 Open Access
 Total Downloads : 55
 Authors : Wantong Chen , Xingli Sun
 Paper ID : IJERTV6IS120134
 Volume & Issue : Volume 06, Issue 12 (December 2017)
 Published (First Online): 26122017
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
A Novel QuasiOpen Loop Architecture for GNSS Vector Tracking Based on Collective Detection
Wantong Chen
Tianjin Key Lab for Advanced Signal Processing Civil Aviation University of China
Tianjin 300300, Peoples Republic of China
Xingli Sun
School of Information and Communication Engineering North University of China
Taiyuan 030051, Peoples Republic of China
Abstract In past several decades, the classical closed loop architectures, such as phase locked loops and delay locked loops, have been utilized for tracking global navigation satellite system (GNSS) signals. However, in harsh environments, such as high dynamic and weak signal applications, navigation with closed loop architectures is challenging. This paper proposes a novel architecture for tracking the frequency of the incoming GNSS signal which combines the good properties of both open loop and closed loop architectures. This novel architecture is based on the collective detection technique, which is shown to be able estimate the receiver position prior to entering vector tracking loops. Since collective detection combines the information from all satellites in view to enable a direct navigation solution, it is useful to provide a rapid initialization to the positiondomain navigation filter. The proposed architecture has a better robustness and accuracy.
Keywords GNSS; open loop; vector tracking; collective detection; weak signal navigation

INTRODUCTION
Global Navigation Satellite System (GNSS) is the general concept used to identify a system which allows user position computation based on a constellation of satellite such as GPS, Galileo and BDS. The user computes its position from measured distances between the receiver and a set of inview satellites. However, the relative motion of both satellites and the user causes a Doppler effect, which results in a large frequency shift in the carrier and in the code of the received signal [1]. Only an accurate tracking of the carrier frequency and Doppler shift allows the receiver to estimate the propagation time that transmitted signals take from each satellite to the receiver, thus enabling reliable estimates of position by trilateralization [2]. In any GNSS receiver, the acquisition stage provides an initial coarse estimation of the frequency shift, and then the subsequent tracking systems refine the coarse values of code phase and frequency and to keep track of these as the signal properties change over time. It indicates that the tracking stage contains two parts, code tracking and carrier frequency/phase tracking, which are generally implemented in the form of closed loops.
The code tracking is most often implemented as a delay lock loop (DLL) where three local codes (replicas) are generated and correlated with the incoming signal. These three replicas are referred to as the early, prompt, and late replica, respectively. The three codes are often separated by a halfchip length. The carrier wave tracking can be done in two
This project is supported by National Natural Science Foundation of China (Grant No. 61401468).
ways: either by tracking the phase of the signal or by tracking the frequency, that is, phase lock loops (PLL) and frequency lock loops (FLL), which track respectively the phase and the frequency of the incoming carrier [2]. All these traditional GNSS signal tracking schemes are based on the scalar strategy, in which the signal of each satellite is tracked separately. For this scheme, the implementation is the relative ease, but the relevance of the signals via the receiver position and velocity is completely ignored.
More recently, many studies have focused on vector tracking, which uses the receivers position and velocity estimates to close the tracking loops. An extensive body of knowledge has well documented vector trackings superiority under harsh environments such as indoor, foliage and deep urban canyons. Instead of the classic scalarbased DLL, PLL and FLL, the corresponding vectorbased delay lock loops (VDLL), vectorbased phase lock loops (VPLL) and vector based frequency lock loops (VFLL) have been employed in vectorbased GNSS receivers. Although a vectorbased tracking loop provides significant performance improvement over traditional techniques, the benefits of vector tracking loops are still hindered by the stability problems. This is because the measurement residuals are independently estimated by discriminators for each satellite signal and the navigation feedbacks are still utilized in the form of closed loops. It is well known that the measurement error from individual discriminators increase very rapidly with the increase in attenuation and interference, thus making the performance deteriorated [3]. Moreover, because of their closed loop structure, they need a long acquisition time before attaining the loop lock, and the combination of receiver clock instability and user dynamics have to be sufficiently low in order to maintain the coherence of the received signal.
In this paper we propose an alternative solution, based on a novel quasiopen loop architecture, which relieves the stress on the vector filter in terms of stability. In the proposed architecture, an open loop tracking strategy is combined with the vectortracking structure based on the maximum likelihood (ML) navigation solution. The benefit is that measurements are generated based on ML criterion, indicating that the performance of these measurements will asymptotically approach the CramerRao lower bound. The ML navigation solution is achieved by the collective detection technique, which projects the signal power from all correlators from all satellites into the navigation domain [4]. The final navigation solution is then obtained from the point in the navigation domain that has the most power. In this way, navigation sensitivity will be superior to conventional
navigation solution processor, even all signals are too weak to be acquired and tracked by the traditional framework.
The rest of this contribution is organized as follows. Section II provides a brief review on the closed loop architecture to signal acquisition and tracking, including both ScalarBased tracking loop and the VectorBased tracking loop. Section III introduces an open loop tracking strategy and the ML navigation solution. Section IV proposes the collective detection algorithm and its assumptions. Section V elaborates on the feasible open loop tracking scheme and the final quasiopen loop architecture. Section VI verifies the correctness of architecture for GNSS vector tracking and the accuracy of coarse position solution. Section VII concludes the overall contributions of this work.

THE CLOSED LOOP ARCHITECTURES

The ScalarBased Tracking Loop
The classical closed loop architectures, such as PLL, FLL and DLL, have been used for many years for GNSS signal tracking. These architectures are usually tracked on a satellite bysatellite basis, indicating that the signal of each satellite is acquired and tracked individually. As is shown in Figure 1, ScalarBased tracking loop consists of independent discriminator, loop filter and numerically controlled oscillator (NCO) for each channel.
Fig.1 The architecture of ScalarBased tracking loop
The instantaneous phase is estimated by the phase discriminator. The output of the discriminator, which is the phase error (or a function of the phase error), is then filtered by the loop filter and used as a feedback to the numerically controlled oscillator (NCO), which adjusts the frequency of the local signal (code or carrier). By this closedloop architecture, the local signal could be an almost precise replica of the input signal. As is well documented in many literatures, the loop filter is the most critical block, whose function is providing a degree of noise rejection and supporting the processing of higher order dynamics [5]. The filtered output can thus be used to drive the NCO for the signal generation. In a closed loop manner, the phase estimate of each channel is then progressively updated using the information provided by the new correlator output.

The VectorBased Tracking Loop
The traditional GNSS signal tracking is based on a scalar strategy, in which the signal of each satellite is tracked separately. The benefits of this architecture is the relative ease of implementation and a level of robustness that is gained by not having one tracking channel corrupt another tracking channel [6]. However, this scalarbased tracking ignores the fact that the signals are in essence related via the receiver
position and velocity. Instead of tracking each satellites signal separately, the VectorBased tracking in GNSS receivers is based on the idea that all signals are ultimately related to the position and velocity of the user antenna and thus can be tracked collectively.
Fig.2 The architecture of VectorBased tracking loop
As is shown in Figure 2, the code phase and Doppler of each signal are calculated by incorporating the current navigation solution (i.e., position, velocity, and time) and available ephemeris into a centralized Kalman filter and then used to drive the NCO that generate the local signals. Next, the tracking loops estimate the errors/residuals in the NCO, which are used to correct the measurements and update the navigation solution.
Code NCO
Code
Discriminator
Correlator 1
Loop Filter
Phase Discriminator
Code tracking loop centralized filter
Incoming IF Data
Carrier NCO
Code NCO
Phase Discriminator
Loop Filter
Correlator 2
Code
Discriminator
Carrier NCO
Phase
Discriminator
Loop Filter
Carrier NCO
Code NCO
Code
Discriminator
Correlator N
Fig.3 The architecture of VDLL
The benefit of this VectorBased tracking loop is that the tracking errors for low carriertonoise ratio satellites can be reduced by the good satellites whose signals are accurately tracked. Because the position and velocity of the receiver can be estimated by the good satellites and they can then be used to set the signal parameters for all satellites. However, in challenging applications, the benefits of the vector tracking loops are still hindered by insufficient signaltonoise ratio, resulting in stability problems in the local tracking loops [7]. This happens because an adequate number of good satellites must be available to assist the tracking of the channels with bad satellites. Due to the cascaded and feedback architecture, the navigation solution and tracking performance may be deteriorated by the more bad satellites. In the past two decades, many architectures have been reported such as
VDLL for code tracking and VFLL or VPLL for carrier tracking. Taking the VDLL as an example, individual delay lock loops are not present in it. Instead, a centralized filter is used to track the satellite signals, as is shown in Figure 3. The VDLL in general uses an extended Kalman filter (EKF) as the centralized filter for code tracking. The state vector of the EKF contains all the output of the discriminators as the elements of its state vector.


THE OPEN LOOP ARCHITECTURES
A.Open Loop Tracking Strategy
For the closed loop tracking strategy, the local tracking loops should be welldesigned to balance processing gain, dynamic range and stability. For the challenging applications, such as for weak signal tracking and extremely high dynamic applications, the loop design procedure becomes intricate. First, the integration time is limited by the databit transitions and good receiver clock performance is required over the integration interval. Second, the stability and robustness are still challenging in harsh environments, for example, it is a known fact that PLLs/VPLLs are vulnerable to fading effects, typically associated with cycle slips [8]. The use of open loop architectures can solve these problems, which operate on batches of the incoming signal. That is, an input signal batch is correlated with batches of a signal replica in order to obtain an entire 3D image of the signal, whose dimensions are the code shift, the Doppler shift, and the signal energy [9]. The open loop approach does not separate clear acquisition and tracking stages. Thus, there is no loop filter, which is replaced by the navigation solution.
B.The Maximum Likelihood Navigation Solution
The ML estimator jointly estimates receiver position and velocity, which is equivalent to a leastsquares estimate which minimizes the integral:
algorithm, namely SAGE (SpaceAlternating Generalized Expectation Maximization) has been investigated for the maximum likelihood position estimation in [13].
Note that open loop tracking strategy is in essence a batch processing strategy and both the pseudorange and Doppler measurements are generated based on maximum likelihood criterion. That is, measurements are generated based on the location of the strongest correlator output and navigation solution is computed from the point in the navigation domain that has the most power. Thus, based on the ML navigation solution, GNSS tracking loop can be designed in a navigation domain.
C.The ML Vector Tracking
By projecting the signal power from all correlators from all satellites into the navigation domain, vector tracking can be implemented based on ML navigation solution estimation.
It has been shown that the direct ML estimation of position and velocity can significantly improve weaksignal tracking capability [13]. Recently, collective detection is proposed as a direct position estimation method, which combines the received power from all GPS satellites in view onto a geographical search space in order to acquire weak signals, and as a byproduct, the ML position estimate is also provided [7]. Figure 4 demonstrates the detail architecture of maximum likelihood vector tracking (MLVT), in which the ML navigation solution takes outputs from correlator arrays and the residuals with respect to the previous solution is passed into the navigation filter. The signal parameters such as code phase and Doppler shift are then computed by the updated navigation solution.
IF signal
Open Loop Tracking
Correlator Array Output
T0 +T N 2
Fault Diagnosis
Navigation Filter
ML Navigation Solution
PVT
Residual
i i,n
r t m t a e jit i ,n dt
T0 i1
(1)
PVT
where r(t) is the complexvalued baseband received signal plus noise, N is the number of satellites observed, T0 is the beginning of the captured signal, T is the duration of the captured signal, and t is GPS time as generated by the receiver. The index i is a satellite index and n is a data bit index; mi(t) is the C/A code modulation normalized to unity magnitude and i is the radian/sec frequency; the signal amplitude and its phase of satellite i and bit n are denoted as ai,n and i,n, respectively. Here, the amplitudes and phases are receivergenerated baseband replica of a noiseless signal for satellite, which are considered to be nuisance parameters involved in minimizing (1) and the minimizer can be given in the equivalent maximization process as follows:
2
Fig.4 The architecture of MLVT

COLLECTIVE DETECTION AND ITS IMPLEMENTATION

Collective Detection
The purpose of acquisition is to determine visible satellites and coarse values of carrier frequency and code phase of the satellit signals. For a successful acquisition, a distinct peak appears in the correlogram bin that corresponds to the true GPS signals code delay and Doppler frequency. Different from the traditional scalarbased acquisition scheme, collective detection is in essence a vectorbased acquisition approach, which combines the received power from multiple satellites onto a geographical search space, and a navigation
N D

jit

(2)
domain correlogram is created. The grid point that has the
J r t mi t ai,ne dt
i1 n1 Ii ,n
where D is the number of the spanned data bits as received from satellite k and Ii ,n is the time interval containing the nth of the D data bits from satellite i. Note that the parameters
involved in the maximization, which are implicit in the
functions mi(t) and frequencies i, are the receiver position and receiver velocity [10]. The ML parameters estimation of
(3) is studied in many literatures [11, 12]. As optimization
highest combined correlation power denotes the ML navigation solution. Thus, the sensitivity will be superior to conventional navigation method and it applies to the weak signal applications such as indoor and urban canyon. Note that the original purpose is to acquire weak signal and the ML position estimation is provided as a byproduct [14]. However, for MLVT architecture, it will drive the open loop tracking. Figure 5 shows a basic principle of collective detection.
Geographical
Search Space
North[m]
rx u
rx u
where dti (tGPS i ) is the satellite clock bias at the GPS time (tGPS i ) . Thus, the received codephase at GPS time
Sig
East[m]
nal Parameter Estimation of SV1
GPS
t
rx
SVN
can be computed by the following equation:
SV2
D i tGPS = mod ti ,T
SV2
ation Power of SV1
Correl
(6)
SVN
rx
Noncoherent combination
where
D i tGPS
rx tx c
is the computed codephase and Tc is the
Doppler
Doppler
Doppler
period of the ranging code. Next, the received baseband
rx
frequency fi tGPS from satellite i is closely approximated by
Code Delay
Code Delay
Code Delay
i GPS
1 GPS GPS
GPS
(7)
Fig.5 The basic principle of collective detection
f trx
= v trx
vk trx
uk trx
In order to formulate a viable position domain search space, a certain position uncertainty should be given, which is
With the estimated codephase
D i tGPS
and the Doppler
rx
typically provided via AGPS, WiFi or cellular position methods [14]. That is, a priori knowledge, the approximate position of the receiver and the current GPS time are often required for the collective detection technique.
B.The Implementation of Collective Detection
Once the navigation solution is obtained by the receiver,
frequency fi tGPS , a local replica signal will be generated.
rx
The correlation power is then calculated by the multiplication of the locally generated signal and the sampled signal from the receivers RF front end. By combining the power from multiple satellites, the total correlation power is given by
N
the receiver clock can be corrected to be synchronous with
E k , k , k Ei k , k , k
(8)
GPS time and the clockbias could be ignored in a short span.
1 2 3
i1
1 2 3
For each candidate grid, the corresponding code phase and the Doppler frequency of each satellite are thus calculated as follows:
rx rx u rx rx u
rx
u
u tGPS s i (tGPS i ) T i u tGPS I i u tGPS c i (3) where u denotes the user position coordinate; t GPS is the GPS time for the received signal; i is the propagation delay
where N is the number of visible satellites; ki is the index of the grid in the different dimension of the geographical search space and Ei k , k , k is the correlation power related to satellite i for the grid k1, k2 , k3 . By traversing all candidate
1 2 3
grids, the point that has the highest combined correlation power is assumed as the user position. The grid with a distinct peak in the correlogram bin is giving by
rx u
1 2 3
from the satellite i to the candidate grid u ; s i (tGPS i ) is
k , k , k arg max
E k , k , k
(9)
the satellite position when the electric signal is sent from the satellite, which can be computed with the known GPS
1 2 3
k ,k ,k
Priori informationephemerides, receiver clock bias, approximate user position
N i
i1
1 2 3
ephemerides;
I i u
is ionosphere error and
T i u
is the
Given a candidate
With the approximate user position, define a geographical search space
Define the size of grid
troposphere error; c denotes the speed of light. Equation (3) can be solved iteratively by the dichotomy algorithm, see the reference [15]. However, the propagation delay can also be closely approximated by
u
i
s i (tGPS )u tGPS
rx rx
rx rx
c v tGPS e tGPS
(4)
Are all the grids traversed? N
Find the grid with a distinct peak in the correlogram bin
Y
Satellite index i=0
Compute the total correlation power
i=i+1
Combining the power from
where at the received GPS time
GPS
t
rx
the vectors
all satellites
s i (tGPS ) , v tGPS , e tGPS are respectively the position of
Are all the satellites Y traversed?
Convert this grid into latitude/longtitude/latitude
rx rx rx
satellite i, the velocity of satellite i, and the unit vector pointing from the most recent estimated receiver position to the position of satellite i. These vectors are computed from received satellite ephemeris data. Assuming that the
N
Generate local replica signal
Compute the code phase and Doppler
u
propagation delay i
is resolved, the signal transmitted time
Compute the correlation power
of satellite clock is given by
ti tGPS i dti (tGPS i )
(5)
tx rx u rx u
Fig.6 The flow chart of collective detection algorithms
Figure 6 shows the flow chart of the collective detection algorithm, which summarizing the steps given by (3) to (9). Given that a priori knowledge of satellite ephemeris and approximate user location is known, collective detection combines the correlator output of all satellite channels and projects them onto the positionclock space to enhance the overall signal detection probability [16].
C.Performance Assessment
Collective detection is an enhance a powerful approach to enhance the sensitivity of receiver, where the gains acquired can be leveraged on longer coherent/noncoherent integration periods, making it feasible even at about 20dBHz C/N0[17]. However, traditional collective detection techniques are computationally intense due to the normal fourdimension scale search space, say NorthEastDownClock bias [18].
In order to accelerate the traditional collective detection, a
the fine estimator to achieve an exact tracking. As depicted in Figure 7, in order to estimate the time delay and the carrier frequency from the incoming signal, the open loop schemes usually operate on batches of the incoming signal. Thus, this approach allows using longer integration time, and both code phase and Doppler measurements are generated based on the location of the strongest correlator output, indicating that the performance of the measurements asymptotically approaches the ramerRao lower bound. The loop stability problem can be well circumvented by the proposed open loop tracking strategy. Note that, there is no feedback loop in the open loop control, thus the key issue of the whole open loop control is How to obtain high precision tracking results from the input signal?
Carrier Frequency
practical strategy is to cut down the dimension of the search space. Since the clockbias dimension is much larger than the
Input Signal
Code Phase
Collective Detection
Local Signal Generator
Replica Signal y'(k)
other three dimensions, it is assumed to be the major factor
y(k)
Estimation
that increases the complexity of the collective detection algorithm [19]. Thus, a very finetime assistance could ease the computational load. For example, if successful position estimation has been done, the receiver clock can be corrected to GPS time and the clockbias is assumed insignificant in a short period of time. Or else, the estimation of receiver clock
Tracking Results
Priori Information Update
Coarse Navigation Navigation Solution Filter
Fine Estimator
PVT
Error
Integrate and Dump Filter
bias is too coarse but as least one satellite is strong enough to be acquired individually by the acquisition block. In this case, the reference [19] proposes one method to estimate the clockbias. The basic principle is that the clock bias can be computed by the difference between the measured code phase and the geometric codephase. The former can be extracted by the strong satellites correlogram and the latter is calculated by the computed geometric range.
Note that both the position accuracy and computational load are also determined by the step sizes or the density of grid partition. That is, the computational load decreases as the step size decreases, however, the accuracy of position also decreases. Hence, a balance should be achieved for the step size between the computing time and accuracy.


QUASIOPEN LOOP ARCHITECTURE
A. Open Loop Tracking With Collect Detection Strategy
In MLVT the navigation solution actually takes outputs from correlator arrays as measurements to directly detect a navigation solution with a maximum likelihood criterion. Based on the collective detection technique, the maximum likelihood navigation solution and the corresponding code phase and Doppler of each satellite are all provided. Although the direct navigation solution is coarse, the accuracy of navigation solution can be improved by a navigation filter and the output of navigation filter could be used to update the priori information for the next estimation. Both the code phase and Doppler are used to generate the local replica signal. That is, the local carrier replica and the local code replica, by comparing with the incoming signal, the discriminator compute the signal estimation error, which enters the following open loop tracking. Since the discriminator output is corrupted by thermal noise, the filter is also required to provide a degree of noise rejection, see the Integrate and Dump Filter unit in Figure 7. Next, the filtered error enters
Fig.7 Open loop architecture with collect detection strategy
B.The QuasiOpen Loop Vector Tracking Architecture
Although the precision of open loop tracking results can be improved by some mathematical approaches such as the weighted multiple regression estimation and the nonlinear least squares, the actual tracking precision is significantly worse than the vector tracking with closed loop tracking strategy. Thus, in this paper we propose a novel tracking scheme, based on a novel quasiopen loop architecture, which relieves the stress on the vector tracking in terms of stability in harsh environment and also keeps a high precision tracking under normal circumstances.
Figure 8 demonstrates the quasiopen loop vector tracking architecture, which combines the good properties of both open loop and closed loop architectures. The combined approach of the block processing and centralized vectorbased tracking can be utilized for robust indoor/outdoor navigation. In other words, when the receiver is under opensky conditions, the closed loop vector tracking is used; when the signal power drops, signal fading level increases, or the Kalman filter tracking loops have difficulty to keep lock, the open loop tracking is enabled. A contextaware approach can be used to rapid recognize the signal environment and optimize the processing load of the receiver and the measurement weighting [20].
As is depicted in Figure 8, for the open loop tracking mode, the incoming IF data will first enter the collective detection algorithms, in which the ML position estimation is given, by product, the code phase and carrier frequency are also provided to the local code NCO and carrier NCO. The local replica signal can thus be generated for each tracking channel.
IF
Signal
Channel 1 I1 Q1
2
I
Channel 2
Q2
Fault detection
Fault detection
Prefilter1
Innovation sequence detection
Prefilter2
Innovation sequence detection
1 f1
2 f2
Centralized Vector Tracking
H Filter
In
Channel N
Qn
PrefilterN
Fault Innovation
n
f
Position Velocity Time
detection
sequence detection
n
Closed Loop
VDLL VFLL
Open Loop
Code NCO
Carrier NCO
f
Closed Loop
Collective Detection
f
Maximum Likehood Navigation Solution
Priori Information
Update
Navigation Filter
PVT
Fig.8 The quasiopen loop vector tracking architecture
Next, by performing the multiplication between the local replica and the incoming signal for each channel, both the in phase (I) and quadrature (Q) correlation outputs can be calculated.
Different from the open loop tracking, the local code
system (INS). Considering the optimal information fusion, the proposed quasiopen loop vector tracking architecture is extended as follows.
ae,an,au Ve,Vn,Vu
NCO and carrier NCO are controlled by the increments of code and carrier frequency respectively, which are the
IMU
Channel 1 I1
SINS
Prefilter1
1
Centralized
outputs of VDLL and VFLL in the centralized vector tracking filter. That is, the feedbacks of navigation filter control the code NCO and carrier NCO to generate the local replica signal.
In the architecture above, the navigation and positioning
Q1
2
I
Channel 2
Q2
In
Fault detection
Fault detection
Innovation sequence detection
Prefilter2
Innovation sequence detection
f1
2 f2
Vector Tracking
H Filter
Position Velocity
function is realized by using a main filter and multiple
Channel N
Qn
Fault
PrefilterN
Innovation
n f
Time
parallel tracking prefilters. In the fault detection section of the
prefilter, the residual chi square test is utilized to detect the
detection
sequence detection
n
VDLL
VFLL
fast abrupt fault, and the slow variation fault of the system is detected by the following innovation sequence detection. The combinations of both detection methods greatly reduce the influence of channel faults on the data fusion and maintain a high precision tracking in the case of failures. Each channel is processed with a prefilter, and all the prefilters works in parallel prior to entering the main filter.
Note that, instead of a Kalman filter, the main filter is designed based on H infinity robust filtering, which can improve the robustness and guarantee the accuracy and real time. Thus, the proposed architecture is an inherently stable architecture, and this architecture can work with extended intgration times to improve the sensitivity. Also, to a certain extent, the dynamic can be compensated by VFLL.
C. GNSS/INS Tightly Coupled Vector Tracking Architecture
However, for high dynamic applications, the proposed architecture should be further improved. The compensation of the user dynamics is necessary. For example, the tracking loops can be assisted by a tightly coupled inertial navigation
Fig.9 Vector tracking architecture with tightly integrated system (The part that is not displayed is the same as Figure 8)
As is shown in Figure 9, the raw measurements of IMU (inertial measurement unit) enter all the prefilters and SINS and the outputs of all prefilters and SINS are input into the centralized vector tracking filter. The final navigation solution is used to assist the receiver tracking loop and make the correction on the SINS. The acceleration information provided by the inertial navigation can effectively improve the loop's equivalent bandwidth and the receiver's anti interference ability and reduce the error caused by dynamic stress. Thus, the proposed architecture has high accuracy, high stability and strong fault tolerance and robustness.

RESULTS AND DISCUSSION

The Position Domain Projected Correlogram
In a typical weak signal environment like deep urban canyon, individual acquisition correlograms cannot be used to decide on the codephase and Dopplerfrequency, since there is no a distinct peak, see Figure 10.
Fig.10 The correlogram for a weak GPS signal acqusitiuon in a deep urban canyon (IF Frequency:4.309MHz;Sampling Frequency:12MHz)
However, in collective detection process, the discrete correlation values, selected by calculating its codephase and Doppler based on the hypothesized position in the position domain, are noncoherently combined. By repeating all position domain search cell, the correlation surface is named the position domain projected correlogram, see Figure 11. Note that even if all signals are too weak to be acquired and tracked by the traditional approach, a quick and coarse position solution can be directly estimated by combining the received signal power from each satellite.
Fig.11 Position domain projected correlogram (The step sizes of both the north component and east component are 50m, using seven satellites and the priori height; the local receiver clock is synchronized with GPS time)
Fig.12 Top view of the position domain projected correlogram in Fig.11
Figure 12 demonstrates the top view of the position domain projected correlogram. The black center area indicates the sharpness of the peak.
Fig.13 Position domain projected correlogram (The step sizes of both the north component and east component are 50m, using three satellites and the priori height; the local receiver clock is synchronized with GPS time)
Fig.14 Top view of the position domain projected correlogram in Fig.12
In Figure 11 and Figure 12, the correlogram of seven satellites are combined. For comparison purposes, Figure 13 and Figure 14 demonstrate the combined correlogram with only three satellites, indicating that the position estimation can be achieved with poor visible satellites. It is also shown that the peak in Figure 11 is sharper than that in Figure 13 and the black center area in Figure 14 is larger than that in Figure 12, representing that the accuracy decreases as the number of satellites decreases.

Assessment of Proposed GNSS/INS Tightly Coupled Vector Tracking Architecture
In order to evaluate the proposed GNSS/INS tightly coupled vector tracking architecture, the simulation experiment has been performed based on the following scheme:
Trajectory
GPS Signal Simulator
IMU Signal Simulator
GPS IF Data
IMU Raw
Measurement
GPS Software Defined Receiver
Fig.15 The scheme of simulation experiment
The simulated trajectory is provided in Table 1 and initial position is (5ÂºN, 5 ÂºE, 5m) and the initial attitude is (0Âº,0Âº,0Âº).The moving direction of vehicle is north. The simulation parameters of IMU is given in Table 2. The initial alignment error of SINS is not taken into account. Both the GPS IF data and the raw measurements of IMU are entered into the GPS SoftwareDefined Receiver, in which the proposed vector tracking architecture is implemented, see Figure 9. The important parameters for the signal processing are

Sampling frequency: 12MHz

Intermediate frequency: 3.563 MHz, and

Sampling bits: One bit

Integration time: 10ms

Bandwidth of code loop:1Hz

Bandwidth of carrier loop:10Hz
TABLE I. TRAJECTORY SIMULATION
Time(s)
Simulation Parameters
Speed(m/s)
Acceleration(m/s2)
Jerk(m/s3)
0
0
0
0
035
0
0
0
3545
10
4555
100
0
5570
20
7077
200
0
7792
20
92102
100
0
102112
10
112125
0
0
For comparison purposes, the noise is assumed as colored in the simulation. The extended Kalman filter and H infinity robust filtering are both implemented in the centralized vector tracking filter. The error curve of position and velocity in X and Z directions are provided respectively, see Figure 16, 17, 18 and 19. The error curves of position and velocity in Y direction are similar with those in X direction.
TABLE II. SIMULATION PARAMETER OF IMU
Parameter
Gyro
Accelerometer
Constant
Drift (Âº/h)
Random
Drift(Âº/h)
Zero Offset
(m/s2)
Random
Drift(m/s2)
Accuracy
0.1
0.1
1e4
1e4
Fig.16 The position error in the X direction
Fig.17 The velocity error in the X direction
Fig.18 The position error in the Z direction
Fig.19 The velocity error in the Z direction
As is shown in the figures above, in the presence of colored noise, the position and velocity measuring errors of EKF filter divergence with time. However, for the H infinity filter, both the position and velocity measuring errors slightly worse compared to the EKF filtering. Hence, it can be concluded that H infinity filtering has very strong robustness and even if the input signal contains a constant and random drift and colored noise, the filter could eventually converge.


CONCLUSIONS
In this contribution, a novel quasiopen loop architecture has been proposed for robust tracking in harsh environments such as weak signal and high dynamic environments. The proposed architecture works with the combination of open loop tracking scheme and the closed loop tracking scheme. The open loop tracking is achieved based on ML navigation solution, which is provided by collective detection technique. The closed loop tracking is performed with VDLL plus VFLL architecture and both fault detection and H infinity filter are also utilized. The proposed architecture could provide a feasible way to GNSS/INS tightly coupled vector tracking. It indicates that th stability, robustness, dynamics and sensitivity can be well taken into account at the same time.
ACKNOWLEDGMENTS
This work is supported by the National Natural Science Foundation of China (Grant No. 61401468)
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