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Precise Rail Track Surveying

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					      APPLICATION CHALLENGE                                                                              RTK GPS




                                                                                       Precise Rail
                                                                                       Track Surveying




                                                                                                                                      All photos courtesy of terra international ltd


This article describes the Swiss Trolley, a multisensor measurement system incorporating real-time
kinematic (RTK) GPS, and its use to survey sections of railroad track in the United Kingdom.

                                              ince May 2003, large track renewal projects                      The survey companies support several aspects of re-

                                    S         have taken place within the West Coast Route
                                              Modernization Program (WCRM), de-
                                    manding a major effort from the United Kingdom rail
                                                                                                            quired measurement activities during construction
                                                                                                            — such as monitoring progress of build-up of ballast
                                                                                                            and to check passing and structural clearances — that
                                    industry. Survey companies play a key role in this pro-                 bring the new track and other systems to within their
                                    gram by providing spatial information for the design-
                                    ers of through alignment, tunnels, platforms, drainage,
                                    and overhead lines — in other words, for the whole of
                                    the rail system’s infrastructural assets.
RALPH GLAUS holds a degree in geodesy and has more than 10 years experience working in the domain
of surveying engineering. He is employed by the Institute of Geodesy and Photogrammetry of the Swiss
Federal Institute of Technology, Zurich (ETH).

GERARD PEELS has a degree in civil engineering. He has worked as a consultant in the international rail
industry for more than 14 years and is employed by terra international surveys ltd., Zurich, Switzerland.

URS MÜLLER is a managing director of terra international surveys ltd.

ALAIN GEIGER received his diploma in physics from Swiss Federal Institute of Technology, Zurich (ETH).
He holds a Ph.D. in engineering sciences also from ETH. Presently, he lectures in satellite geodesy and
navigation at the Institute of Geodesy and Photogrammetry (ETH) and serves as president of the Swiss          FIGURE 1 The West Coast Route Modernization Program
Institute of Navigation.
12    GPS World MAY 2004                                                                                                                       www.gpsworld.com
design specifications in the track
corridor.
   The scope of work for Route Sec-
tion 12, covering about 80 kilome-
tres (Colwich Junction to Cheadle
Hulme, Figure 1) was the first in a se-
ries of track renewal projects that was
crowded with activities coming from
different disciplines and departments.
   As the project got under way, sur-
vey companies were soon stretched to
their limits using the traditional rela-
tive techniques of datum rails and mea-
suring offsets, simply because the con-
struction program allowed for
“floating” roads. These relative mea-
surement methods are common prac-
tice within UK surveying companies.
Offsets are measured and marked up
against spatial, fixed objects (pegs, nails,
permanent ground markers, datum
rails). For instance, the six-foot dis-
tances between up and down line tracks
are measured back to back, or as run-
ning edge to running edge of the rail-
heads. These measurements are
marked up (with chalk or other more The Swiss Trolley with total station (background)
sustainable paints, adding or subtracting both the re- struction efforts of the other track. This works as long
quired horizontal shift and required lift as per design) as the so-called datum rail is not touched, moved by
on that rail, which is intended to stay during the con- tamping operations or rail stressing, or in the worst


 Glossary
 Ballast:
                                                                                                                     1500 mm
 Angularly shaped crushed stone used to support sleepers, timbers, and bearers,                                                       717.5 mm
                                                                                                                                        .
 both laterally and vertically.
                                                                                            Cant
 Cant (or super elevation):
 The amount by which a rail, usually the outside rail of a curved track, rises above                          0 - 14 mm
 the lower rail on the same piece of track.The cant is referred to a nominal base
 (for example, 1.500 meters). See accompanying figure.                                                                                        Track gauge
                                                                                                                         Track axis
 Chaining:
 Path length.
 Gauge:                                                                                Slew:
 Smallest distance between the rails measured 0–14 millimeters below the rail top      Horizontal displacement of the actual railway track from the nominal track.
 edge.A nominal measure is, for example, 1.435 meters. See accompanying figure.        Through Alignment:
 Gradient:                                                                             Expression used for an alignment that covers a project from start to end.The
 Longitudinal inclination of a railway track.                                          three components of alignment are horizontal, vertical and cant, regardless of the
                                                                                       typical cross sections encountered.
 Lift:
 Vertical displacement of the actual railway track from the nominal track.             Track Axis:
                                                                                       Line parallel to the reference rail at the distance of the half nominal gauge. See
 Passing and Structural Clearances:                                                    accompanying figure.
 Passing clearance allows trains to pass on two parallel tracks without hitting each
 other. Structural clearance is the clearance required for trains passing structures   Twist:
 such as tunnels, bridges, platforms, and signal posts.                                Cant rate.


www.gpsworld.com                                                                                                                         MAY 2004       GPS World 13
      APPLICATION CHALLENGE                                                                                                  RTK GPS




                                                                                                                                             case taken out.
                                                                                                                                                terra international surveys ltd. of Zurich,
                                                                                                                                             Switzerland was invited by NetworkRail to
                                                                                                                                             demonstrate its real-time kinematic (RTK)
                                                                                                                                             track-measuring device, called Swiss Trolley,
                                                                                                                                             which incorporates optical kinematic total sta-
                                                                                                                                             tions and RTK GPS receivers. Soon it became
                                                                                                                                             evident that this absolute way of surveying was
                                                                                                                                             the solution required to provide accurate, dense
                                                                                                                                             and multi-parameter information. This arti-
                                                                                                                                             cle features the Swiss Trolley and describes the
                                                                                                                                             performance and limitations of GPS-based
                                                                                                                                             track surveying that we encountered.

                                                                                                                                             System Design
                                                                                                                                           The University of Applied Sciences Burgdorf
                                                                                                                                           developed the trolley in collaboration with terra
                                                                                                                                           international surveys, Switzerland, as part of a
                                                                                                                                           project financed by the Commission for T     ech-
Swiss Trolley
                                                                                                                                  nology and Innovation by the Swiss Office for Profes-
                                                     PosData / PPS                                                                sional Education and Technology. The trolley origi-
                                                                                                                                  nally was designed for kinematic applications with
                 GPS
                (XAPC)
                                                                                                                                  subcentimeter accuracy. However, Grunder Ingenieure
                                                PPS
                                                                                                                                  AG of Burgdorf, Switzerland, another project partner,
                                                                                                                                  successfully applied it in combination with high-pre-
  Incremental Encoder (Odometer 1)                                                                                                cision optical total stations for the alignment of 18 kilo-
                                                                                                                                  meters of slab track in the Zurich-Thalwil-tunnel of
                                                                                                                                  the Swiss Federal Railways. Alignment of the slab track
  Incremental Encoder (Odometer 2)
                                                                                                                                  required millimeter-level accuracies. Apart from
                                                                                                                                  GPS/RTK and total stations, further sensors are needed
          Angular Transducer                                                                                                                                                  wo
                                                                                                                                  to completely measure track geometry. T tiltmeters,
            (dGauge Left)
                                                                     Serial Port RS232                                            a track gauge measuring system, and odometers enable
                                                                                                                                  the assessment of the key parameters: cant, gradient,
          Angular Transducer
           (dGauge Right)
                                                                     Data Acquisiton Card                                         track gauge, and chaining. (See Glossary on page 13.)
                                                                                                                                     The modular design of the trolley allows for the use
                                                                     Serial Port RS232                                            of further sensors such as laser scanners or ground pen-
              PT100
       (Ambient Temperature)



                                                                                Cant tiltmeter
                                                                                                                                                             Synchronization, reduction to reference rail and




                           PT100
                                                                                                                                                                                                                   Forward / backward filter and smoother
                                                                                                     Integrity tests and blunder detection




                     (Box Temperature)
                                                                                                                                                                  comparison with nominal geometry




                                                                                                                                                                                                                                                            Results (Steering file for tamping




                                                                                                                                             Corrected
                                                                                                                                                                                                                                                             machines, charts, and so forth)




                                                                                Gradient tiltmeter                                           gradients
                        Inclinometer
                      (Superelevation)
                                                                                Odometers
                                                                                (cen terline path)

                         Inclinometer
                          (Gradient)                                                                                                          Track
                                                                                Transducer left                                               gauge

                                         Sensorbox

             Total Station                                                      Transducer right
               (XPrism )


                                                                                GPS/RTK
     FIGURE 2 Block diagram of track surveying
trolley                                                                    FIGURE 3 Post-processing concept

14    GPS World MAY 2004                                                                                                                                                                                        www.gpsworld.com
etrating radars (GPR). The operation of the trolley
in a stand-alone mode also can provide “path”-“twist”
and “path”-“track gauge” charts showing twist and track
gauge as a function of the covered chaining. Stand-
alone surveys represent a quick method for track mon-
itoring during construction.

Use of Sensors
Cants and gradients are measured by two fluid damped
inclination sensors. Typical precisions in the static
mode reach about 0.2 mrad. Biases are checked at the
beginning of a survey. This is done by means of in-
clination measurements in two faces over a reference
plane.
   The track gauge is determined by a function of the
angular position of two dragging mechanical scan-
ners. Angular transducers measure the deflection of
the scanners. Calibration is accomplished by com-
paring the track gauge reading with a measuring
rod reading. The precision of the track gauge mea-
suring system reaches about 0.5 millimeters.
    wo
   T odometers provide path measurements of the
                                                           Track surveying trolleys in use
parallel rails, which in turn can generate differential
odometer readings for dead reckoning. The average          affect the tiltmeter readings. The blunder-free data
of the left and right odometer represents the path         allow for a first reduction step. As previously men-
length of the centerline. Systematic errors may arise      tioned, tangential accelerations are removed from
from an inappropriate scale factor (diameter). This        the gradient tiltmeter readings during the prepro-
parameter can be calibrated by comparing the odome-        cessing stage.
ter path against the true path length, using total sta-       We can derive accelerations and velocities from the
tion data and nominal track data. If the odometer data     path measurements by means of fitting polynomials
are free from slippage, precision can reach a level of     and the corresponding derivatives. Cubic parabolas
up to 50 ppm. Figure 2 gives an overview of the sen-       provide good fits with a moderate noise gain. Cen-
sors incorporated into the Swiss Trolley system.           tripetal accelerations are ignored in the data process-
                                                           ing, which is considered acceptable because of the low
Data Processing                                            operation speed and the large track radii. The angular
In order to benefit from the “past” and “future” cor-      transducer data are used for the assessment of the track
relations of adjacent measurements, data are post-         gauge and to determine the trolley wobble between the
processed. Kinematic data postprocessing uses a cas-       rails. Wobble rates are smooth due to the inertia of the
cading filter concept. In contrast to a tightly coupled    trolley.
filter, we preprocess parameters such as tangential ac-       Synchronization, Data Reduction II. Synchro-
celerations that have minor effect on the final results.   nization of the trolley data with the positioning sensor
This produces a simpler and more stable filter formu-      data only must be done if no pulse per second (PPS)
lation. The cascading data processing is divided into      signal is available from the GPS receiver. In contrast
these steps:                                               to use of a GPS-RTK with PPS, optical total station
      blunder detection / data reduction I                 data are not synchronized with the trolley data dur-
      synchronization / data reduction II                  ing data acquisition.
      filter / smoother                                       For the GPS case, the 20 Hz trolley data are down-
    Figure 3 illustrates the cascading postprocessing      sampled to the typically used 1 Hz GPS data. A rate of
concept.                                                   1 Hz is effective to accurately record all track patterns.
    Blunder Detection, Data Reduction I. Data              NMEA sentences sent to the serial port contain the
processing includes integrity checks and blunder de-       GPS-RTK solutions that use carrier phase integer am-
tection for each data channel. The reasons for blun-       biguity resolution methods. The postprocessing soft-
ders are manifold. For example, welding seams on           ware allows for a refinement of the geoid undulations
the rails can cause nuisance accelerations that mainly     and use of a separate geoid model along a railway track
www.gpsworld.com                                                                                                    MAY 2004   GPS World 15
       APPLICATION CHALLENGE                                                           RTK GPS




                                being measured. Within the UK project, we refined         terline of the track. The centerline in general does not
                                the existing geoid model by comparing the leveling and    correspond to the actual track axis, because the track
                                GPS data along the tracks. In order to compare the        gauge can deviate from its nominal value (for example,
                                measurements with nominal data, the GPS antenna           1.435 meters). However, reducing to the track axis
                                phase center coordinates have to be reduced to the cen-   would result in a discontinuous curve disturbing the
                                                                                          subsequent filter step. Therefore, corrections due to
                                                                                          deviations from the nominal gauge are applied after
                                                                                          filtering.
                                                                                              For track renewal projects, a nominal geometry de-
                                                                                          scribing the track as a series of analytical functions —
                                                                                          straight lines, circles, or clothoids — is available. In this
                                                                                          pre-filter step, residual displacements of the nominal
                                                                                          trajectory are evaluated and submitted to the filter.
                                                                                              Filter and Smoothing. An 11-state Kalman filter
                                                                                          estimates the track parameters. The filter states include
                                                                                          displacements from the nominal track, the cant, and
                                                                                          the track gauge. The model assumes a non-accelerated
                                                                                          motion, incorporates rates for the horizontal and
                                                                                          vertical displacements as well as the cant, and intro-
                                                                                          duces additional adaptive parameters for the odome-
     FIGURE 4 Influence of the tiltmeter on the lift parameter
                                                                                          ter scale and the gradient tiltmeter bias. The latter two
                                                                                          parameters can be fixed if they remain constant for a
                                                                                          single survey, which allows a more reliable filter
                                                                                          formulation.
                                                                                              For the filter update, seven observations are used per
                                                                                          time batch. The measured horizontal and vertical dis-
                                                                                          placements from the nominal track, path, cant, track
                                                                                          gauge, gradient, and odometer path can be submitted
                                                                                          to the Kalman filter. The postprocessing allows for fil-
                                                                                          tering in forward and backward directions and smooth-
                                                                                          ing. This reduces the variance of a forward-only fil-
                                                                                          tered solution and minimizes lags caused by a
                                                                                          forward-only filtered series.
                                                                                              The filtered data are represented on charts that show
                                                                                          relevant parameters for track renewals as horizontal
                                                                                          and vertical displacements (“slew” and “lift”), cant, twist
     FIGURE 5 Slew of a forward and a backward run                                        and track gauge. The postprocessing software gener-
                                                                                          ates steering files for machine guidance systems of tam-
                                                                                          pers. By using the slew and lift parameters, the tampers
                                                                                          can correct the track to the desired position.
                                                                                              If no nominal track data is available, as sometimes
                                                                                          occurs with older railway lines, an absolute trajectory
                                                                                          is estimated. The filter states in this case include po-
                                                                                          sition, velocity, azimuth, curvature, cant, gradient, track
                                                                                          gauge, odometer scale, and gradient tiltmeter bias. The
                                                                                          filtered data are a basis for track regressions in which
                                                                                          analytical functions have to be fitted through the
                                                                                          trajectory.
                                                                                              The typical work flow contains several track runs
                                                                                                                          o
                                                                                          with overlapping sections. T improve accuracy, a track
                                                                                          is normally surveyed in both directions. A further tool
                                                                                          within the postprocessing software allows for merging
     FIGURE 6 Lift of a forward and a backward run
                                                                                          data from all runs and takes into consideration the even-
                                                                                          tual remaining biases. The merge process takes into
16     GPS World MAY 2004                                                                                                     www.gpsworld.com
account the covariance matrix of the single filtered so-     tion of track. The twist describes the cant rate. Cant
lutions. Sections with poor GPS reception or with GPS        rates acquired by the trolley are almost bias-free. How-
outages (tunnels, bridges) are supplemented by total         ever, the noise gain can be considerable due to the de-
station measurements in combination with the trolley.        rivation of the (noisy) cant measurements. By means
The total station data are tied to the GPS reference         of the postprocessing, smooth estimates for the twist
frame.

Survey Example
The performance of the Swiss Trolley was evaluated
by comparing a forward and a backward run on a 250-
meter-long section of track as part of the UK project.
The survey was done at walking speed of 1.2 me-
ters/second, with all GPS phase ambiguities solved for
both runs. Apart from an accuracy assessment, forward
and backward runs allow for a determination of most
sensor biases and for a boresight calibration. We
processed the data according to the previously described
scheme and computed displacements with respect to a
nominal geometry.
    Figure 4 shows the influence of the gradient tiltmeter
                                                               FIGURE 7 Track gauge of a forward and a backward run
measurements on the GPS heights. Tiltmeter mea-
surements smooth a pure GPS solution with a conse-
quent improvement in accuracy by a factor of up to 1.5
compared to the GPS-only solution. In the given ex-
ample, an a priori standard deviation of 20 millimeters
was chosen for the GPS heights, while the gradient a
priori standard deviation was set to 3 mrad. How-
ever, gradients have to be weighed carefully, since resid-
ual nuisance accelerations can result in local trajectory
tilts.
    Table 1 summarizes the differences between the for-
ward and backward run in slew, lift, cant, track gauge,
gradient, and twist, showing very small mean values as
the result of first removing the remaining biases. For
the comparison, backward points were interpolated on
the corresponding forward chaining. The covariance
information of smoothed solutions was used for the             FIGURE 8 Cant of a forward and a backward run
evaluation of the differences.
    Figure 5 shows the progression of the slew parame-
ter. A standard deviation better than 4 millimeters re-
sults for the differences. Figure 6, which shows the tilt-
meter-smoothed GPS heights, reflects the ballast tracks’
typical cyclic patterns. Most of these cycles can be as-
signed to previous relative track alignments. The
area below the curve with respect to the cross section
gives an idea of the missing ballast. If lift parameters
reach such high magnitudes, we might well consider a
local adaptation of the nominal geometry.
    Apart from the slew and lift charts, cant and track
gauge parameters are also generally of interest in a
track survey (Figures 7 and 8). A crucial parameter
concerning safety aspects of a track is the twist. Al-
lowable tolerances for twist mainly depend on the               FIGURE 9 Twist of a forward and a backward run
foreseen operation speed of trains traversing the sec-
www.gpsworld.com                                                                                                    MAY 2004   GPS World 17
     APPLICATION CHALLENGE                                                                RTK GPS




                          TABLE 1                                                                                              meter-long section of dou-
                                                                                                                               ble track, repeated runs using
                                                      Mean           Std            Min          Max               Units
                                                                                                                               the trolleys produced more
                                  Slew                  0.1          3.9            -9.9          9.4               mm         than 135 kilometers of mea-
                                   Lift                 0.6          8.1           -22.6         19.8               mm         surements. Often and when-
                                  Cant                  0.0          0.3            -0.9          1.0               mm         ever possible, measuring was
                              Track gauge              -0.4          0.3            -1.1          0.6               mm         done with two trolleys on
                                Gradient                0.0          1.8            -4.8          8.0              mrad        both tracks simultaneously,
                                  Twist                 0.0          0.1            -0.5          0.4             mm/m         thus minimizing interference
                                                                                                                               with other construction ac-
                          The mean, standard deviation (std), minimum and maximum in track parameters between the forward and
                          the backward run.The cant is referred to a 1.5-meter base.                                           tivities and traffic on the
                                                                                                                               track.
                                                                                                      The dense and accurate track information, captured
                                                                                                  on the last measurement run, enabled terra interna-
                                                                                                  tional surveys to guarantee a precise description of the
                                                                                                  actual physical track position. Subsequent applied re-
                                                                                                  gression analysis techniques resulted in a new geo-
                                                                                                  metrical description of the two lines, which can now
                                                                                                  be used to mark up the track and maintain it to this new
                                                                                                  definition. Moreover, any wave patterns or irregular-
                                                                                                  ities were corrected.
                                                                                                      As a result of their involvement in these projects in
                                                                                                  the UK, terra international surveys has become more
                                                                                                  aware of the extreme demand for additional, highly ac-
                                                                                                  curate spatial data of the environment both above and
                                                                                                  under the track. More research and development has
                          Sensor box with two tiltmeters                                             been put into finding a solution for combining and
                          are obtained as seen in Figure 9. The data shown from registering more parameters together, such as ground
                          the forward and backward runs has already been penetration data and laser scans, in order to achieve a
                          smoothed separately.                                                    complete three-dimensional zone around the track.
                              The accuracies demonstrated here fulfill most track
                          alignment requirements. Long periodic track position Acknowledgments
                          errors, such as the ones induced by relative chord tech- The authors would like to thank CTI of the Swiss Of-
                          niques, do not occur using a GPS reference frame. fice for Professional Education and Technology for
                          Thanks to the GPS-based approach, homogenous ac- providing the opportunity to develop the track sur-
                          curacies are obtained along a railway track. An accu- veying trolley. A special thanks also is extended to Net-
                          racy-limiting factor using GPS for track surveying has work Rail, who gave us the opportunity to gain vast ex-
                          multipath effects. New antennas with “stealth” ground perience in optimizing interaction with construction
                          planes will be used in the near future and are expected workflows for the UK market.
                          to produce significant reductions in multipath.
                                                                                             Manufacturers
                          Conclusion                                                         This project used Trimble (Sunnyvale, California).
                          Following completion of the Route 12 project described             GPS RTK receivers 5700/5800 and the Zephyr antenna,
                          earlier, another WCRM Project, Route Section 2, has                Trimble optical total stations ATS600, and a Swiss Trol-
                          been finished. In contrast to the Route 12 project, terra          ley by terra international surveys ltd. (Zurich, Switzer-
                          international surveys was able to install a first-order GPS        land).
                          network. The daily track measurement quantities in-
                          creased considerably as data could now be captured
                          more flexibly at walking speed using the Swiss Trolley
                          in GPS mode, regardless of day- or night-time work-
                          ing hours. By using the coordinated second-order fixed-
                          point network (studs in every second stanchion) and the
                          Swiss Trolley in optical total station mode, GPS-ob-
                          scured areas were efficiently covered. On this 27-kilo-
18   GPS World MAY 2004                                                                                                            www.gpsworld.com

				
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