By Tom Brinkoetter

About 4 million people visit Yosemite National Park each year, arriving via train, bus, and personal vehicles. Once there, many travel the park using one of the four free shuttle lines or the YARTS (Yosemite Area Regional Transportation System) to see the breathtaking sites Yosemite offers. However, most still take private transportation, a situation that has led to congestion and frustration for visitors and park administrators alike. 

To help alleviate the backlog, the National Park Service has conducted public scoping that resulted in myriad suggestions. As part of this transportation initiative, the Park Service is overhauling the existing VHF conventional voice radio system with standalone repeaters, replacing it with a Computer Aided Dispatch/Automatic Vehicle Location (CAD/AVL) data system. This new technology backbone will be used for transit, traffic monitoring, and portable roadside signs throughout Yosemite to help make it easier for vehicles to travel the 214 miles of road within the park confines.

Like most CAD/AVL systems, the one designed for Yosemite will send and receive data packets to and from vehicles anywhere in the park without knowing the precise location of the vehicle. An on-board IP interface will be designed into each of the buses and other authorized park transportation vehicles.

At the heart of the system will be a P25 multisite simulcast system with receiver voting. The P25 simulcast system was selected over a trunked system, because it works well in environments with a combination of high levels of multipath and significant non-capture overlap. P25 simulcast systems often use Linear Simulcast Modulation (LSM), which is more forgiving of multipath.

Less-than-perfect environment

Yosemite National Park might be great for camping and sightseeing, but it is far from perfect for network deployment. First and foremost, the mountainous terrain posed more than a few concerns. Due to the difficulties associated with the topography, a new approach had to be developed that would save time and money.

Teams from IBI Group, as well as Hatfield and Dawson Consulting Engineers, were commissioned to study the installation of the P25 radio system for visitor buses. Initial software predictions indicated two transmitter locations would serve the entire bus route. However, the engineering team was not confident in those predictions, due to reflections off vertical rock walls.

Spot signal-strength measurements from existing YNP repeater sites were made along the bus routes. In many locations, the measured signal strength was significantly stronger than the predictions, confirming the engineers’ initial concerns.