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[goran]

На самом деле GPS - довольно сложная система и "на пальцах" понять ее не получится наверно.
Вот на этой страничке есть информация о принципах определения скорости по GPS, там же есть ссылки на исходные документы:
http://www.geofaq.ru/forum/index.php...orum=2&topic=6

Вот еще цитата о точности определения скорости из NAVSTAR GPS USER EQUIPMENT INTRODUCTION:

3.3 RECEIVER VELOCITY ACCURACY
GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, but 0.2 m/sec per axis (95 % ) is achievable for PPS receivers.
SPS velocity accuracy is the same as PPS when SA is off. When SA is on, SPS velocity accuracy is degraded. The amount of degra dation of the velocity is classified.
However, although not guaranteed, SPS velocity accuracies around 0.4 m/sec 95% have been observed by civilian users for the typical level of SA associated with normal peacetime operations and 100 metres 95% horizontal position ing accuracy.
Velocity accuracy can be effectively tested in a laboratory environment, but field testing can be difficult since a tracking system with 0.05 m/sec or better accuracy is required. The reader is urged to carefully consider the methods of testing if velocity accuracy is an important mission requirement.