Lexus RX300 (MCU15). Service manual - part 66

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5. GPS (Global Positioning System) Voice Navigation

General

The GPS voice navigation function combines the radiowave navigation system that determines the present
position through the GPS signals, and the self-contained navigation system that detects the driven distance
and the direction of travel through the speed sensors and the gyro sensor that is contained in the navigation
ECU. The GPS voice navigation function is a high-precision navigation system that indicates the vehicle
position on the map display on the DVD (Digital Versatile Disc) drive and guides the route from the
present position to the destination on a map and pictogram and through voice instructions.
The multi display shows the data that has been calculated by the radiowave navigation system and the
self-contained navigation system.

Construction and Operation

1) General

The GPS voice navigation function consists of the following components:

D DVD-ROM

D GPS Antenna

D Speed Sensor

D Speaker

D Navigation ECU

D GPS Receiver (contained in the navigation ECU)

D Gyro Sensor (contained in the navigation ECU)

2) DVD (Digital Versatile Disc)

The DVD, which uses a smaller laser beam diameter than the CD (Compact Disc), is able to record
and play back a greater amount of data because it can handle pits, or signal grooves, at a higher density.
The volume of data that a 12 cm-diameter DVD can store is equivalent to approximately 7.5 times that
of a CD-ROM, totaling 4.7 gigabytes. The navigation system has adopted a dual-layer DVD, which
has two layers per side to store a large capacity of signal data, totaling 8.5 gigabytes.

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Second Layer
First Layer

Bit Size Comparison

CD

DVD

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3) GPS Antenna

The GPS function receives, via an GPS antenna, the signals that are transmitted from the GPS satellites
located in space at an approximate altitude of 20,000 km, in order to determine the vehicle’s present
position.

4) Speed Sensor

The navigation ECU will receive the vehicle speed signal directly from A/C and combination meter
integrated ECU.

5) Speaker

Outputs the sound signals that are transmitted from the audio unit. Also outputs the navigation voice
instructions via the driver’s side speaker.

6) Navigation ECU

General

Based on the map data on the DVD, signals from the GPS satellites, signals from the built-in gyro
sensor, and signals from the A/C and combination meter integrated ECU, this ECU calculates the ve-
hicle’s present position, direction of travel, and driven distance, and transmits the data on the multi
display. In addition, it outputs navigation voice instructions.
The GPS receiver and GPS sensor are contained in the navigation ECU.

GPS Receiver

The GPS receiver demodulates the signals that are received by the GPS antenna from the satellites
and outputs them to the navigation ECU.

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Gyro Sensor

The gyro sensor is designed to detect the yaw rate of vertical axis turn of the vehicle and installed
in the navigation ECU.
The gyro sensor has a turning-fork shape type piezoelectric ceramic piece inside. This piezoelectric
ceramic piece deforms by charging voltage and generates voltage by deforming with force.
The piezoelectric ceramic piece inside the gyro sensor is vibrated by the driving circuit and when the
vehicle turns (when the detection portion turns to the axis direction), coriolis force is added to the detec-
tion portion. With this force, the detection portion is twisted. The voltage generated by this twisting
is signal-processed inside the gyro sensor and outputted.
Navigation ECU receives this signal and judges the yaw rate of the vehicle.

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Coriolis Force

Turning Axis Center

Vibration Direction

Detection Portion

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Detecting the Vehicle Position

The navigation ECU calculates the position based on the principle of a 3-point measurement.
The GPS satellites are equipped with high-precision clocks. Thus, the satellites are able to transmit
continuous orbit signals and radiowave transmission time signals.
The navigation ECU also contains a clock, which can understand the radiowave time signals that are
received from the satellites.
As a result, the length of time that is taken by the radiowaves to arrive from the satellites to the antenna
can be determined. Thus, the lengths of time that elapse for the radiowaves of the 4 satellites to reach
the antenna are measured. Each of these lengths of time are multiplied by the luminous flux (the rate
of transmission of luminous energy: approximately 300,000 km per second), the results of which are
the distances from the satellites to the antenna. Because the positions of the GPS satellites are known
by their signals, the receiving point (vehicle position) can be rendered as the point in which the 4 spheres
(of which the centers are the respective satellites) converge.
However, due to the differences that exist between the clocks of the satellite and the ECU, the 4 spheres
do not converge at a single point. Therefore, the ECU uses another satellite to calculate the point at
which the 4 spheres converge at a single point and corrects its internal clock. As a result, the ECU
determines the vehicle position and adjusts its internal clock to the clocks of the satellites.

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Difference

GPS Satellites

Difference

Difference

Difference