Design Considerations

Objectives:
      The GPS location of a shuttle will be transmitted to one of the stops via the radio-modems. The radio-modems are expected to broadcast a signal approximately 2-3 miles. The GPS location will be received by the respective stop where it will be converted to a wait time and displayed on a sign.

Constraints:
      We will need to determine the best placement for the antennae on the shuttles. Possible mounting constraints include whether the antennae need to be inside or on top of the shuttle. We will also need to determine antennae placement at the campus and Bart station shuttle stops. The antennae may need to be installed in an elevated location (i.e. top of building). Depending on disability compliance restrictions, we may need to also install an audio option with our visual displays at the shuttle stops.

Standards:
      The radio-modem transmissions will need to be in compliance with any FCC regulations regarding broadcast frequencies4. Our visual displays may also need to be in compliance with the disability regulations in Section 36.303 of the Americans with Disabilities Act5 (i.e. audio announcement of ETA).

Design Requirements:
Onboard Shuttle Transmitter:
• Must be able transmit data up to 2.5 miles. The transmitter will be farthest away from the receiver unit when the shuttle is in the opposite end of the route. Therefore, to successfully predict the ETA, the shuttle must be able to transmit its coordinates from any point of the route. If the shuttle is unable to transmit its coordinates, then the system’s prediction can potentially lose accuracy.

• Uses a transmitter antenna that does not need to be permanently mounted to the shuttle. Mounting an antenna to a flat metal surface of the shuttle will increase its effectiveness, but if physical alterations to the vehicle are not realizable, the antenna must still function even if it does not become a permanent part of the shuttle.

• Uses a shuttle’s standard 12V DC power socket. Relying on the shuttle’s battery eliminates the need for an extra power supply and will help bring the cost down.

• Fits in an 8”x9”x4” enclosure. A smaller form factor is better so that the transmitter unit does not take up a lot of room in the dashboard or glove box of the bus.

Reciever Unit:
• Antenna must function in any weather condition. Standard San Francisco weather does vary drastically, but the antenna should be able to withstand, or at the very least, be able to handle slight interference with reception.

• Derives power from a standard 120V AC outlet. The system can derive power from common outlets.

• Has a display that can be seen from approximately 25 feet away and readable during day and night.

General Requirements:
Cost:
• The whole system must cost no more than $3000. A higher upfront cost of around $4000 to $5000 is acceptable especially if there would be no recurring fees in the future. However, a nominal recurring fee of around $100 to keep the whole system operational would still be acceptable.

Accuracy:
• The ETA displayed must accurately represent actual wait time, and be within the error margin of 1-2 minutes. If the shuttle is running late, the system should be able to adjust the ETA displayed. The system should also account for the time of day for which the shuttle is running since traffic varies throughout the day.

Ease of Use:
• The overall system must be user friendly. Anyone should be able to read and understand the user-facing portion of the system without instruction. The only input to the transmitter unit required from the driver should only be plugging in the transmitter unit to the shuttle’s power socket. The system should be able to determine the destination of the shuttle automatically.

• The system must not need regular maintenance. If any part of the system breaks, the broken portion should be easy to replace and not require the assistance of an engineer.

Initial Possible Solutions
      Of the different solutions to our design the only ones that were deemed feasible and fit our design criteria were the use of cellular data, Wi-Fi or Radio Frequency Modems. The proposed possible solutions are explained below, and compared against our design criteria in Table 1.

Software Model for All Possible Solutions
      Using Arduino microcontrollers, we would utilize GPS data-collecting units aboard each shuttle to gather data, calculate an ETA and transmit the ETA serially through a network. Through the network, the Arduino receivers would output the ETA to attached LED displays. The receiver units would be located at both the SFSU and BART shuttle stops.

Cellular Data:
• Using either a Boost Mobile™ cell phone or a GSM SIM card and Arduino shield we would interface our microcontrollers with these devices and always be connected to the Internet. Our software model would not change, only the method of data transmission. In order to comply with our no monthly cost requirement the use a donated data plan or advertising would be used to offset any monthly costs. This alternative would allow for online expansion, such as a web application to show the shuttle’s location, because of constant connectivity to the Internet.

Wi-Fi:
• This method would require creating a Wi-Fi network, using off-the-shelf components, between Daly City BART and SFSU. Similar to our proposed solution, this alternative requires creating a highly modified network using high gain antennas and possibly repeater stations. This solution is not ideal as 2.5 miles is an EXTREME distance for a semi-urban Wi-Fi network. Again, this alternative would fit into our software model, with minimal change in coding to direct the data to its destination. This alternative would also allow for online expansion, such as a web application to show the shuttle’s location.

RF Modems:

• Similar to the Wi-Fi method, an implementation using Satel™ RF modems would entail creating a data communication network that would span the entire route the shuttles would take. The radio modems would operate in the Ultra High Frequency (UHF) spectrum. The benefits of this implementation is these frequencies are much more penetrating through trees and other obstacles, as well as being able to travel further distances because of greater FCC-allowed output power. This network would interface.

Chosen Implementation:

      Use regulated FCC radio frequency band, UHF 450-470 MHz, to create a method of transferring serialized GPS coordinates from the shuttle to the ETA calculating microcontroller.

      This design utilizes a 450MHz radio modem to transmit serialized GPS coordinates. The radio modem should be able to transmit the desired distance as long as a proper antenna configuration is used.

      Upon activation of the transmitter modules located in the SFSU shuttles, a GPS unit will start acquiring the shuttle’s position. Once the shuttle’s coordinates have been determined, the serialized data will go on the microcontroller. The microcontroller will then transmit the shuttle coordinates using a radio modem to the receiver modules in either the SFSU Campus or Daly City BART stops. Figure 1 shows the basic signal transmission path.

      The receiver modules at the SFSU Campus or Daly City Bart will be comprised of another radio modem acting as a receiver and a computer that will calculate a shuttle’s ETA based on coordinates received. The computer software will also determine the direction a shuttle is travelling based on two consecutive coordinates received. The software will also be able to determine which shuttle the coordinates are coming from since each transmitter radio modem transmits using a unique identifying signal.

      A lower frequency range was chosen to transmit data because of the many benefits of the UHF spectrum. There will not be many other wireless communication devices operating that frequency that might cause transmission interference, since it is a regulated spectrum. Physical obstacles common in an urban environment such as trees or houses that commonly hinders high frequency wireless transmissions would also be less of an issue because 450 MHz can more easily penetrate physical objects. If the receiver antenna is placed on the tallest building on campus, even concrete structures will not be a problem.

Key Features of the Design:

• Can easily accommodate shuttle routes > 2.5 miles with proper antenna configuration.

• Near real-time tracking

• No monthly fee

• A pair of radio modems which can be configured to work as receivers or transmitters cost around $700.

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