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Joint L-/C-Band Code and Carrier Phase Linear Combinations for Galileo
Linear code combinations have been considered for suppressing the ionospheric error. In the L-band, this leads to an increased noise floor. In a combined L- and C-band (5010–5030 MHz) approach, the ionosphere can be eliminated and the noise floor reduced at the same time. Furthermore, combinations that involve both code- and carrier-phase measurements are considered. A new L-band code-carrier combination with a wavelength of 3.215 meters and a noise level of 3.92 centimeters is found. The double difference integer ambiguities of this combination can be resolved by extending the system of equations with an ionosphere-free L-/C-band code combination. The probability of wrong fixing is reduced by several orders of magnitude when C-band measurements are included. Carrier smoothing can be used to further reduce the residual variance of the solution. The standard deviation is reduced by a factor 7.7 if C-band measurements are taken into account. These initial findings suggest that the combined use of L- and C-band measurements, as well as the combined code and phase processing are an attractive option for precise positioning.
The integer ambiguity resolution of carrier-phase measurements has been simplified by the consideration of linear combinations of measurements at multiple frequencies. Early methods were the three-carrier ambiguity resolution (TCAR) method introduced by Forssell et al. , as well as the cascade integer resolution (CIR) developed by Jung et al. . The weighting coefficients of three-frequency phase combinations are designed either to eliminate the ionosphere at the price of a rather small wavelength or to reduce the ionosphere only by a certain amount with the advantage of a larger wavelength.
The systematic search of all possible GPS L1-L2 widelane combinations has been performed by Cocard and Geiger . The L1-L2 linear combination of maximum wavelength ( m) amplifies the ionospheric error by a factor . Collins gives an overview of reduced ionosphere L1-L2 combinations with wavelengths up to cm ( widelane) in .
The authors have extended this work to three-frequency (3F) Galileo combinations (E1-E5a-E5b) in . A 3F widelane combination with a wavelength of m and an ionospheric suppression of dB was found. Furthermore, a 3F narrowlane combination with a wavelength of cm could reduce the ionospheric error by as much as dB. Sets of linear carrier-phase combinations that are robust against residual biases were studied in . The integer ambiguities of the linear combinations can be estimated by the least-squares ambiguity decorrelation adjustment (LAMBDA) algorithm developed by Teunissen . The method includes an integer transformation which can also be used to determine optimum sets of linear combinations .
In this paper, the authors used code- and carrier-phase measurements in the linear combinations for obtaining ionospheric elimination, large wavelengths, and a low noise level at the same time. The E5a and E5b code measurements are of special interest due to their large bandwidth ( MHz) and their low associated Cramer-Rao bound of cm . The C-band phase measurements are particularly interesting due to their small wavelength and their thus reduced phase noise. The properties of code-carrier linear combinations are optimized by including both L- and C-band measurements. The cost function is defined as the ratio of half the wavelength and the noise level of the ionosphere-free code-carrier combination. It is called combination discrimination and it is a measure of the radius of the decision regions expressed in units given by the standard deviation of the noise. The L-Band signals of Galileo are defined in the Galileo-ICD . The C-band signals are foreseen in a band between MHz to MHz . The signal propagation and tracking characteristics in the C-band have been analyzed by Irsigler et al. . The larger frequencies result in an additional free space loss of dB that has to be compensated by a larger transmit power.
The paper is organized as follows: the next section introduces the design of code-carrier linear combinations. The underlying trade-off between a low noise level and strong ionospheric reduction turns out to be controlled by the weighting coefficients of E5a/E5b code measurements.
In Section 3, code-carrier linear combinations are computed in a way that include both L- and C-band measurements. An ionosphere-free code-only combination is determined that benefits from a 4.5 times lower noise level than a pure L-band combination. Furthermore, a pure L-band ionosphere-free code-carrier combination with a wavelength of 3.215 m and a noise standard deviation of 3.9 cm is found. The combined use of the two reduces the probability of wrong fixing of the latter solution by 9 orders of magnitude with respect to a pure L-band solution.
The use of C-band measurements for ionosphere-free carrier smoothing is discussed in Section 4: an ionosphere-free code-carrier combination of arbitrary wavelength is smoothed by a pure phase combination. The low noise level of C-band measurements provides a linear combination that benefits from an dB lower noise level as compared to the equivalent L-band combination.
2. Code-Carrier Linear Combinations
Linear combinations of carrier-phase measurements are constructed to increase the wavelength (widelane), suppress the ionospheric error, and to simplify the integer ambiguity resolution. The properties of the linear combinations can be improved by including weighted code measurements into the pure phase combinations. Figure 1 shows a three frequency (3F) linear combination where the phase measurements are weighted by , , , and the code measurements are scaled by , , . The weighting coefficients are generally restricted by a few conditions: first, the geometry should be preserved, that is Moreover, the superposition of ambiguities should be an integer multiple of a common wavelength , that is which can be split into three sufficient conditionswith denoting the space of integers. These integer constraints are rewritten to obtain the weighting coefficients
Mixed code-carrier combinations weight the phase part by and the code part by . The border cases are pure phase () and pure code () combinations. The parameter has a significant impact on the properties of the linear combination, and it is optimized later in this section. Replacing the weighting coefficients in by (4) yields the wavelength of the code-carrier combinationThe generalized widelane criterion is given for by . Equivalently, it can be expressed as a function of , and asThe linear combination scales the ionospheric error byThe thermal noise of the elementary carrier phase measurements is assumed Gaussian with the standard deviation given by Kaplan and Hegarty where denotes the loop bandwidth, the carrier-to-noise ratio, and the predetection integration time. The overall noise contribution of the linear combination is written aswith being the noise variance of the code measurements. Table 1 shows the Cramer-Rao bound (CRB) for some Galileo signals as derived by Hein et al. . A DLL bandwidth of Hz has been assumed. The 4 MHz receiver bandwidth for E1 has been chosen to avoid sidelobe tracking.
For E1, E5a, E5b, E6 phase measurements, the wavelength scaling of can be neglected due to the close vicinity of the frequency bands. However, it plays a major role when C-Band measurements are included.
Figure 2 shows the benefit of the code contribution to the (E1), (E5b), (E5a) linear combination: a slight increase in noise level results in a considerable reduction of the ionospheric error. The phase weighting has been fixed to so that , , , and are uniquely determined. The E5b and E5a code weights are adapted continuously and the ionosphere is eliminated in the border caseE1 code measurements have not been taken into account due to the increased noise level but might be included with a low weight.
The combination discrimination—measured by the ratio of half the wavelength and the noise level —is proposed as a cost function to select linear combinations due to its independence of the geometry. It is shown for multiple ionosphere-free code-carrier combinations in Figure 3. The strong dependency on the phase weighting suggests an optimization with respect to this parameter. Note that the legend refers to the elementary wavelengths which have to be scaled by .
The computation of the optimum takes again only E5a/ E5b code measurements into account as the benefit of the E1 code measurement is negligible (). The notation is simplified bywith , and implicitly given by (5), (7), and (9). The E5a/E5b code weights are determined from the ionosphere-free and geometry-preserving conditions as
The combination discrimination becomes from (5), (11), and (12): Setting the derivative to zero yields the optimum weightingwhich is independent of both and . Table 2 contains the weighting coefficients and characteristics of the code-carrier combinations shown in Figure 3.
Figure 4 shows the benefit of adaptive code and phase weighting for the code-carrier linear combination with (E1), (E5b), and (E5a). Obviously, the wavelength increases linearly with and the ionosphere can be eliminated with any . The noise amplification depends on the level of ionospheric reduction: a linear increase can be observed near the pure phase combination, while the increase becomes negligible for the ionosphere-free combination. Thus, the combination discrimination of the ionosphere-free code-carrier combination is increased by almost the same factor as is risen.
3. C-Band Aided Code-Carrier Linear Combinations
The MHz wide C-Band () has been reserved for Galileo. The higher frequency range has a multitude of advantages and drawbacks: an additional free space loss of dB occurs which has to be compensated by a larger transmit power. The ionospheric delay is approximately times lower than in the E1 band. The small wavelength of complicates direct ambiguity resolution but results in an approximately times lower standard deviation of phase noise. Moreover, the C-Band offers additional degrees of freedom for the design of linear combinations.
3.1. Reduced Noise Ionosphere-Free Code-Only Combinations
The design of three frequency code-only combinations that preserve geometry and eliminate ionospheric errors is characterized by one degree of freedom used for noise minimization. The weighting coefficients are derived from the geometry preserving and ionosphere-free constraints in (1), (7) asMinimization of yieldsIonosphere-free code-only combinations with more than three frequencies are obtained by a multidimensional derivative. Table 3 shows that the pure L-band E1-E5b-E5a combination is characterized by a noise level of . If the E5 signal is received with full bandwidth, the CRB is reduced to cm but the number of degrees of freedom is reduced by one so that the noise level of the E1-E5 combinations are lightly increased (Table 4). These combinations will play a role in conjunction with code-carrier combinations.
The C-band is split into bands of MHz bandwidth centered at and allows a significant reduction of the noise level. Note that the noise of any contributing elementary combination is reduced by a weighting coefficient smaller than one.
3.2. Joint L-/C-Band Widelane Combinations
Code-carrier linear combinations can also include both L-band and C-band measurements. Therefore, (1)–(7) are extended to include the additional measurements. The weighting coefficients and are computed such that the discriminator output of (13) is maximized for a given set of integer coefficients .
Table 5 contains ionosphere-free joint L-/C-band code-carrier widelane combinations The E1-E5 pure L-band combination benefits from a noise level of only which simplifies the resolution of the integer ambiguities. In contrast to the code-only combinations, the use of the full-bandwidth E5 signal is advantageous compared to separate E5a and E5b measurements. The C-band offers no benefit for these wavelengths. In the last row of Table 5, a linear combination with a pure L-band code and pure C-band phase part is described. The combination discrimination equals but the noise level is also increased to .
Figure 5 shows the tradeoff between wavelength and noise level for joint L-/C-band ionosphere-free linear code-carrier combinations with and . The E1-E5 combination is of special interest but the maximum combination discrimination is obtained for a joint L-/C-band combination.
3.3. Joint L-/C-Band Narrowlane Combinations
There exists a large variety of joint code-carrier narrowlane combinations where C-band measurements help to reduce the noise substantially. Figure 6 shows the tradeoff between wavelength and noise level for and . For , the consideration of C-band measurements reduces the noise level by a factor of compared to a pure L-band combination (Table 6).
3.4. Reliability of Ambiguity Resolution
The integer ambiguity resolution is based on the linear combination of four different variable types: double-difference measurements for eliminating clock errors and satellite/receiver biases; multifrequency combinations for suppressing the ionosphere; code and carrier phase measurements for reducing the noise level; and finally, L-/C-band combinations for noise and discrimination characteristics.
Two joint L-/C-band code-carrier ionosphere-free combinations are chosen for real-time (single epoch) ambiguity resolution. The , combination of Table 5 and one further combination of Table 4. The double difference (DD) ionosphere-free combinations are modeled aswithand the DD geometry matrix , the baseline and the integer ambiguities . The double-differenced troposphere is assumed to be negligible or known a priori (e.g., from an accurate continued fraction model).
Note that the troposphere has the same impact on all geometry-preserving combinations and does not affect the optimization of the mixed code-carrier combinations. The noise vector is Gaussian distributed, that is,where denotes the Kronecker product. models the linear combination induced correlation and includes the correlation due to double difference measurements from visible satellites. The standard deviation of the most critical ambiguity estimate can be written asand the probability of wrong fixing follows asIn the following analysis, the location of the reference station is at N, E with a baseline length of km.
Figure 7 shows the benefit of C-band measurements for integer ambiguity fixing. If the E1-E5 pure L-band combination is used as second combination in (17), the failure rate varies between and due to its poor noise characteristics. The use of two additional C-band measurements reduces the maximum probability of wrong fixing to . For three C-band frequencies, the failure rate is at most which corresponds to a gain of to orders of magnitude compared to the pure L-band combination.
The reliability of ambiguity resolution can be further improved by using the LAMBDA method of Teunissen . The float ambiguity estimates are decorrelated by an integer transformation and the ambiguity covariance matrix is written as with the decomposition into a lower triangular matrix and a diagonal matrix . The probability of wrong fixing of the sequential bootstrapping estimator is given by Teunissen  aswith . It represents a lower bound for the success rate of the integer least-square estimator and is depicted in Figure 8. Obviously, the use of joint L-/C-band linear combinations reduces the probability of wrong fixing by several orders of magnitude compared to pure L-band combinations.
3.5. Accuracy of Baseline Estimation
After integer ambiguity fixing, the baseline is re-estimated from (17). The covariance matrix of the baseline estimate in local coordinates is given bywith the rotation matrix . Figure 9 shows the achievable horizontal and vertical accuracies for the two optimized joint L-/C-band combinations.
The pure L-band combinations in the first row of Tables 4 and 5 have been again selected as reference scenario. It can be observed that the use of joint L-/C-band linear combinations enables a slight improvement in position estimates compared to the significant benefit for ambiguity resolution.
4. Joint L-/C-Band Carrier Smoothed Carrier
Ionosphere-free code-carrier linear combinations are characterized by a noise level that is one to two orders of magnitude larger than of the underlying carrier-phase measurements (Table 5). Both noise and multipath of the code-carrier combinations can be reduced by the smoothing filter of Hatch  which is shown in Figure 10. The upper input can be an ionosphere-free code-carrier combination of arbitrary wavelength. The lower input is a pure ionosphere-free phase combination that is determined by three conditions: the first ensures that the geometry is preserved, the second eliminates the ionosphere, and the third minimizes the noise, that is,
Note that the superposition of ambiguities of the pure phase combination is not necessarily an integer number of a common wavelength. The respective ambiguities are not affected by the low pass filter and do not occur in the smoothed output due to different signs in the addition to (Figure 10).
Table 7 shows an ionosphere-free E1-E5a-E5b phase combination that increases the noise level by a factor . However, the low noise level of C-band measurements suggests the use of the second combination with . In this case, the noise level is not only reduced by smoothing but also by the coefficients of the pure phase combination.
The variance of the smoothed combination is given bywith the low-pass filtered noise (e.g., Konno et al. )and the smoothing time . Setting (27) into (26), and using the definition of a geometric series yields For long smoothing times, only the low noise of the joint L/C pure carrier-phase combination remains (Table 7).
In this paper, new joint L-/C-band linear combinations that include both code- and carrier-phase measurements have been determined. The weighting coefficients are selected such that the ratio between wavelength and noise level is maximized. An ionosphere-free L-band combination (IFL) could be found at a wavelength of m with a noise level of cm.
The combination of L- and C-band measurements reduces the noise level of ionosphere-free code-only combinations by a factor compared to pure L-band combinations. This increases the reliability of an ambiguity resolution option for the IFL combination by 9 orders of magnitude.
The residual variance of the noise can be further reduced by smoothing. An L-/C-band carrier combination can smooth the noise with a residual variance below the L-band phase noise variance. The smoothed solution can either be used directly or can be used to resolve the narrowlane ambiguities. The variance is basically the same in both cases. The resolved ambiguities, however, provide instantaneous independent solutions.
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