VLBI Intensive Oturumlarının Analizi ile Evrensel Zamanın (UT1) İzlenmesi

Yıl 2017, Cilt: 17 Sayı: 3, 1004 - 1018, 29.12.2017

Kamil Teke Mehmet Fikret Öcal Metin Nohutcu Gonca Okay Ahi

Öz

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.

Anahtar Kelimeler

Evrensel Zaman, UT1, gün-uzunluğu, troposfer gradyanları, VLBI, GNSS

Kaynakça

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