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Computer Cab Data Systems

Computer Cab home page About Computer Cab Data Systems

Computer Cab is an offshoot of the London Taxi Driver's Association (LTDA), a co-operative formed to combat the growth in minicabs. It controls a fleet of around 2500 taxis used mainly to service account customers, and this offers the drivers a higher quality of work than they would otherwise expect.

The Mobistar system went live in mid-1996. It uses a highly accurate satellite navigation technology, together with a radio data system to determine the position of each member of the fleet within a few metres every few seconds. Accurately tracking the vehicles produces a fast and speedy service to customers, as drivers are automatically allocated the jobs that are closest to them. Drivers may also be guided towards areas where there is the most work.

Billing of account customers is straighforward, with a direct connection between the taxi meter and the central billing facility. Charge card and credit card customers are also catered for, with a card reader designed for the purpose installed in each taxi.

The Mobistar system communicates with the taxi fleet anywhere within the area of the London Orbital motorway (the M25) using a number of base stations situated on high-rise buildings within the city.

Project Information

We were involved with Computer Cab from May 1994 until June 1996, developing the hardware for the Mobistar automatic dispatch system.

The company had full responsibility for all aspects of the hardware development, from initial specification, through to detailed design and prototype development, manufacturer selection and final production.

Technical documentation was generated for all of the system elements. This enabled the design process to be monitored and reviewed, and allowed the product to be manufactured and supported by third parties.

The products developed for the Mobistar project were:

  • A mobile data terminal for installation in taxis. The hardware was designed for a production run of 3000 mobile systems, using a third party manufacturer. It employed surface mounted technology and was designed for a service life of ten years in the harsh physical and electrical environment of a London taxi. It was fully protected against vehicle-induced electrical transients and complied with stringent EMC regulations. All interfaces were designed to withstand the conditions found in vehicles that operate UHF transmitters (high electric fields) and to withstand the seemingly arbitrary connection schemes often dreamed up by the vehicle fitters.

    This compact unit used a 16-bit Hitachi H8-3003 microcontroller, field-upgradeable FLASH PROM, battery backed static RAM, a membrane keypad and a vacuum fluorescent graphics display, with interfaces to several other vehicle systems.

    The rear panel of the data terminal had interfaces to a variety of taxi meters, a Motorola GPS receiver, a card reader, the radio modem and an emergency foot switch, together with several expansion ports and product servicing facilities. All interfaces employed either low slew-rate RS232, RS422 or RS485 devices for data signals, or opto-couplers for control I/O.

    The vacuum fluorescent display was powered by a switched mode power supply. This generated a high current filament supply and a high voltage rail (+84V) for the anode. The design employed a Unitrode UC2524 controller connected in a flyback converter configuration. Attention to detail ensured that the EMC characteristics of this section of the unit were kept under control.

    Software for automatically downloading new firmware into FLASH, together with production test software for the mobile data terminal was written.

  • Intelligent radio data modems for installation in taxis. Two modem variants were designed for use with existing speech radios already in service in the taxi fleet. Both used an 8-bit Hitachi H8-337 microcontroller and a CML FX589 4800 bits per second GMSK modem, built with surface mounted technology.

    The modem was designed to carry data between the mobile data terminal and the fixed base stations via a UHF radio link using a TDMA protocol with forward error correction. Each type was designed to pass EMC type approval, since it was situated inside the radio casing and required modifications to the radio circuitry.

    Production test software for each modem was written.

  • An RF base station controller. The fixed infrastructure of the Mobistar system comprised a number of 20W Tait RF base stations with high reliability dual-processor control systems. Hardware support allowed the system protocol to support a large number of taxis.

    An STE-bus single-board PC handled high level communications with both the central system processor and a differential GPS correction system via Kilostream links. The PC communicated with an embedded modem controller board via bi-directional FIFOs with supporting signalling interrupts. This board used an 8-bit Hitachi H8-337 and a CML FX589 GMSK modem, and was responsible for controlling the bit-rate timing, the forward error correction and the real-time aspects of the TDMA protocol.

    The modem controller also monitored alarm conditions within the communications site. Since such sites are relatively inaccessible and are electrically noisy, great attention was paid to reliability and EMC during the design process, with extensive self-test capability being added.

  • Test electronics to allow the products to be tested automatically in the factory. Production test software was written for all products. Each set-up comprised a menu driven PC program and a program running in the equipment under test, connected via a serial link. Both programs were written in C.

    The PC initiated all tests, automatically controlled the test harness electronics and various GPIB instruments and logged results. This allowed a test history to be associated with each individual data terminal during the entire manufacturing process.

    The program running in the equipment under test comprised a command interpreter which ran the specified test procedures and returned results to the PC. Many of the tests required loopbacks to be connected and this was controlled by the PC. This approach to system testing assumes that most of the boards will work first time, because the processor must be able to run the test program. However, with a modern automated manufacturing process, this was judged to be an acceptable risk.

  • Technical documentation was generated for all of the system elements. This enabled the design process to be monitored and reviewed, and allowed third party manufacture and technical support.
Mobistar technology

The LTDA's first weapon in the war against minicabs was the speech radio, which allowed drivers to be directed to waiting customers. Eventually, every taxi was given a computer, but this system suffered from a number of drawbacks.
  • The use of a speech system required drivers to observe a strict code of conduct when bidding for jobs via a shared speech channel.
  • Taxis were not always where their drivers said they were. In an attempt to gain more lucrative work, incorrect positional information was sometimes given. This practice caused the service offered to the cutomers to be degraded due to the increased waiting time incurred.
  • The taxi meter had to be manually read and the fare manually entered on the data terminal. This presented the possibility of human error.
  • The mobile data terminal looked dated and the technology was obsolete, making expansion of the fleet difficult.

The Mobistar system was designed to solve these problems and to provide a clear technology gap between the Computer Cab brand and other rival fleets.

  • It uses a UHF radio link to transfer digital information between the control room and the taxi. This allows the pickup and destination address, together with other relevant job information to be presented on a graphics screen. This results in improved dispatch reliability, because of the better performance of data links in the presence of noise on the radio channel. Use of a data link allows a high level of sophistication to be designed into the data terminal. A range of external interfaces allows connection to other vehicle subsystems.
  • It uses a GPS (Global Positioning System) receiver mounted within each taxi to determine the position of all of the vehicles in the fleet. The GPS system is capable of great accuracy, but with a positional uncertainty (selective availability) introduced by the operators (the United States military), the system is potentially degraded in an erratic way. In order to eliminate the errors introduced by selective availability, differential GPS is employed. This uses a fixed GPS receiver at a precisely known location, allowing positional corrections to be transmitted to each vehicle. Each vehicle is then able to transmit its true position, with the degree of accuracy depending upon how many satellites it can see at the time. Using this technology, the position of each vehicle can be determined without having to rely on information provided by the driver.
  • The mobile data terminal connects directly to the taxi meter. This ensures that a fair system of charging is operated, with no possibility of the driver altering the metered fare. Central billing and auditing of cash jobs produces a secure environment.
  • With a stylish new mobile data terminal and the anti-cheating measures described, the drivers are much more enthusiastic. With the removal of the possibility that their colleagues can cheat the system, they now feel more relaxed and able to concentrate on providing a high quality service.
  • The central control system, written by Logica, allows up to 18,000 jobs per hour to be dispatched. The computer also analyses the location of bookings, allowing drivers to be moved to where there is the most work. Account billing is also performed for many thousands of clients.

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