to sample aircraft reference positions.

1.2. Part provided by EUROCONTROL

The Figure No. 2 shows the part provided by EUROCONTROL of the overall Mode-Sconfiguration in greater detail.

The discrete data required by the transponder is input via switches (i.e. Max air speed,air/ground switch, altitude type, number of antennas, Mode-S address).

A barometric altimeter outputting Gilham coded altitude data provides Mode-C data.

A control unit inputs the Mode-A code and aircraft ident to the transponder.

The DLP on PC has three special I/O cards to interface with the transponder andairborne GPS unit.

3


2. EXTENDED SQUITTER

The Mode-S Extended Squitter messages provide a means to obtain independentsurveillance of aircraft both in the air and on the ground. Highly-accurate GPS-derivedposition information enables precise aircraft tracking for surveillance, planning, andcollision-avoidance applications. The Compact Position Reporting (CPR) compressionalgorithm provides an efficient and unambiguous means to provide uniformly-preciseand bit-efficient encoding of GPS-derived latitude and longitude.

All Mode-S Extended Squitter messages contain 56 bits as required by the Mode-S SLMprotocols. Differential GPS signals are uplinked using Mode-S COMM-A messages. The internal coding of these messages is recalled in Section 2 for ease of reference. Detailed specifications can be found in references /3/ and /4/.

2.1. Format Type Codes

The first 5-bit field in every Mode-S Extended Squitter message contains the messagetype. The message type differentiates the messages into three classes : airborne,surface, and identity. In addition, the message type encodes the measurementprecision category (the ICAO RNP classification) into four classes: 5 meter, 100 meter,0.25 nautical mile and 1.0 nautical mile. The message type also differentiates theairborne messages as to the precision of their altitude measurements. There are 3altitude precision classifications: 25 foot, 100 foot, and GPS-derived. The 5-bitencodings for message type are given in the following table. Note that all the possiblecombinations of message classes, RNP, and altitude precisions are given typeencodings.

4


CODING FORMAT R N P ALTITUDE

01-345678910111213141516

17-3

No dataUnassignedIdentity formatSurface formatSurface formatAirborne formatAirborne formatAirborne formatAirborne formatAirborne formatAirborne formatAirborne formatAirborne formatAirborne formatAirborne formatUnassigned

5 meter RNP100 meter RNP5 meter RNP100 meter RNP0.25 nmi RNP1.0 nmi RNP5 meter RNP100 meter RNP0.25 nmi RNP1.0 nmi RNP5 meter RNP100 meter RNP

25 foot barometric altitude25 foot barometric altitude25 foot barometric altitude25 foot barometric altitude100 foot barometric altitude100 foot barometric altitude100 foot barometric altitude100 foot barometric altitudeGPS heightGPS height

2.2. Airborne Format Coding

The airborne format messages begin with the 5-bit type codes 4 to 16 defined in section2.1. above, depending on the measurements RNP and altitude precision available. Theremainder of the airborne format message consists of 6 fields as given in the followingtable :

SpareSurv/StatusTurnAltitudeTimeLat/lon

2 bits 2 bits 1 bit

11 bits 1 bit

34 bits

5


2.2.1. Surveillance Status

The surveillance status field in the airborne message format encodes information fromthe aircraft's ATCRBS code as follows :

Encoding Meaning

01

23

No information

Emergency/loss of Comm. (ATCRBS codes :7500/7600/7700 octal)SPIChange in ATCRBS code

2.2.2. Turn

The turn field in the airborne message format indicates that the aircraft is performing aturn. The turn field is set to 1 if the aircraft is turning at a rate greater than or equal to 1degree per second. The turn field is set to 0 if the turn rate is less than 1 degree persecond.

2.2.3. Altitude

The altitude field in the airborne message format contains the aircraft altitude. Thedefinition of the altitude precision is determined from the message format type (25 feet,100 feet, GPS-derived).

2.2.4. Time

The time in the airborne message format is a 1-bit field containing the low-order bit ofthe seconds value of the GPS time of position. A time value of 0 indicates an evensecond measurement, while a time value of 1 indicates an odd second measurement.

6


2.2.5. Lat/Lon

The latitude/longitude field in the airborne message format is a 34-bit field containing thelatitude and longitude of the aircraft's surface position. The latitude and longitude eachoccupy 17 bits. The surface latitude and longitude encodings contain the high-order17 bits of the 19-bit CPR-encoded values defined in Section 5 below. The positionalaccuracy maintained by the airborne CPR encoding is approximately 5.1 meters. Notethat the Lat/Lon encoding is also a function of the time value described in 2.2.4. above.

2.3. Identity Format Coding

The identity format message begins with the 5-bit type code 4, as defined in Section 2.1.above. The remainder of the 56-bit message consists of a 3-bit type/wake field and a48-bit ICAO identifier field.

2.3.1. Type/Wake Field

The next 3 bits are assigned the value binary zero.

2.3.2. ICAO Identifier Field

The remaining 48 bits comprise the ICAO identifier. This consists of up to eight 6-bitcharacters whose encoding is given in Table 6 of Section 3.8.2. of Chapter 3,Annex 10.

2.4. Latitude Longitude Coding

The Mode-S Extended Squitter applications uses the Compact Position Reporting (CPR)encoding algorithm to convert an aircraft's known latitude (-90 to +90 degrees) andlongitude (-180 to +180 degrees) into a pair of 19-bit encoded values - Ref. /5/ /6/. TheCPR algorithm uses a different encoding for latitude and longitude depending onwhether the encoding time is an even or odd second. The CPR algorithm providesseveral benefits in the Mode-S Extended Squitter application : a) The encoded positions are nearly uniform in precision for all latitudes and

longitudes. b) A single encoded position report may be unambiguously decoded over a range

of 90 nautical miles from the receiving sensor ( for surface format messages) or360 miles (for airborne format messages).

c) A pair of encoded airborne position reports (one even-second and one odd-

second) separated by less than 10 seconds may be unambiguously decodedglobally.

7


2.4.1. CPR Algorithm Parameters and Internal Functions

The CPR algorithm uses the following parameters whose values are set as follows forthe Mode-S Extended Squitter application :

a) The number of bits used to encode a position co-ordinate, Nb, is be set to 19. b) The number of geographic latitude zones, NZ, is be set to 60.

These parameters settings determine the unambiguous range for decoding(360 nautical miles) and the encoded position precision (approximately 1.25 meters). Note that the airborne Lat/Lon encoding (Section 2.5. above) uses only the high-order17 of the 19 CPR encoded positions, so the effective precision for airborne positionreports is one-fourth of the CPR precision. Note also, that the surface Lat/Lon encoding(Section 3.4. above) truncates the high-order 2 bits of the 19-bit CPR encodings, so theeffective unambiguous range for surface position reports is one-fourth of the CPRunambiguous range.

The CPR algorithm defines some internal functions to be used in the encoding anddecoding processes :

a) The "convert to integer" function denoted Int() accepts a single argument, andreturns the largest integer value less than or equal to that argument.

b) The "modulus" function denoted MOD() accepts two arguments that represent

angles. The MOD() function returns the remainder of its first argument dividedby its second argument. If the first argument is negative, the MOD() functionadds 360 degrees to the first argument before performing the division by thesecond argument.

c) The "number of longitude zones" function denoted NL() accepts one argument

that represents a latitude angle. The NL() function returns the value of thefollowing computation :

NL arccosNZ

lat= −

−

−

intcos

cos2 1

12

2

1802

1

π

π

π

where lat denotes the latitude argument. If the NL() argument lat is plus or minus90 degrees (North or South pole); the NL() functions returns 1.

Note : This equation for NL() is impractical for a real time implementation. A table oftransition latitudes can be pre-computed using the following equation :

8


lat arccos 2NZ

NL

for NL 2 to 4 *NZ=−

−

=1801

12

0 25

π

π

π

cos

cos

.

and a table search procedure used to obtain the return value for NL(). The table valuefor NL=1 is 90 degrees.

2.4.2. CPR Position Encoding Process

The CPR encoding process calculates the encoded 19-bit position values Xzi and Yz forthe airborne or surface Lat/lon field from the global position latitude (Lat), longitude(Lon ), and the position time parity, (i) (0 for even second and 1 for odd second), byperforming the following sequence of computations :

a) ∆lati is computed from the equation :

∆ l a tN Z

i

o

i=

−

9 0

4

b) Yzi is then computed from ∆lati and Lat using the equation :

( )Yz =

MOD Lat, lat

latRounded to nearest integeri

i

i

2Nb ∆∆

c) Rlati is then computed from LAT , YZi, and ∆lati using the equation :

Rlat = latYz

2Int

Lat

lati ii

Nbi

∆∆

+

d) ∆loni is then computed from Rlati using the equation :

( )∆lonNL Rlat ii

o

i

=−

360

9


e) Xzi is then computed from Lon and ∆loni using the equation :

( )Xz = 2

MOD Lon, lon

lon Rounded to nearest integeri

Nb

i

i

∆∆

If the position time parity is odd (i=1), the CPR encoding process performs thefollowing additional steps (f) and (g) :

f) The boundary adjustment, A, is computed using the equation :

A = Sign (Rlat0) [NL(Rlat0) - NL(Rlat1)]

where Rlat0 is computed using steps (a) through (c) for (i=0). g) If the boundary adjustment, A, is non zero, subtract A from the value of Yzi

calculated in step (b) and redo steps (c) through (e).

The Lat/lon encoding for airborne message formats utilises only the upper17 bits of Xzi and Yzi.

10


3. DESCRIPTION OF GPS UPLINK FORMATS

3.1. RF Formats

Figure No. 4 shows the Comm-A Broadcast RF Format.

Bits 1 to 112 are described as follows :

The UF field is set to 20 or 21 decimal. The PC field is not used.

The RR, DI, SD and MA fields are used to transmit the correction data.

The Mode-S Address field is set to all ones (broadcast).

The RR, DI, SD and MA fields are used to transmit the correction data.

The Mode-S Address field is set to all ones (broadcast).

3.2. Message Bloc Format

Figure No. 4 shows how the uplink Comm-As are arranged in a table using the UBI andGI fields as an index to compose the total correction message.

The useful data then starts with the Block Identifier (BI) and finishes with the CRC bytes.

3.3. Message Block Header

Figure No. 5 shows the correction message fields in detail.

The Message Block Identifier (BI) is set to 99 hexadecimal. The Station ID is set to theident code of the nearest aerodrome to the ground transmitter (4 ISO 6 characters or 24bits). The next 2 bits are reserved for future implementations. The message type (6bits) is set to 1 for differential corrections and the message length (8 bits) is the numberof bytes in the message from the BI fields to and including the CRC field (24 bits) but notthe UBI and GI fields.

3.4. Message Data Format

The Modified Z-count (13 bits) gives the reference time at which the parameters of thecorrection message was validated.

11


The Acceleration Error Bound gives the appropriate acceleration errors for the pseudo-distance corrections.

The satellite ID gives the satellite number 1 to 64 where 64=0 binary.

The pseudo range correction (PRC) is a twos complement value, the resolution = 2 cmand the range is + -655.34 metres.

Issue of Data (IOD), the pseudo-distance correction is only possible if the IOD of thesatellite and that of the correction are the same.

The Range Rate Correction (RRC) is a two complement value where the resolution is0.002 m/s and the range + 4.094 m/s.

The User Differential Range Error (UDRE) is an approximation of the differential error atthe reference station calculated by the reference station.

The resolution is 0.2 m and the range 0 to 12.4 m, where code 111111 binary is invaliddata.

3.5. Cyclic Redundancy Check

This is a 24 bit CRC transmitted by the ground station to ensure message integrity

12


4. SPECIFICATIONS OF MODIFICATIONS TO TRANSPONDER SOFTWARE

The THOMSON-TRT transponder software shall be modified as follows :

1) The short squitter shall be maintained as it is at present except that no short squittershall be transmitted when the aircraft is on the ground (squat switch activated). Thisis a 56 bit squitter DF 11 which is transmitted each 1 second (+ 200 ms) alternatelyon top then bottom antenna if antenna diversity is enabled or each 1 second on thebottom antenna only if only one antenna cabled.

2) An extended ADS squitter (112 bit DF 17) shall be transmitted each 500 + 200 ms, as

follows : a) When airborne, the GPS position data shall be squittered from BDS 5 of the

transponder.

The squitter shall be transmitted alternately on top and bottom antenna if antennadiversity is enabled or only on bottom antenna, if not.

b) An ident extended squitter (112 bit DF 17) shall be transmitted each 5 seconds

(+ 200 ms) with the aircraft identification extracted from BDS 20 (hex). Thissquitter is on the top antenna only if the aircraft is on the ground.

The main 6809 microprocessor program was modified as follows :

Two extra time counters and flags were added for the extended position squitter and theextended ident squitter.

The timers are set using a random number generator algorithm to 500 + 200 ms and5000 + 200 ms respectively.

The timers are decremented by the transponder system clock interrupt. When thetimer(s) reach zero a flag is set in common memory to indicate to the TMS 320 signalprocessor that a squitter must be transmitted.

At the TMS 320 level the extended squitter is transmitted after various discrete signalshave been verified.

Figures Nos. 6 to 9 show the extended squitter timing /7/ and detailed flow charts.

13


5. SPECIFICATION OF THE DATA LINK PROCESSOR MODIFICATIONS

The PC based DLP shall interface with the transponder over the ARINC 718 lines. Tothis effect, an interface card shall be installed in the PC. The PC shall interface with theairborne GPS receiver using RS 422 and ARINC 429 interface boards installed in thePC.

The software proposed is based on a real time operating system "Real Time Kernel"where both the system control and the application programmes are written in PASCAL.

There will be the following tasks in order of priority :

High 1) Receive transponder data2) Send data to transponder3) Receive data from GPS receiver4) Send data to GPS receiver5) Display DLP status on PC screen

Low 6) Record data for analysis

The application programme shall receive the corrections from the transponder in theform of broadcast Comm As, extract the correction message and send it to the GPSreceiver using the RS 422 protocol.

Also, the PC shall receive the corrected position from the GPS receiver via anARINC 429 interface and after reformatting pass it to the transponder via the ARINC 718interface.

14


6. DLP SOFTWARE DESCRIPTION

6.1. Implementation Presentation

Transponder - GPS receiver interface (DGPS).

A ground differential GPS unit of known position measures its latitude and longitudefrom satellites.

The deviation between the known and measured positions give the differentialcorrections which are sent to the aircraft as Mode-S Broadcast - Comm-A messages(see Figure No. 2

The DLP receives these corrections from the transponder via the ARINC 718Transponder-DLP bus. These data are tested and filtered before being sent to the on-board GPS receiver in RS 422 format.

In the other direction, the on-board GPS receiver transmits three satellite data blocks onan ARINC 718 bus to the transponder.

6.2. Hardware Support

The computer is a ruggedised portable PC (DASSAULT) in which two ARINC cards /8/and a standard serial interface RS 422 are installed.

The ARINC cards communicate with the host PC via interrupts and dual ported RAM.

The main data exchange with the PC is through a dual port RAM of 128 Kbytes.

A driver program is loaded onto the ARINC card and executed by the local on-boardprocessor. Data exchange between ARINC cards and the Transponder/GPS is byARINC 718/429 respectively.

The cards are configurated by jumpers :

Example of configuration :

• ARINC card 1 used for the Uplink process.Input/Output port address : 280 HexInterrupt : IRQ 5Start address of the dual port memory : A00000 Hex (12 Mbytes)

• ARINC card 2 used for the Downlink process.

Input/Output port address : 300 HexInterrupt : IRQ 10Start address of the dual port memory : C00000 Hex (12 Mbytes)

15


• RS422 serial board for transmission to the DGPS.

Interrupt : IRQ 3Serial port : COM4

6.3. Software Description

There are three separate programs.

Two are assembler programs which are loaded onto the ARINC cards by the PC tocontrol the ARINC 718 and 429 protocols.

The third is the main application program written in PASCAL which controls the functionsof the GPS-DLP.

This program runs under control of a real-time multi-tasking system called RTKernel 4.0from ON-TIME GmbH Hamburg.

This system which controls applications on MS-DOS computers, offers many attractivefeatures (unlimited number of tasks, fast inter-task switch time, priorities, interruptsupport, semaphores, mailboxes, MS-DOS re-entrance problem solved, support ofperipheral hardware ...).

The use has been divided in two distinct parts which correspond to the Uplink andDownlink processes.

6.3.1. Uplink Chain

An assembler program (XPDR.ASM) is loaded onto the ARINC card 1 from the PC. This driver controls the ARINC 718 protocol between the Transponder and the DLP.

The uplink section processes the differential corrections received in the form ofBroadcast -Comm-As via the ARINC 718 channel. The application detects and storesthese GPS - Comm A/Bs in a table which when complete is sent to the satellite receivedvia the RS 422 interface.

6.3.2. Downlink Chain

The ARINC card 2 is loaded with an Assembler driver program (GPSRD.ASM) whichreads the blocks of data sent by the GPS unit on the ARINC 429 bus /9/. The driververifies the checksum of each block and transmits only the useful data to the mainapplication.

16


6.4. Detailed Description

The higher priorities have been affected to the differential corrections, with which we willbegin the explanation.

6.4.1. Uplink Process (see Figure No. 10 Uplink Process)

Conditions :

The corrections are transmitted twice a second in two pulse-trains of 100 ms. Eachblock may contain up to eleven Broadcast - Comm-As.

The program is divided into several tasks.

When the ARINC card 1 receives a Data Link message on the ARINC 718 channel, the« Interrupt 5 » task of the main application detects an IRQ 5 which generates a signal(semaphore) for the ARINC 718 task.

This task reads the message on the card and analyses it.

A Broadcast - Comm-A will be put in a mailbox whereas the other message types will bediscarded.

Remark : The semaphores and mailboxes are synchronisation tools; they are usedrespectively to exchange signals and data between tasks.

Next, a « GPS Comm-AB » task reads the contents of mailbox 1 and verifies the fieldsGI, UBI to check if the data is a GPS Broadcast - Comm-A.

The validated message is put in a second mailbox.

The following function reads mailbox 2.

At the first GPS message, the application activates by means of a signal No. 2 abackground timer of 350 ms, « Delay 350 » corresponding to a lapse of time greaterthan the two pulse-trains together ([train 1 = 100 ms], gap of 100 ms, [train 2 = 100 ms].

During this time, « GPS Table » sorts and stores the new incoming messages. Anindividual counter linked to each message is set to 1 at the first passage (pulse-train 1).

It is incremented to 2 during the second pulse train, if and only if both correspondingmessages are identical, otherwise the value is set to 0 and the data discarded.

After 350 ms, « Delay 350 » sends a signal No. 3 to « Corrections » task which fixesand checks the table of data from individual counters attached to each message.

The verifications are the following :

17


• All the segments have been received once (counter >=1), • At least half the segments have been received twice (counter = 2), • Segments received twice are identical, • The message count in segment 1 corresponds to the number of bytes of the total

correction message.

If these conditions are true, the table is displayed on the screen and converted to ASCIIcharacters to be transmitted to the satellite receiver via the RS 422 bus.

The task ends with the transmission of a signal No. 4 to activate « Send RS 422 » whichtransmits the data under interrupt to the COMM4 port.

6.4.2. Downlink Process (see Figure No. 11 Downlink GPS)

Conditions :

The satellite receiver generates three pulse trains on an ARINC 429 bus. Train 1 is sentout every 100 ms and trains 2 and 3, once each second.

The ARINC card 2 verifies the checksum of each received pulse train and extracts theuseful information for the « Extended Squitter Position » broadcast.

When the process is finished, the « Interrupt 10 » task of the main program detectsIRQ 10 and sends a signal A to « ARINC 429 » task.

This task reads the data on the ARINC card 2 and stores it in an array. The present taskneeds a period of initialisation corresponding to a first reception on pulse trains 1, 2 and3 in order to generate a coherent Extended Squitter composed with the Latitude,Longitude, Altitude, Time and Heading information, located in the three pulse trains.

These conditions achieved, « ARINC 429 » loads the array into mailbox A.

« BDS 5 Process » reads the mailbox A and processes the data to build a BDS 05.

To do this, it must detect the odd/even second for the Compact Position Reportalgorithm, calculate the Latitude and Longitude CPR co-ordinates and the Altitude, Timeand Turn indicator values.

The formatted BDS is then put in mailbox B. « Send BDS 5 » task activated each250 ms then sends it to the transponder.

The choice of 250 ms is transponder dependant, because it transmits an ExtendedSquitter at a random interval between 300 and 700 ms. With this value, we are surethat a message will be ready for each squitter.

18


« Send BDS 5 » receives 2 or 3 BDS updates for each activation; it selects the latestone and checks if the ARINC card 1 is busy before sending the message on the 718bus. The BDS sent is displayed on the screen.

These two main processes consisting of about 15 tasks are performed simultaneously,task activation depending on the interrupts from the I/O cards. They are illustrated byFigures No. 10 & 11 .

The control window enables several options such as the on-line recording on hard diskof the RS 422 and BDS 05 data, a status of tasks and interrupts or the CPU load, to bechosen.

An example of the DLP PC screen during experimentation is shown in Figure No. 12 .

Figures Nos. 13 to 15 show the equipment rack, aircraft and installation.

19


7. GLOSSARY

ATCRBS ATC Radio Beacon System

BDS Binary Data Store

CPR Compact Position Report

DLP Data Link Processor

GPS Global Positioning System

ICAO International Civil Aviation Organisation

RCC Cyclic Redundancy Check

RNP Required Navigational Performance

SLM Standard Long Message

SPI Special Pulse Identification

STNA Service Technique de la Navigation Aérienne (France)

STNACPR Compact Position Report

20


8. REFERENCES

• /Ref. 1/ ICAO Annexe 10

• /Ref. 2/ Air Traffic Control Quarterly, Wiley, 1994, Volume 1, Number 4

• /Ref. 3/ Mode-S Extended Squitter for the Mode-S Specific Services Manual

• /Ref. 4/ ORLANDO and G.H. KNITTEL« GPS-Squitter Concept, Performance and Status »ICASP WP/1 2 April, 1994

• /Ref. 5/ BAYLISS« Compact Position Reports for Efficient Data Link Usage »Lincoln Laboratory Project Report (preliminary draft)

16 March, 1994

• /Ref. 6/ GRAPPEL and V.A. ORLANDO« An algorithm for Compact Position Reporting (CPR) »SICASP/WG-1 WP/1 26 April, 1994

• /Ref. 7/ Mesures de SpectresH.P. ENGLMEIER/L. DUTTONote Technique CEE No. 29/94

• /Ref. 8/ Advanced PC ARINC Card Version 2H.P. ENGLMEIEREEC Technical Note No. 17/94

• /Ref. 9/ SEXTANT AVIONIQUE -Spécification des Trames d’Instrumentation du Récepteur GPSSEXTANT AVIONIQUE DV2 - 10 canaux pour expérimentations DGPSRef. DHI/N/SN/94/06082

21


9. FIGURES

Figure No. 1 : Overall On-Board Configuration

Figure No. 2 : EUROCONTROL Part (Detail)

Figure No. 3 : Extended Squitter Formats

Figure No. 4 : Format of Uplink GPS Correction Message

Figure No. 5 : DGNSS Message Format

Figure No. 6 : Extended Squitter Timing

Figure No. 7 : Transponder Main Programme Flow Chart

Figure No. 8 : Transponder Ident Squitter Flow Chart

Figure No. 9 : Transponder Signal Processor Flow Chart

Figure No. 10 : GPS Data Link Processor Flow Chart (Uplink)

Figure No. 11 : GPS Data Link Processor Flow Chart (Downlink)

Figure No. 12 : Example of PC Display

Figure No. 13 : Photo of EUROCONTROL Equipment

Figure No. 14 : Photo of Aircraft Rack

Figure No. 15 : Photo of Pilatus aircraft

PR EAM PLI

G PS AN TEN NA

VH F M IN ILIRM IN IL IRD ECO D ER

M O D E-STRA NS PO ND ER

D LPU O N PC

G P S RA CK

PC G PS

PC STNA

RS 422

EXPER IM EN TAL O U TPU T

VH F A NTEN N A

M O D E-S AN TEN NA

OVERALL ON-BOARD CONFIGURATION

FIGURE 1

EUROCONTOL PART

GPS CORRECTIONS COMMA BROADCASTS

MODIFIED MODE-S

TRANSPONDER

ALTIMETER BARO

CONTROL UNIT

MODE-SANTENNA

AIRCRAFT POSITIONEXTENDED SQUITTER

DLP ON PC

ARINC 429 I/O CARD

RS 422 I/O CARD

ARINC 718 I/O CARD

KEYBOARD DISPLAY

SEXTANTGPS RACK

429

RS 422

GILLHAM 429

718 DISCRETES

SATELLITES

GPSANTENNA

FIGURE 2

1 112

TYPE ALTITUDESpareSurv.Status

TIME

TURN

LATITUDE CPR CODING LONGITUDE CPR CODING

11 1 17 175 2 2 1

EXTENDED SQUITTER FORMATS USED IN TRIALS

GPS AIRBORNE DATA FORMAT

TYPE WAKE

5 3

AIRCRAFT IDENT

48

AIRCRAFT IDENTITY FORMAT

PARITYDF FS DR DI SD MB

56 24165 3 5 3

SHOWING NUMBER OF BITS IN EACH FIELD

FIGURE 3

FORMAT OF UPLINK GPS CORRECTION MESSAGE

CommA BROADCAST RF FORMAT

Bit 1 112Field UF PC RR DI SD1 SD2 MA1 MA2 MA3 MA4 MA5 MA6 MA7 Mode-S AddressLength 5 + 3 5 + 3 8 8 8 8 8 8 8 8 8 24

BLOCK of CommA BROADCAST MESSAGES

GPS MODE-S BLOCK MESSAGE FORMAT

GPS CORRECTION MESSAGE

Fields CommA MA1 MA2 MA3 MA4 MA5 MA6 MA7 RR+DI SD1 SD2GPS UBI GI b1 b2 b3 b4 b5 b6 b7 b8

1 01 00 BI Station ID 4 * 6 bit ISO r type Len Zc ErrorCommA 2 01 01 m1 m1 m1 m1 m1 m1 m2 m2

3 01 02 m2 m2 m2 m2 m3 m3 m3 m3Message 4 01 03 m3 m3 m4 m4 m4 m4 m4 m4

5 01 04 m5 m5 m5 m5 m5 m5 m6 m6Numbers 6 01 05 m6 m6 m6 m6 m7 m7 m7 m7

7 01 06 m7 m7 m8 m8 m8 m8 m8 m88 01 07 m9 m9 m9 m9 m9 m9 m10 m109 01 08 m10 m10 m10 m10 crc1 crc2 crc3

Figure 4

DGNSS MESSAGE FORMAT

General Message Format

Message Block Header 48 bitsMessage Data VariableCyclic Redundancy Check 24 bits

Message Block Header Format

Parameter Bits BytesMessage Block Identifier 8Reference Station ID 24Reserved 2 6Message Type 6Message Length 8

Message Data Format

Parameters Bits BytesModified Z-count 13Acceleration Error Bound 3 2Satellite ID 6Pseudo Range correction 16Issue of data 8 6 Repeated for N satellitesRange Rate correction 12UDRE 6

Acceleration Error Bound Format

AEB Field Meaning000 0.000m/s² < AEB < 0.002 m/s²001 0.002m/s² < AEB < 0.004 m/s²010 0.004m/s² < AEB < 0.006 m/s²011 0.006m/s² < AEB < 0.008 m/s²100 0.008m/s² < AEB < 0.010 m/s²101 0.010m/s² < AEB < 0.015 m/s²110 AEB > 0.015 m/s²111 Station not working

Figure 11

Ident Squitter Timing 200 events

0

1

2

3

4

5

6

7

8

9

10

4800

4840

4880

4920

4960

5000

5040

5080

5120

5160

5200

Time in milliseconds

Num

be

r of e

vent

s

Long Squitter Timing 1000 events

0

5

10

15

20

25

30

35

300

350

400

450

500

550

600

650

700

Time in milliseconds

Num

be

r of e

vent

s

FIGURE 6

NO

Modifications to MMONI.ASM

Long Squitter 6809 Main Loop

12:43

11/5/94

A:\MMONI.AF2

100 ms Elapsed?

E.AUTO

M.RESU

E.LIGT

E.DISC

M.ACID

E.CHOS

E.TFR

E.SQUI

E.SQUIL

E.SQUID

M.COMA

M.COMB

M.FLID

Short Squitter Routine

Long Squitter Routine

Ident Squitter Routine

Yes

Yes

Yes

Yes

No Yes

0 1

No

Yes

Program ESQD.ASM

Ident Squitter Flowchart

14:17

25/3/94

Page 1

A:\ESQD.AF2Start test oscillator Enable IRQ Set TMS flag Inhibit antennas

Squat switch? Set CH1 = 0 Force Top only

Select Top Antenna Select Bottom Antenna

Diversity?

Wait 37 µsec Inhibit IRQ FIRQ Select antenna

Set CH1 = 1 Force Bottom only

Send Interrogation P1 P3 P4L

Sent to TOP?


P4L Validated?

P4L Validated?

Add Fail P4L Validation

CH1 = ?


P4L Validated?

P4L Validated?

Add Fail P4L Validation

1 2

No

No

Program ESQD.ASM

Ident Squitter Flowchart

14:17

25/3/94

Page 2

A:\ESQD.AF2

1

Antenna select OK ?

Add fail Antenna selection

Inhibit bottom antenna Set CH1 = 1

No Antenna select OK ?

Add fail Antenna selection

Wait 187 µsec.

P4L validation reset ?

Enable antennas Halt test oscillator Enable IRQ FIRQ Tell TMS squitter finished Clear IRQ

Add fail validation reset

Inhibit top antenna Set CH1 =0

2

no

yes

yes

yes

yes

yes

Save context

Decode UF

Sync. phase

detected ?UF subroutine

Short squitter flag ?

Long squitter flag ?

Ident squitter flag ?

TD timeout ?

Rate OK ?

E$A500

Reset

INT,IRQ,RAM

Restore context

E$A600 E$A600

Build DF 17

Message

Build DF 17

Message

1

1

ENTRY14:11

10/5/94

A:\TMS1.AF2

E$A501

Build DF 11

Message

EXIT

no = P4L

TMS 320 FLOWCHART - LONG SQUITTER

Inte rrupt5 Task

set semaphore 1

ARINC 718 interrupt from transponder (IRQ5)

ARINC718 Task

wait semaphore 1

read message on ARINC card1

if Coma_Broadcast put in mailbox 1

else exit

GPS Table Task

get mailbox 2

if first Comab set semaphore 2

put Comab in a global Table

Delay 350ms Task

wait semaphore 2

delay

set semaphore 3

Correc tion s Task

wait semaphore 3

check corrections Table

if complete set semaphore 4

Uplin k GPS

Send RS422 Task

wait semaphore 4

send corrections to GPS in RS422

format

Option: set semaphore 5

GPS Comab Task

get mailbox 1

if GPS data put in mailbox 2

else exit

MAIN TASK

wait commands

RS422 record to disk

Read uplink interrogations from Transponder. Keep CommA broadcasts only.

Process CommA broadcasts. Keep GPS broadcasts only. Check GPS correction data received.

Transfer GPS correction data to GPS receiver.

Record data on hard disk

Inte rrupt1 0 Task

set semaphore A

ARINC429 Task

wait semaphore A

read message on ARINC card2

put ARINC 429 data in mailbox A

BDS Process Task

get mailbox A

prepare BDS5 data

Put in mailbox B

Send BDS Task

get mailbox B find last BDS5

check ARINC card1 if available send BDS5

delay 250 ms

Option:set semaphore B

ARINC 429 interrupt from GPS receiver (IRQ10)

Downli nk GPS

BDS5 record to disk

Read ARINC 429 output from GPS receiver. Get LAT, LONG, Altitude, Time, rate of Turn.

Code LAT, LONG in Compact Position Report Prepare Altitude Time bit and Turn bit.

Send data to Transponder for Extended Squitter.

Record data on hard disk.

1

FRENCH RESUME

Cette note décrit le travail réalisé par EUROCONTROL pour obtenir la position desaéronefs en utilisant le GPS différentiel (DGPS) et le transpondeur Mode-S.

Cette expérimentation a été initialisée par le Service Technique de la Navigation Aérienne(STNA) et réalisée en collaboration avec les Sociétés DASSAULT et THOMSON.

Les modifications apportées par le Centre Expérimental EUROCONTROL sur letranspondeur THOMSON-CNI, les différents formats de messages utilisés ainsi que lastructure et la méthode de programmation employées sur PC sont présentés.

Description générale

Depuis quelques mois, de nombreuses expérimentations utilisant des squitters longs etcourts ont été proposées et réalisées.

Aux Etats-Unis, la FAA a déjà effectué plusieurs tests avec un transpondeur COLLINSmodifié qui émet chaque seconde la position GPS de l’avion. Cela permet à un systèmesol de suivre une grande précision les déplacements d’un aéronef au sol ou au contrôleurde vérifier la position de l’avion et cela indépendamment des conditions météorologiques.

En Europe, le STNA réalise une expérimentation en vol pour évaluer les reports de positionémis par un avion équipé de DGPS et de transpondeur Mode-S. Du fait de son expertisedans le domaine des transpondeurs Mode-S et Processeurs de liaisons de données(DLPU), le Centre Expérimental EUROCONTROL (CEE) a été invité à contribuer à cesexpérimentations qui appartiennent au domaine « Futurs Concepts » (FCO) de EATCHIP.Des mesures infrarouges effectuées à partir du sol servent de référence pour apprécier lesécarts de trajectoire.

Les équipements sol ont été réalisés et fournis par les Sociétés DASSAULT et THOMSON.Ce travail ne sera pas décrit dans cette présente note. Les équipements de bord sontfournis par le STNA et EUROCONTROL.

EUROCONTROL a, d’une part, modifié un transpondeur THOMSON-TRT pour latransmission des squitters longs et, d’autre part, fourni une version spéciale du DLP (DataLink Processor). Ce dernier consiste en un PC avionable (délivré par le STNA) équipé decartes d’interface ARINC 718/429 et RS 422 et pouvant dialoguer d’un côté avec letranspondeur et de l’autre avec le récepteur GPS.

Le récepteur GPS SEXTANT a été fourni par le STNA.

Les équipements de bord ont été installés sur un avion expérimental de type PILATUS.Les essais sont conduits par le STNA à Blagnac, près de Toulouse.

2

Rappel opérationnel

L’expérimentation Mode-S - GPS différentiel vise à étudier le report au sol de la positionGPS différentiel de l’aéronef.

La position GPS accessible par les civils (environ 100 mètres) n’est pas suffisammentprécise pour le contrôle aérien. L’idée du GPS différentiel consiste à corriger lesinformations délivrées par le GPS de bord avant qu’il ne les retransmette au sol.

Ces informations de correction sont calculées par un système sol qui effectue lacomparaison entre les informations déterminées par la réception de plusieurs satellitesGPS (identiques à ce que reçoit le GPS de bord) et les coordonnées géodésiques du siteparfaitement connues.

Ces corrections étant déterminées, il suffit de les « monter » à l’aéronef par le canal DataLink Mode-S. Les corrections sont ensuite fournies au récepteur GPS de bord qui corrigeces informations et retransmet la position corrigée au sol via l’émission de squitter Mode-S.Il en résulte un gain de précision important (précision = 10 m) et cela dans un rayon de 60miles nautiques autour de l’installation sol.

Le système sol doit pouvoir transmettre à l’avion en moyenne 2500 bits par seconde etcela de façon omnidirectionnelle.

Travail réalisé par l’Agence

EUROCONTROL a pris en charge la réalisation de la maquette embarquée. Cette dernièrecomprend un rack métallique 19 pouces qui supporte un alticodeur Ghillam, untranspondeur TRT modifié, une boîte de commande et un ensemble de discrets (Max AirSpeed, Mode-S address...). Cette maquette est connectée à un DLP développé sur PC quiassure le traitement des informations montantes et descendantes entre le transpondeur etle récepteur GPS dans l’aéronef.

Le transpondeur Mode-S modifié

Les informations montantes de correction GPS sont émises par le sol sous forme demessages Mode-S Comm-A. Pour transmettre la quantité de bits nécessaires qui dépendde la couverture satellite à cet instant, un maximum de 22 messages Comm-A peut êtreenvoyé chaque seconde. Le traitement de ces messages est de base dans letranspondeur qui n’a subi aucune adaptation particulière.

Le squitter court a été conservé. Rappelons que ce dernier constitue une émissionspontanée d’une réponse Mode-S, transmise aléatoirement toutes les secondes ou deuxsecondes dans le cas d’un avion équipé de la diversité d’antennes, et formant un messagede 56 bits. Ce message appelé squitter court contient l’adresse de l’aéronef.

Dans l’expérimentation GPS, l’idée consiste à ajouter d’autres émissions spontanéesémises toutes les 500 msec dont un message donnant les informations de temps et deposition (Altitude, Longitude, Latitude). Un message Mode-S long de 112 bits a été choisiavec un DF format égal à 17. Par ailleurs, l’information d’identification du vol estégalement transmise par squitter toutes les cinq secondes.

3

Pour ce faire, le logiciel du transpondeur TRT a été modifié en ajoutant les compteurs de500 msec et de 5 secondes ainsi que les indicateurs nécessaires. Cette modification a étéminime et représente moins de 1 % du logiciel.

Le DLP

Le DLP a été développé sur un PC avionable. Il gère le traitement des informationsmontantes (Corrections DGPS) et les délivre au GPS ainsi que les informations fourniespar le GPS (Position, Flight Ident, etc...). Ces dernières informations sont traitées pouroptimiser le codage et formatées en code compatible avec le réseau Mode-S.

Les échanges Transpondeur - PC sont au standard ARINC 718. Les échanges PC - GPSsont au format RS 422 dans le sens montant et ARINC 429 dans l’autre sens.

Trois interfaces ont été installées dans le PC pour être compatible avec ces protocoles. lelogiciel utilisé fonctionne avec un noyau temps réel et a été écrit en PASCAL.

Le PC reçoit les informations de correction GPS du transpondeur sous la forme demessages Comm-A en ARINC 718, extrait les corrections et les envoie au GPS en utilisantle protocole RS 422.

Le PC reçoit à son tour les informations de positions corrigées par le récepteur GPS sur unbus ARINC 429 et après optimisation les transmet au transpondeur par le protocole ARINC718. Le transpondeur les transmet au sol par l’émission de squitter.

Documents

002 Aircraft Position Report Using DGPS Mode-S