1 // FGGround - a class to provide ground control at larger airports.
3 // Written by David Luff, started March 2002.
5 // Copyright (C) 2002 David C. Luff - david.luff@nottingham.ac.uk
7 // This program is free software; you can redistribute it and/or
8 // modify it under the terms of the GNU General Public License as
9 // published by the Free Software Foundation; either version 2 of the
10 // License, or (at your option) any later version.
12 // This program is distributed in the hope that it will be useful, but
13 // WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 // General Public License for more details.
17 // You should have received a copy of the GNU General Public License
18 // along with this program; if not, write to the Free Software
19 // Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
21 #include <simgear/misc/sg_path.hxx>
22 #include <simgear/math/sg_random.h>
23 #include <simgear/debug/logstream.hxx>
24 #include <simgear/misc/sgstream.hxx>
25 #include <simgear/constants.h>
26 #include <Main/globals.hxx>
33 #include "ATCutils.hxx"
34 #include "ATCdisplay.hxx"
35 #include "AILocalTraffic.hxx"
37 SG_USING_STD(ifstream);
44 for(unsigned int i=0; i < arcs.size(); ++i) {
49 // Make sure that a_path.cost += distance is safe from the moment it's created.
54 FGGround::FGGround() {
55 ATCmgr = globals->get_ATC_mgr();
57 networkLoadOK = false;
58 ground_traffic.erase(ground_traffic.begin(), ground_traffic.end());
59 ground_traffic_itr = ground_traffic.begin();
61 // Init the property nodes - TODO - need to make sure we're getting surface winds.
62 wind_from_hdg = fgGetNode("/environment/wind-from-heading-deg", true);
63 wind_speed_knots = fgGetNode("/environment/wind-speed-kt", true);
65 // TODO - get the actual airport elevation
69 FGGround::FGGround(string id) {
70 ATCmgr = globals->get_ATC_mgr();
72 networkLoadOK = false;
73 ground_traffic.erase(ground_traffic.begin(), ground_traffic.end());
74 ground_traffic_itr = ground_traffic.begin();
77 // Init the property nodes - TODO - need to make sure we're getting surface winds.
78 wind_from_hdg = fgGetNode("/environment/wind-from-heading-deg", true);
79 wind_speed_knots = fgGetNode("/environment/wind-speed-kt", true);
81 // TODO - get the actual airport elevation
85 FGGround::~FGGround() {
88 void FGGround::ParseRwyExits(node* np, char* es) {
92 const char delimiters[] = "-";
93 token = strtok(estr, delimiters);
94 while(token != NULL) {
96 //cout << "token = " << token << endl;
97 //cout << "rwy number = " << i << endl;
98 //runways[(atoi(token))].exits.push_back(np);
99 runways[i].exits.push_back(np);
100 //cout << "token = " << token << '\n';
101 token = strtok(NULL, delimiters);
106 // Load the ground logical network of the current instances airport
107 // Return true if successfull.
108 // TODO - currently the file is assumed to reside in the base/ATC directory.
109 // This might change to something more thought out in the future.
110 // NOTE - currently it is assumed that all nodes are loaded before any arcs.
111 // It won't work ATM if this doesn't hold true.
112 bool FGGround::LoadNetwork() {
117 int gateCount = 0; // This is used to allocate gateID's from zero upwards
118 // This may well change in the future - probably to reading in the real-world
119 // gate numbers from file.
122 SGPath path = globals->get_fg_root();
123 //string taxiPath = "ATC/" + ident + ".taxi";
124 string taxiPath = "ATC/KEMT.taxi"; // FIXME - HARDWIRED FOR TESTING
125 path.append(taxiPath);
127 SG_LOG(SG_ATC, SG_INFO, "Trying to read taxiway data for " << ident << "...");
128 //cout << "Trying to read taxiway data for " << ident << "..." << endl;
129 fin.open(path.c_str(), ios::in);
131 SG_LOG(SG_ATC, SG_ALERT, "Unable to open taxiway data input file " << path.c_str());
132 //cout << "Unable to open taxiway data input file " << path.c_str() << endl;
140 // Node, arc, or [End]?
141 //cout << "Read in ground network element type = " << buf << endl;
142 if(!strcmp(buf, "[End]")) { // TODO - maybe make this more robust to spelling errors by just looking for '['
143 SG_LOG(SG_ATC, SG_INFO, "Done reading " << path.c_str() << endl);
145 } else if(!strcmp(buf, "N")) {
148 np->struct_type = NODE;
150 np->nodeID = atoi(buf);
152 np->pos.setlon(atof(buf));
154 np->pos.setlat(atof(buf));
156 np->pos.setelev(atof(buf));
157 fin >> buf; // node type
158 if(!strcmp(buf, "J")) {
160 } else if(!strcmp(buf, "T")) {
161 np->type = TJUNCTION;
162 } else if(!strcmp(buf, "H")) {
165 SG_LOG(SG_ATC, SG_ALERT, "**** ERROR ***** Unknown node type in taxi network...\n");
169 fin >> buf; // rwy exit information - gets parsed later - FRAGILE - will break if buf is reused.
171 fin >> ch; // strip the leading " off
174 fin.unsetf(ios::skipws);
176 if((ch == '"') || (ch == 0x0A)) {
178 } // we shouldn't need the 0x0A but it makes a nice safely in case someone leaves off the "
181 fin.setf(ios::skipws);
182 network.push_back(np);
183 // FIXME - fragile - replies on buf not getting modified from exits read to here
184 // see if we also need to push it onto the runway exit list
185 //cout << "strlen(buf) = " << strlen(buf) << endl;
186 if(strlen(buf) > 2) {
187 //cout << "Calling ParseRwyExits for " << buf << endl;
188 ParseRwyExits(np, buf);
190 } else if(!strcmp(buf, "A")) {
192 ap->struct_type = ARC;
198 if(!strcmp(buf, "R")) {
200 } else if(!strcmp(buf, "T")) {
203 SG_LOG(SG_ATC, SG_ALERT, "**** ERROR ***** Unknown arc type in taxi network...\n");
209 if(!strcmp(buf, "Y")) {
211 } else if(!strcmp(buf, "N")) {
212 ap->directed = false;
214 SG_LOG(SG_ATC, SG_ALERT, "**** ERROR ***** Unknown arc directed value in taxi network - should be Y/N !!!\n");
221 fin.unsetf(ios::skipws);
224 if((ch == '"') || (ch == 0x0A)) {
226 } // we shouldn't need the 0x0A but it makes a nice safely in case someone leaves off the "
228 fin.setf(ios::skipws);
229 ap->distance = (int)dclGetHorizontalSeparation(network[ap->n1]->pos, network[ap->n2]->pos);
230 //cout << "Distance = " << ap->distance << '\n';
231 network[ap->n1]->arcs.push_back(ap);
232 network[ap->n2]->arcs.push_back(ap);
233 } else if(!strcmp(buf, "G")) {
235 gp->struct_type = NODE;
238 gp->nodeID = atoi(buf);
240 gp->pos.setlon(atof(buf));
242 gp->pos.setlat(atof(buf));
244 gp->pos.setelev(atof(buf));
245 fin >> buf; // gate type - ignore this for now
246 fin >> buf; // gate heading
247 gp->heading = atoi(buf);
251 fin.unsetf(ios::skipws);
254 if((ch == '"') || (ch == 0x0A)) {
256 } // we shouldn't need the 0x0A but it makes a nice safely in case someone leaves off the "
258 fin.setf(ios::skipws);
259 gp->id = gateCount; // Warning - this will likely change in the future.
261 network.push_back(gp);
262 gates[gateCount] = gp;
265 // Something has gone seriously pear-shaped
266 SG_LOG(SG_ATC, SG_ALERT, "********* ERROR - unknown ground network element type... aborting read of " << path.c_str() << '\n');
275 void FGGround::Init() {
279 // Figure out which is the active runway - TODO - it would be better to have ground call tower
280 // for runway operation details, but at the moment we can't guarantee that tower control at a given airport
281 // will be initialised before ground so we can't do that.
283 //cout << "In FGGround::Init, active rwy is " << activeRwy << '\n';
284 ortho.Init(rwy.threshold_pos, rwy.hdg);
286 networkLoadOK = LoadNetwork();
289 void FGGround::Update(double dt) {
290 // Each time step, what do we need to do?
291 // We need to go through the list of outstanding requests and acknowedgements
292 // and process at least one of them.
293 // We need to go through the list of planes under our control and check if
294 // any need to be addressed.
295 // We need to check for planes not under our control coming within our
296 // control area and address if necessary.
298 // Lets take the example of a plane which has just contacted ground
299 // following landing - presumably requesting where to go?
300 // First we need to establish the position of the plane within the logical network.
301 // Next we need to decide where its going.
303 if(ground_traffic.size()) {
304 if(ground_traffic_itr == ground_traffic.end()) {
305 ground_traffic_itr = ground_traffic.begin();
308 //Process(*ground_traffic_itr);
309 GroundRec* g = *ground_traffic_itr;
310 if(g->taxiRequestOutstanding) {
311 double responseTime = 10.0; // seconds - this should get more sophisticated at some point
312 if(g->clearanceCounter > responseTime) {
314 // TODO - move the mechanics of making up the transmission out of the main Update(...) routine.
316 trns += g->plane.callsign;
317 trns += " taxi holding point runway "; // TODO - add the holding point name
318 // eg " taxi holding point G2 runway "
319 trns += ConvertRwyNumToSpokenString(activeRwy);
321 globals->get_ATC_display()->RegisterSingleMessage(trns, 0);
323 g->planePtr->RegisterTransmission(1); // cleared to taxi
324 g->clearanceCounter = 0.0;
325 g->taxiRequestOutstanding = false;
327 g->clearanceCounter += (dt * ground_traffic.size());
329 } else if(((FGAILocalTraffic*)(g->planePtr))->AtHoldShort()) { // That's a hack - eventually we should monitor actual position
330 // HACK ALERT - the automatic cast to AILocalTraffic has to go once we have other sorts working!!!!! FIXME TODO
331 // NOTE - we don't need to do the contact tower bit unless we have separate tower and ground
332 string trns = g->plane.callsign;
333 trns += " contact Tower ";
334 double f = globals->get_ATC_mgr()->GetFrequency(ident, TOWER);
336 sprintf(buf, "%f", f);
339 globals->get_ATC_display()->RegisterSingleMessage(trns, 0);
341 g->planePtr->RegisterTransmission(2); // contact tower
342 delete *ground_traffic_itr;
343 ground_traffic.erase(ground_traffic_itr);
344 ground_traffic_itr = ground_traffic.begin();
346 ++ground_traffic_itr;
350 // Figure out which runways are active.
351 // For now we'll just be simple and do one active runway - eventually this will get much more complex
352 // Copied from FGTower - TODO - it would be better to implement this just once, and have ground call tower
353 // for runway operation details, but at the moment we can't guarantee that tower control at a given airport
354 // will be initialised before ground so we can't do that.
355 void FGGround::DoRwyDetails() {
356 //cout << "GetRwyDetails called" << endl;
358 // Based on the airport-id and wind get the active runway
361 double hdg = wind_from_hdg->getDoubleValue();
362 double speed = wind_speed_knots->getDoubleValue();
363 hdg = (speed == 0.0 ? 270.0 : hdg);
364 //cout << "Heading = " << hdg << '\n';
367 bool rwyGood = globals->get_runways()->search(ident, int(hdg), &runway);
369 activeRwy = runway.rwy_no;
370 rwy.rwyID = runway.rwy_no;
371 SG_LOG(SG_ATC, SG_INFO, "In FGGround, active runway for airport " << ident << " is " << activeRwy);
373 // Get the threshold position
374 double other_way = runway.heading - 180.0;
375 while(other_way <= 0.0) {
378 // move to the +l end/center of the runway
379 //cout << "Runway center is at " << runway.lon << ", " << runway.lat << '\n';
380 Point3D origin = Point3D(runway.lon, runway.lat, aptElev);
381 Point3D ref = origin;
382 double tshlon, tshlat, tshr;
383 double tolon, tolat, tor;
384 rwy.length = runway.length * SG_FEET_TO_METER;
385 geo_direct_wgs_84 ( aptElev, ref.lat(), ref.lon(), other_way,
386 rwy.length / 2.0 - 25.0, &tshlat, &tshlon, &tshr );
387 geo_direct_wgs_84 ( aptElev, ref.lat(), ref.lon(), runway.heading,
388 rwy.length / 2.0 - 25.0, &tolat, &tolon, &tor );
389 // Note - 25 meters in from the runway end is a bit of a hack to put the plane ahead of the user.
390 // now copy what we need out of runway into rwy
391 rwy.threshold_pos = Point3D(tshlon, tshlat, aptElev);
392 Point3D takeoff_end = Point3D(tolon, tolat, aptElev);
393 //cout << "Threshold position = " << tshlon << ", " << tshlat << ", " << aptElev << '\n';
394 //cout << "Takeoff position = " << tolon << ", " << tolat << ", " << aptElev << '\n';
395 rwy.hdg = runway.heading;
396 // Set the projection for the local area based on this active runway
397 ortho.Init(rwy.threshold_pos, rwy.hdg);
398 rwy.end1ortho = ortho.ConvertToLocal(rwy.threshold_pos); // should come out as zero
399 rwy.end2ortho = ortho.ConvertToLocal(takeoff_end);
401 SG_LOG(SG_ATC, SG_ALERT, "Help - can't get good runway in FGTower!!");
406 // Return a random gate ID of an unused gate.
407 // Two error values may be returned and must be checked for by the calling function:
408 // -2 signifies that no gates exist at this airport.
409 // -1 signifies that all gates are currently full.
410 int FGGround::GetRandomGateID() {
411 // Check that this airport actually has some gates!!
416 gate_vec_type gateVec;
421 gatesItr = gates.begin();
422 while(gatesItr != gates.end()) {
423 if((gatesItr->second)->used == false) {
424 gateVec.push_back(gatesItr->second);
430 // Check that there are some unused gates!
431 if(!gateVec.size()) {
435 // Randomly select one from the list
437 thenum = (int)(sg_random() * gateVec.size());
438 ID = gateVec[thenum]->id;
443 // Return a pointer to an unused gate node
444 Gate* FGGround::GetGateNode() {
445 int id = GetRandomGateID();
454 // WARNING - This is hardwired to my prototype logical network format
455 // and will almost certainly change when Bernie's stuff comes on-line.
456 // Returns NULL if it can't find a valid node.
457 node* FGGround::GetThresholdNode(string rwyID) {
458 // For now go through all the nodes and parse their names
459 // Maybe in the future we'll map threshold nodes by ID
460 //cout << "Size of network is " << network.size() << '\n';
461 for(unsigned int i=0; i<network.size(); ++i) {
462 //cout << "Name = " << network[i]->name << '\n';
463 if(network[i]->name.size()) {
464 string s = network[i]->name;
465 // Warning - the next bit is fragile and dependent on my current naming scheme
466 //cout << "substr = " << s.substr(0,3) << '\n';
467 //cout << "size of s = " << s.size() << '\n';
468 if(s.substr(0,3) == "rwy") {
469 //cout << "subsubstr = " << s.substr(4, s.size() - 4) << '\n';
470 if(s.substr(4, s.size() - 4) == rwyID) {
480 // Get a path from a point on a runway to a gate
483 // Get a path from a node to another node
484 // Eventually we will need complex algorithms for this taking other traffic,
485 // shortest path and suitable paths into accout.
486 // For now we'll just call the shortest path algorithm.
487 ground_network_path_type FGGround::GetPath(node* A, node* B) {
488 return(GetShortestPath(A, B));
491 // Get a path from a node to a runway threshold
492 ground_network_path_type FGGround::GetPath(node* A, string rwyID) {
493 node* b = GetThresholdNode(rwyID);
495 SG_LOG(SG_ATC, SG_ALERT, "ERROR - unable to find path to runway theshold in ground.cxx\n");
496 ground_network_path_type emptyPath;
497 emptyPath.erase(emptyPath.begin(), emptyPath.end());
500 return GetShortestPath(A, b);
503 // Get a path from a node to a runway hold short point
504 // Bit of a hack this at the moment!
505 ground_network_path_type FGGround::GetPathToHoldShort(node* A, string rwyID) {
506 ground_network_path_type path = GetPath(A, rwyID);
507 path.pop_back(); // That should be the threshold stripped of
508 path.pop_back(); // and that should be the arc from hold short to threshold
509 // This isn't robust though - TODO - implement properly!
513 // A shortest path algorithm from memory (ie. I can't find the bl&*dy book again!)
514 // I'm sure there must be enchancements that we can make to this, such as biasing the
515 // order in which the nodes are searched out from in favour of those geographically
516 // closer to the destination.
517 // Note that we are working with the master set of nodes and arcs so we mustn't change
518 // or delete them - we only delete the paths that we create during the algorithm.
519 ground_network_path_type FGGround::GetShortestPath(node* A, node* B) {
521 shortest_path_map_type pathMap;
522 node_array_type nodesLeft;
525 int pathsCreated = 0;
527 // Initialise the algorithm
528 nodesLeft.push_back(A);
529 pathPtr = new a_path;
531 pathPtr->path.push_back(A);
533 pathMap[A->nodeID] = pathPtr;
534 bool solution_found = false; // Flag to indicate that at least one candidate path has been found
535 int solution_cost = -1; // Cost of current best cost solution. -1 indicates no solution found yet.
536 a_path solution_path;
538 node* nPtr; // nPtr is used to point to the node we are currently working with
540 while(nodesLeft.size()) {
541 //cout << "\n*****nodesLeft*****\n";
542 //for(unsigned int i=0; i<nodesLeft.size(); ++i) {
543 //cout << nodesLeft[i]->nodeID << '\n';
545 //cout << "*******************\n\n";
546 nPtr = *nodesLeft.begin(); // Thought - definate optimization possibilities here in the choice of which nodes we process first.
547 nodesLeft.erase(nodesLeft.begin());
548 //cout << "Walking out from node " << nPtr->nodeID << '\n';
549 for(unsigned int i=0; i<nPtr->arcs.size(); ++i) {
550 //cout << "ARC TO " << ((nPtr->arcs[i]->n1 == nPtr->nodeID) ? nPtr->arcs[i]->n2 : nPtr->arcs[i]->n1) << '\n';
552 if((solution_found) && (solution_cost <= pathMap[nPtr->nodeID]->cost)) {
553 // Do nothing - we've already found a solution and this partial path is already more expensive
555 // This path could still be better than the current solution - check it out
556 for(unsigned int i=0; i<(nPtr->arcs.size()); i++) {
557 // Map the new path against the end node, ie. *not* the one we just started with.
558 unsigned int end_nodeID = ((nPtr->arcs[i]->n1 == nPtr->nodeID) ? nPtr->arcs[i]->n2 : nPtr->arcs[i]->n1);
559 //cout << "end_nodeID = " << end_nodeID << '\n';
560 //cout << "pathMap size is " << pathMap.size() << '\n';
561 if(end_nodeID == nPtr->nodeID) {
562 //cout << "Circular arc!\n";
563 // Then its a circular arc - don't bother!!
564 //nPtr->arcs.erase(nPtr->arcs.begin() + i);
566 // see if the end node is already in the map or not
567 if(pathMap.find(end_nodeID) == pathMap.end()) {
568 //cout << "Not in the map" << endl;;
569 // Not in the map - easy!
570 pathPtr = new a_path;
572 *pathPtr = *pathMap[nPtr->nodeID]; // *copy* the path
573 pathPtr->path.push_back(nPtr->arcs[i]);
574 pathPtr->path.push_back(network[end_nodeID]);
575 pathPtr->cost += nPtr->arcs[i]->distance;
576 pathMap[end_nodeID] = pathPtr;
577 nodesLeft.push_back(network[end_nodeID]); // By definition this can't be in the list already, or
578 // it would also have been in the map and hence OR'd with this one.
579 if(end_nodeID == B->nodeID) {
580 //cout << "Solution found!!!" << endl;
581 // Since this node wasn't in the map this is by definition the first solution
582 solution_cost = pathPtr->cost;
583 solution_path = *pathPtr;
584 solution_found = true;
587 //cout << "Already in the map" << endl;
588 // In the map - not so easy - need to get rid of an arc from the higher cost one.
589 //cout << "Current cost of node " << end_nodeID << " is " << pathMap[end_nodeID]->cost << endl;
590 int newCost = pathMap[nPtr->nodeID]->cost + nPtr->arcs[i]->distance;
591 //cout << "New cost is of node " << nPtr->nodeID << " is " << newCost << endl;
592 if(newCost >= pathMap[end_nodeID]->cost) {
593 // No need to do anything.
594 //cout << "Not doing anything!" << endl;
596 delete pathMap[end_nodeID];
599 pathPtr = new a_path;
601 *pathPtr = *pathMap[nPtr->nodeID]; // *copy* the path
602 pathPtr->path.push_back(nPtr->arcs[i]);
603 pathPtr->path.push_back(network[end_nodeID]);
604 pathPtr->cost += nPtr->arcs[i]->distance;
605 pathMap[end_nodeID] = pathPtr;
607 // We need to add this node to the list-to-do again to force a recalculation
608 // onwards from this node with the new lower cost to node cost.
609 nodesLeft.push_back(network[end_nodeID]);
611 if(end_nodeID == B->nodeID) {
612 //cout << "Solution found!!!" << endl;
613 // Need to check if there is a previous better solution
614 if((solution_cost < 0) || (pathPtr->cost < solution_cost)) {
615 solution_cost = pathPtr->cost;
616 solution_path = *pathPtr;
617 solution_found = true;
627 // delete all the paths before returning
628 shortest_path_map_iterator spItr = pathMap.begin();
629 while(spItr != pathMap.end()) {
630 if(spItr->second != NULL) {
631 delete spItr->second;
637 //cout << "pathsCreated = " << pathsCreated << '\n';
638 if(pathsCreated > 0) {
639 SG_LOG(SG_ATC, SG_ALERT, "WARNING - Possible memory leak in FGGround::GetShortestPath\n\
640 Please report to flightgear-devel@flightgear.org\n");
643 //cout << (solution_found ? "Result: solution found\n" : "Result: no solution found\n");
644 return(solution_path.path); // TODO - we really ought to have a fallback position incase a solution isn't found.
649 // Randomly or otherwise populate some of the gates with parked planes
650 // (might eventually be done by the AIMgr if and when lots of AI traffic is generated)
652 // Return a list of exits from a given runway
653 // It is up to the calling function to check for non-zero size of returned array before use
654 node_array_type FGGround::GetExits(string rwyID) {
655 // FIXME - get a 07L or similar in here and we're stuffed!!!
656 return(runways[atoi(rwyID.c_str())].exits);
659 void FGGround::RequestDeparture(PlaneRec plane, FGAIEntity* requestee) {
660 // For now we'll just automatically clear all planes to the runway hold.
661 // This communication needs to be delayed 20 sec or so from receiving the request.
662 // Even if display=false we still need to start the timer in case display=true when communication starts.
663 // We also need to bear in mind we also might have other outstanding communications, although for now we'll punt that issue!
664 // FIXME - sort the above!
666 // HACK - assume that anything requesting departure is new for now - FIXME LATER
667 GroundRec* g = new GroundRec;
669 g->planePtr = requestee;
670 g->taxiRequestOutstanding = true;
671 g->clearanceCounter = 0;
674 // TODO - need to handle the next 3 as well
675 //Point3D current_pos;
677 //node* last_clearance;
679 ground_traffic.push_back(g);
683 void FGGround::NewArrival(plane_rec plane) {
684 // What are we going to do here?
685 // We need to start a new ground_rec and add the plane_rec to it
686 // We need to decide what gate we are going to clear it to.
687 // Then we need to add clearing it to that gate to the pending transmissions queue? - or simply transmit?
688 // Probably simply transmit for now and think about a transmission queue later if we need one.
689 // We might need one though in order to add a little delay for response time.
690 ground_rec* g = new ground_rec;
691 g->plane_rec = plane;
692 g->current_pos = ConvertWGS84ToXY(plane.pos);
693 g->node = GetNode(g->current_pos); // TODO - might need to sort out node/arc here
696 ground_traffic.push_back(g);
700 void FGGround::NewContact(plane_rec plane) {
701 // This is a bit of a convienience function at the moment and is likely to change.
702 if(at a gate or apron)
708 void FGGround::NextClearance(ground_rec &g) {
709 // Need to work out where we can clear g to.
710 // Assume the pilot doesn't need progressive instructions
711 // We *should* already have a gate or holding point assigned by the time we get here
712 // but it wouldn't do any harm to check.
714 // For now though we will hardwire it to clear to the final destination.
717 void FGGround::AssignGate(ground_rec &g) {
718 // We'll cheat for now - since we only have the user's aircraft and a couple of airports implemented
719 // we'll hardwire the gate!
720 // In the long run the logic of which gate or area to send the plane to could be somewhat non-trivial.