There is an example for the radio signal for Twin Paradox from the book "Classical Dynamics of Particle and Systems, Thornton, Marion". Refer "1.jpeg", example 14.5, it said that "However, Frank continues to receive a signal every 3 years for the 8 years it takes the last signal Mary sends when she reaches the planet to travel to Frank". I cannot understand, why there is another 8 years Frank receive signal for every 3 years? Anyone can explain more about this? Thanks.