Sunday, May 19, 2013

ICAO vs. 7233-1

We all grew up using the plain old FAA flight plan form (7233-1) that was in the AIM or in the manual we got in ground school. It is the information that the FAA says we need in the order they want it right?
Yes, it is still in the AIM, and it will allow us to get flight following, and all kinds of services. It works in the US.

The 7233 form is in need of updates. This form will still let the FAA know if you have a LORAN equipped aircraft, just use the /I, /C, or /Y. The current set of suffix codes allows the controllers to know if the aircraft is RNP capable, using the /R suffix, but doesn't show how the aircraft meets the requirements, of if the aircraft is RNP10, RNP4 or RNP0.1.

How about that fancy question regarding equipment? If the aircraft GPS equipped, a /G should be filed, or still and older plane with only an ILS and DME, what should be filed. The FAA has plans to change many of the equipment suffix codes August 2013. Mostly the FAA isn't going to care about any performance based navigation (PBN) using the 7233 form. Most of the changes only affect aircraft in the flight levels, that are RVSM capable. The big changes are:

All Mode C transponders (at least, including mode S)
  • /Q - RNP (obsolete)
  • /W - RVSM no RNAV (no change)
  • /Z - RVSM and RNAV with no GNSS (new)
  • /L - RVSM GNSS (any GNSS capability is new)
  • /J - RVSM DME/DME/IRU (obsolete, similar to /W)
  • /K - RVSM FMS with DME (obsolete, similar to /W)

The reality is, and the FAA folks in the know will tell you this, the time has come to retire this old friend. Controllers are like pilots, they grew up on this format, and their flight strips will still use some of this format for a couple years, but mostly, they are being trained on something else.

ICAO form

If you have ever flow to Mexico or Canada you probably had to fill out the ICAO flight plan. Canada calls it the Nav Canada form. It looks intimidating, but it leaves a lot of the guess work out of the above form.

Everything before the "FPL" is not needed. There are links to various instructions for filling out this form. Everything after the remarks (Item 18) is optional. The ICAO has a document 4444 "Rules of the Air and Air Traffic Services" about 4000 pages similar to the FAA AIM for both ATC and pilots. Appendixes 2 and 3 cover the flight plan form, and what text to put where. If the above links are followed the form is easy to deal with. 

The big advantage to using this form is specifying your equipment. If your aircraft has at least one Com radio you put in a V (VHF Radio Telephone), if you have a DME, you put a D, if you can fly in RVSM airspace you put in a W. The suffix is either a C for a mode C transponder, S for mode S transponder or an N for no transponder. Then the suffix beyond that would be for ADS/B. PBN levels can be specified using the R in field 10, but the PBN details must be entered in field 18.

You get to tell the FAA exactly what equipment you want to use for your flight, without interpretation and the FAA will pay attention, and let you use it.

That does mean the air traffic controller may put an aircraft on a GPS approach without asking. That should be a good thing, since it is a little more efficient. You can negotiate the ILS or VOR approach if you prefer still. The ATC computers are reading the flight plan, and offering controllers the most efficient reroute based on capabilities specified.

Going Forward

To start using the ICAO flight plan, most of the flight plan filing services will offer ICAO plans is specified. Select that option, and fill out the plan as before. Specific details for the aircraft will need to be specified when setting up the aircraft, but once set, they will continue to be used for plans going forward.

It would be good it the FAA quit publishing the 7233 form, but that doesn't seem to be in the cards any time soon. There are many publications that still refer to the 7233 form, and they also will need to be changed. We are well past the transition phase, most of the FAA employees are familiar with the ICAO form, and are capable of processing instructions in ICAO format.

Wednesday, May 8, 2013


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.


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.

Friday, May 3, 2013

GPS helpers

This is part 2 to the Next Gen article. This article will reference details from the last article. If you don't know the details of how GPS works, you might want to review that article. As in the previous article, when I say GPS I mean all GNSS systems.

Sometimes the GPS signal is not reliable, due to varoius conditions including atmospheric interference, reflections off of terrain and building or satellite maintence. To keep the GPS signal consistent, various groups have come up with augmentation systems. There are two basic kinds of augmentation methods, satellite based and ground based.

The augmentation systems all basically operate the same way. The augmentation system has one or more GPS receivers at a known locations. The receivers calculate the position as best it can. The system then compares the calculated GPS position with the actual location. The difference from the actual position is broadcast to GPS receivers that are in the area, so they can compensate their calculated position the proper amount making the resolved location more accurate.

Satellite Based

In the US, the satellite based augmentation system (SBAS) is called wide area augmentation system (WAAS). In Europe they have European Global Navigation Overlay Service (EGNOS). Wikipedia GNSS augmentation page has a great map that outlines the various systems proposed for the rest of the world:

The WAAS system uses several ground based stations at known locations throughout North America. The calculated difference is then broadcast to a master station that calculates the Deviation Correction (DC). The DC message is sent to the WAAS satellite. The DC signals are broadcast from the satellite to the appropriately equipped GPS receivers. The WAAS receivers are the ones certified to TSO-C145/C146 standard.

WAAS signals should allow the GPS receiver to resolve the position of the receiver to within 25ft vertically and horizontally 95% of the time. Usually the resolution is closer to 2ft. This accuracy will allow the the receiver to be used for precision approaches similar to the traditional ILS system.

Ground Based

There are two major types of ground based augmentation systems in the US, DGPS and LAAS. DGPS is differential GPS. DGPS users are normally survey crews, and boats. On boats, DGPS navigation is provided by the Coast Guard. Local area augmentation system (LAAS) is the FAA's version. 

LAAS uses multiple (at least 4) receivers around an airport at known locations broadcasting correction signals in all directions. The LAAS correction signal will be broadcast on VHF navigation frequencies using a normal data link.

The advantage of LAAS over and ILS is that all the LAAS transmitters transmit on the same frequency. ILS will require a separate radio, antenna array and maintenance for each runway that the ILS is available for. The LAAS receiver
will calculate the approach path allowing for standard ILS like approaches. The accuracy should be better than WAAS since the difference is focused to a 30 mile diameter.

LAAS isn't not available at many airports today. The experimental LAAS installations have proven the system is quite useful. The system at Memphis has been proven safe since fall of 2006. The Memphis tests have been used for testing RNAV like approaches.


GPS approaches are being added to many airport, almost daily. The typical GPS approach will not be as straight as and approach requiring VORs. Most aviation GPS receiver systems will be able to calculate the approach path waypoint to waypoint, where the waypoints are RNAV type locations.

Localizer Performance with Vertical guidance (LPV) is the highest precision approach level below RNP Special Aircraft and Aircrew Authorization Required (RNP SAAAR) approaches. LPV approaches are equivalent to ILS approaches. WAAS can provide adequate accuracy for LPV approaches. LAAS can also provide accuracy for LPV approaches. 

The FAA is able to survey and publish LPV approaches at airports and runways without adding any extra equipment. Many runways that are not suitable for ILS equipment have LPV approaches today, and more are being surveyed all the time. The LPV is typically depicted on the GPS approach chart.

LNAV is a non-precision approach that can be accomplished with almost any GPS. Augmentation is not required for the LNAV approach provided the receiver is able to maintain resolution to 1800ft (0.3nm).LNAV approaches, are similar to VOR approaches for the minimum descent altitude (MDA). The LNAV is also typically depicted on the GPS approach chart.

Next up, we will talk about ADS/B and RNP and how they help keep us out of each others way.