availability of the third civil frequency
has obvious advantages to instantaneous
carrier phase accuracy and ambiguity resolution
for cm level measurements in the short base-line
will expand the area of positioning service and
will increase its convenience. Particularly, the
third civil frequency namely L5, which will be
provided by both modernized GPS and QZSS, is expected
to improve the performance of precise positioning
using carrier phase observation. This paper reports
an experimental result regarding the convenience
of additional L5 signal. Actual navigation signals
generated from GPS simulator were received by
triple frequency GPS receiver in the experiment.
Navigation data, pseudo-range and carrier phase
observation have been obtained and used for the
analysis. This experiment verifies the improvement
of ambiguity fixing time and multi-path mitigation
using three frequency signals.
A GPS simulator
designed for the modernized three GPS signals
and a GPS/ QZSS receiver developed for the high
accuracy positioning experiment were used for
this experiment. These equipments have been developed
by Japan Aerospace Exploration Agency (JAXA) as
a part of QZSS navigation test bed. Equipments
used for this experiment are listed in table 1
with their specifications. Figure 1 shows the
data flow of this experiment. A cesium clock was
employed to provide stable external reference
for GPS simulator in order to generate GPS-like
high quality navigation signals. The data analysis
software was developed by Tokyo University of
Marine and Science Technology. DGPS and RTK solution
as well as usual navigation solution can be obtained
with this software. It is also possible to evaluate
important parameters for precise positioning such
as ambiguity fix rate using this software. Data
generation by the simulator and the data recording
by the receiver have been performed in real time
while data analysis was performed off-line.
of the high accuracy positioning with triple frequencies
was investigated by using pseudo-range and carrier
phase measurements. The base-line between reference
station and rover station is set about 3.7km because
we want to assume that the effects of the atmosphere
can be disregarded. Particularly, the effect of
L5 frequency in the ambiguity resolution was investigated
in the specular multipath condition. The algorithm
used in this test is general code and carrier
DGPS using double difference. Table 2 shows the
details of the data. Table 3 also shows the details
of the scenarios. All of the parameters were set
GPS simulator. The reason why there are two rovers
is to analyze the case of the different multipath
delay. Since the band of L5 frequency has high
chip rate compared with L1 frequency, the improvement
in accuracy is expected in the case that the multipath
delay is over 20 m. Figure 2 shows the configuration
between the antenna and the wall in the case of
large multipath error respectively.
|Algorithm of precise
algorithm to analyze observation data is described
in this section. The overall flow of analysis
is as follows.
1) Read observation data from reference and rover
2) Cycle slip detection 3) Calculation of satellite
4) DGPS and RTK
DGPS was carried out respectively by using L1
pseudo-range and L5 pseudorange. RTK was carried
out by using general double difference data. Since
there are triple frequency observation data, ambiguity
resolution was carried out by using the combination
of L1- L2 (wide-lane) and L2-L5 (extrawide- lane).
The reason why we choose these combinations is
that the noise level is not so large and the effect
of ionosphere is same order compared with L1.
Although this linear combination technique in
order to resolve ambiguities is not the best way,
the object in this paper is to analyze the effects
of the three frequencies in the multipath condition.
So the conventional method in order to resolve
ambiguities is used in this paper. Ambiguity resolution
was initialized when the cycle slip was detected
in each frequency. In this experiment, cycle slip
due to the band of L2 was
dominant. This is because that the power of L2
by GPS simulator was set lower than the power
of L1 and L5.The measurement errors of L1, L2
and L5 carrier phase is assumed 0.5~1cm.
The flow of ambiguity resolution algorithm is
1) DGPS position estimate using non smoothed pseudo-range
2) Initial estimate of the ambiguity
3) Ambiguity search (position estimate using all
4) Rejection of candidates due to criteria in
both measurements and positioning domains (statistical
5) After the one candidate is retained, receiver
position is computed with each ambiguity candidate
The algorithm for wide-lane is same as the algorithm
for extra-wide-lane. Table 4 shows the summary
of the parameters in the ambiguity resolution.
results are shown in the following sections.
shows the horizontal positioning error of the
base-line between the rover and the reference
station for three different multi-path scenarios.
Each figure contains the errors for both L1 and
L5 DGPS results. In case of no multipath environment,
the horizontal positioning error is less than
Both Fig.3 and Fig.4 are the experimental results
for scenarios in which the distance between the
antenna and the wall is 20 m. Although the distance
between the antenna and the wall is identical
in both cases, the difference of obstruction condition
makes a significantly different result. The positioning
error for scenario sc1m is smaller than that of
scenario sc1b since the obstruction condition
for the latter is worse than the former. However,
if we employ L5 measurements, the accuracy degradation
was improved by 50% or more (Fig.3 and Fig.4).
The Fig.3 and Fig.5 are the result of the same
obstruction condition. The difference of the both
figures is based on the different distance of
an antenna and a wall. In Fig.3, a reflected signal
is delayed 20 to 30 m. On the other hand, the
delay of reflected signal is within 20 m in Fig.5.
Therefore, even if we used L5 signal, which chip
rate is high, the multipath errors couldn’t
be reduced in scenario sc2b.
And all results of positioning error in multipath
environment are significantly worse than that
of usual DGPS with no multipath environment. For
comparison, the relationship between the delay
of reflected signal and the multipath error is
shown in Fig.6 (reflection loss is -6dB), where
the receiver bandwidth is 20 MHz. In these experiments,
a GPS receiver equivalent to 0.1 chip narrow correlator
results using carrier phase
section, we show the positioning results using
carrier phase observation for both latitude and
longitude error components of baseline vector
between the reference station and the rover in
two scenarios (sc1b and sc2m). Fig.7 and Fig.8
are the results for scenario sc1b, which are obtained
by wide-lane (WL) and extrawide- lane (EWL), respectively.
In the ambiguity resolution, WL utilizes the L1
DGPS positioning result for ambiguity search and
EWL utilizes the L5 DGPS positioning result. Fig.7-Fig10
shows that the experimental results using L5 signal
are slightly noisier. But the ambiguity fixing
success rate is very high in L5 case evidently.
In this paper, WL ambiguity resolution based on
the EWL was not carried out. However, it will
be easy for us to improve the success rate by
using the results of EWL in the WL ambiguity resolution.
Moreover, in case of using EWL, the most of ambiguity
resolution were determined at 1 epoch. These results
mean that L5 signal is really effective for a
precise positioning, especially in the urban area.
shows the number of successful ambiguity fix and
its success rate, for both WL and EWL, respectively.
In this table, the right side number shows the
number of times with five or more visible satellites.
Based on this table, in case of scenario sc1b,
in which multipath effect is large, the ambiguity
fixing success rate is only 25% and even in case
of scenario sc2m, in which mutlipath effect is
medium, that rate is 69%. On the other hand, when
using EWL, the rate of that is 95% or more. Therefore,
it is confirmed that EWL in ambiguity resolution
introduced the effectiveness of new L5 frequency
for precise positioning based on the experiment
using three frequency GPS simulator and three
frequency QZSS/GPS proto-type receiver. Data analysis
result shows that the new L5 signal contributes
to the improvement of positioning accuracy for
DGPS and RTK under the severe multipath environment.
It is also verified that the ambiguity fix rate
of RTK is significantly improved.
|Authors wish to acknowledge
the contributions provided by
Furuno Electric to this study.
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