|
| |
Curious how GPS works? (Click here -> GPS
Definition)
|
GPS TRACKING AND WIRELESS RAILWAY
NETWORKING
Most industry standard GPS receivers do a very good
job of calculating where you are , but additional methods are required for transmitting this
information to another location -- so that other people also know where you are.
Generally this is accomplished using an existing Cellular network (such as
AT&T). The GPS information from the receiver is transmitted to the
Operations Center via the cellular provider -- were the data is then collected
from multiple GPS receivers -- and deciphered such that it can be displayed on a
graphical track plan on a central office system workstation.
|
|
B&C Transit office systems have developed such
a graphical interface where GPS information is deciphered and shown on a
track plan (graphical computer display), and have already solved the
problems that exist on dual tracks -- where it is usually difficult (due to
close proximity) to determine exactly which track the rail car is on.
It can be expensive using a Cellular provider to
transmit GPS data to the central control center, since the cellular transmission
cost for each GPS receiver on a rail car can run upwards of $50 per month.
However, there are alternatives. One such solution is Wireless networking.
|
Wireless
Networking to Rail Cars
 |
|
|
In the ultimate system, each rail car
would have it’s own internal LAN (local area network). The connection to the
outside world from the rail car would be accomplished via a wireless router
mounted on the rail car. This router would in turn communicate with mounted
wireless repeaters located every quarter of a mile on the wayside track. These
repeaters would then bounce the communications to the nearest station platform
to a wireless access point router, which would then physically connect the
wireless network to the land based fiber optic network routed into each station
on the railway. The Operations Control Central, also connected to the fiber
network, would have visibility to each rail car in the transit system,
regardless of where it resides at any given moment.
|
|
GPS (Global Positioning
System) tracking receivers are light weight and rugged.
A GPS receiver monitors satellite positions to
triangulate its position on the Earth's surface via a
Latitude and Longitude vector. Speed and Direction are
calculated based upon new positions relative to previous
positions.
As mentioned earlier, the real
challenge in any transit system is getting this
information for a vehicle to the OCC (Operations Control
Center). There are three methods that can be used to do
this. The first and easiest is using an existing
cellular network provider, however, it is the least cost
effective when many units are needed. The second option
is using your own cellular network, and the third is
using a wireless (radio) LAN on the wayside.
|
GPS Receiver
 |
|
|
GPS Receiver to Base Station
 |
Cellular Base
Stations
There are industry providers available to handle
all GPS transmission over the cellular network and provide this data directly to
an Operations Control Center via an Internet connection to their in-house
servers and databases. However, an ambitious railway agency can bypass this
intermediate provider by investing in their own cellular base station.
A
base station is the cellular relay station (or cell tower) that a cell phone
talks to when initiating or receiving a wireless call. A base station transmits
calls to devices over the Forward Control Channel (FOCC). Mobile devices
transmit calls to the base station over the Reverse Control Channel (RECC).
|
|
|
Radio
transmissions use a bi-directional (full duplex) configuration, transmitting and
receiving on separate frequencies. A mobile device transmits on the radio
frequency the base station is tuned to, and the base station transmits on the
frequency the mobile device is tuned to.
Base stations can be setup to
receive data from the remote GPS receivers. The format
of such a message generally exists as a comma delimited
ASCII string with the Identification Number of the
remote unit along with the data, such as the GPS
latitude, longitude, speed, and direction. The OCC
system servers decode this information and transmit to
the workstations so the rail cars (and other vehicle
types such as buses and cars) can be mapped to the
workstation screens. Refresh rates are generally
configurable up to several seconds. |
Base Station Hardware
 |
|
|
Wayside
Wireless Network
If the transit system already has a fiber LAN network between its
stations and the OCC, then the more exciting approach to
transmitting GPS (and other rail car information) to the OCC is via
the use of wayside wireless LAN networks. To send this railcar
information to the station, a wireless modem would be placed in the
railcar and then transmitted to various wireless repeaters on the
wayside and finally to the Station, where it can be translated onto
the existing fiber network and then sent to the OCC. This is a two
way transmission. OCC can also send date to the rail car via the
wireless network.
|
|
Rail Car Wireless modem
 |
Compact and ruggedly designed, the Vehicle Mounted
Modem (VMM) wireless router turns a vehicle into a
mobile office. Mobile Data Terminals (MDT), IP video
cameras, and other IP ready devices can access a
high-speed, mobile broadband network via a standard RJ45
Ethernet Port. This low cost, high performance, wireless
modem supports up to 6 Mbps burst data rates at speeds
of over 100 mph.
The VMM provides high bandwidth access
to mission-critical information on the move. Remote database inquiries, on-scene
report submission, multi-megabyte file transfers, and live video streams will
make field personnel more efficient. The VMM also acts as a wireless
router/repeater – automatically extending the range, robustness and performance
of the wireless network.Refer to the illustration below for a scheme that will
allow GPS information (and other rail car data) to be
transmitted to the Central Control Center. |
|
Such components could consist of:
A GPS receiver that monitors exact position,
speed, direction, and altitude of the rail car.
A public address system comprising of a message
board and loud speaker to keep passengers updated on arrival times and current
events.
Real time video feeds for security monitoring.
Real time audio for direct communications with
train operators (eliminating the need for trunk radio systems).
Real time diagnostic and environmental
monitoring of equipment on the rail car -- such as oil levels, temperature,
equipment wear, etc.
Pay per view access points for
passengers to access the Internet to bring in additional revenue to the transit
authority and increase passenger satisfaction.
|
|
For maximum reliability, repeaters
would need to be placed on the wayside -- close enough such that if one repeater
failed, the adjacent repeaters could leap frog over the failed repeater. If this
was not possible due to cost constraints, the worst scenario is that
communication would be lost with the rail car only in the area near the failed
repeater, since the access points at stations would pick up the communication
from the rail car once it arrived close enough, and could then re-establish
communication and transmit the data between the rail car and the OCC over the
fiber optic land line.
Disadvantages to a wireless network
are security, cost, and providing power to wayside repeaters. Many strides have
been taken over recent years in security encryption, and this problem can be
overcome quite easily. Cost, however, is still a factor based upon the number of
repeaters required. However, if installed in phases, this expense can be
minimized and balanced over time.
|
|
A comprehensive wayside wireless
network would allow the OCC to stay in touch with each rail car at all times,
regardless of where it is on the railway. Wireless repeaters, mounted on poles
or other structures adjacent to the rail way would echo communication between
the rail car, each repeater, and the nearest station structure -- where the land
based access point resides. The major advantage of such a network
is that it would be reliable and wouldn’t require the transit agency to rely on
outside agencies to provide the network for GPS receivers. The network would be
under the full control of the transit agency. This network also allows
continuous communication with rail cars anyway on the railway, and most
importantly, allows the internal car network to exist. |
Wireless Routers
 |
|
|
Access
Point Routers
 |
|
|
|
Public message boards and public
announcement systems would tie in directly to the fiber network at each station.
|
|
