Navigation System: Description and Operation
NAVIGATION SYSTEM
System Description
- For Navigation System operation information, refer to Navigation System Owner's Manual.
- Each control unit that comprises the system is connected with a communication circuit. It transmits/receives data signals including request signals and response signals, and controls the system.
- The display control unit transmits/receives data signals to/from each control unit with CAN communication. It performs an arithmetical operation on fuel information values by using data obtained from the control units, and then displays the calculated values on the screen.
- The display control unit receives door switch signals from the BCM with CAN communication, and displays a warning on the screen when driving over the set speed with a door half-shut.
- The display control unit receives vehicle speed signals that are transmitted from the unified meter and A/C amp., performs an arithmetical operation on drive information values, and then displays the calculated values on the screen.
- The images displayed on the monitor screen contain NAVI control unit-generated RGB images.
- The display control unit controls image switching and image quality adjustments by communications with the display.
NAVIGATION SYSTEM
Location Detection Principle
The navigation system periodically calculates the vehicle's current position according to the following three signals:
- Travel distance of the vehicle as determined by the vehicle speed sensor
- Turning angle of the vehicle as determined by the gyroscope (angular velocity sensor)
- Direction of vehicle travel as determined by the GPS antenna (GPS information)
The current position of the vehicle is then identified by comparing the calculated vehicle position with map data read from the DVD-ROM, which is stored in the DVD-ROM drive (map-matching), and indicated on the screen as a current-location mark. More accurate data is judged and used by comparing vehicle position detection results found by the GPS with the result by map-matching.
The current vehicle position will be calculated by detecting the distance the vehicle moved from the previous calculation point and its direction.
- Travel distance
Travel distance calculations are based on the vehicle speed sensor input signal. Therefore, the calculation may become incorrect as the tires wear down. To prevent this, an automatic distance correction function has been adopted.
- Travel direction
Change in the travel direction of the vehicle is calculated by a gyroscope (angular velocity sensor) and a GPS antenna (GPS information). They have both advantages and disadvantages.
More accurate traveling direction is selected because priorities are set for the signals from these two devices according to the situation.
Map-Matching
Map-matching compares a current location detected by the method in the "Location Detection Principle" with a road map data from DVD-ROM stored in DVD-ROM drive.
NOTE: The road map data is based on data stored in the DVD-ROM.
The vehicle position may not be corrected under the following circumstances and after driving for a certain time when GPS information is difficult to receive. In this case, the current-location mark on the display must be corrected manually.
- In map-matching, alternative routes are prepared and prioritized in addition to the road judged currently driven. Due to the distance and/or direction error, the incorrect road may be prioritized and current-location mark may be repositioned to the wrong road.
When two roads are running in parallel, they are judged to the same priority. Therefore, the current-location mark may appear on either of them alternately, depending on maneuvering of the steering wheel and configuration of the road.
- Map-matching does not function correctly when a road on which the vehicle is driving is new and not recorded in the DVD-ROM, or when road pattern stored in the map data and the actual road pattern are different due to repair.
When driving on a road not present in the map, the map-matching function may find another road and position the current-location mark on it. Then, when the correct road is detected, the current-location mark may change to it.
- Effective range for comparing the vehicle position and travel direction calculated by the distance and direction with the road data read from the DVD-ROM is limited. Therefore, when there is an excessive gap between current vehicle position and the position on the map, correction by map-matching is not possible.
GPS (Global Positioning System)
GPS (Global Positioning System) was developed for and is controlled by the US Department of Defense. The system utilizes GPS satellites (NAV-STAR), sending out radio waves while flying on an orbit around the earth at an altitude of approximately 21,000 km (13,100 miles).
The GPS receiver calculates the vehicle's position in three dimensions
(latitude/longitude/altitude) according to the time lag of the radio waves received from four or more GPS satellites (three-dimensional positioning). If radio waves were received only from three GPS satellites, the GPS receiver calculates the vehicle's position in two dimensions (latitude/longitude), and utilize the altitude data calculated previously with radio waves from four or more GPS satellites (two-dimensional positioning).
Position correction by GPS is not available while the vehicle is stopped.
Accuracy of GPS will deteriorate under the following conditions:
- In two-dimensional positioning, GPS accuracy will deteriorate when altitude of the vehicle position changes.
- The accuracy can be even lower depending on the arrangement of the GPS satellites utilized for the positioning.
- Position detection is not possible when vehicle is in an area where radio waves from the GPS satellite do not reach, such as in a tunnel, parking lot in a building, and under an elevated highway. Radio waves from the GPS satellites may not be received when some object is located over the GPS antenna.
NOTE:
- Even a high-precision three dimensional positioning, the detection result has an error about 10 m (30 ft).
- Because the signals of GPS satellite is controlled by the Tracking and Control Center in the United States, the accuracy may be degraded lower intentionally or the radio waves may stop.
Component Description
NAVI CONTROL UNIT
- The gyro (angular speed sensor) and the DVD-ROM drive are built-in units that control the navigation functions.
- Signals are received from the gyro, the vehicle speed sensor, and the GPS antenna. Vehicle location is determined by combining this data with the data contained in the DVD-ROM map.
Locational information is shown on liquid crystal display panel.
DVD-ROM Drive
Maps, traffic control regulations, and other pertinent information can be easily read from the DVD-ROM.
DVD-ROM
- The DVD-ROM has maps, traffic control regulations, and other pertinent information.
- To improve DVD-ROM map matching and route determination functions, the DVD-ROM uses an exclusive Nissan format. Therefore, the use of a DVD-ROM provided by other manufacturers cannot be used.
Gyro (Angular Speed Sensor)
- The oscillator gyro sensor is used to detect changes in vehicle steering angle.
- The gyro is built into the navigation (NAVI) control unit.
GPS ANTENNA
The GPS antenna receives and amplifies the radio waves from the GPS satellites, and then transmits the GPS signal to NAVI control unit.
DISPLAY CONTROL UNIT
- Display control unit draws a status of the audio and air conditioner, a TRIP screen, a FUEL ECONOMY screen, etc., and transmits the image signals to the display screen.
- It receives operation signals of audio and air conditioner from A/C and AV switch, and transmits the operation signal of audio to the audio unit via the communication line and transmits the operation signal of air conditioner to the meter and A/C amp. via CAN communication.
DISPLAY
- Images on the display include RGB image such as map screen.
- Display control unit controls images on the display.
A/C AND AV SWITCH
- A/C and AV switch, an integrated combination of audio and air conditioner switches, are adopted.
- Operation signal of audio is transmitted to the audio unit through display control unit with the communication line. Operation signal of air conditioner is transmitted to meter and A/C amp. through display control unit with CAN communication.
CAN Communication Unit
Refer to LAN-32, "CAN Communication Unit". Description and Operation