GPS TAKEN TO A HIGHER LEVEL
NAVAMN'S NEW DEAD RECKONING (DR) TECHNOLOGY ADDS A POWERFUL DIMENSION TO GPS APPLICATIONS. THE SOLUTION PERMITS GPS POSITIONS TO BE CALCULATED THROUGHOUT THE ENTIRE RECEPTION AREA, EVEN DURING SITUTATIONS WHRE THE RECEIPT OF GOOD GPS SIGNALS IS NOT POSSIBLE. WITH ITS MANY SPECIAL FEATURES, THE NEW NAVMAN VARIANT HAS TAKEN GPS TECHNOLOGY TO A DECIDEDLY HIGHER LEVEL.
To fix an exact GPS position, it normally requires four satellites in clear line of sight. However, conventional solutions face problems in areas where the line of sight becomes blocked: e.g., in tunnels or in the urban canyons of larger cities, where GPS signal-quality degrades rapidly. A loss of reliable navigation is the result. However, thanks to NAVMAN's new dead reckoning technology, this common problem is now under control.
NAVMAN DR modules are connected to a GPS antenna, a speed sensor (wheel-tick sensor) and a gyro. The piezoelectric gyro design is based on a technology similar to that used in miniature quartz tuning forks, which are used in low-frequency oscillators for watches. The fork construction used in the DR modules comprises four piezo units: two used for generating the fork vibration and two used to detect rotating motion. The algorithm used in the DR receiver has been optimized for this type of gyro.
DEAD RECKONING SENSORS CALIBRATED DURING NAVIGATION
When the NAVMAN receiver is connected to the dedicated DR components, it essentially operates the same as does a GPS receiver without the added DR functionality. However, once the receiver commences navigation using GPS signals, the DR sensors are calibrated. The calibration process starts by saving the various initial parameters, either in the program itself (ROM) or in a memory storage unit (RAM), with the latter being used to save the continuously updated parameters. Beginning with the initial parameters, the software compares the navigation data provided by the DR sensors with the continuously received GPS data, and proceeds to update these parameters accordingly. Calibration of the wheel-tick sensors is performed when the required distance is traveled for achieving adequate GPS accuracy (roughly 1 km or less in most cases). The precise duration and distance of travel required for calibrating the DR sensors depends on the GPS signal quality and on receiver temperature stability.
GYRO CALIBRATED IN TWO STEPS
Calibration of the gyro is performed in two steps, with the first step determining the gyro's orientation and the second its sensitivity. The gyro's orientation (zero turn rate) is determined when the vehicle remains in a stationary position, while the GPS receiver connects with a satellite and switches to the navigation mode.
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To determine the sensitivity, the vehicle must be in motion and passing through a turn. Normally this requires two changes of direction, one turn to the right and one to the left, or a complete 360° turn in one direction and then the other.
This procedure makes for a more precise calibration from the very start, and it enables the GPS receiver to calibrate the gyro's original turn radius information and the actual changes in heading, as well as checking the accuracy of the latter.
NAVMAN develops and manufactures a series of GPS receiver products, for example the Jupiter family, which recently added the new Jupiter 20 to its ranks. The Jupiter 20 includes the DR functionality and is based on SiRF and SiRF Star II chipset technology. The new receiver is thus ideally suited for very demanding applications, such as vehicle tracking in dense urban areas. The 12-channel Jupiter 20 receiver is a Surface Mount Device (SMD) and comes in a chip-scale package, thereby enabling its use in portable devices.
The Jupiter 20 will be offered in three versions: a standard version, an extremely sensitive version that will ship with XTrack software for highly reliable navigation in areas where only low-level GPS signals are available, and thirdly, a DR version equipped with connections for the wheel-tick and gyro sensors for installation in vehicles. In addition to the proven, highly reliable GPS navigation signals, the Jupiter 20 can also deliver an extremely accurate time signal that is based on the atomic clocks aboard the Navstar satellites. As a result, it can also be used in time-critical applications, for example to synchronize telecom equipment in mobile communications. Despite the compact form factor of 1x1 inch (25.4 x 25.4mm), NAVMAN has managed to incorporate antenna amplifiers into the module to enable the use of passive antennas in addition to the GPS chipset. What's more, the GPS chipset is equipped with in-system programmable program memory that permits software updates in the field. NAVMAN specifies a horizontal position accuracy of better than 3m for the Jupiter 20, while the time signal it provides is accurate to better than 1µs. In addition to the small footprint, the supply voltage of 2.4 - 3.6V also makes the Jupiter 20 ideally suited for use in mobile, battery-operated devices. The operating temperature range of -40°C to +85°C meets the requirements of industrial and automotive applications.
The piezoelectric gyro sensor comprises four piezo units: two used for generating the fork vibration and two used to detect rotating motion.
Ivan Mitic, EXT 85
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