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Air and Space Traffic Control
by JR


The purpose of an air and space traffic control system is to provide for the safe, orderly and expeditious flow of traffic in a specified area. The minimum requirements to successfully operate an ASTC system are: a reliable, dedicated and reasonably secure means of two-way communications, a common, published set of measurements and navigation references, and a navigation system which allows accurate position fixes to be derived in a timely manner. In modern systems, a method of tracking and identifying air and space traffic, such as radar, is also employed.

Communications

Reliable two-way communications is required between the ASTC controllers and the aircraft or spacecraft being controlled, and between the controllers themselves. The inter-controller comms may be simple telephone lines, or any other appropriate system. Dedicated frequencies are usually set aside for ASTC use. Also, recording devices are tied into all communications lines and systems.

The communications system used plays a large role in determining how many aircraft a single controller can work at any one time. Using VHF simplex radio (and radar), a controller can handle up to twenty aircraft at any one time. After that, the workload and frequency congestion becomes too much for one person to handle. The solution is to divide the airspace into sectors that a single controller can monitor. Flight plan information is passed from controller to controller as the aircraft moves along through the different sectors.

As the tech level of the ASTC system increases, more and more functions are automated, including coordination between controllers and, at higher tech levels, actual remote control. The remote functions have been discontinued since Virus, however.

Navigation

Currently, air navigation is accomplished by several different systems, reflecting the advance in technology over the past 30 years. The most basic navigation reference in use is a system of simple AM radio ground stations from which an aircraft can derive a bearing with a direction finder.

The backbone of air navigation, and the air route system, is based on a VHF Omnidirectional Range or VOR. This radio ground station emits coded radio signals in all directions, and the corresponding receiver in an aircraft can determine which "radial", or bearing line from the station, the aircraft is on at the time. For example, if two VORs are located on a north-south line, a course may be plotted to follow the 360 degree radial from the southern VOR and then, after changing the receiver to the northern VOR's frequency, to follow the 180 degree radial to the northern VOR. The receiver displays whether the aircraft is left or right of the chosen radial, or on course, thus allowing precise navigation. Charts are published depicting numbered airways which have been established between VORs, much like highways between cities.

Other navigation methods are also used, such as INS or GPS systems, for direct navigation between the departure point and the destination. Intermediate waypoints may also be programmed, allowing these systems to emulate the VOR system and use the airway structure. In the above example, if the GPS waypoints were plotted in the same location as the VORs, then the route of flight would be the same.

In areas with more traffic, published arrival and departure routes, showing the exact route, speed and altitudes the aircraft are to fly, are also used to reduce the controllers' workload. These routes are named and given a revision number to ensure that the aircrews have the latest information. For example, an arrival route into the Houston area from the southwest is called the GLAND 7 Arrival. The controller can simply tell an aircraft: "Cleared to Houston Intercontinental Airport via the GLAND Seven Arrival" and the aircrew has the arrival route, altitude and other instructions, plus they know that they have the current edition of the route (version 7).

Higher tech level ASTC systems can use space-based radar systems to monitor traffic within a star system. Laser and meson communicators would replace radio as a primary means of communications. Any preplanned arrival or departure routes would be transmitted to incoming spacecraft as needed. Satellite navigation stations would replace most ground stations used for navigation.

Preplanned routes for both arrivals and departures can be specified. These routes are used to avoid restricted areas (government buildings, population centers, military installations, etc.) and to regulate traffic flow (ex: keep inbounds and outbounds away from each other). These routes can be in deep space, orbital, or planetary, or any combination of the three. They are usually named and given a version number. Feel free to be creative, the FAA has certainly come up with some winners. (ex: LEMIG1 into San Antonio, pronounced "Lemming").

Traffic Regulations and Standards

Air traffic systems always have a set of rules that mandate the minimum separation distances between aircraft. These distances vary widely depending on the system being used for navigation and/or surveillance. For example, a fast-turning, short range airport radar allows for three miles in-trail separation between two aircraft, compared to thirty minutes in-trail separation on mid-oceanic routes with no radar coverage.

Altitude, as determined by barometric altimeters, is a preferred method of separation, since aircraft that maintain different altitudes cannot collide no matter what their route. The usual minimum is 1000 feet of vertical separation, though this is increased to as much as 5000 feet at very high altitudes.

Standard phraseology is also set forth to reduce confusion and possible errors. Flight crews are usually required to repeat back control instructions given by the controller to further reduce errors in communication. Some examples of standard phraseology:

"Delta twelve heavy, runway eight, fly heading two six zero, cleared for takeoff."

Reply: "Cleared for takeoff, heading two six zero after departure, Delta twelve heavy."

"Piper one charlie lima, traffic, twelve o'clock, three miles, northbound, type and altitude unknown, Mode C indicates three thousand unverified."

Reply: "Looking for traffic, one charlie lima."

In Traveller ASTC systems, the local ASTC system parameters (method of altitude determination, planetary `north', etc) would have to be transmitted to any new arrivals, along with current weather information and local navigation routes and charts.

There may be some variances in local phraseology, but the general rule is to keep communications short and concise. The fewer words used to convey a meaning, the better. Also, most words should mean only one thing in most cases.

When developing phraseology, remember that the terms should fit the operation. Controller-issued directions are either altitudes, speeds or headings based on planetary or galactic directions, as appropriate. A vector is specifically a heading issued by a controller who has taken over navigation for an aircraft, either to avoid traffic or to place the aircraft on a desired route of flight. After the aircraft has completed the desired maneuvers, the controller then returns the aircraft to its own navigation. Certain orbits may be specified (a pox on you if you issue the instruction "standard orbit") as needed, such as a polar orbit designed to avoid the equatorial geosychronous satellite belt. Numbers are usually spoken singly, and letters are spelled out using a phonetic alphabet.

Also remember that controllers are a lazy bunch, so they will usually do things the easiest way possible. Don't make up a complex system if a simple one will do.


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