P2 Coding Research Articles

Tsinghua Scientific Satellite Precise Orbit Determination Using Onboard GNSS Observations with Antenna Center Modeling

The Tsinghua scientific satellite is a Chinese spherical micro satellite for Earth gravity and atmospheric scientific measurements. The accurate orbits of this satellite are the prerequisites to satisfy the mission objectives. A commercial off-the-shelf dual-frequency GNSS receiver is equipped on the satellite for precise orbit determination (POD). The in-flight performances of the receiver are assessed. Regular long-duration gaps up to 50 min are observed in GNSS data, and the typical data availability is about 60–70% each day. The RMS of code noises is 0.24 m and 0.30 m for C1 and P2 codes, respectively. The RMS of fitting residuals of the carrier phase geometry-free L1–L2 combination is 2.4 mm. The GNSS receiver antenna center offsets (ACOs) and antenna center variations (ACVs) maps are estimated using in-flight data for both dual-frequency and single-frequency POD. Significant improvements in POD performances are obtained when the measurement models are updated by using the ACO and ACV maps’ corrections. With the updated measurement model, the RMS of the orbit overlap differences is 1.23 cm in three dimensions for dual-frequency POD, which is reduced by 27%. Meanwhile, two different empirical acceleration types are employed and compared for dual-frequency POD, and the results show that consistency on the 5 cm level is demonstrated for orbit solutions obtained with the updated measurement model. After correcting the ACO and ACV maps, the precision of single-frequency orbit solutions is better than 10 cm, which is improved by 32%. The results indicate that the antenna center modeling can significantly improve the consistency of Tsinghua scientific satellite precise orbits, which will be conducive to the realization of the mission objectives.

In-flight performance analysis and antenna phase center calibration of MEMS GPS receiver on-board TianQin-1 in the nadir-pointing and Sun-pointing modes

The TianQin-1 satellite, which was launched on 20 December 2019 into a Sun-synchronous orbit with an altitude of 628 km, is the first technology demonstration satellite for the TianQin project. A micro-electro-mechanical system (MEMS) global positioning system (GPS) receiver is installed on the satellite for precise orbit determination (POD). In addition to the conventional nadir-pointing mode, a specific Sun-pointing mode was used by the satellite owing to the limitation of system resources. The in-flight performance of the MEMS GPS receiver was assessed. The average numbers of available satellites for dual-frequency navigation in the nadir-pointing and Sun-pointing modes are 7 and 5, respectively. The qualities of the P2 code and carrier phase observations are basically equal for both modes. The average root mean square of the multipath errors for C1 observations in the Sun-pointing mode is 0.29 m, slightly larger than that in the nadir-pointing mode. In the Sun-pointing mode, the body of the TianQin-1 satellite rotates around the Sun-pointing axis, which may provide favourable geometric conditions for the estimation of the antenna phase center offset (PCO) corrections in the X, Y, and Z directions. Its feasibility is demonstrated using the simulated data. The GPS measurements of TianQin-1 are then used to estimate the PCO corrections in all three directions while in the Sun-pointing mode and in the Z direction while in the nadir-pointing mode. Satellite laser ranging validation shows that the orbit accuracy of TianQin-1 is approximately 5.90 cm in the nadir-pointing mode. In the Sun-pointing mode, the three-dimensional position difference of 5-h overlap decreased from 8.87 cm to 8.73 cm when the antenna PCO corrections were used in the POD.

Intrapulse modulation type recognition for pulse compression radar signal

The existing modulation recognition algorithms for a pulse compression radar (PCR) signal can hardly adapt to complex modulation types and low signal-to-noise ratio (SNR). To solve the problems, with respect to the seven kinds of widely used PCR signals—including linear frequency modulation signal, Baker code, Frank code, P1 code, P2 code, P3 code, and P4 code—a modulation type recognition algorithm based on integrated quadratic phase function (IQPF) and fractional Fourier transform (FrFT) is proposed. First, signals are preclassified according to their chirp rates (CRs) estimated through IQPF. Then, FrFT is carried out depending on the order, which is correlated to the estimated CR. Finally, signals in each class are subdivided and modulation recognition is accomplished according to the features of the FrFT spectrum. The simulation results validate the feasibility of the algorithm. They also demonstrate that, compared against existing research, the proposal achieves better correct recognition performance for various modulation types under low SNR condition.

Analysis of the Ionospheric Effect for Time Offsets per GPS Code Measurements

This paper shows the analysis results of time offsets using P1, P2 and P3 code in GPS time transfer. The r2cggtts program has been used in GNSS Time Transfer and use the P3 ionosphere free combination of P1 and P2 code measurements. We modified the r2cggtts program in order to calculate and write all kinds of time offsets P3 as well as P1 and P2. The result of the modified r2cggtts program shows that time offsets using P1 and P2 are of wide distribution comparing to time offsets using P3. In order to Figure out the error factors for wide distributions of P1 and P2, we analyze and present the relation between the elevation angle and time offsets in this paper.

Performance of the L2C civil GPS signal under various ionospheric scintillation effects

As GPS is modernizing, there are currently fourteen satellites transmitting L2C civil code and seven satellites transmitting L5 signal. While the GPS observables are subject to several sources of errors, the ionosphere is one of the largest error sources affecting GPS signals. Small irregularities in the electrons density along the GPS radio signal propagation path cause ionospheric scintillation that is characterized by rapid fluctuations in the signal amplitude and phase. The ionospheric scintillation effects are stronger in equatorial and high-latitude geomagnetic latitude regions and occur mainly due to equatorial anomaly and solar storms. Several researchers have analyzed the L2C signal quality since becoming available in December, 2005. We analyze the performance of L2C using GPS data from stations in the equatorial region of Brazil, which is subject of weak, moderate and strong ionospheric scintillation conditions. The GPS data were collected by Septentrio PolaRxS---PRO receivers as part of the CIGALA/CALIBRA network. The analysis was performed as a function of scintillations indexes S4 and Phi60, lock time (time interval in seconds that the carrier phase is tracked continuously without cycle slips), multipath RMS and position variation of precise point positioning solutions. The analysis shows that L2C code solutions are less affected by multipath effects than that of P2 when data are collected under weak ionospheric scintillation effects. In terms of analysis of positions, the kinematic PPP results using L2C instead P2 codes show accuracy improvements up to 33 % in periods of weak or strong ionospheric scintillation. When combining phase and code collected under weak scintillation effects, the results by applying L2C against P2 provide improvement in accuracy up to 59 %. However, for data under strong scintillation effects, the use of L2C for PPP with code and phase does not provide improvements in the positioning accuracy.

Accurate simulations of TEPC neutron spectra using Geant4

Abstract A Geant4 model of a tissue-equivalent proportional counter (TEPC) has been developed in which the calculated output spectrum exhibits unparalleled agreement with experiment for monoenergetic neutron fields at several energies below 20 MeV. The model uses the standard release of the Geant4 9.6 p2 code, but with a non-standard neutron cross section file as provided by Mendoza et al., and with the environment variable options recommended by the same authors. This configuration was found to produce significant improvements in the alpha-dominated region of the calculated response. In this paper, these improvements are presented, and the post-processing required to convert deposited energy into the number of ion pairs (which is the quantity actually measured experimentally) is discussed.

New Piecewise Linear Polyphase Sequences Based on a Spectral Domain Synthesis

A new class of piecewise linear polyphase codes of perfect square length, named PAT codes, is introduced. PAT codes derive from an innovative design procedure and present excellent performance in terms of both merit factor and peak to side-peak ratio with respect to previously proposed sequences such as Frank, P1, P2, and Px codes. In particular, for odd lengths, the merit factor of PAT codes is larger than that of all other polyphase sequences under analysis, and the peak to side-peak ratio is as large as that of the best of these sequences (namely the Frank and P1 codes). For even lengths, the merit factor and the peak to side-peak ratio performance of PAT codes coincide with those of Px and P2 codes, which are the best polyphase sequences under analysis. PAT code synthesis is based on a rigorous analytical method in the code spectral domain and leads to a simple recursive formula for the generation of sequences of arbitrary length.

On stochastic modeling of the modernized global positioning system (GPS) L2C signal

In order to take full advantage of the modernized GPS L2C signal, it is essential that its stochastic characteristics and code bias be rigorously determined. In this paper, long sessions of GPS measurements are used to study the stochastic characteristics of the modernized GPS L2C signal. As a byproduct, the stochastic characteristics of the legacy GPS signals, namely C/A and P2 codes, are also determined, which are used to verify the developed stochastic model of the modernized signal. The differential code biases between P2 and C2, DCBP2-C2, are also estimated using the Bernese GPS software. It is shown that the developed models improved the precise point positioning (PPP) solution and convergence time.

Comparisons of Maize pericarp color1 Alleles Reveal Paralogous Gene Recombination and an Organ-Specific Enhancer Region

The maize (Zea mays) p1 (for pericarp color1) gene encodes an R2R3 Myb-like transcription factor that regulates the flavonoid biosynthetic pathway in floral organs, most notably kernel pericarp and cob. Alleles of the p1 gene condition distinct tissue-specific pigmentation patterns; to elucidate the molecular basis of these allele-specific expression patterns, we characterized two novel P1-rw (for red pericarp/white cob) alleles, P1-rw1077 and P1-rw751Ac. Structural analysis of P1-rw1077 indicated that this allele was generated by recombination between p1 and the tightly linked paralogous gene, p2. In the resulting gene, the p1 coding sequence was replaced by the p2 coding sequence, whereas the flanking p1 regulatory sequences remained largely preserved. The red pericarp color specified by P1-rw1077 suggests that the p1- and p2-encoded proteins are functionally equivalent as regulatory factors in the flavonoid biosynthesis pathway. Sequence analysis shows that the P1-rw1077 allele lacks a 386-bp sequence in a distal enhancer region 5 kb upstream of the transcription start site. An independently derived P1-rw allele contains an Ac insertion into the same sequence, indicating that this site likely contains cob glume-specific regulatory elements.

ESTIMATION OF THE STOCHASTIC MODEL FOR GPS CODE AND PHASE OBSERVABLES

The processing of data gathered with a geodetic measurement technique, like GPS surveying, requires the specification of a mathematical model, consisting of a functional model and a stochastic model. The functional model for GPS surveying is already largely known, but the stochastic model still remains virtually uncharted territory. Hence, in this article a procedure is presented for the estimation of elements of the stochastic model by means of relatively simple formulae.The procedure is used to analyze time series of GPS pseudo-range and carrier phase observations collected at a 1-second interval in a precise relative positioning set up with some state-of-the-art geodetic GPS receiver pairs. It is indicated what effects an adequate stochastic model should comprise. The results show that the variance of the code and phase observables per channel/satellite depends on the elevation angle and that significant mutual correlation exists between C1 and P2 code and between L1 and L2 phase observables. It is therefore concluded that refinements to the stochastic model currently in use are definitely necessary.

ESTIMATION OF THE STOCHASTIC MODEL FOR GPS CODE AND PHASE OBSERVABLES

The processing of data gathered with a geodetic measurement technique, like GPS surveying, requires the specification of a mathematical model, consisting of a functional model and a stochastic model. The functional model for GPS surveying is already largely known, but the stochastic model still remains virtually uncharted territory. Hence, in this article a procedure is presented for the estimation of elements of the stochastic model by means of relatively simple formulae.The procedure is used to analyze time series of GPS pseudo-range and carrier phase observations collected at a 1-second interval in a precise relative positioning set up with some state-of-the-art geodetic GPS receiver pairs. It is indicated what effects an adequate stochastic model should comprise. The results show that the variance of the code and phase observables per channel/satellite depends on the elevation angle and that significant mutual correlation exists between C1 and P2 code and between L1 and L2 phase observables. It is therefore concluded that refinements to the stochastic model currently in use are definitely necessary.

Differences in cultivar response and complete sequence analysis of two isolates of wheat yellow mosaic bymovirus in China

Seeds of selected European and Japanese winter wheat cultivars were grown at two experimental sites in China, namely Yaan, Sichuan province (YA), and Yangzhou, Jiangsu province (YZ), where wheat yellow mosaic bymovirus (WYMV) was severe. There were some differential responses of the cultivars to the virus isolates present at the two sites. The complete nucleotide sequence of both RNAs of both virus isolates was determined. Their genome organization was identical to that reported for a Japanese isolate and the sizes were very similar. Nucleotide comparisons demonstrated that parts of the CI and NIa coding regions on RNA1 and the N-terminal part of the P2 coding region on RNA2 were particularly variable, while substantially conserved regions occurred in the 3' UTR of RNA2, the 7K, one part of the CI and parts of the NIb and coat protein. It seems unlikely that differences in the 7K and NIa-VPg proteins are responsible for virulence differences and the CI and NIb regions were considered the most promising for further study.

The role of Gag in human immunodeficiency virus type 1 virion morphogenesis and early steps of the viral life cycle.

The phenotypes of a series of mutant human immunodeficiency virus type 1 proviruses with linker insertion and deletion mutations within the gag coding region were characterized. These mutants, with mutations in the matrix, capsid, and p2 coding regions, produced replication-defective virion particles with defects in the early steps of the viral life cycle. To investigate this phenotype further, the abilities of mutant virion particles to enter T cells, initiate and complete reverse transcription, and transport the newly transcribed proviral DNA were investigated. Only 4 of 10 of the mutants appeared to make wild-type levels of viral DNA. Biochemical analyses of the mutants revealed the middle region of CA as being important in determining virion particle density and sedimentation in velocity gradients. This region also appears to be critical in determining the morphology of mature virion particles by electron microscopy. Particles with aberrant morphology were uninfectious, and only those mutants which displayed cone-shaped cores were capable of carrying out the early steps of the viral life cycle. Thus, the normal morphology of human immunodeficiency virus type 1 appears to be critical to infectivity.

Identification of the Active-Site Residues of the 3C Proteinase of Foot-and-Mouth Disease Virus

To identify the active-site residues of the 3C proteinase of foot-and-mouth disease virus (FMDV), we introduced mutations into the 3C coding region and examined the activity of mutant enzymes on various substrates. Based on alignment of FMDV 3C with other picornavirus 3C proteinases and with the trypsin family of serine proteinases, mutations were introduced at residues presumed to be part of the catalytic triad, involved in substrate binding, or present in nonconserved regions. Wild-type and mutant 3C proteins were expressed inEscherichia coliand tested for their ability to cleave synthetic substrates corresponding to different portions of the viral genome. Substitutions at His-46 (catalytic triad), Asp-84 (catalytic triad), or His-181 (substrate binding) produced enzymes unable to process P1, P2, or P3 substratesin trans,whereas a change in the conserved Asp-98 had no effect on enzyme activity. Substitution of Ser for Cys-163 (catalytic triad) yielded an enzyme that retained activity on some substrates, while a substitution of Gly at this position resulted in a completely inactive enzyme. The kinetics oftransprocessing of translation products from a transcript encoding the P1 and P2 coding regions and the 2C/3A cleavage site with wild-type 3C or a transcript encoding P1 with 3C mutants revealed that the order of cleavage was VP3-VP1, VP0-VP3, VP1-2A, 2C-3A, and 2B-2C. Mutations in 3C that resulted in a partially active enzyme were individually introduced into full-length FMDV cDNA and RNA transcripts were translated in a cell-free system and used to transfect cells. In all cases the virus that was rescued had reverted to the wild-type 3C codon.

Sequences of members of the human gene family for the c subunit of mitochondrial ATP synthase

Subunit c is an intrinsic membrane component of ATP synthase, and in mammals it is encoded by two expressed nuclear genes, P1 and P2. Both genes encode the same mature c subunit, but the mitochondrial import pre-sequences in the precursors of subunit c are different. The DNA sequences of the human P1 and P2 genes are described. They occupy about 3.0 and 10.9 kb respectively of the human genome, and both genes are split into five exons. The human genome also contains about 14 related spliced pseudogenes, and the sequence of one such pseudogene related to P2 is described. Sequences flanking the 5' ends of the human P1 and P2 coding sequences each contain a CpG-rich island. Potential promoter elements (TATA and CCAAT boxes) are present in the 5' sequences of the P1 gene, but not that of P2, although there is no direct experimental evidence to show the involvement of these sequences in transcription of the genes.