Hi, if I understand you correclty you want to verify the distance between two positions of the point and prefferbaly have it constant? Note that this distance will depend on the input angle (i.e. LowerTheta).
In the code below I modified your code a bit to work vectorized, I plotted the trajectory of the point and evaluate the distance between the consecutive positions.
Looking at your original code, I think the mistake is that you do not update point 'b'. You are always evaluating the length with respact to the origin (i.e. [0,0,0]).
L = 9;
% define some random angles, OP did not provide them
LowerTheta = linspace(0,2*pi,50);
% evaluate the position of the point
ax = 8*cos(LowerTheta);
az = 8*sin(LowerTheta);
ay = L*cosd(asin(az./L));
% create a figure of the trajectory of the point
figure
scatter3(ax(:),ay(:),az(:),'*r')
xlabel('X'); ylabel('Y'); zlabel('Z')
title('Point Trajectory')
grid on
% merge the coordinates into a grid
a = [ax(:) ay(:) az(:)];
% now determine the distance between the consecutive points
SegLength = sqrt( sum( (a(1:end-1,:) - a(2:end,:)).^2 ,2 ))
