KTU-GEOD IVS Analysis Center Annual Report 2012

This report summarizes the activities of the KTU-GEOD IVS Analysis Center (AC) in 2012 and outlines planned activities for 2013. The analysis of the EUROPE sessions is one of our specific interests, and the combination of different AC solutions for continuous VLBI campaigns, e.g. CONT11, will be investigated

Monitoring Universal Time (UT1) from the Analysis of VLBI Intensive Sessions

Yer yönelim parametrelerinden zamana göre en hızlı değişkenlik göstereni Evrensel Zaman, UT1’dir. Yapay uydu ve gezegenlerin yörünge parametrelerinin belirlenmesi ve uzay navigasyonu amaçlı çalışmalar için UT1 vazgeçilmez bir öneme sahiptir. Uluslararası VLBI (çok uzun baz interferometrisi) Jeodezi ve Astrometri Servisi (IVS), UT1’i belirleyebilmek için “Intensive” oturumlar olarak adlandırılan VLBI oturumlarını her gün planlamak ve gerçekleştirmekle yükümlüdür. Ekonomik ve operasyonel anlamdaki güçlüklerden ve her gün gerçekleştirilme zorunluluğundan ötürü IVS, VLBI Intensive oturumlarını bir veya en fazla iki saat süre ile ve iki veya en fazla üç VLBI radyo teleskobu ile gerçekleştirmektedir. Bu kısıtlayıcı faktörler VLBI Intensive oturumlarında ölçü sayısının az olması (15-55 ölçü) sonucunu doğurur. Ölçü sayısının az olması ise, en-küçük-kareler (EKK) yöntemi ile kestirilebilecek bilinmeyen parametre setini kısıtlar. VLBI ölçü modelinde öncül değerlerine sabit alınan parametrelerdeki hatalar veya modele dâhil edilemeyen parametreler (örneğin, troposfer gradyanları) UT1 parametresinin kestirim sonuçlarına olumsuz olarak yansırlar ve UT1 doğruluğunun azalmasına yol açarlar. UT1 parametresinin Intensive oturumlarının IVS standart analizi ile belirlenmesinde yukarıda belirtilen problemlerden hareketle, bu çalışmada yeni bir analiz stratejisi önerilmektedir. Önerilen IVS Intensive oturumu analizi ile, Küresel Navigasyon Uydu Sistemleri (GNSS) ölçülerinden elde edilen troposfer gradyanları VLBI ölçülerinin analizine dahil edilerek doğruluğu IVS standart analizinden daha yüksek doğruluklu UT1 (YeniUT1) kestirimi gerçekleştirilmiştir. 2008 yılı başından 2017 yılı Ocak ayı sonunu kapsayan Intensive oturumlarının analizlerinden kestirilen gün-uzunlukları ile GNSS gün-uzunluklarının istatistiksel karşılaştırılmaları sonucu, YeniUT1 gün-uzunluklarının doğruluklarının IVS standart analizi ile elde edilenlere kıyasla 2 ila 3 mikrosaniye/gün daha yüksek olduğu ortaya konulmuştur.Universal Time (UT1) is the most variable parameter with respect to time among the Earth orientation parameters. UT1 has utmost importance for determining the orbital parameters of the artificial satellites and the planets, and for space navigation. International VLBI (Very Long Baseline Interferometry) Service for Geodesy and Astrometry (IVS) is in charge of planning and realizing exclusive VLBI sessions everyday, so-called Intensive sessions. Due to the drawbacks arising from economic and operational issues and the necessity of daily realization of VLBI Intensive sessions, IVS operates these sessions with two to three VLBI radio telescopes over one or two hours. These restrictions result in a limited number of observations (15-55 observations) in an Intensive session. Thus, the few number of observations constraint the number of parameters to be estimated by the Least-Squares method. The errors of the a priori fixed parameters and the parameters that cannot be included in the observation model (e.g. troposphere gradients) propagate into UT1 estimate and cause UT1 accuracy to decrease. Concerning the above-mentioned restrictions for the UT1 estimation of IVS standard analysis, a new analysis strategy is recommended in this study. In the scope of the new analysis strategy of IVS Intensive sessions, the troposphere gradients from Global Navigation Satellite Systems (GNSS) are assimilated into the analysis of VLBI Intensive sessions that result in more accurate UT1 estimates, i.e. New UT1, than those derived by IVS standard analysis. Based on the statistical comparisons of the length-of-day (LOD) estimates of the Intensive sessions, observed from the beginning of 2008 till the end of January 2017, with the GNSS LOD, it is proved that the LOD computed from New UT1 estimates have 2 to 3 microseconds/day better accuracies than those estimated from the IVS standard analysis

Sub-daily parameter estimation in VLBI data analysis

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M2 constituent of ocean tide loading displacements from VLBI CONT14 hourly sessions

Several studies prove that ocean tide loading (OTL) displacements can be observed with space geodetic techniques. In this study, the amplitudes and Greenwich phase lags for each coordinate component, i.e., radial, west, and south of the principal lunar semidiurnal tide, M2 of OTL displacements were estimated at the very long baseline interferometry (VLBI) sites of the 15 days long continuous VLBI campaign, CONT14, carried out by the International VLBI Service for Geodesy and Astrometry (IVS). In the estimation of the amplitudes and Greenwich phase lags of the M2 tidal constituent, hourly VLBI station coordinate time series were used as observations derived through analyzing 1 hour VLBI sessions of the CONT14 campaign. In the analysis of hourly sessions of the CONT14 campaign, to derive accurate hourly station coordinates, troposphere delays estimated from daily sessions were reduced from the observations a priori to the analysis. The estimated amplitudes and Greenwich phase lags of the M2 constituent of OTL displacements were compared with the predictions the state-of-the-art ocean tide models, among others, FES2012 (Lyard et al. 2006, Carrère at al. 2012), FES2014 (Carrère et al. 2016) and TPXO8 (Egbert and Erofeeva 2002, Egbert et al. 2010). Both the amplitudes and the phases between CONT14 estimates and ocean tide models agree well for the M2 tide at all the sites and in most of the coordinate components. The RMS misfits of the M2 tide of OTL displacements in all coordinate components between CONT14 and ocean tide models over coastal sites are found about two times larger than those of inland sites. This result confirms the modeling insufficiencies in shallow waters of ocean tide models which cause an accuracy restriction of OTL displacement predictions around coastal regions

Estimation of High-Frequency Earth Rotation Parameters from the Analysis of IVS-CONT14 Campaignand Comparison with IERS2010 Model

Bu çalışmada, Uluslararası Çok Uzun Baz İnterferometrisi (VLBI) Jeodezi ve Astrometri Servisi (IVS) tarafından 6-21 Mayıs 2014 tarihleri arasında 6 kıtadaki 17 VLBI istasyonunun katılımıyla gerçekleştirilen 15 günlük sürekli VLBI oturumlarından (IVS-CONT14) elde edilen ölçülerin analiz edilmesi ile yüksek-frekanslı Yer dönme parametreleri kestirimi yapılmıştır. Kestirilen Yer dönme parametreleri ile Uluslararası Yer Dönme ve Referans Sistemleri Servisi (IERS) 2010 konvansiyonlarında önerilen, günlük ve yarı-günlük (yüksek-frekanslı) okyanus gel-gitleri kaynaklı Yer dönme değişim modeli (IERS2010 modeli) değerleri, spektral analiz ve tanımlayıcı istatistik yöntemleri ile karşılaştırılmıştır. Ayrıca, IERS2010 okyanus gelgitleri kaynaklı yüksek frekanslı Yer dönme modelindeki başlıca gelgitlerin sinüs ve kosinüs genlikleri kestirilmiş, Yer dönme ekseninin ileri-giden (prograde) ve geri-giden (retrograde) kutup gezinmesi hareketlerine etkileri hem model hem de kestirilen parametreler için fazör gösterimi üzerinden yorumlanmıştır. IERS2010 modelinden hesaplananve IVS-CONT14 ölçülerinden kestirilen yüksek frekanslı Yer dönme parametrelerinin genliklerinin 12 saat periyodunda iyi uyum gösterdiği belirlenmiştir.In this study, high-frequency Earth rotation parameters were estimated by means of analysing the observations of the 15 days continuous Very Long Baseline Interferometry (VLBI) sessions, participating 17 VLBI stations located at 6 continents (IVS-CONT14), carried out from 6th to 21st of May 2014 by the International VLBI Service for Geodesy and Astrometry (IVS). The estimated Earth rotation parameters were compared to themodel values of the diurnal and semi-diurnal (high-frequency) Earth rotation variations caused by ocean tides (IERS2010 model), recommended by the International Earth Rotation and Reference Systems Service (IERS) 2010 conventions, by means of spectral analysis and descriptive statistics. Moreover, sine and cosine amplitudes of the major tidal terms of the IERS2010 highfrequency Earth rotation model due to the ocean tides were estimated and their effects on prograde and retrograde polar motions of the Earth rotation axis were interpreted upon the phasor plots for both estimated and model values.It is inferred that the amplitudes of high-frequencyEarth rotation parameters with 12 hours period computed from the IERS2010 model and those of estimated from the IVS-CONT14 observations are in a good agreement

GNSS zenith delays and gradients in the analysis of VLBI Intensive sessions

Very Long Baseline Interferometry (VLBI) is the only space geodetic technique which is capable of estimating Universal Time (UT1 = UTC + ΔUT1). So-called VLBI Intensive sessions of the International VLBI Service for Geodesy and Astrometry (IVS) are dedicated to the rapid production of ΔUT1. However, the accuracy achieved with those sessions is still below what could be expected from formal uncertainties of the estimates and one of the reasons is the inappropriate modeling of azimuthal asymmetries of the troposphere delays, because usually no gradients are modeled or estimated. To overcome that deficiency, we introduced troposphere zenith delays and horizontal total gradients estimated from the observations of Global Navigation Satellite Systems (GNSS) i.e. the solution of the Center for Orbit Determination in Europe (CODE) in the analysis of VLBI Intensive sessions carried out from the beginning of 2008 till the end of 2014. We compared our results with the GNSS-derived length-of-day (LOD) estimates of CODE and the International GNSS Service (IGS) and find slight improvements of agreement by up to 1 μs for both INT1 and INT2 sessions with gradients from CODE. We do not see any additional significant improvement of LOD agreement when GNSS zenith delays are introduced

Combination of Two Radio Space-Geodetic Techniques with VieVS during CONT14

Unlike CONT11, CONT14 does not have official information on common frequency standards for co-located sites. Nevertheless, according to Kwak et al. (2015) [1], we have a possibility to find the co-located sites, which used the same clocks, through comparing clock rates from single technique solutions. Moreover, CONT14 includes co-located VLBI radio telescopes, i.e. HOBART26 and HOBART12. Therefore, it is also a good test bed to develop the analysis strategy for future twin/sibling telescopes. In this study, we compute VLBI-like GNSS delays (GNSS single differences) between the ranges from two stations to a satellite, using phase measurements with most of the errors correctedby the c5++ software. We estimate station coordinates and site common parameters, i.e. zenith wet delays, troposphere gradients and clock parameters, with the Vienna VLBI Software. Common clock parameters are limited to the sites sharing the same frequency standard and having good performance of it during CONT14. Local tie vectors are introduced as fictitious observations for co-located instruments, GNSS-VLBI and even VLBI-VLBI, i.e. at Hobart. In this paper, we show the comparison results between the combination solutions and the single technique solutions in terms of station position repeatability during 15 days

Combination of Two Radio Space-Geodetic Techniques with VieVS during CONT14

Unlike CONT11, CONT14 does not have official information on common frequency standards for co-located sites. Nevertheless, according to Kwak et al. (2015) [1], we have a possibility to find the co-located sites, which used the same clocks, through comparing clock rates from single technique solutions. Moreover, CONT14 includes co-located VLBI radio telescopes, i.e. HOBART26 and HOBART12. Therefore, it is also a good test bed to develop the analysis strategy for future twin/sibling telescopes. In this study, we compute VLBI-like GNSS delays (GNSS single differences) between the ranges from two stations to a satellite, using phase measurements with most of the errors correctedby the c5++ software. We estimate station coordinates and site common parameters, i.e. zenith wet delays, troposphere gradients and clock parameters, with the Vienna VLBI Software. Common clock parameters are limited to the sites sharing the same frequency standard and having good performance of it during CONT14. Local tie vectors are introduced as fictitious observations for co-located instruments, GNSS-VLBI and even VLBI-VLBI, i.e. at Hobart. In this paper, we show the comparison results between the combination solutions and the single technique solutions in terms of station position repeatability during 15 days