Measurement of Line Characteristics and of Track Irregularities ... - UIC

World Congress on Railway Research, Köln
November 2001
Special Poster Presentation (SP) “Technical Diagnosis”
Measurement of Line Characteristics and of Track Irregularities
by Means of DGPS and INS
SUMMARY
Dipl.-Ing. Thorsten Lück, IFEN Neubiberg
Prof. Dr.-Ing. Peter Meinke, C+K GmbH Starnberg
Prof. Dr.-Ing. Bernd Eisfeller, IFEN Neubiberg
Dipl.-Ing. Christian Kreye, IFEN Neubiberg
Dipl.-Ing. Johannes Stephanides, ÖBB Wien
Dipl.-Ing. Laszlo Tordai, ERRI Utrecht
The knowledge of geometric track irregularities is important for dynamic safety of
railways. It becomes evident with rising use of tilting trains, as irregularities may lead
to train derailment. Especially to identify isolated track defects, a new measuring technique
using differential GPS and an inertial navigation system to determine the position
of the track measuring system was developed.
The goal is to measure long wavelength track irregularity up to 150 m wavelength
within mm accuracy. The length of the track segment under surveillance is assumed 40
km ( 20 km) and the intended measuring speed is 30 - 50 km/h with approximately
15 cm data point spacing on the track.
This report discusses the system design and integration technique and presents experimental
results from the prototype measurement vehicle. Also critical aspects of
high-precision DGPS/INS integration are outlined.
1 OVERALL SYSTEM
It is the task of the Track Irregularity Measurement System to provide highly accurate
positioning data for both of the railheads. From the instrumental conditions and
operational requirements the following parameters for the Track Irregularity Measurement
System were defined:
(1) General parameters:
Measurement Segment 20 km (equals 40 km)
Data point spacing d = 20 cm
Measurement speed v = 30 – 50 km/h

(2) Navigation and positioning system requirements:
Horizontal position 1 mm
Vertical position 1 mm
The system design is based on the integration of Differential GPS (DGPS) with an Inertial
Navigation System (INS) for positioning. The primary task of the system is to
acquire high-precision data for post-processing.
Using ultrasonic sensors, in addition the horizontal and vertical distances between the
platform and the railheads is determined. From this, with the knowledge of the absolute
position of the platform, the absolute geodetic position of the railheads can be derived.
1.1 Positioning of measurement platform
The platform positioning system consists of three basic sub-systems:
Platform measurement equipment to be mounted on a draisine (Figure 1),
containing
Sensor subsystem
Data acquisition and archiving system
Time synchronization module
DGPS reference station
Off-line processing system
Figure 1: Measurement Vehicle of the Austrian railways (ÖBB) with upset measurement
platform

The sensor subsystem consists of both the positioning system to determine the absolute
position of the platform and the distance measurement system to determine the
position of the railheads relative to the platform.
To determine the higher dynamics of the platform, a precise inertial navigation system
is used. The INS position is then augmented using carrier phase positions of the differential
GPS system to eliminate inertial errors.
Figure 2 shows the block diagram of the measurement system.
Figure 2: Block diagram of platform measurement system
Figure 3: Principal of track geometry and irregularity measurements

1.2 Measurement of the railheads
Surveying of the railheads is done by use of ultrasonic sensors in two perpendicular directions.
The distance between platform and railheads is measured in horizontal and
vertical direction. Considering the attitude and position of the platform determined by
the positioning system, the absolute position of both railheads could be derived.
Figure 4: Definition of track irregularity parameters
To determine the track defects, the track parameters in accordance to Figure 4 are determined
and are investigated for specific defects e.g. by means of wavelet analysis.
2 HARDWARE COMPONENTS
The system to determine the inertial position (positioning system) uses the following
hardware components:
Inertial Navigation System (INS)
GPS L1/L2 receiver
Choke-Ring Antenna
Multiport Serial Card
Event timer
Dual-Pentium personal computer
For the reference station the following hardware components are used:
GPS L1/L2 receiver
Choke-Ring Antenna
Pentium personal computer

2.1 Synchronization
The timing board is triggered with the 1 PPS output of the GPS receiver, which is synchronized
to GPS time. The INS uses it's own time base but provides a trigger signal
for each message it sends to the serial port on a 100Hz basis for raw inertial data and
on a 50Hz basis for navigation data. These trigger signals are time-tagged by the
event-timing card and are read from the navigation software nearly in real time.
2.2 GPS Receiver
The GPS receiver provides dual frequency GPS performance. Featuring Narrow Correlator
and P-code Delayed Correlation Technologies, the receiver outputs pseudorange
and full wavelength carrier phase observations for both L1 and L2 frequencies.
It provides two serial data channels, assigned as COM 1 and COM 2. Both channels
can be used for data logging and receiver programming. Real pseudorange and
carrier phase are provided at up to 4Hz. Table 1 provides the specifications of the receiver.
Features
mm level post-processed accuracy
L1-C/A code and carrier tracking
L2-P code and carrier tracking
4 Hz position output rate
4 Hz raw data output rate (Pseudorange)
Specifications
Position accuracy
Stand alone
SA off 15m CEP
SA on 40m CEP
Differential
Code (L1, C/A) 0.75m
Measurement precision
L1 carrier phase
Single channel 3mm RMS
Differential channel 0.75mm RMS
Dynamics
Acceleration 6g
Velocity 515m/s
Table 1: Specification of GPS-receiver
The GPS data (pseudoranges and doppler velocity) is captured during mission and
processed in offline mode (post processing). For data analysis and processing, several
software packages are applicable, PHARAO (Phase Ambiguity Resolution On-The-
Fly), which is a high-precision DGPS/DGLONASS/DAPL navigation system that has
been developed over the last few years at the institute (IfEN) and GeoGenius from
Terrasat.

To suppress multipath errors, the antenna of the reference station as well as the rover
antenna is equipped with a choke ring. To minimize effects due to shadowing and
multipath on the draisine, the antenna is build up on a mast so that the antenna phase
center is above the vehicle’s roof.
2.3 Inertial Navigation System (INS)
To measure the higher dynamics of the measurement platform, a so-called strapdown
INS is used. This systems uses ring laser gyros to determine the rotation angel. Table 2
shows some technical data of the used system.
Performance
Position
Altitude
Azimuth
Pitch and Roll
Characteristics
Size
Weight
Power requirements
Cooling
Periodic calibration
Operating conditions
Latitude range
Temperature
Vibrations
Shock
Environment

To avoid unsteady position solutions of the INS, the ZUPT mode can be deactivated
after the initial alignment.
Figure 5: Strapdown INS together with GPS choke ring antenna. This right figure shows
the left sided lever arm of the measurement platform with two ultrasonic sensors to
measure the vertical and horizontal distance to the rail head.
2.4 Ultrasonic Distance Measurement Unit
To measure the distance between the platforms lever arms and the railheads, four ultrasonic
sensors are used. They measure the horizontal and vertical distance on both
sides of the rail. Measurements are synchronous to the inertial measurements and are
triggered by the INS 100Hz synchronization signal.
Using a switch, the trigger signal can be applied to the sensors by the operator. The
timing card determines the time when the switch is used. Thus a precise correlation to
the inertial measurement data is possible. Figure 5 shows one lever arm and the corresponding
ultrasonic sensors. The horizontal distance is measured from the inner side to
the railhead.

3 KALMAN FILTER DESIGN
To integrate INS and DGPS measurements, a KALMAN filter is used. This optimal
filter estimates the systematical errors of the inertial system due to precise DGPS observations
and thus can correct for these errors to calculate the absolute position, attitude
and velocity.
Integration of GPS and INS can be done on different levels (so called loose, tight and
deeply coupling).
The system specification requires a resolution on the track of about 20 cm. At measurement
speed of 30km/h this leads to a position rate of at least 42 Hz. As the INS delivers
navigational information at a 50Hz data rate, loose coupling is feasible.
As a basis for the KALMAN Filter, the filter concept first described by Schmidt is
used, which models the basic dynamic errors of a free inertial system. Adding other
states derived from the error analysis of the INS then augments this system. As the
used INS is of high precision, only biases of the accelerometers and gyros are used to
augment the KALMAN filter whereas scale factor errors are neglected.
3.1 State Vector
The state vector of the filter consists of the following 27 elements:
X äÈ , äv,
är,
d,
b
INS/GPS
The first nine states represent the INS navigation errors:
Vector of attitude error with respect to north, east and down axes
v Vector of velocity error in latitude rate, longitude rate and altitude rate
r Vector of position error in latitude, longitude and altitude
The next 15 states result from the error analysis and express the major INS sensor error
sources:
d Vector of uncompensated gyro drift
b Vector of uncompensated accelerometer bias
3.2 Dynamic Coupling and State transition
Dynamic coupling between states is expressed by the dynamic matrix F with white
process noise input:
x
F
The transition matrix, necessary for prediction of states and covariance is obtained by
x
w

Ö
I
F
1
t F
2
The dynamic matrix F is shown below, with brief explanation of the sub matrices following:
F
0
0
FFree
[ 9x9]
0
0
0
0
The sub matrix F free is taken from Schmidt (1978) and is not shown in detail here.
C
n
b
0
0
I
0
g
2
I
0
t
DC
Correlation coefficients of gyros (index g) and accelerometers (index a)
I 3x3 unit matrix
n
The C b Matrix is the transformation matrix between body and navigation frame and is
defined as
with
Roll
Pitch
n
Cb
Azimuth.
cos
cos
sin
cos
sin
cos
sin
sin cos
sin sin sin
cos cos
cos sin
sin
0
0
2
n
b
a
cos
sin
cos
This matrix is needed to relate sensor errors, normally defined in body coordinates to
the navigation errors defined in local horizon frame.
The scaling matrix D is defined by
D
1
R
0
0
0
1
Rcos
0
0
0
1
sin
cos
sin
sin
sin
cos
sin
cos
cos
and is necessary to transform linear velocity to latitude or longitude rate.

F b and b are diagonal matrices containing the gyros and accelerometers output.
b
Ù
b
F
b
x
0
0
b
x
f
0
0
With respect to process noise w we add white noise to each state in order to keep the
KALMAN filter adaptive.
3.3 Observations
Six different observations are used to update the KALMAN filter:
Three dimensional position
Three dimensional velocity
The observation, used to update the KALMAN filter is the difference between the INS
and GPS computed position vectors, thus yielding the simple error equation
with
r position vector
rGPS rINS
0
b
y
0
0
f
b
y
0
är
är vector of position error (states)
n white noise.
The way to observe the velocity error is to compute the difference between INS and
GPS derived velocity. The velocity difference has to be related to the velocity error
state via scaling matrix D, described before. This equation is also simple, because velocity
error can be observed directly.
3.4 Smoothing
v
GPS
v
INS
Post processing gives the opportunity, to take all data of a survey mission into account
to calculate position, attitude and velocity at a certain place. With the so-called
smoothing KALMAN filter all data are first processed in positive time direction and
afterwards – under consideration of the already processed data – in negative direction.
This allows another reduction of the variance of the navigation solution.
D
1
f
0
0
0
0
b
z
b
z
n
äv
n

4 EVALUATION OF THE PROTOTYPE OF THE TRACK IRREGULARITY
SYSTEM
In September 2000 several test runs were performed on an auxiliary line of the Austrian
railways (ÖBB) near St. Pölten from Spratzern to Lilienfeld. Below first of all the
setup and surveying of the measurement platform will be described following some results
from the test runs.
4.1 Setup and surveying of the measurement platform
4.2 Trials
Measurements of the track geometry respectively their defects with millimeter accuracy
requires a survey of all sensors with at least sub millimeter accuracy. Due to platform
scale and big distances between the sensors, a survey of the system in the laboratory
was not possible. Surveying the platform with the required accuracy could
therefore only be managed using geodetic techniques. To do so, the antenna phase
center as well as the ground plate of the INS and three reference points on each lever
arm where marked to be easily located by a theodolith.
Particularly the positions of the ultrasonic sensors were not visible from observation
points at only one side of the measurements vehicle. Therefore it was necessary to
build up a six-point network around the draisine with legs between 5 and 25 meters.
Because of the high accuracy requirements the essential distances needed to fix the
network scale were determined by the parallactic angle concerning a 1-meter subtense
bar. Then the coordinates of the target points defining the mutual 3D-position of the
three different sensors could be determined by the spatial intersection of the lines of
sight from the observing theodolites (measurement of horizontal and vertical angles
concerning two observation points) in the local network system.
An additional stabilization of the network geometry was possible by the double-sided
determination of the target points defining the INS ground plane and the phase center
of the GPS antenna. Using a free network 3D-adjustment optimized concerning the
object points with a reliability if 0.61 the vectors between the different sensors could
be determined with an accuracy of 0.3 to 1.7 mm depending on the particular observation
geometry. Finally the transformation of the vectors in the coordinate system of the
INS was carried out.
Data of a total distance of approximately 120 km was acquired by successive travels
over the test range, which had a total length of more than 20 km. Figure 6 show a part
of the test range between rail station Traisen/Markt via Traisen and Wilhelmsburg to
Spratzern. In addition the position of the reference station is marked.

Figure 6: Part of the test line near St. Pölten /Austria
Figure 7: Detail of the curved line between Traissen and Wilhelmsburg

Figure 8: The figure show the ultrasonic signals measured in the lateral channel (upper
part) and the derived gauge (lower part) within the marked area of figure 7. The oscillations
found in the ultrasonic signal can be explained as a motion of the track guided
vehicle, as both signals are phase-delayed by and the signal is therefore eliminated in
the superposition.
Figure 9: Cross-level and gauge within curvatures along the line

As an example, the track geometry within the time frame between 374737.29 and 374739.97
GPS-seconds is enlarged (Figure 9).
Figure 10: Absolute position of the railheads. The centerline shows the position of the
barycenter of the inertial navigation system.
The accuracy of the GPS carrier phase position solution was in the range of

Under close consideration of all these factors, the positioning of the platform with accuracy in
the millimeter range as well as the derivation of the absolute railhead position from the platform
reference point is feasible.
Acknowledgement
The authors gratefully acknowledge, that the work described here was funded by the European
Community in the frame of the Project DYSAF under the contract BRPR-CT-97-0558
(DG12-HIAS).
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