The spectrum of GPS measurement errors and the accuracy of airborne gravity measurements

Geophysics ◽  
1990 ◽  
Vol 55 (8) ◽  
pp. 1101-1104 ◽  
Author(s):  
D. R. Bower ◽  
J. Kouba ◽  
R. J. Beach
Keyword(s):  
Measurement Errors ◽  
Regional Scale ◽  
Vertical Motion ◽  
Airborne Gravity ◽  
Width Ratio ◽  
Noise Power ◽  
Power Spectral ◽  
Gps Measurement ◽  
Gravity Measurements ◽  
Allowable Error

Recent observations (Georgiadou and Kleusberg, 1987; Kleusberg et al., 1989) suggest that errors in GPS carrier phase observations at frequencies within the gravity passband of airborne gravity systems may be due mainly to multipath interference. Further, the power spectral density (PSD) of these errors has been found to fall off rapidly with increase in frequency throughout the anticipated gravity passband, in the manner of a red spectrum rather than a white (which remains constant). It is shown that this implies a much greater allowable error in GPS‐derived altitude reference than would be the case if the PSD of altitude errors (1) was white, (2) had the same shape as that of typical aircraft vertical motion, or (3) was dominated by a sinusoidal wave located near the high frequency limit of the gravity passband. This enhances the feasibility of airborne gravity for regional scale surveys and perhaps explains why actual measurements have been better than predicted. For example, given a uniform [Formula: see text] distribution of spectral noise power and a speed to grid‐width ratio of 60 per hour, an rms altitude error as large as 12 cm will still allow the computation of acceleration correction with an accuracy of 2 mGal. For the same conditions, the allowable rms altitude error given a white distribution of spectral noise power is 1.5 cm.

scholarly journals Characterizing the γ-ray long-term variability of PKS 2155−304 with H.E.S.S. and Fermi-LAT

2017 ◽  
Vol 598 ◽  
pp. A39 ◽  
Author(s):  
◽  
H. Abdalla ◽  
A. Abramowski ◽  
F. Aharonian ◽  
F. Ait Benkhali ◽  
...  
Keyword(s):  
Flicker Noise ◽  
High Energy ◽  
Long Term Results ◽  
Noise Power ◽  
Quiescent State ◽  
Physical Processes ◽  
Bl Lac Objects ◽  
Power Spectral ◽  
Bl Lac

Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term variability of the BL Lac object PKS 2155−304 is analyzed in the high (HE, 100 MeV < E < 300 GeV) and very high energy (VHE, E > 200 GeV) γ-ray domain. Over the course of ~9 yr of H.E.S.S. observations the VHE light curve in the quiescent state is consistent with a log-normal behavior. The VHE variability in this state is well described by flicker noise (power-spectral-density index βVHE = 1.10+0.10-0.13) on timescales larger than one day. An analysis of ~5.5 yr of HE Fermi-LAT data gives consistent results (βHE = 1.20+0.21-0.23, on timescales larger than 10 days) compatible with the VHE findings. The HE and VHE power spectral densities show a scale invariance across the probed time ranges. A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established. These long-term-variability properties are discussed and compared to the red noise behavior (β ~ 2) seen on shorter timescales during VHE-flaring states. The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes, while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection.

scholarly journals The Assessment of Hydrologic- and Flood-Induced Land Deformation in Data-Sparse Regions Using GRACE/GRACE-FO Data Assimilation

2021 ◽  
Vol 13 (2) ◽  
pp. 235
Author(s):  
Natthachet Tangdamrongsub ◽  
Michal Šprlák
Keyword(s):  
Data Assimilation ◽  
Gravity Data ◽  
Regional Scale ◽  
Vertical Motion ◽  
Vertical Displacement ◽  
Spatial Coverage ◽  
Vertical Displacements ◽  
Land Deformation ◽  
Gravity Recovery ◽  
Central South

The vertical motion of the Earth’s surface is dominated by the hydrologic cycle on a seasonal scale. Accurate land deformation measurements can provide constructive insight into the regional geophysical process. Although the Global Positioning System (GPS) delivers relatively accurate measurements, GPS networks are not uniformly distributed across the globe, posing a challenge to obtaining accurate deformation information in data-sparse regions, e.g., Central South-East Asia (CSEA). Model simulations and gravity data (from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO)) have been successfully used to improve the spatial coverage. While combining model estimates and GRACE/GRACE-FO data via the GRACE/GRACE-FO data assimilation (DA) framework can potentially improve the accuracy and resolution of deformation estimates, the approach has rarely been considered or investigated thus far. This study assesses the performance of vertical displacement estimates from GRACE/GRACE-FO, the PCRaster Global Water Balance (PCR-GLOBWB) hydrology model, and the GRACE/GRACE-FO DA approach (assimilating GRACE/GRACE-FO into PCR-GLOBWB) in CSEA, where measurements from six GPS sites are available for validation. The results show that GRACE/GRACE-FO, PCR-GLOBWB, and GRACE/GRACE-FO DA accurately capture regional-scale hydrologic- and flood-induced vertical displacements, with the correlation value and RMS reduction relative to GPS measurements up to 0.89 and 53%, respectively. The analyses also confirm the GRACE/GRACE-FO DA’s effectiveness in providing vertical displacement estimates consistent with GRACE/GRACE-FO data while maintaining high-spatial details of the PCR-GLOBWB model, highlighting the benefits of GRACE/GRACE-FO DA in data-sparse regions.

Airborne gravity measurements over mountainous areas by using a LaCoste & Romberg air‐sea gravity meter

Geophysics ◽  
2002 ◽  
Vol 67 (3) ◽  
pp. 807-816 ◽  
Author(s):  
Jérôme Verdun ◽  
Roger Bayer ◽  
Emile E. Klingelé ◽  
Marc Cocard ◽  
Alain Geiger ◽  
...  
Keyword(s):  
Data Reduction ◽  
Classical Method ◽  
Gravity Data ◽  
Airborne Gravity ◽  
Mountainous Area ◽  
Vertical Acceleration ◽  
Mountainous Areas ◽  
Free Air ◽  
Gravity Measurements ◽  
Free Air Anomaly

This paper introduces a new approach to airborne gravity data reduction well‐suited for surveys flown at high altitude with respect to gravity sources (mountainous areas). Classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western French Alps by using a LaCoste & Romberg air‐sea gravity meter. The part of nongravitational vertical accelerations correlated with gravity meter measurements are investigated with the help of coherence spectra. Beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft. This finding is theoretically argued by solving the equation of the gravimetric system (gravity meter and stabilized platform). The transfer function of the system is derived, and a new formulation of airborne gravity data reduction, which takes care of the sensitive response of spring tension to observable gravity field wavelengths, is given. The resulting gravity signal exhibits a residual noise caused by electronic devices and short‐wavelength Eötvös effects. The use of dedicated exponential filters gives us a way to eliminate these high‐frequency effects. Examples of the resulting free‐air anomaly at 5100‐m altitude along one particular profile are given and compared with free‐air anomaly deduced from the classical method for processing airborne gravity data, and with upward‐continued ground gravity data. The well‐known trade‐off between accuracy and resolution is discussed in the context of a mountainous area.

scholarly journals Dry and moist dynamics shape regional patterns of extreme precipitation sensitivity

2020 ◽  
Vol 117 (16) ◽  
pp. 8757-8763 ◽  
Author(s):  
Ji Nie ◽  
Panxi Dai ◽  
Adam H. Sobel
Keyword(s):  
Global Warming ◽  
Extreme Precipitation ◽  
Large Scale ◽  
Climate Model ◽  
Regional Scale ◽  
Vertical Motion ◽  
Precipitable Water ◽  
Small Scale ◽  
Climate Projections ◽  
Regional Patterns

Responses of extreme precipitation to global warming are of great importance to society and ecosystems. Although observations and climate projections indicate a general intensification of extreme precipitation with warming on global scale, there are significant variations on the regional scale, mainly due to changes in the vertical motion associated with extreme precipitation. Here, we apply quasigeostrophic diagnostics on climate-model simulations to understand the changes in vertical motion, quantifying the roles of dry (large-scale adiabatic flow) and moist (small-scale convection) dynamics in shaping the regional patterns of extreme precipitation sensitivity (EPS). The dry component weakens in the subtropics but strengthens in the middle and high latitudes; the moist component accounts for the positive centers of EPS in the low latitudes and also contributes to the negative centers in the subtropics. A theoretical model depicts a nonlinear relationship between the diabatic heating feedback (α) and precipitable water, indicating high sensitivity of α (thus, EPS) over climatological moist regions. The model also captures the change of α due to competing effects of increases in precipitable water and dry static stability under global warming. Thus, the dry/moist decomposition provides a quantitive and intuitive explanation of the main regional features of EPS.

scholarly journals Evaluating quasi-periodic variations in the γ-ray light curves of Fermi-LAT blazars

2020 ◽  
Vol 634 ◽  
pp. A120 ◽  
Author(s):  
F. Ait Benkhali ◽  
W. Hofmann ◽  
F. M. Rieger ◽  
N. Chakraborty
Keyword(s):  
Flicker Noise ◽  
Timing Analysis ◽  
Statistical Significance ◽  
Light Curves ◽  
Noise Power ◽  
Care Needs ◽  
Radiation Physics ◽  
Power Spectral ◽  
Timing Properties ◽  
Log Normal

Context. The detection of periodicities in the light curves of active galactic nuclei (AGNs) could have profound consequences for our understanding of the nature and radiation physics of these objects. At high energies (HE; E >  100 MeV), five blazars (PG 1553+113, PKS 2155−304, PKS 0426−380, PKS 0537−441 and PKS 0301−243) have been reported to show year-like quasi-periodic variations (QPVs) with significance > 3σ. As these findings are based on only a few cycles, care needs to be taken to properly account for random variations that can produce intervals of seemingly periodic behavior. Aims. We present results of an updated timing analysis for six blazars (adding PKS 0447−439 to the above), using suitable methods to evaluate their long-term variability properties and to search for QPVs in their light curves. Methods. We generate γ-ray light curves covering almost ten years, study their timing properties, and search for QPVs using the Lomb-Scargle Periodogram and the Wavelet Z-transform. Extended Monte Carlo simulations are used to evaluate the statistical significance. Results. (1) Comparing their probability density functions, all sources (except PG 1553+113) exhibit a clear deviation from a Gaussian distribution, but are consistent with being log-normal, suggesting that the underlying variability is of a nonlinear, multiplicative nature. (2) Apart from PKS 0301−243, the power spectral density for all investigated blazars is close to flicker noise (power-law slope −1). (3) Possible QPVs with a local significance ≳3σ are found in all light curves (apart from PKS 0426−380 and PKS 0537−441), with observed periods in the range (1.7 − 2.8) yr. The evidence is strongly reduced however if evaluated in terms of a global significance. Conclusions. Our results advise caution as to the significance of reported year-like HE QPVs in blazars. Somewhat surprisingly, the putative redhift-corrected period all cluster around ∼1.6 yr. We speculate on possible implications for QPV generation.

scholarly journals Reducing the Susceptibility of Lumped-Element KIDs to Two-Level System Effects

2020 ◽  
Vol 200 (5-6) ◽  
pp. 239-246
Author(s):  
A. L. Hornsby ◽  
P. S. Barry ◽  
S. M. Doyle ◽  
Q. Y. Tang ◽  
E. Shirokoff
Keyword(s):  
Dielectric Materials ◽  
Kinetic Inductance Detectors ◽  
Kinetic Inductance ◽  
Noise Power ◽  
Microwave Background ◽  
Superconducting Resonators ◽  
Lumped Element ◽  
Noise Power Spectral Density ◽  
Power Spectral ◽  
Materials Used

Abstract Arrays of lumped-element kinetic inductance detectors (LEKIDs) optically coupled through an antenna-coupled transmission line are a promising candidate for future cosmic microwave background experiments. However, the dielectric materials used for the microstrip architecture are known to degrade the performance of superconducting resonators. In this paper, we investigate the feasibility of microstrip coupling to a LEKID, focusing on a systematic study of the effect of depositing amorphous silicon nitride on a LEKID. The discrete and spatially separated inductive and capacitive regions of the LEKID allow us to vary the degree of dielectric coverage and determine the limitations of the microstrip coupling architecture. We show that by careful removal of dielectric from regions of high electric field in the capacitor, there is minimal degradation in dielectric loss tangent of a partially covered lumped-element resonator. We present the effects on the resonant frequency and noise power spectral density and, using the dark responsivity, provide an estimate for the resulting detector sensitivity.

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