The Telesat Mobile Incorporated (TMI) Mobile Data System (MDS) was developed to apply satellite technology to the transportation industry's requirement for a fleet management system. It will provide two-way messaging and automatic position reporting capabilities between dispatch centers and customers' fleets of trucks. The design was based on the Inmarsat L-Band space segment with system link parameters and margins adjusted to meet the land mobile satellite channel characteristics. The system interfaces with the Teleglobe Des Laurentides earth station at Weir, Quebec. The signaling protocols were derived from the Inmarsat Standard C packet signalling system with unique trucking requirements incorporated where necessary

Kent, John D. B.

English

NASA Technical Reports Server

N92 24087
A Land Mobile Satellite Data System
John D.B.Kent
Canadian Astronautics Limited
1050 Morrison Drive
Ottawa, Ontario K2H 8K7
Phone: 613-820-8280
FAX: 613-820-8314
ABSTRACT
The Telesat Mobile Incorporated
(TMI) Mobile Data System (MDS) has
been developed to apply satellite
technology to the transportation
industry's requirement for a fleet
management system. It will provide
two-way messaging and automatic
position reporting capabilities
between dispatch centres and
customers' fleets of trucks. The
design has been based on the
Inmarsat L-Band space segment with
system link parameters and margins
adjusted to meet the land mobile
satellite channel characteristics.
The system interfaces with the
Teleglobe Des Laurentides earth
station at Weir, Quebec. The
signalling protocols have been
derived from the Inmarsat Standard
C packet signalling system I with
unique trucking requirements
incorporated where necessary.
SYSTEM DESCRIPTION
The system topology is
illustrated in Figure i. and the
HUB equipment connectivity in
Figure 2. Fleets are connected to
the central HUB via terrestrial
(leased lines, dialup facilities
or X.25 data accesses) or VSAT
systems. The system connectivity
maps each dispatch centre to a
fleet or subfleet(s) of mobiles.
Each mobile has an association
with only one dispatch centre. The
mobile earth terminal (MET)
identifier then becomes the only
routing indicator needed to
address or identify the originator
or addressee of a one-to-one
message.
Four classes of messages are
utilized in the MDS: general
messages of up to 121 bytes for
outbound messages and 129 bytes
for inbound messages, coded
messages which are a set of 32
pre-selected messages, 32 pre-
formatted message with blank
fields to be filled in by the
operator and broadcast messages.
Fleet or subfleet broadcast
messages from the dispatch centre
in the outbound direction are
routed to groups of METS whose
association is established when
the MET logs-on the system.
SYSTEM ELEMENTS
The major hardware elements in
the HUB system are the STARMASTER
system switch (SS), redundant VAX
3300 Network Control Processors
(NCP) and Single Board Computers
(SBC) Satellite Protocol
Processors (SPP), the PACX 200
Remote Switch and the Channel
Units. Messages from dispatch
centres are routed from the
incoming terrestrial circuits to
the on-line NCP via the SS. The
NCP contains the dispatch centre
records, fleet and terminal
information and system
configuration data. Historical
records are also maintained in the
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NCP for billing and network
management purposes. Incoming
messages are processed by the NCP
and forwarded to two queues
(Priority and Routine) to the SPP.
The NCP also maintains the
position reporting register and
assigns channels, slots and slot
intervals to the MET's at log-on
for position reporting messages.
The two elements of the HUB are
interconnected by four 9.6 kbps
terrestrial routed lines, backed
up by a single redundant line. The
remote switch configuration is
controlled by the NCP and lines
and channel unit grouping are
changed via a 2.4 kbps dialup
orderwire.
PROPAGATION AND SYSTEM MARGIN
The land mobile satellite
channel is characterized by a
line-of-sight (LOS) component, a
multipath-path component and
shadowing or blockage of the LOS
component. The LOS combined with
the multi-path form the fast
fading component which follows a
Rician distribution and the
shadowing results in a slow fading
component with a log-normal
distribution. Figure 22 shows the
cumulative probability
distribution function for typical
land mobile satellite paths in the
Ottawa, Ontario area.
The limited satellite EIRP
available at L-Band (21.6 dBW) and
the relatively wide bandwidth
available per channel (5 KHz), in
relation to the channel data rate,
indicated that coding should be
utilized to correct errors
introduced by the rapidly changing
channel characteristics. To
improve the performance of the
coding the outbound frame is
interleaved in a 64x162 matrix
thus spreading burst errors
throughout the frame. The selected
coding system was rate 1/2
convolution coding with a
constraint length of 7 with 3 bit
soft decision Viterbi decoding.
The inbound frame is not
interleaved due to its short
duration (527 ms); however, it is
coded as per the outbound channel.
The margin and data rates on the
inbound channels are adjusted for
the lack of interleaving.
FRAME STRUCTURE AND MESSAGING
The outbound and inbound channel
and packet structure is shown in
Figure 4. A single outbound TDM
channel is used on the forward
link with a frame period of 8.64
seconds. A 128 bit unique word
indicates the start of frame and
is followed by a 112 bit bulletin
board which includes the system
identifier and network
information. The inbound channel
descriptors (ICD) provide the
acknowledgement status of the
previous frame (ACK/NACK) register
and the reservation status of the
14/28 slots in the position
reporting and messaging channels
three frames ahead. This is
necessary due to the depth of
interleaving and the time required
for decoding and descrambling. The
message packet length is
established by the length of the
message. In the event of a message
overlapping two frames, the
message is carried over to the
first information packet of the
next frame. I f the frame has
excess capacity, the unused space
is padded out with dummy zeros.
The outbound message includes the
reservation of an inbound channel
and slot on which the MET sends
the message acknowledgement.
Optimization of the system
throughput is accomplished by
utilization of a combination of
slotted Aloha and reservation
Aloha for inbound messages. The
inbound channels, up to a total of
14 messaging and position
reporting channels, are divided
98
International Mobile Satellite Conference, Ottawa, 1990
into 14/28 slots depending on the
channel data rate of 300 or 600
bps. The channels are divided into
position reporting and general
messaging channels which are
subdivided into retry and priority
message slots. Initial log-on to
the system and the first packet of
a single or multi-packet message
utilize a slotted Aloha protocol
with subsequent packets sent in
reserved slots controlled by the
satellite protocol processor (SPP)
through the ICD. If a fade or
other loss of the inbound packet
occurs, indicated by a cyclic
redundancy code (CRC) error, the
ICD is set to NACK and the
reservation is extended for
another packet by the SPP. The MET
acknowledges the NACK by repeating
the lost packet and continuing the
message on a three frame multi-
slot basis adding a slot for the
lost packet. The relatively long
message delay for a multi-packet
inbound message is indicated in
Figure 5, which assumes no CRC
errors for the inbound packets. An
ARQ system is used in conjunction
with the FEC with the last two
bytes of packets reserved for the
CRC.
Position report information is
forwarded from the Loran-C board
(latitude and longitude to .01
degree along with a two digit
relative reliability indicator) to
the MET processor board on the
schedule established at log-on.
This information is sent in the
reserved slots and channel every
104, 208, 416 or 832 frames
corresponding to position
information every 15, 30, 60 or
120 minutes approximately. If the
position report is not received by
the HUB, indicated to the MET by
the ICD status the MET takes no
further action. If the user has
subscribed to a retry service then
the HUB interrogates the MET
requesting a position report and
assigning one of a number of
reserved slots in a position
reporting channel for the retry.
Broadcast messages to fleets and
subfleets are not acknowledged by
addressees to avoid congestion on
the inbound channels. They are
repeated at least three times in
successive outbound frames to
ensure reception by vehicles
subject to nominal propagation
conditions.
DEMODULATOR PERFORMANCE OBJECTIVES
The MET demodulator operates in
a synchronous half duplex mode at
1200 sps. During transmission the
MET must retain frequency and bit
timing so that it can re-acquire
the outbound TDM channel after the
first unique word. With additive
white Gaussian noise and a
specified received phase noise the
demodulator performance shall be:
C/No Pe
dB-Hz xE-05
33.0 7.8
33.5 2.0
Initial acquisition of the carrier
with a frequency offset at the
demodulator of +/-850 Hz shall be
achieved over a range of C/No of
33.0 to 35.0 dB-Hz within 25
seconds with a probability of
failure not greater than 0o01.
The inbound TDMA burst
demodulators will operate at
600/1200 sps (HUB selectable) with
632/316 TDM symbol periods per
burst. The burst to burst
frequency uncertainty after
frequency correction at the Des
Laurentides earth station shall
not be greater than +/- 650 Hz and
the maximum rate of change of
frequency during a burst shall not
exceed +/- 65 Hz per second for
six seconds. The packet error
probability (PEP) for a 15 byte
packet shall be less than the
International Mobile Satellite Conference, Ottawa, 1990
99
following at 300 bps:
C/No PEP
dB-Hz
32.3 0.I
34.6 0.02
Digital signal processing is
used in all modems and provides
the flexibility to rapidly
reconfigure the network and
reduces the hardware required for
redundancy.
SUMMARY
The MDS system is currently
undergoing system and acceptance
tests and is scheduled to be in
operational service in the Spring
of 1990. The satellite link
performance has proven to be quite
robust and to meet the messaging
objectives even under relatively
dense shadowing. The Loran-C
position reporting accuracy has
been excellent in rural areas and
the quality indicator has alerted
to possible errors in high noise
environments.
Final test results will be
presented at the conference and
overall system performance will be
discussed.
REFERENCES
I. INMARSAT, Standard-C, System
Definition Manual, Release 1.3,
July 1989.
2. BUTTERWORTH, J. AND MATT, E.
1983. The Characterization of
Propagation Effects for Land
Mobile Satellite Services. IEE
Conference Publication No. 222.
IDISPATCH
LCENTRE2
DISPATCH
CENTRE 1 L
/
HUB
/
!
/
I/
DISPATCH F TERRESTRIALCENTRE J LINKS
FLEET1
/"
/
//
///
\\
"\
FLEET2
MET1
/
./
./
.._______,J . MET 2
"x
"-. METM
MET1/-
/
,/ , MET 2
\\
". MET N
\
\\ , i, MET 1
\ //
FLEET L . < . MET 2
SATELLITE LIN KS
FIGURE 1 MDS NETWORK TOPOLOGY
\
", METP
100
International Mobile Satellite Conference, Ottawa, 1990
I NCP 1 / NCP 2[
v,x v,x 1
I TERMINAL
E'THERNET
TERMINAL
SERVERS
SPP 1
SBC
18 K]BP$
t
SPP 2
SBC
T
0 C DATAPAC
E
D N )(-25
I T
S R I)IALIN/OUT
P E FACILITY
A S
T LEASED
C UNE_'VSAT
H
DATA
SWITCH
9.6 _BPS
t0 ICDell
18 KBII'II
ITERMINALt J
FIGURE2 MDS HUB
DATA
SWITCH
C-BAND
IF OUT
CHANNEL I
IC-BAND
UNITS tlF IN
( 70 MHZ ) IL-BANO
IF IN
O
I--
UJ
_z
O3
a
b
LU
n-
L-BAND DATA
8- FOR 19" ELEVATION ANGLE
4-
_ "_-_. _,. -..
':1
>
_- -8:1 /-I\\ ',
"r
(3 ..1 SURBURBAN DATA f \ \ \
_"_) 12_ (0.45 MIL.(.ION PO,NTS, 100Hz BW) \'_. \\
,,,, _-
z -16
" RURAL/FORESTED "I _'_ \
20" _ON POINTS, 300Hz BW) J '_,_,_ _\
" _ \_,, '
- RURAL/FARM LAND \,,,
-24- (1.3 MILLION POINTS, 300Hz BW) _ \\
' 1 t I i i f 1 ' I ' I ' I ' I ' I ' f
0.01 0.1 1,0 10 30 50 70 90 99 99.9 99.99
PERCENT OF LOCATIONS WHERE SIGNAL WAS GREATER THAN ORDINATE
FIGURE 3 L-BAND PROPAGATION
International Mobile Satellite Conference, Ottawa, t990
101
• l
8.64 secs
10368 tdm symbols
co MESSAGE PACKETS ANDCOMMANDS
OUTBOUND TDM CHANNEL
_:dc°"v°_VT'O_A%_o"Cs°°'°
}. 632tdm S ,_
740 tdm S
INBOUND TDMA CHANNEL (300 BPS)
FIGURE 4 CHANNEL AND PACKET STRUCTURE
150
U,J
w
z
co
0
r',r
U,,J
Z
120
90
60
30
0
_j--
0 40
FIGURE 5
8o 12o 16o _o 2_o 28o
TIME IN SECONDS
INBOUND PACKET TRANSFER TIME
102
International Mobile Satellite Conference, Ottawa, 1990


A land mobile satellite data system

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