# Soyuz-TM spacecraft traffic control system part 2

In the first part, I began to tell you about the work of the Motion Control System (SUD) at the stage of convergence. We stopped on the method of free trajectories and I promised to tell you about the guidance method along the line of sight and parallel guidance. The post will be small but interesting.

# Sight guidance (LP)

The line of sight (LP) is the line connecting the centers of mass of objects.

This method has a big minus. Since this method does not take into account the laws of orbital motion of spacecraft (SC), therefore, when approaching by this method, it is impossible to predict the relative motion of the ship and station, and to predict the consequences of control actions quite accurately even at relatively short time intervals. In this regard, certain restrictions are imposed on the approach path.

The vector of the relative approach velocity Vrel should be directed strictly along the LP:
V rel = V tk -V ok , where

V tk is the vector of the orbital velocity TK,
Vok - vector of orbital speed OK.

In this case, the angular velocity of the line of sight should be maintained equal to zero:
ω lv = V side / ρ , where

ω lv is the vector of the angular velocity of the drug,
V side is the vector of the lateral velocity of approach,
ρ is the relative distance.

Under this condition, the line of sight in inertial space moves parallel to itself. The control of the ship, at which ω lv = 0 ismaintained,is calledthe parallel guidance method.It is difficult to implement this method in its pure form. The residual angular velocity of approach is always present, which means that the approach comes with some miss. The magnitude of this miss is proportional to the range and magnitude of the angular velocity. The task of implementing the parallel guidance method, both in manual and automatic mode, is to sequentially damp the angular velocity of the line of sight, thereby reducing the miss and keeping the radial velocity of approach within the specified limits.

The method of inertial parallel guidance is preferable in practical terms due to the fact that it is simple to implement and does not require complex computing techniques. The disadvantage of this method is that it is applicable only at a relatively short range and the approach time should be minimal. This reduces the effect of the difference in
gravitational accelerations acting on the ship and station. The parallel guidance method is used both at the final section of the automatic approach of the TK (from a range of 200–100 m in the approach mode), and with manual control in standby modes of approach.

In the next article I will try to talk about the principles of constructing a spacecraft motion control system in the approach mode, describe all coordinate systems used in the Soyuz spacecraft, and also write out a checkmate. ship and station motion models, which are implemented in the Soyuz spacecraft.