Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 47

Fig. 2.12 Satellite
LAGEOS. Source: NASA

Fig. 2.13 Geometric
principle of VLBI

L ¼ c ˄ g , ð 2 : 15 Þ

where c is the velocity of light in vacuum. Suppose ! is the baseline vector from
antenna 1 to antenna 2 and ! is the direction of the electrical source observed; then
we get:

1
c

!

˄ g ¼ : ð 2 : 16 Þ

Due to the movement of the Earth, the direction of vector * relative to the
baseline vector * will change. Suppose

˄ g is a time function, then we can denote its

time derivative as the delay rate ˄ g , namely:

1

c ∂

!

˄ g ¼

: ð 2 : 17 Þ

t

∂

The observed values of VLBI in geodesy are mainly the delay and the delay rate.