Caption "VOLSCAN - Electronic Computer Controls Air Traffic"
From cover of Radio - Electronics Magazine July 1954.

Volscan Speeds Up Air Traffic

By Angie Pascale
When a large number of military aircraft return to a base within a short time, a terrific air traffic control problem is set up. The problem becomes greater when we consider that more and more jets are being used. They have to be landed as quickly as possible because the lower they fly, the more fuel they use. Therefore, if the landing operation takes too long, the jets can very easily run out of fuel and crash. No human being can control more than 40 planes an hour. The problem was how to land 120 planes or more an hour. It was solved with the Volscan Air Traffic Control System, developed under Ben F. Green, Air Force Cambridge Research Center.

Volscan Closed-loop System
To control 120 aircraft an hour Volscan uses two identical Traffic Operator Consoles and one Monitor Console. The Traffic Operators use a common radio channel to answer incoming aircraft. Each console has a PPI scope a panel controlling seven Antrac-Datac channels, and a Volscan Light Gun for assigning Antracs (Auto-matic tracking-while-scanning).

The Monitor's PPI permits him to watch the progress of all controlled aircraft. His time-situation panel shows the type and scheduled arrival time of each aircraft. Push-buttons permit him to change their automatically assigned schedules or to reserve time in the future for take-off's. The control for setting in the wind direction and velocity and entry gate locations are also on his panel.

Volscan Data Flow Diagram



Antrac
An Antrac is assigned to the radar target by pointing the Volscan Light Gun at the corresponding blip on the PPI. When the light gun is pointed at a signal on the PPI, the light from that signal alone enters the barrel, thus effecting a time coincidence which moves the tracking gate to the position on the scope at while the gun is pointed. Once the Antrac is assigned, it isolates the target from all others by throwing a "gate" around it. Since the gate moves, following the blip, the Antrac retains the identity of the target.

An Antrac sort out the data on a specific aircraft by generating two electronic time gates, one representing the predicted range of the aircraft plus an minus the gate width, and the other having the same function for azimuth. Since it is required that an incoming radar echo coincide with both these gates to enter an Antrac, each Antrac received video signals from only aircraft that are within the area of space defined by the coincided range-azimuth gates. When a new echo from the aircraft does enter the Antrac, the error between the gate position and the echo is measured electronically and is utilized to correct the gate';s position and velocity. which are then stored for use until the next echo is received.

As the Antrac, in period of several scans, acquires the velocity of the target, it smoothes the periodic video pulses that are in the input into a gradually changing d.c. output. By holding the target velocity in its memory, the Antrac predicts where the target is during radar fades by continuing to move at the last rate it had memorized. The young lady of the cover has placed four gates on the PPI scope. The Antrac continuously reports to the computer the exact positions of the four targets.

Datac Schedule Computer



A channel of the Volscan computer is called Datac, automatically selects a scheduled arrival time for the aircraft and calculates heading and altitude orders which will make good this schedule.

Datac calculates the aircraft's direct flight time over the shortest possible path, which is the tangent path to the final turn circle. It reserves the earliest possible scheduled arrival time which the aircraft can make without conflict with other inbound traffic.

Throughout the run, Datac computes an error ratio which indicates the aircraft's performance in related to its schedule. If the aircraft is on time, Datac orders a heading which directs it along the tangent path at present velocity. If the aircraft is late, Datac orders a heading which will hold it on the tangent path and increase velocity. If the aircraft is early, Datac orders an offset heading proportional to the magnitude of the error ratio so that the aircraft will be delayed sufficiently to meet its schedule.

Datac does not predict a fixed path for the aircraft, but is always calculating to see what heading it should fly to make good its schedule. It controls the descent of the aircraft so that it will reach the entry gate at correct altitude.

The Datac control orders are relayed to the aircraft by voice or are sent automatically over the Data Link for display on the Zero Reader or for injection into the aircraft's autopilot. As the plane responds to the control orders, the radar reports the change in its position, thus completing the loop.

The tracking and computing equipment making up the Volscan can be utilized in building-block fashion to meet the needs of any particular installation. For instance, if the traffic rate at a base increases, one additional console and seven additional channels can be added.

One Volscan channel contains only 60 tubes. Since a Volscan airbase installation consists principally of a number of such channels, it is not necessary to provide spares; each is a spare for the others.

Volscan can deliver aircraft satisfactorily to terminal points as far away as 20 miles, with some reduction in traffic rate, and as close as 2 miles, with no reduction in rate.

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