| Advanced Communications In Transportation |
| Upgrading MARTA’s Transit Radio Communications |
| By Curtis T. Legg, Atlanta, Georgia, 1-404-870-3231,
c/o mjohnson@ogc.itsmarta.com |
| Upgrading MARTA·s radio system to an 800
MHz trunk system was a logical choice for providing an expandable
and field-proven system that will handle new technology now and into
the future. |
|
MARTA’s Current System
of Radio Communications
Today, MARTA comprises a heavy rail system,
bus fleet, transit police force and miscellaneous services, for
which MARTA has approximately 2,900 mobile and portable radios.
Rail. The MARTA rail system consists of:
- 73 km (45.4 miles) of heavy rail,
16 km (10 miles) of which are underground
- 36 rail stations (two more under construction)
- 238 rail cars (100 more under contract)
- 2 rail yards (another on the drawing
board).
Radio Communication: Two UHF
repeater systems and a yard radio channel. Primary channels use
satellite receivers and have radiating transmission lines in tunnels,
stations and parking decks.
Bus. MARTA’s bus fleet comprises nearly 800
vehicles, 250 of which are equipped with automatic vehicle location
(AVL) systems. Plans call for all buses to be equipped with AVL
in the future.
Radio Communication: Six UHF repeater channels, two of
which are used for data communications primarily.
Transit Police. The MARTA transit police force
operates out of four police precincts (a fifth is in the planning
stage). It has approximately 50 police vehicles and 287 sworn police
officers, including a Special Weapons and Tactics (SWAT) team and
an investigative section.
Radio Communication: Two UHF repeater channels that use
satellite receivers and have radiating transmission lines in tunnels,
stations and parking decks.
Miscellaneous Services. Other parts of the MARTA
organization that use radio communications include a number of maintenance
functions, station managers and operational support personnel.
Radio Communication: Two UHF repeater channels. |
More than twenty years ago, PB engineers designed
a state-of-the-art radio communications system for the newly created Metropolitan
Atlanta Rapid Transit Authority (MARTA). The system was to provide radio
communications for the heavy rail transit system being constructed in
Atlanta. The design was based on narrow-band frequency modulation (FM)
radio, which was new at the time. It consisted of multiple ultra high
frequency (UHF)—450 MHz—repeaters utilizing satellite receivers
and receiver voting selectors.
On June 30, 1979 rail service was opened on the MARTA East Line with service
from Avondale Station to Georgia State Station (seven stations). The newly
installed radio system provided above ground coverage to rail maintenance
and support personnel and extended underground coverage to rail operations
and security police in stations and tunnels.
New Radio Communications Systems
MARTA’s current radio communications systems (see box) have become
overcrowded and suffer from increasing interference. Expansion of the
systems in the UHF band is not feasible because of a lack of available
frequencies.
MARTA’s radio communications systems will be upgraded to an 800
MHz trunk system serving all MARTA operations. Recent changes in Federal
Communications Commission (FCC) rules governing the future narrow banding
of UHF channels also played a part in this decision. The 800 MHz frequencies
were found to be available in the 821-824/866-869 MHz bands.
We started the design process by seeking out technologies that provide
for currently envisioned requirements and that are expandable so they
can include future growth. Consideration was given to new time division
multiple access technologies as might be applied to the existing MARTA
UHFs, but equipment was neither readily available nor time-tested for
this application, and FCC license application appeared difficult and time
consuming. Because the 800 MHz frequencies were found to be obtainable,
this seemed to be the only logical choice for a new system that was expandable
and field proven. (Outsourcing communications to local specialized mobile
carriers is also under consideration by MARTA and may be used for some
support functions.)
The new system will provide services for the more than 2,900 radios and
support for the following:
- All voice communications
- Voice encryption
- Telephone interconnect
- Automatic vehicle location
- Mobile data terminals
- Police mutual aid radio.
Command and control systems will be upgraded to include
computer aided dispatch technologies. Existing station and underground
distributed antenna systems will be upgraded and combined using bi-directional
amplifiers and/or fiber optic radio frequency transmission technologies.
Infrastructure Equipment
Five remote radio sites will provide radio coverage to all bus, rail,
police and maintenance/support personnel in the two county MARTA service
area. Each of these sites will be equipped with a 20-channel 800 MHz radio
system and connect to a master site via T1 digital links that will provide
for control, monitoring and audio transmission to/from each remote site.
The system will be designed to simultaneously transmit (“simulcast”)
the same radio frequencies from each of the five remote sites (20 channels
total). The transmitters will be synchronized using global positioning
system (GPS) satellite receivers and high stability oscillators.
Four of the remote sites will be “above ground” sites and
provide for radio coverage above ground. The remaining remote site will
be installed “underground” to provide coverage to the core
underground portions of the MARTA rail system.
Conclusion
The system design will allow for future expansion beyond the existing
MARTA service area by adding sites or by adding new areas of operation
with additional sites/frequencies and integrating the new frequencies
into the system in a “cellular” fashion.
The installation of the new system should begin in about August 2000
and be completed in December 2001. |
|
| Curtis Legg is an electrical engineer with specialization
in the communications field. He has been involved in radio communications
for more than 30 years. |
|
