With laser interferometry, an instrument consisting of three small steerable mirrors at the vertices of a 10 meter triangular frame will have an angular resolution for a laser point source of $5 \times 10^{-10}$ radians. That corresponds to $2.5 \times 10^{-4}$ meters, or $0.25$ mm at 500 km. That will be the uncertainty of position in the plane normal to the line of sight from the instrument to an arriving shuttle when the shuttle is still 500 km out. Star sensors, integrated with horizon sensors and laser ring gyroscopes, can provide similar accuracy of measurement for the axis of the instrument. Lidar pulses delivered to a corner reflector, can measure the distance to that reflector to within millimeters in a single pulse. With a feasible stream of thousands of measurements per second and correlation between measurements set by Newtonian mechanics, the trajectory of the shuttle can be tracked with near absolute precision from the moment the shuttle emerges from the lower atmosphere.

The free fall trajectory required for a smooth catch of the shuttle can be computed by integrating the equations of motion backward from the intended instant of capture. When the approaching shuttle has matched its actual trajectory to this ideal free fall trajectory, it is on course to coast to a perfect catch with all thrusters off. Before the shuttle makes its rendezvous with the catcher sled, it makes a virtual rendezvous with this ideal free fall trajectory. If the shuttle's control system fails to achieve that virtual rendezvous within an allowed time limit, then something is wrong. The catch is aborted and the shuttle will fall back toward Earth on a continuation of its suborbital trajectory.

The instruments and control systems for achieving this type of fast rendezvous are critical to the feasibility of the orbiting spaceport concept. They will almost certainly need to be demonstrated before a serious commitment to building an orbiting spaceport can be made. Interestingly, demonstration of the capability need not be a long, expensive project. And significantly, development and validation of the capability would have substantial value to the space program in general, independent of its operational role in the orbiting spaceport.

The instruments and control algorithms for achieving this type of ultra-fast rendezvous are independent of the specific velocity of the arriving shuttle. They can therefore be tested and validated for any arrival velocity. For validation of the sensors and control algorithm, it isn't even necessary to perform an actual catch. A small satellite containing the sensor system, the trajectory control computer, and the communications line to an arriving vehicle are sufficient to demonstrate compliance of the arrival trajectory. After that, a somewhat larger test satellite might carry a prototype catcher sled and "landing track" designed to handle an arrival velocity of 30 mps or so. The track would only need to be a few meters long.

Although a small prototype system of this sort would only increase the payload capacity of an arriving shuttle vehicle by a few percent, it would have a significant impact on transit time. The precise trajectory control and the tolerance for a substantial arrival velocity could reduce the launch to docking interval from a current minimum of four orbits (~6 hours) to under half an orbit (~45 minutes). That could be important to mission logistics and emergency response capabilities.