ans =

From the Tips section in the documentation page -

"assume removes any assumptions previously set on the symbolic variables. To retain previous assumptions while adding an assumption, use assumeAlso."

syms Phi_geod(t) lambda_geod(t)

syms Phi_dot_geod(t) lambda_dot_geod(t)

assumptions(Phi_geod)

assume(Phi_geod(t),"real");

assumptions(Phi_geod)

assumeAlso(lambda_geod(t),"real");

assumptions(Phi_geod)

assumeAlso(Phi_dot_geod(t),"real");

assumeAlso(lambda_dot_geod(t),"real");

assumptions(Phi_geod)