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EUROPEAN ORGANISATION FOR THE SAFETY OF AIR NAVIGATION EUROCONTROL EXPERIMENTAL CENTRE AIRCRAFT POSITION REPORT USING DGPS & MODE-S Subdivision B2.2. - Communications EEC Task No. AT58 EEC Note No. 01/95 Approved for publication by the Head of Division B2 Issued : FEBRUARY 1995 The information contained in this document is the property of the EUROCONTROL Agency 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 : Security Classification : EEC Note No. 01/95 Unclassified Originator Code : Originator (Corporate Author) Name/Location : EEC Division B2 EUROCONTROL Experimental Centre B. P. 15 F - 91222 BRETIGNY SUR ORGE Cedex Telephone 33 (1) 69 88 75 00 Sponsor Code : Sponsor (Contract Authority) Name/Location : EATCHIP Development Directorate EUROCONTROL Agency Rue de la Fusée, 96 B - 1130 BRUSSELS Telephone 32 (2) 7299011 Title : AIRCRAFT POSITION REPORT USING DGPS AND MODE-S Author : Mr. P. HUNT Mr. L. CROUZARD Det. Task Specification AT 58 Distribution Statement : (a) Controlled by : (b) Special limitations : (c) Copy to NTIS : Descriptors (keywords) : Date Pages 02/95 21 Period 2nd Semester 1994 Figs 15 Refs Annexes 7 Task No. Sponsor FCO.ET2.ST08 Task No. Originator AT 58 Head of Division B2 None NO DGPS, Extended Squitter, Mode-S Abstract : 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. EEC Task No. AT58 EEC Note No. 01/95 Issued : February 1995 AIRCRAFT POSITION REPORT USING DGPS AND MODE-S by P. HUNT L. CROUZARD SUMMARY 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. Aircraft Position Report using DGPS & Mode-S C O N T E N T S 1. GENERAL DESCRIPTION.....................................................................................................................................1 1.1. OVERALL ON-BOARD CONFIGURATION ......................................................................................................................2 1.2. PART PROVIDED BY EUROCONTROL......................................................................................................................2 2. EXTENDED SQUITTER.........................................................................................................................................3 2.1. FORMAT TYPE CODES ................................................................................................................................................3 2.2. AIRBORNE FORMAT CODING......................................................................................................................................4 2.2.1. Surveillance Status............................................................................................................................................5 2.2.2. Turn...................................................................................................................................................................5 2.2.3. Altitude..............................................................................................................................................................5 2.2.4. Time ..................................................................................................................................................................5 2.2.5. Lat/Lon..............................................................................................................................................................6 2.3. IDENTITY FORMAT CODING .......................................................................................................................................6 2.3.1. Type/Wake Field ...............................................................................................................................................6 2.3.2. ICAO Identifier Field........................................................................................................................................6 2.4. LATITUDE LONGITUDE CODING .................................................................................................................................6 2.4.1. CPR Algorithm Parameters and Internal Functions ........................................................................................7 2.4.2. CPR Position Encoding Process.......................................................................................................................8 3. DESCRIPTION OF GPS UPLINK FORMATS ..................................................................................................10 3.1. RF FORMATS ...........................................................................................................................................................10 3.2. MESSAGE BLOC FORMAT .........................................................................................................................................10 3.3. MESSAGE BLOCK HEADER .......................................................................................................................................10 3.4. MESSAGE DATA FORMAT .........................................................................................................................................10 3.5. 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 ..........................................................................................................................14 6.2. HARDWARE SUPPORT ...............................................................................................................................................14 6.3. SOFTWARE DESCRIPTION .........................................................................................................................................15 6.3.1. Uplink Chain...................................................................................................................................................15 6.3.2. Downlink Chain ..............................................................................................................................................15 6.4. DETAILED DESCRIPTION...........................................................................................................................................16 6.4.1. Uplink Process (see Figure No. 10 Uplink Process).......................................................................................16 6.4.2. Downlink Process (see Figure No. 11 Downlink GPS)...................................................................................17 7. GLOSSARY .............................................................................................................................................................19 8. REFERENCES ........................................................................................................................................................20 9. FIGURES..................................................................................................................................................................21 Aircraft Position Report using DGPS & Mode-S 1 1. GENERAL DESCRIPTION Extended Squitter Experimentation with the TRT Mode-S Transponder ICAO Annex 10 /Ref. 1/ specifies that Mode-S transponders send spontaneous transmissions called « squitters » on a regular basis. LINCOLN Laboratory in the USA has proposed to extend those messages to carry additional information such as present position. This initiative opens the way to very innovative applications such as passive surveillance. See /Ref. 2/ for more information. Recently, several experiments have been conducted or proposed using extended squitters. In the USA, the FAA already conducted some tests using modified COLLINS transponders which squitter the aircraft GPS position. This enables a ground system to track the aircraft with high precision whilst taxiing at the airport, hence allowing the ground controller to monitor the position of the aircraft even in adverse weather conditions. In Europe, the French DGAC/STNA is conducting flight trials to evaluate the airborne position reports received from an aircraft equipped with DGPS and Mode-S equipment. Due to its expertise in Mode S transponders and Data Link Processors, the EUROCONTROL 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 described here. The airborne equipment is provided by EUROCONTROL and STNA and is described in this document. A THOMSON-TRT Mode-S transponder modified to transmit Extended Squitter messages is provided by EUROCONTROL. A special version Data Link Processor is also provided by EUROCONTROL. This DLP consists of a ruggedised PC (provided by STNA) with ARINC 718/429 and RS 422 interface 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 at Toulouse-Blagnac. Aircraft Position Report using DGPS & Mode-S 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-S configuration 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 and airborne GPS unit. Aircraft Position Report using DGPS & Mode-S 3 2. EXTENDED SQUITTER The Mode-S Extended Squitter messages provide a means to obtain independent surveillance of aircraft both in the air and on the ground. Highly-accurate GPS-derived position information enables precise aircraft tracking for surveillance, planning, and collision-avoidance applications. The Compact Position Reporting (CPR) compression algorithm provides an efficient and unambiguous means to provide uniformly-precise and bit-efficient encoding of GPS-derived latitude and longitude. All Mode-S Extended Squitter messages contain 56 bits as required by the Mode-S SLM protocols. 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 message type. The message type differentiates the messages into three classes : airborne, surface, and identity. In addition, the message type encodes the measurement precision 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 the airborne messages as to the precision of their altitude measurements. There are 3 altitude precision classifications: 25 foot, 100 foot, and GPS-derived. The 5-bit encodings for message type are given in the following table. Note that all the possible combinations of message classes, RNP, and altitude precisions are given type encodings. Aircraft Position Report using DGPS & Mode-S 4 CODING 0 1-3 4 5 6 7 8 9 10 11 12 13 14 15 16 17-3 2.2. FORMAT No data Unassigned Identity format Surface format Surface format Airborne format Airborne format Airborne format Airborne format Airborne format Airborne format Airborne format Airborne format Airborne format Airborne format Unassigned RNP 5 meter RNP 100 meter RNP 5 meter RNP 100 meter RNP 0.25 nmi RNP 1.0 nmi RNP 5 meter RNP 100 meter RNP 0.25 nmi RNP 1.0 nmi RNP 5 meter RNP 100 meter RNP ALTITUDE 25 foot barometric altitude 25 foot barometric altitude 25 foot barometric altitude 25 foot barometric altitude 100 foot barometric altitude 100 foot barometric altitude 100 foot barometric altitude 100 foot barometric altitude GPS height GPS height Airborne Format Coding The airborne format messages begin with the 5-bit type codes 4 to 16 defined in section 2.1. above, depending on the measurements RNP and altitude precision available. The remainder of the airborne format message consists of 6 fields as given in the following table : Spare Surv/Status Turn Altitude Time Lat/lon 2 bits 2 bits 1 bit 11 bits 1 bit 34 bits Aircraft Position Report using DGPS & Mode-S 5 2.2.1. Surveillance Status The surveillance status field in the airborne message format encodes information from the aircraft's ATCRBS code as follows : Encoding 0 1 2 3 Meaning No information Emergency/loss of Comm. (ATCRBS codes : 7500/7600/7700 octal) SPI Change in ATCRBS code 2.2.2. Turn The turn field in the airborne message format indicates that the aircraft is performing a turn. The turn field is set to 1 if the aircraft is turning at a rate greater than or equal to 1 degree per second. The turn field is set to 0 if the turn rate is less than 1 degree per second. 2.2.3. Altitude The altitude field in the airborne message format contains the aircraft altitude. The definition 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 of the seconds value of the GPS time of position. A time value of 0 indicates an even second measurement, while a time value of 1 indicates an odd second measurement. Aircraft Position Report using DGPS & Mode-S 6 2.2.5. Lat/Lon The latitude/longitude field in the airborne message format is a 34-bit field containing the latitude and longitude of the aircraft's surface position. The latitude and longitude each occupy 17 bits. The surface latitude and longitude encodings contain the high-order 17 bits of the 19-bit CPR-encoded values defined in Section 5 below. The positional accuracy maintained by the airborne CPR encoding is approximately 5.1 meters. Note that 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 a 48-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-bit characters 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) and longitude (-180 to +180 degrees) into a pair of 19-bit encoded values - Ref. /5/ /6/. The CPR algorithm uses a different encoding for latitude and longitude depending on whether the encoding time is an even or odd second. The CPR algorithm provides several 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) or 360 miles (for airborne format messages). c) A pair of encoded airborne position reports (one even-second and one oddsecond) separated by less than 10 seconds may be unambiguously decoded globally. Aircraft Position Report using DGPS & Mode-S 7 2.4.1. CPR Algorithm Parameters and Internal Functions The CPR algorithm uses the following parameters whose values are set as follows for the 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-order 17 of the 19 CPR encoded positions, so the effective precision for airborne position reports 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 the effective unambiguous range for surface position reports is one-fourth of the CPR unambiguous range. The CPR algorithm defines some internal functions to be used in the encoding and decoding processes : a) The "convert to integer" function denoted Int() accepts a single argument, and returns 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 divided by its second argument. If the first argument is negative, the MOD() function adds 360 degrees to the first argument before performing the division by the second 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 the following computation : −1 π 1 − cos 2 NZ NL = int 2 π arccos 1 − 2 2π cos lat 180 where lat denotes the latitude argument. If the NL() argument lat is plus or minus 90 degrees (North or South pole); the NL() functions returns 1. Note : This equation for NL() is impractical for a real time implementation. A table of transition latitudes can be pre-computed using the following equation : Aircraft Position Report using DGPS & Mode-S 8 0.25 π 1 − cos 2NZ 180 lat = arccos for NL = 2 to 4 * NZ π 2π 1 − cos NL and a table search procedure used to obtain the return value for NL(). The table value for 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 for the 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), by performing the following sequence of computations : a) ∆lati is computed from the equation : ∆ la t i = b) 90o NZ − i 4 Yzi is then computed from ∆lati and Lat using the equation : MOD(Lat, ∆lat i ) Yz i = 2 Nb Rounded to nearest integer ∆lat i c) Rlati is then computed from LAT, YZi, and ∆lati using the equation : Yz Lat Rlat i = ∆lat i Nbi + Int ∆lat i 2 d) ∆loni is ∆lon i = then computed from Rlati using the equation : 360 o NL(Rlat i ) − i Aircraft Position Report using DGPS & Mode-S 9 e) Xzi is then computed from Lon and ∆loni using the equation : MOD(Lon, ∆lon i ) Xz i = 2 Nb Rounded to nearest integer ∆lon i If the position time parity is odd (i=1), the CPR encoding process performs the following 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 upper 17 bits of Xzi and Yzi. Aircraft Position Report using DGPS & Mode-S 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 and GI 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 the ident code of the nearest aerodrome to the ground transmitter (4 ISO 6 characters or 24 bits). The next 2 bits are reserved for future implementations. The message type (6 bits) is set to 1 for differential corrections and the message length (8 bits) is the number of bytes in the message from the BI fields to and including the CRC field (24 bits) but not the UBI and GI fields. 3.4. Message Data Format The Modified Z-count (13 bits) gives the reference time at which the parameters of the correction message was validated. Aircraft Position Report using DGPS & Mode-S 11 The Acceleration Error Bound gives the appropriate acceleration errors for the pseudodistance 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 cm and the range is + -655.34 metres. Issue of Data (IOD), the pseudo-distance correction is only possible if the IOD of the satellite and that of the correction are the same. The Range Rate Correction (RRC) is a two complement value where the resolution is 0.002 m/s and the range + 4.094 m/s. The User Differential Range Error (UDRE) is an approximation of the differential error at the 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 invalid data. 3.5. Cyclic Redundancy Check This is a 24 bit CRC transmitted by the ground station to ensure message integrity Aircraft Position Report using DGPS & Mode-S 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 squitter shall be transmitted when the aircraft is on the ground (squat switch activated). This is a 56 bit squitter DF 11 which is transmitted each 1 second (+ 200 ms) alternately on top then bottom antenna if antenna diversity is enabled or each 1 second on the bottom 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 antenna diversity 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). This squitter 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 the extended ident squitter. The timers are set using a random number generator algorithm to 500 + 200 ms and 5000 + 200 ms respectively. The timers are decremented by the transponder system clock interrupt. When the timer(s) reach zero a flag is set in common memory to indicate to the TMS 320 signal processor that a squitter must be transmitted. At the TMS 320 level the extended squitter is transmitted after various discrete signals have been verified. Figures Nos. 6 to 9 show the extended squitter timing /7/ and detailed flow charts. Aircraft Position Report using DGPS & Mode-S 13 5. SPECIFICATION OF THE DATA LINK PROCESSOR MODIFICATIONS The PC based DLP shall interface with the transponder over the ARINC 718 lines. To this effect, an interface card shall be installed in the PC. The PC shall interface with the airborne GPS receiver using RS 422 and ARINC 429 interface boards installed in the PC. 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 Low 1) 2) 3) 4) 5) 6) Receive transponder data Send data to transponder Receive data from GPS receiver Send data to GPS receiver Display DLP status on PC screen Record data for analysis The application programme shall receive the corrections from the transponder in the form of broadcast Comm As, extract the correction message and send it to the GPS receiver using the RS 422 protocol. Also, the PC shall receive the corrected position from the GPS receiver via an ARINC 429 interface and after reformatting pass it to the transponder via the ARINC 718 interface. Aircraft Position Report using DGPS & Mode-S 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 longitude from satellites. The deviation between the known and measured positions give the differential corrections 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 718 Transponder-DLP bus. These data are tested and filtered before being sent to the onboard GPS receiver in RS 422 format. In the other direction, the on-board GPS receiver transmits three satellite data blocks on an 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-board processor. Data exchange between ARINC cards and the Transponder/GPS is by ARINC 718/429 respectively. The cards are configurated by jumpers : Example of configuration : • ARINC card 1 used for the Uplink process. Input/Output port address : Interrupt : Start address of the dual port memory : 280 Hex IRQ 5 A00000 Hex (12 Mbytes) • ARINC card 2 used for the Downlink process. Input/Output port address : Interrupt : Start address of the dual port memory : 300 Hex IRQ 10 C00000 Hex (12 Mbytes) Aircraft Position Report using DGPS & Mode-S 15 • RS422 serial board for transmission to the DGPS. Interrupt : Serial port : 6.3. IRQ 3 COM4 Software Description There are three separate programs. Two are assembler programs which are loaded onto the ARINC cards by the PC to control the ARINC 718 and 429 protocols. The third is the main application program written in PASCAL which controls the functions of the GPS-DLP. This program runs under control of a real-time multi-tasking system called RTKernel 4.0 from ON-TIME GmbH Hamburg. This system which controls applications on MS-DOS computers, offers many attractive features (unlimited number of tasks, fast inter-task switch time, priorities, interrupt support, semaphores, mailboxes, MS-DOS re-entrance problem solved, support of peripheral hardware ...). The use has been divided in two distinct parts which correspond to the Uplink and Downlink 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 of Broadcast -Comm-As via the ARINC 718 channel. The application detects and stores these GPS - Comm A/Bs in a table which when complete is sent to the satellite received via the RS 422 interface. 6.3.2. Downlink Chain The ARINC card 2 is loaded with an Assembler driver program (GPSRD.ASM) which reads the blocks of data sent by the GPS unit on the ARINC 429 bus /9/. The driver verifies the checksum of each block and transmits only the useful data to the main application. Aircraft Position Report using DGPS & Mode-S 16 6.4. Detailed Description The higher priorities have been affected to the differential corrections, with which we will begin 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. Each block 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 be discarded. Remark : The semaphores and mailboxes are synchronisation tools; they are used respectively to exchange signals and data between tasks. Next, a « GPS Comm-AB » task reads the contents of mailbox 1 and verifies the fields GI, 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 a background timer of 350 ms, « Delay 350 » corresponding to a lapse of time greater than 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. An individual 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 corresponding messages 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 fixes and checks the table of data from individual counters attached to each message. The verifications are the following : Aircraft Position Report using DGPS & Mode-S 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 ASCII characters 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 » which transmits 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 sent out 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 the useful information for the « Extended Squitter Position » broadcast. When the process is finished, the « Interrupt 10 » task of the main program detects IRQ 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 task needs a period of initialisation corresponding to a first reception on pulse trains 1, 2 and 3 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 Report algorithm, calculate the Latitude and Longitude CPR co-ordinates and the Altitude, Time and Turn indicator values. The formatted BDS is then put in mailbox B. « Send BDS 5 » task activated each 250 ms then sends it to the transponder. The choice of 250 ms is transponder dependant, because it transmits an Extended Squitter at a random interval between 300 and 700 ms. With this value, we are sure that a message will be ready for each squitter. Aircraft Position Report using DGPS & Mode-S 18 « Send BDS 5 » receives 2 or 3 BDS updates for each activation; it selects the latest one and checks if the ARINC card 1 is busy before sending the message on the 718 bus. 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 by Figures No. 10 & 11. The control window enables several options such as the on-line recording on hard disk of the RS 422 and BDS 05 data, a status of tasks and interrupts or the CPU load, to be chosen. 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. Aircraft Position Report using DGPS & Mode-S 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 Aircraft Position Report using DGPS & Mode-S 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 Spectres H.P. ENGLMEIER/L. DUTTO Note Technique CEE No. 29/94 • /Ref. 8/ Advanced PC ARINC Card Version 2 H.P. ENGLMEIER EEC Technical Note No. 17/94 • /Ref. 9/ SEXTANT AVIONIQUE Spécification des Trames d’Instrumentation du Récepteur GPS SEXTANT AVIONIQUE DV2 - 10 canaux pour expérimentations DGPS Ref. DHI/N/SN/94/06082 Aircraft Position Report using DGPS & Mode-S 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 Aircraft Position Report using DGPS & Mode-S OVERALL ON-BOARD CONFIGURATION GPS ANTENNA PR EAM PLI VHF ANTENNA V H F M IN IL IR M IN IL IR DECODER GPS RACK R S 422 M O D E -S A N T E N N A M O D E -S TRANSPONDER E X P E R IM E N T A L O U TPU T D LPU ON PC PC GPS PC STNA FIGURE 1 EUROCONTOL PART SATELLITES GPS CORRECTIONS COMMA BROADCASTS AIRCRAFT POSITION EXTENDED SQUITTER GPS ANTENNA MODE-S ANTENNA DLP ON PC 429 ARINC 429 I/O CARD SEXTANT GPS RACK RS 422 MODIFIED MODE-S DISCRETES RS 422 I/O CARD TRANSPONDER GILLHAM ALTIMETER BARO 718 429 ARINC 718 I/O CARD KEYBOARD DISPLAY CONTROL UNIT FIGURE 2 EXTENDED SQUITTER FORMATS USED IN TRIALS SHOWING NUMBER OF BITS IN EACH FIELD 1 112 DF FS DR DI SD MB PARITY 5 3 3 16 56 24 5 GPS AIRBORNE DATA FORMAT TYPE 5 Surv. Spare Status 2 2 T U R N 1 ALTITUDE 11 T I M E 1 LATITUDE CPR CODING 17 LONGITUDE CPR CODING 17 AIRCRAFT IDENTITY FORMAT TYPE WAKE 5 3 AIRCRAFT IDENT 48 FIGURE 3 FORMAT OF UPLINK GPS CORRECTION MESSAGE CommA BROADCAST RF Bit Field Length 1 UF PC RR DI 5 + 3 5 + 3 SD1 8 SD2 8 MA1 8 MA2 8 FORMAT MA3 8 MA4 8 MA5 8 MA6 8 MA7 8 SD1 b7 SD2 b8 Zc m2 m3 m4 m6 m7 m8 m10 crc3 Error m2 m3 m4 m6 m7 m8 m10 112 Mode-S Address 24 BLOCK of CommA BROADCAST MESSAGES GPS MODE-S BLOCK MESSAGE FORMAT GPS CORRECTION MESSAGE Fields CommA GPS 1 CommA 2 Message 4 3 5 Numbers 6 7 8 9 MA1 UBI 01 01 01 01 01 01 01 01 01 MA2 GI 00 01 02 03 04 05 06 07 08 MA3 b1 BI m1 m2 m3 m5 m6 m7 m9 m10 MA4 b2 MA5 b3 MA6 b4 Station ID 4 * 6 bit ISO m1 m1 m1 m2 m2 m2 m3 m4 m4 m5 m5 m5 m6 m6 m6 m7 m8 m8 m9 m9 m9 m10 m10 m10 MA7 b5 RR+DI b6 r type Len m1 m1 m3 m3 m4 m4 m5 m5 m7 m7 m8 m8 m9 m9 crc1 crc2 Figure 4 DGNSS MESSAGE FORMAT General Message Format Message Block Header Message Data Cyclic Redundancy Check 48 bits Variable 24 bits Message Block Header Format Parameter Message Block Identifier Reference Station ID Reserved Message Type Message Length Bits 8 24 2 6 8 Bytes Bits 13 3 6 16 8 12 6 Bytes 6 Message Data Format Parameters Modified Z-count Acceleration Error Bound Satellite ID Pseudo Range correction Issue of data Range Rate correction UDRE 2 6 Repeated for N satellites Acceleration Error Bound Format AEB Field 000 001 010 011 100 101 110 111 Meaning 0.000m/s² < AEB < 0.002m/s² < AEB < 0.004m/s² < AEB < 0.006m/s² < AEB < 0.008m/s² < AEB < 0.010m/s² < AEB < AEB > 0.015 m/s² Station not working 0.002 m/s² 0.004 m/s² 0.006 m/s² 0.008 m/s² 0.010 m/s² 0.015 m/s² Figure 11 Ident Squitter Timing 200 events 10 9 Number of events 8 7 6 5 4 3 2 1 0 5200 5160 5120 5080 5040 5000 4960 4920 4880 4840 4800 Time in milliseconds Long Squitter Timing 1000 events 35 25 20 15 10 5 700 650 600 550 500 450 400 350 0 300 Number of events 30 Time in milliseconds FIGURE 6 A:\MMONI.AF2 11/5/94 12:43 Long Squitter 6809 Main Loop 100 ms Elapsed? NO Modifications to MMONI.ASM E.AUTO M.RESU E.LIGT E.DISC M.ACID M.FLID E.CHOS E.TFR Short Squitter Routine E.SQUI E.SQUIL E.SQUID M.COMA M.COMB Long Squitter Routine Ident Squitter Routine A:\ESQD.AF2 Start test oscillator Enable IRQ Set TMS flag Inhibit antennas 25/3/94 14:17 Ident Squitter Flowchart Program ESQD.ASM Squat switch? Yes Set CH1 = 0 Force Top only Diversity? No Set CH1 = 1 Force Bottom only 1 Select Bottom Antenna Wait 37 µsec Inhibit IRQ FIRQ Select antenna 0 Select Top Antenna CH1 = ? Send Interrogation P1 P3 P4L Yes P4L Validated? Sent to TOP? Yes P4L Validated? Yes Send Interrogation P1 P3 P4L Send Interrogation P1 P3 P4L P4L Validated? No Yes P4L Validated? Add Fail P4L Validation Add Fail P4L Validation 1 2 Page 1 Yes A:\ESQD.AF2 25/3/94 14:17 Ident Squitter Flowchart Program ESQD.ASM 1 Antenna select OK ? 2 No Add fail Antenna selection Antenna select OK ? No Inhibit top antenna Set CH1 =0 Inhibit bottom antenna Set CH1 = 1 Wait 187 µsec. P4L validation reset ? Enable antennas Halt test oscillator Enable IRQ FIRQ Tell TMS squitter finished Clear IRQ Page 2 No Add fail validation reset Add fail Antenna selection A:\TMS1.AF2 10/5/94 TMS 320 FLOWCHART - LONG SQUITTER ENTRY 14:11 Save context Decode UF Sync. phase 1 yes UF subroutine detected ? no = P4L yes Short squitter flag ? yes Long squitter flag ? yes Ident squitter flag ? yes TD timeout ? Build DF 17 Build DF 17 Build DF 11 Message Message Message E$A600 E$A600 E$A501 no Rate OK ? E$A500 1 Reset INT,IRQ,RAM Restore context EXIT Read uplink interrogations from Transponder. Keep CommA broadcasts only. ARINC 718 interrupt from transponder (IRQ5) Interrupt5 Task ARINC718 Task set semaphore 1 wait semaphore 1 read message on ARINC card1 if Coma_Broadcast put in mailbox 1 else exit Process CommA broadcasts. Keep GPS broadcasts only. Check GPS correction data received. Transfer GPS correction data to GPS receiver. Record data on hard disk GPS Comab Task get mailbox 1 if GPS data put in mailbox 2 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 MAIN TASK wait commands Send RS422 Task delay Corrections Task set semaphore 3 wait semaphore 3 Uplink GPS wait semaphore 4 check corrections Table send corrections to GPS in RS422 format if complete set semaphore 4 Option: set semaphore 5 RS422 record to disk Read ARINC 429 output from GPS receiver. Get LAT, LONG, Altitude, Time, rate of Turn. ARINC 429 interrupt from GPS receiver (IRQ10) Interrupt10 Task ARINC429 Task set semaphore A wait semaphore A 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. 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 BDS5 record to disk Downlink GPS FRENCH RESUME Cette note décrit le travail réalisé par EUROCONTROL pour obtenir la position des aéronefs 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 le transpondeur THOMSON-CNI, les différents formats de messages utilisés ainsi que la structure et la méthode de programmation employées sur PC sont présentés. Description générale Depuis quelques mois, de nombreuses expérimentations utilisant des squitters longs et courts ont été proposées et réalisées. Aux Etats-Unis, la FAA a déjà effectué plusieurs tests avec un transpondeur COLLINS modifié qui émet chaque seconde la position GPS de l’avion. Cela permet à un système sol de suivre une grande précision les déplacements d’un aéronef au sol ou au contrôleur de 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 position émis par un avion équipé de DGPS et de transpondeur Mode-S. Du fait de son expertise dans le domaine des transpondeurs Mode-S et Processeurs de liaisons de données (DLPU), le Centre Expérimental EUROCONTROL (CEE) a été invité à contribuer à ces expérimentations qui appartiennent au domaine « Futurs Concepts » (FCO) de EATCHIP. Des mesures infrarouges effectuées à partir du sol servent de référence pour apprécier les écarts 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 sont fournis par le STNA et EUROCONTROL. EUROCONTROL a, d’une part, modifié un transpondeur THOMSON-TRT pour la transmission des squitters longs et, d’autre part, fourni une version spéciale du DLP (Data Link Processor). Ce dernier consiste en un PC avionable (délivré par le STNA) équipé de cartes d’interface ARINC 718/429 et RS 422 et pouvant dialoguer d’un côté avec le transpondeur 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. 1 Rappel opérationnel L’expérimentation Mode-S - GPS différentiel vise à étudier le report au sol de la position GPS différentiel de l’aéronef. La position GPS accessible par les civils (environ 100 mètres) n’est pas suffisamment précise pour le contrôle aérien. L’idée du GPS différentiel consiste à corriger les informations 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 la comparaison entre les informations déterminées par la réception de plusieurs satellites GPS (identiques à ce que reçoit le GPS de bord) et les coordonnées géodésiques du site parfaitement connues. Ces corrections étant déterminées, il suffit de les « monter » à l’aéronef par le canal Data Link Mode-S. Les corrections sont ensuite fournies au récepteur GPS de bord qui corrige ces 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 60 miles nautiques autour de l’installation sol. Le système sol doit pouvoir transmettre à l’avion en moyenne 2500 bits par seconde et cela de façon omnidirectionnelle. Travail réalisé par l’Agence EUROCONTROL a pris en charge la réalisation de la maquette embarquée. Cette dernière comprend un rack métallique 19 pouces qui supporte un alticodeur Ghillam, un transpondeur TRT modifié, une boîte de commande et un ensemble de discrets (Max Air Speed, Mode-S address...). Cette maquette est connectée à un DLP développé sur PC qui assure le traitement des informations montantes et descendantes entre le transpondeur et le récepteur GPS dans l’aéronef. Le transpondeur Mode-S modifié Les informations montantes de correction GPS sont émises par le sol sous forme de messages Mode-S Comm-A. Pour transmettre la quantité de bits nécessaires qui dépend de la couverture satellite à cet instant, un maximum de 22 messages Comm-A peut être envoyé chaque seconde. Le traitement de ces messages est de base dans le transpondeur qui n’a subi aucune adaptation particulière. Le squitter court a été conservé. Rappelons que ce dernier constitue une émission spontanée d’une réponse Mode-S, transmise aléatoirement toutes les secondes ou deux secondes dans le cas d’un avion équipé de la diversité d’antennes, et formant un message de 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 spontanées émises toutes les 500 msec dont un message donnant les informations de temps et de position (Altitude, Longitude, Latitude). Un message Mode-S long de 112 bits a été choisi avec un DF format égal à 17. Par ailleurs, l’information d’identification du vol est également transmise par squitter toutes les cinq secondes. 2 Pour ce faire, le logiciel du transpondeur TRT a été modifié en ajoutant les compteurs de 500 msec et de 5 secondes ainsi que les indicateurs nécessaires. Cette modification a été minime et représente moins de 1 % du logiciel. Le DLP Le DLP a été développé sur un PC avionable. Il gère le traitement des informations montantes (Corrections DGPS) et les délivre au GPS ainsi que les informations fournies par le GPS (Position, Flight Ident, etc...). Ces dernières informations sont traitées pour optimiser le codage et formatées en code compatible avec le réseau Mode-S. Les échanges Transpondeur - PC sont au standard ARINC 718. Les échanges PC - GPS sont 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. le logiciel 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 de messages Comm-A en ARINC 718, extrait les corrections et les envoie au GPS en utilisant le protocole RS 422. Le PC reçoit à son tour les informations de positions corrigées par le récepteur GPS sur un bus ARINC 429 et après optimisation les transmet au transpondeur par le protocole ARINC 718. Le transpondeur les transmet au sol par l’émission de squitter. 3