Gate to Gate

Over the last couple years, I have written about many bits of technology that aircraft use. I really haven't discussed too much what bits are used when, and for what. This post, I will try and tie all the items and their use into a comprehensive post. I'll use a commercial airliner (Part 121) for the discussion, both because they typically use more technology, and because that is what my background is. I will also base most of this on flights in the US, to keep it simpler.

We can start a few hours before the flight actually leaves. As the flight approaches it's scheduled time to leave, a group of folks have started planning for the flight. Sure, there is network planning, they set up the schedules and try to be sure the flight will make money, and provide continuity, and such, but that is usually done months in advance. There are also the simulators that the pilots use for proficiency checks, and training, but that is on going and not related to a particular flight, but important none the less.

Dispatchers and meteorologists are considering the situation between the origin and the destination of the flight. The meteorologists are generalists, looking at the weather over the country, where dispatchers are more concerned with the weather along the route between the cities. The dispatcher needs to consider the situation at the specific airports, for runway closures, and other challenges unrelated to weather. There is a tool called Collaborative Decision Making (CDM), where the dispatchers work with the other airlines so everyone can utilize the airports and airspace as optimally as possible.

The dispatchers can use CDM to look for areas to avoid when selecting a route between the two cities. If there is a bad thunderstorm along the optimum route and all the other airlines are avoiding it to the south, the dispatcher may pick a northern route to stay out of everyone elses way. Once the dispatcher selects a route, they need to build the rest of the flight plan. The dispatcher will build a flight plan using many tools. The dispatcher may allow the flight planning engine to select routes, or the amount of fuel. Depending on aircraft maintenance situation, and MEL deferrals and such the flight planning engine can accurately predict the fuel burn based on weather and route.

Once the dispatchers are happy with route and fuel selected, they will file a flight plan. The flight plan will be filed with the Air Navigation Service Provider (ANSP) for both the origin and the destination. For the US the ANSP is the FAA, in Canada is is NavCanada, and in the UK it is NATS. The ANSP handles all the RADAR and air traffic control (ATC) functions. The flight plan will give the ATC controllers a heads up on what the aircraft was planning on doing once in the air.

As the pilots get to the aircraft, one will typically do a walk around of the outside of the aircraft making sure the aircraft looks safe and no damage is visible. The other pilot will typically go to the cockpit and begin setting things up. There may be a the initialization of the FMS, maybe a RAIM check of the GPS, and entering the flight plan prepared by the dispatcher into the FMS. The dispatcher provided flight plan will usually include weather information for the route, and any other non weather realted information for the route (IE ATC changes, etc), The flight plan will also contain fuel and time information that the pilot can double check, insuring the dispatcher hasn't made any mistakes.

When the pilot know the fuel situation, they may confer with the fueler to adjust any fuel amounts to be put on board. The pilot will also need to know how many bags and passengers are on the flight, so they may make proper weight and balance calculations. Some airlines have a load planner who takes care of the weight and balance, others still let the pilot take care of this. Depending on the aircraft, it may be necessary to have a person dedicated to making the load calculations.

As all the passengers are seated, and the pilot is about to move the aircraft, they will ask for permission to move. There may be a ground controller dedicated to the gate area, and there will need to be taxi clearances and such needed from them. Other airports everything is controlled from the tower, and any movement must be cleared through the tower controllers. An ACARs message may be sent requesting the Pre Departure Clearance (PDC), that will be a version of the flight plan sent to the ATC with any ATCneeded changes to the plan. The PDC will also contain the code the pilot needs to enter into the transponder. The pilot must acknowledge receipt of the message.

After the aircraft is taxied to the runway, the pilot will ask the tower for the final airport clearance, by announcing "ready for takeoff". Once the ATC controller gives the pilot final instructions the pilot can access the runway and start the takeoff. The ATC instructions will be the route the pilot should take to get from the runway to the beginning of the flight plan route. Every bit of the instructions and plans for the takeoff are there in case there is a failure. If there is a radio failure either from ATC or the Aircraft, the instructions given are good enough for the pilot to take off, fly the planned route, and approach the destination. It is a safety situation, should the plan be the safest and most expeditious way to fly the route.

Once the aircraft is above about 300 ft, depending on the  airport, the aircraft will appear on RADAR. The first RADAR that will show the aircraft is the TRACON, who will control the aircraft after tower hands off the aircraft. The TRACON controller will control the aircraft until it is more than 30-50 miles from the airport. The TRACON will hand the aircraft off the enroute controllers who will control the flight until it is 30-60 miles from the destination airport. The RADAR data will be collected and sent to the FAA command center for others to view, and use the ADSI information. As we move into NextGen, there may be more ADS/B position reporting, instead of RADAR.

Once the aircraft is on the route, the pilots will typically engage the autopilot. The autopilot will help maintain the route of flight, altitude and throttle settings to insure the aircraft flies the route planned, and uses the fuel planned. The pilot must monitor the autopilot to be sure it is engaged, and doing the right thing the whole flight. Occasionally pilots will hand fly the aircraft, for practice. Once in a while the autopilot will fail, and the pilots must had fly the aircraft. The systems in the aircraft are designed for certain reliability levels.

During the enroute portion of the flight, there may be messages the pilots need to send to the company operations center. The pilots will usually send a text message over ACARS if they don't have a lot of urgency to the message. The pilots also have an option for voice communication using company assigned frequencies. If there were to be a medical emergency, the voice communications will be used, if a pilot is looking for a weather report for 400 miles ahead, they will use ACARS.

As the aircraft gets closer to the destination, ATC will typically begin having the aircraft start to descend. Newer approaches follow a continuous descent profile, where the pilots set the throttles to idle at altitude, and basically use the potential energy to glide the aircraft to the runway, reducing noise, fuel burn and pollution.

Current approaches typically are designed for the aircraft to provide it's own guidance. That is there are airs on the ground (or satellite) to provide the aircraft the information it needs to know where it is, and fly to the runway. Features like DME and ILS radions are on the ground, and GPS satellites are in the air.

The enroute controller will hand the flight off to the TRACON controller about 50 miles from the airport, where the aircraft will be below about 10,000ft. The TRACON controller will clear the flight for the approach that it will use to get to the airport. At about 5 miles out, the pilot will be told to contact the tower, and the tower and the pilot will make the final checks and be cleared for the runway to land on. Once on the ground, the pilot will talk to the ground controllers to get to the proper parking area, and maybe a gate controller for certain airports. Once the wheels are chocked, and the engines shutdown, the pilots are mostly done with the flight.

Yes, there is a bit of technology going on between each gate, and a little before. Ever think about that before.

Transponders

Surveillance RADAR is very useful, and can be augmented like GPS. RADAR alone can only tell range and azimuth. If the transponder is added, the range and azimuth can be augmented with altitude and identification.

Transponders are transceivers, like DME. When the RADAR interrogation is received, the transponder transmits a specific response. For most GA aircraft, the response will be the identification (mode A), and the altitude (Mode C). The transponder concept comes from a technology developed around WWII, used for positively identifying friend or foe targets (IFF). Early targeting RADAR on aircraft could identify targets, but not who they were. Occasionally people will still call a transponder  an IFF box.

If the transponder is Mode S, the response will include quite a bit more information. The mode  S transponder has a payload capable of holding identification, altitude, and various other information, depending on mode. The message can be 56 or 112 (extended squitter or ES) bytes and include a 24 digit ICAO identifier assigned to each unique aircraft. Location and speed can also be encoded in the response. There are various modes the Mode S transponder will work in. (For a good article that covers much of the ModeS modes, see this EETimes article)

Usually aircraft transponders will transmit on 1090MHz. TCAS receivers and RADAR antennas will all be expecting to receive messages on 1090MHz. The transponder is mostly listening, but can be quite busy in class B airspace, with several TCAS units pinging traffic in the area. 

The code that is entered in the transponder is asigned by ATC. There are only 4096 unique codes, and some are reserved (IE 1200, 0000, etc). The numbers are limited to 0-7 or octal digits (octal = 8, and 0-7 are 8 distinct values). Octal is a throwback to early computers that were used for Air Traffic Control, and numbers were represented in octal values. On a busy day, there may be more than 4000 aircraft in the air at once, how does air traffic control keep conflicts out? Certain ranges of transponder (squawk) codes are reserved for local traffic (staying in the area, like training, or ferry flights). Other ranges are for long distance flying, some east, some west, depending on origin and destination. Occasionally, something unexpected happens, and two aircraft with the same transponder code appear in the same area, and the RADAR display will alert the controller to that.

The altitude that all the transponders send is pressure altitude. Pilots will set the altimeter on the ground to local barometric pressure. The altitude encoder attached to the transponder is not adjusted to local barometer. ATC will set their scope to the local pressure. Having ATC consistently reading the same uncompensated pressure will allow more consistent readings aircraft to aircraft. Sometimes pilots will forget to change their altimeter, or set it wrong, and this would cause trouble for ATC trying to figure out what everyone's altitude is. If ATC is saying the aircraft reporting altitude is significantly different than the pilot thinks they are flying, ATC may ask the pilot to stop reporting altitude, and the pilot will switch to Mode A.

Most Mode S transponders are capable or working in Mode A/C or just Mode A as well. Mode S transponders, with their large payloads can be used for ADS/B as well. ADS/B will require other  transponders in the area to be sending specific payloads, in order to plot the position on the receiving aircraft's display.

Transponders add a great deal to RADAR. Transponders will stay on aircraft even after the aircraft are switched to ADS/B. Eventually, the need for a transponder will be replaced by the ADS/B system, but that may be many years. 

ADS/B and RNP

There are many uses for the GPS data in and out of the aircraft. The GPS location is very accurate, normally. Knowing where the aircraft thinks it is can help ATC in many instances.

Shortcomings of RADAR

RADAR will send out a radio signal in a cone shape. The farther the aircraft is from the antenna, the larger the target will appear on the radar screen. The location displayed to the air traffic controller isn't as accurate when the aircraft is far from the antenna. The RADAR cannot "see" straight up either, so if the aircraft flies directly over the antenna, the software has to guess where the aircraft will be.

RADAR can only get range and azimuth information. The RADAR cannot determine altitude. Altitude information is sent from the aircraft in  the transponder message. When the aircraft transponder hears the RADAR interrogation, the transponder responds with the transponder code and altitude (with mode-s, there may be more information).

Most short range RADAR has about a 5 second sweep. The long range radars have a 12 second sweep. The sweep time is how long the RADAR antenna takes to turn once. The sweep time is how long it takes between aircraft updates. If the aircraft is going 600knots, and the sweep is 12 seconds, the aircraft moves about 2 miles between sweeps. 

The RADAR software has to do some correlations between the raw RADAR range and  azimuth information, and the transponder code altitude message. Usually, there is only one aircraft that comes into RADAR range at a time at the same point, so it is easy to correlate this, but occasionally two targets may appear at different altitudes at the same place. The software will occasionally get this wrong.

Why ADS/B is Better

ABS/B out messages from the aircraft will usually be the same quality. The GPS accuracy will be pretty consistent in an area, and be very accurate. The target drawn on the air traffic controllers screen will be the same size as the target moves across the screen.

The ADS/B message will contain both location and altitude information in a single message. The software will not need to correlate that data. During times when the ADS/B aircraft are operating in the RADAR environment, correlation will still be done with the raw RADAR and the transponder messages. The ADS/B information will only make correlation more accurate.

The ADS/B location is broadcast about once a second. The controllers screen will update every time it hears the ADS/B signal.

ADS/B will broadcast to the aircraft in the area without relying on ground station. The FAA having two frequencies, 978MHz and1090ES almost requires a ground station for ADS/B to work. There are other benefits to the ground stations, in that they will broadcast weather (FIS/B) and traffic (TIS/B) from non-participating aircraft.

RNP and ANP

In most of this and previous articles I have hesitated on specifying the accuracy of GPS location information. GPS accuracy changes during the day, and in certain locations. The current accuracy is defined as Actual Navigation Performance (ANP) and is measured in miles. Sometimes the ANP will be down to feet in all directions, sometimes it will be in miles.

To fly a GPS approach it is necessary to have an ANP of 0.3 miles or better. 0.3 miles means the receiver is able to pinpoint it's position to less than 1500 feet. The 0.3 mile value is called the Required Navigation Performance (RNP). To fly with any more precision, special training is needed. The FAA has many public RNP approaches with levels as low as 0.1 miles, or about 500ft.

This sounds pretty sloppy, 500ft is bigger than 10 houses.  Remember, the GPS receiver is calculating where it was when it heard the last update from the satellite, but the aircraft is carrying that receiver at 150-200kts on final.  The receiver is throwing values into the Kalman filter as fast as it can, and guessing that the pilot won't turn more that 3 degrees per second, assuming a mostly straight course.  It ain't easy, 500ft is pretty good.

Many commercial aircraft display the ANP and RNP values at the bottom of the navigation display. The display on a 737 is the instrument on the left (see slightly above this link).

NextGen future

There is a lot to NextGen technologies. Alaska has been playing with ADS/B since the late 1990's. The FAA is financing more Capstone work in Alaska even in the time of sequestering. Much of the enroute NextGen requires ERAM, but that is still in process, and is partially on hold until the FAA gets their financial situation in order.

The FAA is currently wanting to require all aircraft to participate in ADS/B out by 2020. There will be some challenges to that. With the current financial situation and things being on hold, will the FAA be ready for all aircraft to be using ADS/B? What about the Luscome 8F that never had an alternator, what will it use to power the GPS and transmitter needed? What will the FAA use to track aircraft with electrical failure?

Some airlines are equipping their aircraft with RNP and ADS/B. Some have had a challenge reaping the benefits from the upgrades. The other airlines are waiting until some indication they will reap some benefits. The FAA and the airlines are still trying to figure out the best time to move forward.