This is the biggest part of controlling any ground position. The overall concept is fairly simple, however traffic loads can make it quite intense. The controller needs to know what each plane is doing at any one time. This is why there are almost always understood routes via which the planes move.
There may be a pre-defined set of taxiways that are used by departing aircraft to get to their runway, and a set by arriving aircraft to get to their gate. Of course this depends on the airport complexity. In smaller airports, there may only be one main taxiway beside the runway, which needs to be used for both. Similarly, there may be times where those pre-defined taxiways mentioned earlier, may need to be used for other uses (eg A plane repositioning from overnight parking to his gate, or a business jet moving to another parking spot to pick up a client, a Coast Guard helicopter repositioning to a different ramp, a lost aircraft needing to find his way, etc.). In these cases it is absolutely essential that the controller/s have a strong plan and understand what everyone is doing at any one point.
a. When authorizing an aircraft to proceed on the movement area, or to any point other than assigned takeoff runway, specify the route/taxi instructions. If it is the intent to hold the aircraft short of any given point along the taxi route, issue the route and then state the holding instructions. Note that the word “cleared” is NOT used in conjunction with an instruction to taxi.
“Cross Runway Two Eight Left, hold short of Runway Two Eight Right.”
“Taxi/continue taxiing/proceed to the hangar.”
“Taxi/continue taxiing/proceed straight ahead then via ramp to the hangar.
“Taxi/continue taxiing/proceed on Charlie, hold short of Runway Two Seven.”
“Taxi/continue taxing/proceed on Charlie, hold short of Runway Two Seven.”
As part of the items mentioned above, Ground Control needs to ensure that the aircraft reads back the hold short instructions. This is imperative, as it ensures that the aircraft understands he is not to cross or enter any area that's not planned for his movement. If you instruct the aircraft to hold short of something, and don't receive such a read back, you need to confirm it with the aircraft(The specific words of type “Will hold short of ”/”Roger, hold short of /etc” are required in order to deem pilots readback valid.
GC: “Experimental five-five-lima, continue via Alpha, hold short of Echo”
GC: “Experimental five-five-lima, read back hold short instructions”
N55L: “Roger, will hold short of echo”
This ensures that both the pilot and controller are absolutely sure that the aircraft will hold short of the instructed point. If you wish for the aircraft to cross any active runway, you need to coordinate with the local control first.
GC: “Fedex three-seventy-two heavy, cross runway seven at Oscar”
b. When authorizing an aircraft to taxi to an assigned takeoff runway, state the departure runway followed by the specific taxi route. Issue hold short restrictions when an aircraft will be required to hold short of a runway or other points along the taxi route.
“Runway (number) taxi via (route as necessary).”
“Runway (number) taxi via (route as necessary)(hold short instructions as necessary).”
“Runway Three Six Left, taxi via Alpha, hold short of Charlie.”
“Runway Three Six Left, taxi via Alpha, hold short of Charlie.”
“Runway Three Six Left, taxi via Alpha, hold short of Runway Two Seven Right.”
“Runway Three Six Left, taxi via Charlie, cross Runway Two Seven Left at Charlie, hold short of Runway Two Seven Right.”
“Runway Three Six Left, taxi via Alpha, Charlie, cross Runway One Zero.”
c. Aircraft must receive a clearance for each runway their route crosses. An aircraft must have crossed a previous runway before another runway crossing clearance may be issued.
“Cross Runway One Six Left at Charlie, hold short of Runway One Six Right.”
d. When an aircraft is instructed to “follow” traffic and requires a runway crossing, issue a runway crossing clearance in addition to the follow instructions and/or hold short instructions, as applicable.
“Follow (traffic), cross Runway Two Seven Right at Delta.”
“Follow (traffic), cross Runway Two Seven Right at Delta, hold short Runway Two Seven Left.”
e. At those airports where the taxi distance between runway centerlines is less than 1,000 feet, multiple runway crossings may be issued with a single clearance.
Your job as the ground controller, also requires you to ensure that the aircraft are not required to put themselves into any immediate threat or danger. This is ensured by not requiring them to use more power than normal nor taxi at greater speeds than they usually would, as well as not requiring smaller aircraft to taxi in close proximity of larger aircraft's jet blast or hovering helicopter's jetwash.
Ground control should taxi departures to the active runways specified by Tower. Whenever the airport is busy, ground control should strive to taxi each departure to the runway most closely aligned with its direction of departure regardless of the aircraft’s initial parking location. The purpose of this procedure is to reduce crossing of departure courses in the air, which would increase the departure controller’s workload and takeoff delays. For example, when KLAX is busy, west- and north-bound departures would be assigned runway 24L, since they make a right turn on course after departure; while south-, east-, and northeast-bound departures would be assigned runway 25L, since they make a left turn on course after departure.
At airports with only one departure runway, taxiing departures to the runway in a sequence alternating directions of departure will also increase the takeoff capacity of the runway. For example, at KSAN, it would be wise to alternate between right turn (PADRZ/PEBLE) and left turn (ZZOOO/BRDR) departures. This would allow less spacing on takeoffs (by use of divergent courses on departure), and increase in-trail spacing on each individual stream (since only every other departure will be turning in the same direction).
Be aware that a pilot may request a departure runway not designated as active (e.g. due to aircraft performance limitations). Ground must honor such a request and coordinate with Tower for when the pilot’s requested runway should be available.
Although not realistic, it’s quite common on VATSIM to see a ground controller online with no tower available. And since the tower controller is the one who’s supposed to define active runways, where should a lone ground controller taxi his departures? The answer is, it depends. Generally, you’ll taxi departures to whatever runway the pilot chooses to depart from, since any runway could be active. If a pilot doesn’t make a specific request, ground may taxi them to the runways that would normally be designated as active.
Normally, the active runway would be the one most nearly aligned with the surface wind of 5 knots or more. When the surface wind is less than 5 knots, “calm wind” runway would be used. Calm wind runways, when they exist, will be specifically designated in the vZLA SOP for the airport. Busier airports have individually developed "runway use" programs aimed to best facilitate the airport’s efficiency and/or noise abatement. Runway use programs may exceed the standard wind alignment requirements before a runway change occurs. The vZLA airports with runway use programs will specify the conditions for selecting active runways in the SOP for the airport.
It’s important to remember that whenever there’s a discrepancy between the pilot’s requested runway and the one that would normally be designated as active, the pilot’s request preempts.