2017 International Conference on Indoor Positioning and Indoor Navigation (IPIN), 18-21 September 2017, Sapporo, Japan

Real-time Monitoring for Structure Deformations
Using Hand-held RTK-GNSS Receivers on the Wall
So TAKAHASHI, Nobuaki KUBO

Norihiro YAMAGUCHI, Takashi YOKOSHIMA

Graduate school of Marine Science and Technology, Courseof Maritime Technology and Logistics
Tokyo University of Marine Science and Technology
Tokyo, Japan
m175009@edu.kaiyodai.ac.jp, nkubo@kaiyodai.ac.jp

Aerospace and Robotics Technology Group, Center for
future technology and design, Institute of Technology
Shimizu Corporation
Tokyo, Japan
yamaguchi.norihiro@shimz.co.jp, yokosima@shimz.co.jp

Abstract— Monitoring the displacements of structures causedby slight or big seismic shakings and landslides the irrespective ofobservation points by simply taking the data using a single
equipment is useful. GNSS antennas and receivers on the roof ofa steel tower to monitor the displacement of the structure causedby the wind etc. has already been reported, but ours is the firststudy on a low-cost GNSS system attached to the wall of abuilding to monitor structure deformations. More specifically,
this research focuses on real time monitoring for structure
deformations using hand-held RTK-GNSS (Real-Time Kinematic
Global Navigation Satellite System) receivers installed onbuilding wall. RTK positioning is normally carried out using
expensive receivers and antennas compatible with dual
frequencies. However, aside from its high cost, this type ofequipment is also not easy to procure. This study addresses thisproblem by using a single-frequency low-cost receiver, withcomparable results in centimeter level. The methods presented
make use of horizontal spread and statistical values to come up
with the positioning results. Our work on the proposed
revolutionary monitoring system with a robust RTK
improvement method will serve as basic research for monitoring
the fluctuations and deformations of structures in the fields ofarchitecture and construction.

In open sky environments, RTK positioning is often used
for surveying on construction sites owing to its highperformance. The Ministry of Land, Infrastructure, Transport
and Tourism of Japan launched a project called “iConstruction” that encourages using and adapting ICT in its
entirety under construction sites. However, receivers
compatible with dual frequencies and can be used in suchsituations are expensive; in the future, therefore, systems will
be emerging that use single-frequency low-cost handheld
receivers that are easy to produce and operate.

Keywords—RTK-GNSS; hand-held GNSS receiver; structure
monitoring

I. RESEARCH BACKGROUND AND OBJECTIVE
With the advent of a variety of smartphone applications
that use location information, the need for accurately
identifying location is increasing. Multi-GNSS positioning is
now possible due to the progress in the deployment of
positioning satellites and the high quality of firmware suppliedby GNSS receiver manufacturers around the world. Therefore,real-time kinematic (RTK) positioning in open sky
environments without structures that affect positioning is nolonger a problem. However, it is also well known that if thesurrounding environment deteriorates slightly due to thepresence of urban buildings, trees and other obstacles, theperformance of the RTK positioning drops markedly in thatarea and problems to be addressed are highlighted.

In this research, we affixed a low-cost handheld GNSS
receiver’s patch antenna to the wall of a building in an adverse
environment shielded to some extent by trees or otherstructures. RTK positioning was performed with the directionof the antenna receiving the radio waves from the satelliteoriented perpendicular in regard to the surface of the wall. An
innovative system that has been proposed for the first time
monitors the building’s shaking using information of
positioning epochs obtained at the centimeter level. In Japan,
which is a highly seismic country, the above system that can
detect the shaking of a building is extremely useful. This study
presents a system which can detect the fluctuation of theexisting structure as well as the building in construction siteand can lead to significant structural measures from theobtained data. In contrast, the conventional method requires
expensive measuring instruments and many personnel tomeasure the shaking and deformation of buildings. On theother hand, in our proposed method, only three elements,
namely power supply (lithium-ion battery, PC etc.), handheld
receiver, and antenna (shown in Fig. 1.), are required. The
equipment is installed on the wall of a building where
measurements need to be performed at a set hour (or longterm). The logged observation data is analyzed by using
software, and this approach has great advantages both in termsof personnel and cost.

2017 International Conference on Indoor Positioning and Indoor Navigation (IPIN), 18-21 September 2017, Sapporo, Japan
Moreover, previous studies such as the monitoring steel
tower’s displacements caused by the wind using RTK
positioning [1] and the monitoring the landslide caused by rainon the land near the river do not employ these systems in an
extreme environment. The aim of this research is to implement
the RTK monitoring system in this kind of environment to test
its efficiency and accuracy.
II. POST-PROCESS BY RTKLIB AND ROBUST RTK METHOD

Fig. 1. Rover equipment

Several challenges exist in realizing the above system. First,
since it is a receiver of single frequency, the restriction of thenumber of required satellites for RTK positioning is sometimes
severe. Therefore, it is very important whether the number ofsatellites required in the limited sky view is satisfied or not.
The time zone with the optimal satellite arrangement (BeiDousatellites in China tend to concentrate in the direction from thenortheast direction to the southwest direction in the sky above
Tokyo), dilution of precision caused by the geometricalarrangement of satellites, and the direction of the place where
the experiment is conducted in relation to the above items arealso factors that greatly influence the performance. Moreover,as compared with receivers compatible with dual frequencies,
since the solution (position information) of RTK obtained byreceivers compatible with single frequency has a characteristicthat reliability and availability are low, technologies that ensure
these requirements are also important.
Such systems will be easier to handle if the existence ofstructures that shield radio wave propagation paths (and hence
deteriorate the surrounding environment) are known in advanceand are available on the database. One way to cope with suchenvironmental obstacles is to optimally apply the elevation
mask and make it easy to receive more than 10 direct signals at
all times. With this kind of technique, the influence ofdiffracted wave signals and reflected wave signals (multipath
waves) from non-line-of-sight satellites can be reduced andepochs obtained at centimeter levels will increase. In theexisting research, we can point out the “GPS-Studio” made by
KOZO KEIKAKU ENGINEERING Inc. (Japan), which is aradio wave propagation simulation that uses both the ray
tracing and 3D map methods and provides precise positions ofbuildings, but it has not been widely put into practical use yet.
Accumulation of these kinds of environmental data throughtime provides an advanced arbitrary measurement using AI.
The easy-to-obtain $100 low-cost handheld receiver canachieve an accuracy of a few centimeters at many measurement
points, and it is possible to introduce several receivers at many
places. In this paper, we propose a low-cost RTK system,
which is not found in existing research, as a monitoring systemof building shaking, as a basic research toward practicalapplications.

A. Using RTKLIB
To process the raw data logged by the receiver, we used
RTKLIB, which is internationally famous as existing free
software, developed by Mr. Tomoji Takasu who is a visiting
researcher in our university. We compared the results of thisexperiment that seem to be best set by “Options” of
RTKPOST, which is a post-processing function in RTKLIB,
with the improvement results by CUI post-processing software
incorporating the proposed method.
Detailed setting in “Options” of RTKPOST is shown (inFig. 2.–Fig. 5.). Currently, it is considered that the favorable
results are obtained with this setting.

Fig. 2. Setting1

Fig. 3. Setting2

2017 International Conference on Indoor Positioning and Indoor Navigation (IPIN), 18-21 September 2017, Sapporo, Japan
C (N) = (N – N^)TWN(N – N^)(3)

where N is the minimized integer ambiguity in the costfunction, is the float solution ambiguities, and W is the inverseof its covariance matrix.
The LAMBDA method is a well-known and efficient integer
AR method based on the least-squares method [2] and wasused to determine the optimal integer ambiguities.
Ambiguity Validation Test

To validate the integer ambiguities determined by the
LAMBDA method, a test was performed. Several tests areavailable for ambiguity validation [4]. A ratio test based on
integer least-squares is a clean and effective method tovilification tests. In this study, the ratio test with a fixed value
is used for short base line RTK. According to the ratio test, theratio of difference from float ambiguity solution residuals
computed by cost function is compared.

Fig. 4. Stats

The signal strength mask is as shown in Fig. 5.

The ratio value of the ratio test is defined as
Ratio = C(N2) / C(N1) > Threshold(4)

Fig. 5. SNR Mask

The results of RTKLIB are shown in Section IV.
B. Robust RTK Method
This section briefly introduces the proposed RTK method
under severe environments.


Float Solution and Ambiguity Resolution



The RTK-GNSS uses a double difference (DD) code andphase observation. The DD measurements efficiently cancel
the receiver and satellite dependent biases. In addition, theatmospheric reflection errors that occur when two receivers
are close are so similar that they can be eliminated. The DD
observation models can be written as follows [3]:DDP = ( Psv1ref – Psv2ref ) – (Psv1rov – Psv2rov) = ρcode + εcode

where C(N1) is the cost function value of the best integer
vector and C(N2) is the cost function value of the second-best
integer vector. If this ratio is large enough, these ambiguities
are acceptable. The threshold value is determined from theempirical results. The ratio test threshold is usually set from 2
to 3 depending on the environment. In this study, the ratio test
threshold was set to 3.

(1)

DDφ = (φsv1ref – φsv2ref ) – (φsv1rov – φsv2rov) = ρphase + λN + εphase (2)where P and Φ are the code and phase observations,
respectively, ρ is the DD geometric distance, λ is the
wavelength, N is the DD integer ambiguity, and ε is the DD
measurement noise including the multipath error.
First, the DD observations are used to determine the 3D
position solution (i.e., float solution) and real number
ambiguity solutions at the same time. Then, the AR(Ambiguity Resolution) method is used to optimize the integer
ambiguity solution. The AR method searches for the integer
ambiguity that minimizes the cost function. The cost functionof the integer vector is given by

Robust RTK under severe condition

To enhance the RTK performance under a severe
condition, we need to modify the algorithm to search correct
ambiguities. In addition to that, selecting the satellites withgood quality is quite important. Generally, Kalman-filter
based RTK has been used for this purpose because we canreduce the noise of float solutions, and the signal strengthbased satellite selection is used. We use the similar satellite
selection method according to the suitable threshold of the
expected signal strength. However, we do not use the Kalmanfilter to reduce the error and noise of float solutions. In some
applications such as determination of structure deformation of
the building, the movement of the structure is usually very
slow and small. Even in the case of a bridge, the amount of
displacement will be less than 10 cm or more. In suchsituations, we can tighten the condition of constraint of theantenna movement according to the dynamics of the target
position. Furthermore, the initial position of the target is very
important and we can know the precise target position
beforehand. Even without the previous target position
information, once we resolve the reliable precise position, we
will use this information as an initial position. By doing so, theperformance of RTK-GNSS under very severe conditions likeas almost half masking of the sky view is improved
dramatically even compared with the geodetic-level GNSS
receiver that cost over $10,000.

2017 International Conference on Indoor Positioning and Indoor Navigation (IPIN), 18-21 September 2017, Sapporo, Japan
III. EXPERIMENTS
Over several days during April 2017, we conducted thepositioning experiments mentioned in Section I, on various
walls of a building at Tokyo University of Marine Science andTechnology for 12 hours. The equipment used in theexperiment, installation location, and installation status areshown below (Table I., Fig. 6.–Fig. 9.).
TABLE I.

EQUIPMENT USED

Fig. 8. Installation status (SE)Fig. 6. Installation location
Fig. 9. Installation status (NW)

Fig. 7. Installation status (SW)This particular experiment makes use of Trimble Inc., an
expensive antenna of the reference, but it is possible to replace
the antenna to a patch antenna to minimize the cost of the
project without the deterioration of the performance. For the
purpose of investigating building movement fluctuations, it isdesirable to be able to analyze data over a span of months or
years. However, for this experiment, we processed 12 hours ofdata. Post-processing of RTK positioning on software was
performed using three of the observation data of the reference
station obtained by positioning, the observation data of thereference station, and the navigation message. We will
investigate the long-term data over one month in the near
future.
IV. RESULTS AND EVALUATIONThe experimental results are shown in the following tables
and graphs: (Table II.-Table IV., Fig. 10.-Fig. 11.). The
standard deviation and the average represent the threedimensional gap from the true value, and the epoch number
represents the number of times RTK positioning was possible
during about 12 hours. The total number of epochs was 43572
in the southwest, 43386 in the southeast and 43676 in the2017 International Conference on Indoor Positioning and Indoor Navigation (IPIN), 18-21 September 2017, Sapporo, Japan
northwest. However, since it is important to judge whether thesolution decided as “Fix state” is a really correct solution, theitem of reliability (%) is also included in these tables.
The reliability of RTK is defined as follows:
Reliability = Number of Reliable solutions / Number of FIX solutions

RTK Fix Rate is defined as follows:
RTK Fix Rate = Number of FIX solutions / Number of all epochs

TABLE II.

THE RESULT OF SOUTHWEST

TABLE III.

THE RESULT OF SOUTHEAST

Fig. 10. Horizontal result (southwest)

TABLE IV.

THE RESULT OF NORTHWEST

Fig. 11. Horizontal result (southeast)

In any direction of these tests, it can be seen that the
proposed method was better than the results of RTKLIB. The
standard deviation and the average were both better by morethan one digit, and the number of epochs that have the RTK
flag set also increased. As for the reliability of the southwest

2017 International Conference on Indoor Positioning and Indoor Navigation (IPIN), 18-21 September 2017, Sapporo, Japanand southeast results with relatively good satellite
constellation because of several BeiDou satellites, thehorizontal results (within ±10 cm from the true value) and theheight result (within ± 20 cm from the true value) can beobtained with a high reliability at a rate of more than 99%. As
can be seen from the horizontal results, as the direction inwhich radio waves can be received and the satellite
constellation were biased, the fixed solutions were elongatedin the direction perpendicular to the surface of the wallcompared to the horizontal direction. On the other hand, in thecase of northwest, the fix rate was lower than the above twocases because of the shortage of the number of usable
satellites simply. With more visible satellite in the near future
like as QZSS and Galileo, the performance will be improved
more.

positioning of M8P was not good. Therefore, the effect of the
proposed method was also confirmed from this experiment.

Based on these test results, it will be possible to detect thevibration of the wall over at least 3-4 cm in the case of south
direction. In a high-rise building, the walls vibrate even more,
and when an earthquake occurs, large shakes can be detected,
and it is possible to know the occurrence time, the amplitudeof shaking precisely. These data are very important for theconstruction company.

Furthermore, the following horizontal results were thoseobtained when the antenna was manually moved on thesouthwest wall’s surface to simulate a shaking. Since it isdifficult to perform positioning experiments of big outdoor
shakings, we decided to manually move the antenna very close
to the wall. It was moved in the lateral and vertical direction
with respect to the wall’s surface, every minute, from left toright and from up to down.

To investigate the performance of the low-cost commercial
receiver with RTK, we took one hour data of NEO-M8P with
the newest firmware (u-blox) which is able to obtain thereceiver’s output (NMEA format) of low-cost RTK, andcompared them with the corresponding data obtained with theproposed method. Horizontal results were shown below (Fig.
12.).

Fig. 13. Quality flag

Fig. 14. Horizontal result (southwest)

Fig. 15. Height result (southwest)
Fig. 12. Horizontal result (southeast)

From the results, it was found that M8P couldn’t obtain afixed solution in this environment. In addition, Fig. 13. below
shows the quality flag in time series. It turned out that there
were almost no fixed solutions and most of them were float
solutions, and hence in this kind of severe environment, RTK

Above figures were the results of sampling at 5Hz and it isassumed that it shakes as it does when an earthquake occurs,in a short time and moving approximately 10 cm at a constant
speed to the left and right. From the elongation of thesolution, it can be seen that the fixed solutions can firmly
follow the direction of the antenna. The fix rate was almost

2017 International Conference on Indoor Positioning and Indoor Navigation (IPIN), 18-21 September 2017, Sapporo, Japan
100%. Even if an external dynamic force works under suchcircumstances, it can detect exactly several centimeters ofmotion, so it was found that it is a method that can be used formonitoring.
In conclusion, from the above results, in an environmentthat satisfies conditions such as the number of satellites andthe surroundings not being completely closed by buildings, it
is possible to monitor the deformations and shakings ofbuildings at low cost by using the proposed method.
V. SUMMARY AND FUTURE SUBJECTSIn this paper, we proposed a method of structure
deformations detection with the concept of low-cost handheld
RTK under severe environments. Although only one buildingat Tokyo University of Marine Science and Technology was
shown as example, we plan to include the following in our
research: experimental results of low-cost handheld receivers
on other buildings and skyscrapers in other areas, and methodsof improvement to obtain more accurate positions andapplications to demonstrate its real world applications.
The fix rate was as high as 80% or more in the southwest
and the southeast, and can be considered to be a practical level.
Regarding the direction of northwest, if the antenna issurrounded by buildings, securing the sufficient number ofsatellites is difficult. However, as the availability increases
due to the increase of the usable satellites in the near future, it
is highly expected that the performance will be improved
regardless of the direction.
Currently, as a post-processing step of RTK, “forward”
which is processing only from the front in time series, isregarded as real-time performance. We need to develop thereal-time software and it is necessary to obtain long-term datasuch as that for several months to put into this system topractical use.
REFERENCES[1] Akihito YOSHIDA, Yukio TAMURA, Masahiro MATSUI and Sotoshi
ISHIBASHI “MEASUREMENT OF WIND-INDUCED RESPONSE OF
BUILDINGS USING RTK-GPS AND INTEGRITY MONITORING”[2] Teunissen PJG “The least-squares ambiguity decorrelation adjustment: amethod for fast GPS integer ambiguity estimation” Journal of Geodesy
70.1-2.1995[3] Misra, P., and Enge, P. “Global Positioning System: Signals,
Measurements and Performance Second Edition”. Lincoln, MA: GangaJamuna Press, 2006.
[4] Verhagen, S. “The GNSS integer ambiguities: estimation and validation”
TU Delft, Delft University of Technology, 2005.