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Integration essential to affordable GPS

Posted: 01 Sep 2002 ?? ?Print Version ?Bookmark and Share

Keywords:gps receiver? telematics processor? rf front-end? automotive electronics? single ic gps?

The important features that a telematics system should include are position awareness, two-way communications ability, and speech recognition.

In most current designs of telematics units, these separate functions are performed by separate modules. For example, a GPS module usually connects to a main processor by a serial interface. The main processor fuses the GPS data with information from other motion sensors, such as the vehicle's odometer and a gyro or compass to produce a robust position in both the presence and absence of GPS signals. The GPS module, as well as the main processor, has its own reference oscillator, RAM, ROM, and CPU.

There is an opportunity to reduce costs by integrating the GPS and dead-reckoning processing onto a single IC. For example, Philips' SAF3100 Telematics Processor combines most of the essential functions required for telematics into a single IC.

To complete a basic telematics system, an RF front-end for the GPS system has to be added as well as a cellphone module suited to the type of network required (CDMA, PCS) plus antennas for the GPS and cellphone as well as controlling software.

Even with GPS and dead-reckoning functions running, a good proportion of the ICs resources, such as the CPU cycles, embedded RAM, and battery-backed RAM, should be kept free for application software to be integrated with the drivers, otherwise, the system simply becomes an overpriced GPS sensor. However, if there is sufficient spare capacity, this IC can function as, for example, the central processor of a midrange or high-end car radio, while still carrying out its positioning and communications functions.

With integration, there is less duplication of components, such as crystals, and more economy of scale by rolling all the RAM and ROM together, smaller PCB footprint and greater reliability in service because there are fewer interconnects and solder joints.

Normally, a GPS receiver for automotive use is a tracking receiver - that is, it continually receives and decodes GPS signals, tracking as many satellites as are visible or as many as it has channel hardware to track. It uses the orbital parameters (ephemeris) of each tracked satellite to calculate the receiver's position. Such a receiver is a collaboration between a hardware and software part.

The hardware part usually operates at RF, IF, and the 1.023MHz GPS chip rate, whereas the software part operates at a scale of 1kHz or slower. The dividing line between hardware and software can be moved in either direction. If less hardware is used, then the receiver should be less expensive (lower total silicon area), but the software will need to function at a higher rate, making the problems of integrating application-layer control code more difficult.

If more hardware is used, the silicon area, and therefore cost, increases but the software task becomes easier, such that a smaller, simpler CPU might be used. A common compromise is to use hardware for the RF and IF sections, and to implement correlators and their associated integrators in hardware. The correlator outputs are then read by software at intervals of 1 millisecond (or longer) and the control loops are closed by software. All the higher-level functions - data extraction, decoding and checking, position and time calculation--are done purely in software.

Fortunately, most application software running on this type of platform is designed to run user interfaces and thus, is keyed to human response times. As a result, a latency of a few hundred microseconds is not usually noticed.

- Torsten Lehmann

Product Marketing Manager, Navigation/Telematics

Philips Semiconductors

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