- Eurocontrol

- Eurocontrol
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
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