International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004
The ICU HW is composed of specialized communications cards
that receive the measurements from the inertial sensors.
Currently, TAG is able to communicate with sensors that use
the following protocols:
* . HDLC/RS-485 (2 ports available)
° RS-232 (8 ports)
» Ethernet (1 port)
e CAN-BUS (2 ports)
These ports can be used not only for reading the IMU
measurements but also for transmitting data to an external
device, for example a Flight Management System (FMS) or for
receiving information from other kind of sensors, as for
example an odometer.
3.3 Synchronization
The Time Synchronization Unit (TSU) module is the clock of
the system. It is a specific HW that is configured with the GPS
time when the system is booted. The function of the TSU is to
register the time when an event has happened in the system.
Events are for example the measurement of a data by a sensor.
This card has a stable clock in order to deliver an accurate time
reference. However, in order to correct the clock drift with the
even more accurate GPS time, the device has a Pulse Per
Second (PPS) input channel. With the PPS signal, coming from
the receiver, the time precision can be set to microsecond
accuracy. The TSU also has some trigger output channels that
are used for some sensors to mark the moment when they have
to take a measurement or for any special signalization. The
current TSU HW used in the TAG is a Brandywine PCI-
Sineclock-32.
3.4 Real time navigation system
There is a large number of definitions for a real time system. It
could be said that a real time system is able to respond in a
timely predictable way to asynchronous events coming from
outside.
Another classification inside the frame of a real time system
according to their properties is, to simplify, between 'hard' real
time systems (strict time constraints) and 'soft real time
systems (non-strict time constraints). An example of a 'hard'
real time system could be a flight control system and an
example of 'soft' real time system a data adquisition system.
The TAG will be discused in terms of 'soft' real time systems.
An important issue is the operating system used. There are
different types of operating systems: on-line OS, batch OS and
real time OS (RTOS). The best option depends on the
requirements of the application being developed.
Some of the desirable characteristics of a RTOS are:
preemption, concurrency, syncronization tools, privacity,
efficiency,etc.Examples of commonly used OS's for real time
applications in the industry are: Windows NT, Linux RT, QNX,
RT, etc. The TAG currently works with a batch OS : Windows
NT.
Windows NT is widely applied, although it was created as a
general purpose operating system . It can not be considered as a
'hard' real time system, but, as it meets some of the
characteristics previously mentioned, it can be seen as a 'soft'
real time system Thus, for small and non complex real time
applications, Windows NT can be used. To improve the
performance of the system, a real time kernel (RTK) can be
introduced within this OS.
The integration of Linux RT is the next goal in the SW
configuration. It is expected that the real time core will give an
improved solution for the navigation and a more stable
behaviour of the whole system.
The Real time Navigation Unit (RNU), that integrates an
INS/GPS navigator SW, is able to process the raw navigation
data acquired from the GCU and the ICU and to provide a
trajectory (positions and velocities) and attitude angles.
After the habitual synchronization step, the application begins
registering events in terms of time from the sensors.
The program has a multithreading performance, allowing
several threads to work concurrently.
3.5 Control and Interfacing
The Operation Interface Unit (OIU) is the coordinator of the
components described above. It is in charge of reading the data
generated by the sensors through the GCU and the ICU and
read the time through the TSU, write the acquired data to output
files, and interact with the user by means a FUI (file user
interface) or/and a GUI (Graphic User Interface).
3.6 Power supply
The power unit (PU) was made to measure for the CU.
One part of the PU, the BPU, is composed of two aeronautical
power sources that generate all the voltages needed by the CU
and the sensors. This unit is able to switch automatically
between the main power and a system of reserve batteries in
case main power fails or is needed to be isolated from the
system.
The other part, the UPU, is a system of reserve batteries to
support to the BPU when the main power is disconnected. It
gives up to 15 minutes of autonomy to the system. It is
composed of a 24V pack of 4Ah Ni-Cd batteries and the charge
circuit and it is intended to work in aeronautical environments.
3.7 Sensors
TAG is capable of operating with various sensors at the same
time (e.g. GPS receiver + multiples IMUs). The IG has
available a pool of sensors, in particular IMUs of distinct
quality and properties, the data of which is captured by TAG.
This year the IG will have 4 IMUs with distinct properties
available. The objective is to have a wide range of sensors at
disposition to meet the different requirements of the applied
research in this field. So, this collection of IMUs will enable the
use of the TAG in configuration well adapted to the actual
requirements of the planned work.
The properties of the IMUs are summarized in table 1
Before using the IMU data, it is necessary to calibrate it [
Salychev, 1998]. The calibration reduces the uncertainty of the
observations of the sensor, which usually depends on its
quality.
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