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.