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. 

Whats wrong with RADAR?

Ever see the news reports about the "World War II RADAR technology"? The headlines are usually provided by the FAA or other vendors when talking about NextGen technologies. RADAR has significantly improved since World War II. It is much more reliable, more consistent, and more accurate. The output now is mostly digital, and requires little adjusting.

All RADAR systems work by sending out a radio signal, and listening for that signal to bounce off a target, and timing the round trip of the signal. A passive RADAR signal is one where dish sends out a signal, and listens for the return. The passive RADAR message can only measure distance from the dish. Knowing the orientation of the antenna when the target distance was measured will allow the operator to know the range and azimuth of the target relative to the antenna.

The radio signal goes about the speed of light through the air, or about one foot per nanosecond, or about 5ms per mile, and remembering to double that for the round trip, will allow the RADAR system to determine the distance.

The RADAR dish is used to focus the transmitted signal, as well as the return signal. The pointy part near the bottom of the dish is the antenna for both the transmitter and receiver. The dish is a parabolic reflector, with the antenna at the focus point. The antennas are aimed at the dish. While the antenna does a good job of focusing the signal, it still goes out in a cone shape.

RADAR will detect various targets. The metal targets reflect the radio signals well. Other material will reflect at different levels. Most aircraft have metal somewhere, including tube and fabric, composite and wooden aircraft. Water also reflects radio signals. A large blob of moisture will show up on RADAR as a target. The processor on the RADAR unit will separate the blobs of moisture from the metal things. The blobs of moisture will be called weather, and the other metal objects will be considered primary targets.  

Many dishes have a secondary surveillance antenna on them as well. Secondary surveillance is used to listen for the transponder that is on many aircraft. The transponder on the aircraft will transmit the aircraft altitude, and some other data. The transponders will automatically transmit when they hear the RADAR interrogation signal.

Mostly there are two types of RADAR in use for civil aviation in the US, enroute and tracon. Enroute RADAR, or ARSR covers a radius of about 250 miles, and the dish rotates in about 12 seconds. Tracon RADAR covers about 60 miles, and the dish rotates in about 4.7 seconds. Both RADAR types can feed computers, that allow different people to see different views of the same data.

Since the RADAR signal go goes out in a cone shape, the exact position of the aircraft is less accurate the farther the target is from the RADAR antenna. The tracon RADAR will be more accurate than the ARSR RADAR since it is turning faster, and only is looking at shorter distances.

The RADAR signal can be blocked by buildings and terrain. Buildings and terrain can also reflect signals. Reflected signals can make the targets appear to be farther away. If an aircraft is opposite terrain relative to the antenna, it won't be picked up by the RADAR. Enroute charts will have a MSA altitude indicating the lowest altitude the RADAR can allow the controllers to see the aircraft.  

The RADAR units will output various channels, weather, secondary, and primary target data.  This data will be collected by computers, and be correlated to determine a track. Correlating the secondary target with the primary data will allow a track to know an aircraft speed, altitude and location. Correlating the signals will also need to remove bogus signals, like reflections, or smallish blobs of weather.

Newer technologies called multilateration is another way to find an aircraft. The multilateration will rely on the transponder on the aircraft. The ground station will have multiple receivers in known locations. A transmitter in the area will send out a signal, the transponder will detect the signal, and respond. The ground stations will measure the time it took to receive the signal,and the difference will tell the range and azimuth of the signal. The signal will contain the altitude.

Building RADAR sites can be expensive, building multilateration sites can be significantly less. If some acreage is available, the multilateral station can be a good choice to cover mountainous terrain, rather than building new RADAR sites in the mountains. The output of the multilateration system can feed the same computer systems that are used for RADAR displays.

Modern RADAR systems are quite flexible. Much different than the "World War II technology" the newscasters present. RADAR also has the advantage of not requiring any technology on the aircraft to work. Should an aircraft have a system failure, or an operator turn off a transponder. The FAA would like to decommission RADAR, but I believe long term, they won't completely. DOD and other organizations will require their existence. 

Is that helpful?

Measuring or Are We There Yet?

I was going to do a post on Kalman Filters, and how great they are. They are great, and completely useful. A Kalman filter will let us take a bunch of error out of a collection of samples and make something useful out of the information. I was reading an article by Jack Crenshaw about Kalman filters, and he said something very profound:

 Whenever I measure any real-world parameter, there is one thing I can be sure of: the value I read off my meter is almost certainly not the actual value of that parameter.

Jack get things. He worked on getting the Apollo spaceships to the moon with little or no computers. He did most of the work prior to Kennedy saying the US will go to the moon. He was working on trajectories and such in the 1958-1960 time frame. He can present the most complex bit of math in ways that almost anyone can understand if they are willing to read what he says.

Where is got me thinking, though, is we need data to do stuff. When we get a location fix off a GPS for instance, are we sure we are there? My phone and tablet all give a DOP value, Dilution Of Precision. The DOP is the current inaccuracy of the reading I have displayed. It  involved the quality of the data we are getting from the satellites right now.

When we look at the instruments in an aircraft, do we take them as gospel? If you are like me, the altimeter only shows 100ft accurately, so I don't try to interpolate to the nearest foot. The value I can read is good enough. Same with the airspeed, I don't really care if I am going 130kts or 131kts, as long as I am maintaining a margin over stall speed.

How do you convince a computer to say close enough. Computers are a big challenge. The computers measure to the nearest bit, what ever that is measuring. If the sensor is outputting perfect information, and the analog to digital converter is converting this perfect data to bits, life is good. The trouble is, the sensors may not return perfect information, or the A/D converter may have some non-linearity, or there whole system may introduce some noise.

All the data the computer can get is the data the sensors are measuring. It probably isn't exactly the value that is real right now, but it is probably close enough. How can we be sure the values are reasonable? We can correlate them to recent events.

If we have a temperature sensor, and we are reading 100 degrees for example, is this right? We can take the most recent 10 or 100 samples, and see if there is a significant change. If the last 100 samples where 45 degrees, we might question the 100 degree sample. If 50 of the last 100 samples were between 90 and 98 degrees, and the other 50 of the last 100 samples were between 102 and 110 degrees then we should keep this sample, and say it is reasonable. However if we are seeing a trend, and the 100 previous samples were showing a steady climb from 90 degrees to 110 and this sample came in at 100, we might again question it.

We need to consider the source as well. If there is a pressure sensor acting as a pitot sensor, measuring airspeed, and it is reading about 180kts. If the indicated pressure increases relative to the static pressure, as the aircraft climbs, there may be a blockage. The trapped air will maintain the surface pressure, but the static pressure will decrease, making the aircraft seem to accelerate as it is climbing. Accidents have been caused by the pitot tube being blocked.

All measuring devices have inherent limitations. Buying one ruler from one manufacturer will probably show differences in another manufacturers ruler. Electronic sensors coming out of the same factory will have slight differences device to device.

Which answer is right? It probably doesn't matter. We just need to be good enough for the situation we are in. We can use math to make questionable information accurate information.


  

DME's

Distance Measuring Equipment (DME) what is it and how does it work? Most large aircraft and some smaller aircraft have DME equipment. It seems to be so easy to use, and when it is there, we just tune to a frequency, and it tells us how far away you are from that station.

The simplest explanation is that the DME box is a transceiver that has a computer in it. The transmitter part sends a signal out to a ground station, usually co-located with a VOR. The ground station then will send a signal back to the DME receiver on the aircraft. The time between the signal being sent, and the return signal being received will be computed by the computer, and converted into miles.

The DME signal isn't actually on the frequency that you tune into. The VOR frequencies and the DME frequencies are paired. The pairing is outlined in the AIM, and other documents. A VOR on 111.0 will be paired with a DME on channel 47 (Aircraft Transmits on 1072MHz, Ground transmits on 1009MHz). The channel concept helps us think about the transmit/receive frequencies.

Each DME transceiver sends a unique set of pulses. The ground station sends the same set of pulses back. If their are several aircraft near the same DME ground station, the aircraft only will do timings on the signal with the matching pulses. The aircraft transmitter will listen for quiet time before transmitting to prevent signals colliding. Most ground stations are capable of servicing about 100 unique aircraft at a time.

The distance measured will be straight line. That means, that if an aircraft is flying at about 10000ft straight over the top of the DME ground station, the DME indicator will read 2miles, not 0. The DME system will try to maintain about a quarter mile, per ICAO requirements. The system will include the ground station and the aircraft equipment.

Some of the RNAV and RNP requirements can be met by using two DME systems along with the inertial reference unit (IRU). Approach plates will sometimes be labeled DME/DME/IRU as needed to meet the requirements of the approach.

GPS is making some of the DME capabilities obsolete. Will the FAA begin decommissioning them any time soon? Probably not. Aircraft upgrades are expensive, and the DME systems are quite reliable, and low maintenance. Potentially when all aircraft are equipped with GPS will the FAA consider removing DMEs from the NAS.

Is this helpful?

ASDI what is it?

When we go to FlightAware.com, FlightExplorer or any of the other flight tracking web sites or apps, they have a lot of good information. The information all comes from the FAA, for free! Ever think about the FAA and how they collect that information? How does is all come together? 

All over the country, there are RADAR sensors. These RADAR sensors are scanning the skies 24 hours a day, 7 days a week. The output of the RADAR sensor is sent to a computer, where the range and azimuth data is correlated to the transponder and altitude data. In the drawing below, the black line coming out of the RADAR dish is the interrogation signal, the black line coming back it the "skin paint" reflection signal, and the blue line is the transponder broadcast from the aircraft.

The computers correlate the transponder code to the flight plan, and take the RADAR returns and calculate a speed, altitude and track that the aircraft would be on. The flight plan helps determine where the airplane will be, based on the speed and time since last sample. 

All of the track information for all of the RADAR systems are sent to the FAA command center where they are made available for display in the various FAA systems that need the information (IE URET, TRACONs, ERAM, TFMA, etc).

One of those system that get the FAA RADAR data is the Aircraft Situation Display to Industry (ASDI). The ASDI data is availble to the airlines and other organizations in the aviation industry. The information includes flight plans, position reports, departure, and cancel flight plans. Using this information sites like FlightAware can present aircraft on maps for the general public. 

The ASDI data only contains non-blocked RADAR and flight plan data for aircraft with IFR flight plans in the US and some of Europe. There is an option available to private aircraft allowing them to block the ASDI data for competitive reasons (IE the president of ATT doesn't want the Verizon corporation to know about some special meetings with a partner or something).  

There are about 4 different kind of feeds of ASDI data. There is the internal FAA feed, the need to know real-time feed, the need to know real-time with European data, and a delayed feed. The real-time feeds are for the airlines and such to use for business reasons. The delayed feed is for the web sites visited by the general public. The delay is like 5 minutes, so it is good enough for people to know when to show up at the airport to get their loved one. 

There is a bit of information that can be derived from the ASDI data. Looking at the ASDI data, someone can determine which airports are taking delays with many aircraft holding. Other things can include looking at the projected track, and weather data to see when it might be best to re-route an aircraft because it is heading toward some convective activity. Airport operators can use the data to count operations relative to other airports, to help improve service. 

The ASDI data feed contains a lot of data. The position reports will be updates of aircraft positions every time the computers are aware of a position update (IE every 12 seconds for enroute RADAR, 5 seconds for TRACON, or 1 second for ADS-B). Typically Monday through Friday in the US there will be 3000-5000 aircraft in the air from 6am to 6pm. 

The FAA has many other services similar to ASDI. Most of it isn't as useful. ASDI is a great resource. 

What do you think.

1500 hours or Nothing

 We seem to be at a weird crossroad. Congress is trying to mandate that the first officers, if they are in the cockpit must have 1500 hours for safety reasons. They are also being pushed to keep both pilots out of the cockpit, and leave them on the ground.

The UAV or drone folks want to keep the people out of the cockpit, while the safety people want more hours for the folks in the cockpit. I kind of get it, I guess, pilots are highly paid people, and technology is getting better, such that UAVs are pretty reliable. If only drones are in the air, then they should all cooperate, and everyone should be happy.

Well, how would you feel about a cockpit with no one in it while you were being whisked on your vacation in the Bahamas? There is someone on the ground paying attention to your airplane, should anything be out of the ordinary. They are paying attention to six or seven other flights as well, heck aircraft on autopilot re all pretty reliable.If the autopilot notices anything unusual, the pilot on the ground will control the aircraft to a landing.

Ice seems to be a common failure mode for recent passenger aircraft crashes. The Colgan 3407 had a captain that had switched aircraft types, and may have been confused about proper action with ice. The Air France 447 crash had ice that caused the autopilots to give up, and ask the less experienced co-pilots to fly the airplane. It is probably good that the FAA mandate more experience to crews, to insure that should something out of the ordinary happen, they will be able to take the proper action.

How much experience should someone on the ground have, if they are needed? Based on recent incidents, they ought to have lots of experience. They will not be dealing with "normal" flights, only abnormal situations. Maybe they will trying to get an ice laden commuter to a safe place at an airport, or a larger transport aircraft through a massive thunderstorm with no reliable airspeed indication. Either way, they will need all the feedback they can get to know what the situation is.

Airplanes are built on many systems. The pilots job is to be able to manage all these system in all situations. Sometimes the indications are providing questionable feedback, and correlating different dis-separate systems can yield hints to the true trouble. The human brain is still better at tasks where the data is really fuzzy.

There are arguments, should pilots be trained in full motion simulators, or are fixed simulators good enough. Well there certainly is a good bit of seat of the pants information that is available in a full motion simulator, but for many situations, the basic procedure trainer will get the normal flying situations covered.

 Should the remote pilot be in a full motion cockpit to help fly this broken airplane? I don't think anyone is considering that. Mostly the remote pilots are going to be expected to fly from a desk in an office somewhere.  Typically it will be a cockpit looking desk, but the chair will probably be on wheels, and just a couple computer displays will be in front of the pilot.

Depending on how bad the broken airplane is broken, it may not be able to provide any feedback. Maybe sensors have gone bad, and that is why the autopilot has given up would be the primary reason the remote motion cockpit will not work. Sometimes the computers in the aircraft don't work, and the remote pilot is going to rely on backups to backups.

Datalinks go bad. We are all used to always on internet, but how often does your internet go out? Your home internet isn't moving, so it should be very reliable. If you have satellite TV when it rains, what happens? Well, imagine flinging through the sky at 40000 feet, in a thunderstorm, 1500 miles from any land, how reliable will the communications link be there? Satellite is pretty reliable, especially in the rain? How about ground links, 1500 miles from the nearest based station, VHF and above won't cut it, and HF is too slow. So the autopilot should be 100% reliable, after getting struck by lightning 2 or 3 times? maybe.

Look I love technology, but I like to relax on my vacations. I don't mind paying a few bucks for the pilot to be sitting up in the front of the airplane. He has some skin in the game. If he messes up, he gets as hurt as me. A guy sitting in an office, might not think things are so important.

What do you think?

Installing Equipment

A few years ago, I bought my wife a Toyota Sienna. It was mostly top of the line, with SatNav, CD Changer, Bluetooth, DVD player and all the bells and whistles one might want. Well, time marched on, and now the SatNav looks old. I don't use CDs in the car, I use Pandora, or MP3s that I have. The Bluetooth doesn't do A2DP, so I can't play music from my cell phone while in the car unless I plug in. The DVD player only plays DVDs, and not Blueray. It isn't even that old, but it seems my cell phone has passed it by.

Looking back to about that same time for avionics, and things have updated there as well. The GNS 430 and 530 were the top of the line radios back then. Today, we kind of admire the aircraft with those radios, but would rather have the G1000 or something newer, given a choice. How about today, well Garmin hasn't been standing still, and are ready to offer the latest better things.

What should someone do who wants the best latest avionics? I believe we are on the edge of the future. What if the (Attitude Heading Reference System) AHRS and Engine Monitor were mounted in the aircraft, but the display you could update? That would allow some lower cost better newer looking panels.

What if we could use our tablets as the display, and update the software and data over the air (OTA)?  Well today people are doing just that. Sporty's sells AHRS's that you can get today that will display the results on your tablet. Today they are only "backup", because they aren't bolted into the aircraft. I'd wager, people with these systems are using them as the primary navigation display, and relying on the less functional panel equipment as the backup.

Say you did a proper bit of engineering to mount the gyros, and added some pitot/static data. Send the data over USB or WiFi to a tablet that would also be mounted in the panel. What is the difference than a panel mount system? It will work for experimentals, and maybe part 91 operations. TSO equipment is required for Part 135, so it won't work on aircraft for hire.

It can be done, or at least something to think about. How far would someone have to go to get one of the tablet systems TSOd. I am sure there are documents that the FAA and others produce that would tell me. That will be my research of the next couple weeks.

Keep reading.