Design of Tracking, Telemetry, Command (TT&C) and Data Transmission Integrated Signal in TDD Mode

For satellite or aircraft networks, tracking, telemetry, and control (TT&C) and data transmission between different nodes are necessary. Traditional measurement mostly adopts the frequency division duplex (FDD) mode and uses a continuous measurement system to achieve high-precision measurement. However, as the number of network nodes increases, the mode suffers from complex frequency domain allocation, and high-cost measurement and data transmission equipment is required. This paper proposes the integrated signal in time division duplex (TDD) mode to improve frequency utilization to address these circumstances. The proposed signal can transmit the TT&C and data at the same frequency. In addition, the high-precision time-frequency synchronization and relative measurement technology in the TDD mode for distributed spacecraft or aircraft networks are studied. The simulation results show that the signal can work normally when the Doppler extrapolation error is less than a quarter of the integration frequency. The distance extrapolation error should be less than a quarter of the length of a chip. The integrated signal reduces the frequency band occupation and realizes the integration of TT&C and data transmission. In addition, the measurement performance is reduced by only 2~3 dB compared with that of the traditional pure TT&C signal.

Monitoring geomagnetic information in the territory of Croatia

In orientation and navigation using compass, reliable map’s marginal information of Earth’s magnetic field declination and its annual variation, namely geomagnetic information (GI), is crucial. Monitoring geomagnetic information means observing declination and its annual variation and checking the reliability of the actual GI model. A typical way of monitoring GI across a national territory involves conducting periodic geomagnetic network surveys to assess and update the model. The objective of the paper was to investigate improving the GI model reliability when an earlier model’s error was raised to standard accuracy, and repeat station network surveys were not yet completed. A series of processing steps in modelling were revised to preserve the original data reliability. The partial 2008.5, 2009.5 and 2010.5 declination solutions were directly reduced to epoch 2015.0, and then to 2016.0, using the IGRF-12 model. The next step was to use 2016 and 2017 quiet daily declination means to estimate corresponding annual variations at surrounding observatories and repeat stations. Normal declination annual variation models were then built for further reductions to epoch 2017.0, and 2018.0, and for forward extrapolations. The quiet days observatory data were analysed to estimate the effect of the input time series length and linear extrapolated time span on forward extrapolation error. Thus, the reliability decline of the initial GI model slowed down in the sequence of models presented. The final GI2018v2 model, valid for 2018.0–2019.0, proved reliable in comparison to the repeat station declination observations of 2018.

Morphology of GPS and DPS TEC over an equatorial station: validation of IRI and NeQuick 2 models

We investigated total electron content (TEC) at Ilorin (8.50∘ N 4.65∘ E, dip lat. 2.95) for the year 2010, a year of low solar activity in 2010 with Rz=15.8. The investigation involved the use of TEC derived from GPS, estimated TEC from digisonde portable sounder data (DPS), and the International Reference Ionosphere (IRI) and NeQuick 2 (NeQ) models. During the sunrise period, we found that the rate of increase in DPS TEC, IRI TEC, and NeQ TEC was higher compared with GPS TEC. One reason for this can be attributed to an overestimation of plasmaspheric electron content (PEC) contribution in modeled TEC and DPS TEC. A correction factor around the sunrise, where our finding showed a significant percentage deviation between the modeled TEC and GPS TEC, will correct the differences. Our finding revealed that during the daytime when PEC contribution is known to be absent or insignificant, GPS TEC and DPS TEC in April, September, and December predict TEC very well. The lowest discrepancies were observed in May, June, and July (June solstice) between the observed values and all the model values at all hours. There is an overestimation in DPS TEC that could be due to extrapolation error while integrating from the peak electron density of F2 (NmF2) to around ∼1000 km in the Ne profile. The underestimation observed in NeQ TEC must have come from the inadequate representation of contribution from PEC on the topside of the NeQ model profile, whereas the exaggeration of PEC contribution in IRI TEC amounts to overestimation in GPS TEC. The excess bite-out observed in DPS TEC and modeled TEC indicates over-prediction of the fountain effect in these models. Therefore, the daytime bite-out observed in these models requires a modifier that could moderate the perceived fountain effect morphology in the models accordingly. The daytime DPS TEC performs better than the daytime IRI TEC and NeQ TEC in all the months. However, the dusk period requires attention due to the highest percentage deviation recorded, especially for the models, in March, November, and December. Seasonally, we found that all the TECs maximize and minimize during the March equinox and June solstice, respectively. Therefore, GPS TEC and modeled TEC reveal the semiannual variations in TEC.

Morphology of GPS and DPS-TEC Over an Equatorial Station: Validation of IRI and NeQuick 2 Models

We investigated total electron content (TEC) at Ilorin (8.50° N 4.65° E, dip lat. 2.95) during a low solar activity 2010. The investigation involved the use of GPS derived TEC, TEC estimated from digisonde portable sounder data (DPS-TEC), the International Reference Ionosphere model (IRI-TEC) and NeQuick 2 model (NeQ-TEC). The five most quietest days of the months obtained from the international quiet days (IQD) from the website http://www.ga.gov.au/oracle/geomag/iqd_form.jsp were used for the investigation. During the sunrise period, we found that the rate of increases in DPS-TEC, IRI-TEC and NeQ-TEC were higher with respect to GPS-TEC. One reason for this can be alluded to an overestimation of plasmaspheric electron content (PEC) contribution in modeled TEC and DPS-TEC. A correction factor around the sunrise where a significant percentage difference of overestimations between the modeled TEC and GPS-TEC was obtained will correct the differences. Our finding revealed that during the daytime when PEC contribution is known to be absent or insignificant, GPS-TEC and DPS-TEC in April, September and December predicts TEC very well. The lowest discrepancies were observed in May, June and July (June solstice) between the observed and all the model values in all hours. There is an overestimation in DPS-TEC that could be due to extrapolation error while integrating from the peak electron density of F2 (NmF2) to around ~ 1000 km in the Ne profile. The underestimation observed in NeQ-TEC must have come from the inadequate representation of contribution from PEC on the topside of NeQ model profile whereas the exaggeration of PEC contribution in IRI-TEC amount to overestimations of GPS-TEC. The excess bite-out observed in DPS-TEC and NeQ-TEC show the indication of overprediction of fountain effect in these models. Therefore, the daytime bite-out observed in these two models require a modifier that could moderate the perceived fountain effect morphology in the models accordingly. Seasonally, we found that all the TECs maximize and minimize during the March equinox and June solstice, respectively. Therefore, GPS-, DPS-, IRI- and NeQ-TEC reveal the semi-annual variations in TEC as reported in all regions. The daytime DPS-TEC performs better than the daytime IRI-TEC and NeQ-TEC in all the months, however, the dusk period requires attention due to highest percentage difference recorded especially for DPS-TEC and the models in March, and November and December for DPS-TEC.

Development of High Accurate Flow Nozzle for Feedwater Flowrate Measurement in Nuclear Power Plant

The high accurate throat tap flow nozzle with four different diameter taps is developed and its discharge coefficients are measured in the Reynolds number range from 1.5×106 to 1.4×107 using the high Reynolds calibration facility of AIST,NMIJ. The discharge coefficient of a throat tap nozzle extrapolated according to ASME PTC 6 are confirmed to deviate 0.37% at Red=1.4×107 from the experimental results. The high accurate flow nozzle developed can reduce this extrapolation error of the discharge coefficient to high Reynolds numbers by using the equations of discharge coefficients, which is determined as a function of Reynolds number and tap diameter based on the experimental results of four different diameter taps. The error of extrapolated discharge coefficient using the derived equations is estimated to be less than 0.1% at Red=1.4×107. The present results show that the throat tap flow nozzle developed is expected to work as a high accurate flowmeter even under the extrapolation of the discharge coefficient toward high Reynolds numbers.

Uncertainty in river discharge observations: a quantitative analysis

This study proposes a framework for analysing and quantifying the uncertainty of river flow data. Such uncertainty is often considered to be negligible with respect to other approximations affecting hydrological studies. Actually, given that river discharge data are usually obtained by means of the so-called rating curve method, a number of different sources of error affect the derived observations. These include: errors in measurements of river stage and discharge utilised to parameterise the rating curve, interpolation and extrapolation error of the rating curve, presence of unsteady flow conditions, and seasonal variations of the state of the vegetation (i.e. roughness). This study aims at analysing these sources of uncertainty using an original methodology. The novelty of the proposed framework lies in the estimation of rating curve uncertainty, which is based on hydraulic simulations. These latter are carried out on a reach of the Po River (Italy) by means of a one-dimensional (1-D) hydraulic model code (HEC-RAS). The results of the study show that errors in river flow data are indeed far from negligible.