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GPS, the Global Positioning System, was built by the US Department of Defense from 1973 for the military. A receiver, like the one in your phone, works out where it is by timing signals from at least four satellites orbiting about 20,200 km up. Since 2000 it has been accurate to a few metres for everyone.

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Each GPS satellite carries very precise atomic clocks and constantly broadcasts the time and its own position. Radio signals travel at the speed of light, so the delay tells the receiver how far away each satellite is: three satellites fix a position, and a fourth corrects the receiver's own, less accurate clock. The clocks even have to allow for Einstein's relativity. Seen from the ground, they run about 38 microseconds a day fast, which would otherwise add kilometres of error every day. After a Korean airliner strayed into Soviet airspace and was shot down in 1983, President Ronald Reagan promised that GPS would be free for civilian use. At first the civilian signal was deliberately degraded to an accuracy of about 100 metres, a policy called Selective Availability; President Bill Clinton ended it on 1 May 2000. Europe's Galileo, Russia's GLONASS and China's BeiDou now work alongside GPS.

Türkçe oku
Küresel Konumlama Sistemi GPS, ABD Savunma Bakanlığı'nın 1973'te askerî amaçla başlattığı bir projedir. Telefonunuzdaki gibi bir alıcı, yaklaşık 20.200 km yükseklikte dönen en az dört uydudan gelen sinyallerin ne kadar sürede ulaştığını ölçerek yerini hesaplar. Sistem 2000'den beri herkese birkaç metrelik doğruluk sağlıyor.

Her GPS uydusunda son derece hassas atom saatleri bulunur; uydu, saatin gösterdiği zamanı ve kendi konumunu sürekli yayınlar. Radyo sinyalleri ışık hızıyla yol aldığı için gecikme, alıcıya her uydunun ne kadar uzakta olduğunu söyler: Üç uydu konumu belirler, dördüncüsü alıcının daha az hassas olan kendi saatini düzeltir. Saatlerde Einstein'ın görelilik kuramı bile hesaba katılır. Yerden bakıldığında uydu saatleri günde yaklaşık 38 mikrosaniye ileri gider; bu düzeltilmeseydi hata her gün kilometrelerce büyürdü. 1983'te rotasından sapıp Sovyet hava sahasına giren bir Kore yolcu uçağı düşürülünce ABD Başkanı Ronald Reagan, GPS'in siviller için ücretsiz olacağını açıkladı. Başta sivil sinyal, seçici kullanılabilirlik (Selective Availability) denen bir uygulamayla bilerek bozuluyor ve doğruluk 100 metre dolayında tutuluyordu. Başkan Bill Clinton bu uygulamayı 1 Mayıs 2000'de kaldırdı. Bugün Avrupa'nın Galileo'su, Rusya'nın GLONASS'ı ve Çin'in BeiDou'su da GPS'le birlikte çalışıyor.

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Background reading: Global Positioning System
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