Sunday, January 29, 2012

VHF propagation from aircraft at 130 MHz

Posted by: Dennis Cimino

First, I want to state for the record that I have been working with high power R.F. transmitters and countermeasures sets, radar, communications, deception repeaters and jammers since 1973, and am quite experienced with in-flight dynamics of VHF signal propagation from both low altitude and high altitude aircraft, both civilian and military. Now, the average person is not too familiar with voltage standing wave ratios, or effective radiated power, insertion or line losses, reflected power, antenna gain factors or for that matter antenna radiation patterns on aircraft, as I am. I have extensive experience in these matters and have the actual aviation experience in a wide variety of aircraft, with regard to everything from H.F. long haul communications to millimeter wave jamming or ECM systems, radar, and navigation equipment. For this discussion we are going to talk strictly about 130 MHz, not 2 MHz, not 17 GHz, and the beam pattern and directionality and effective ranges of these systems on both commercial, military and commercial aircraft both below and above the 12,500 pound weight category.

For those of you who are unfamiliar with path loss nomographs for 130 MHz, I would like you to make it a point to study the diagram below. Notice that no assumption is made for effective radiated power, or system gain factors, and there is a reason for this.

Click on the image to enlarge
This diagram is shown to illustrate the fact that, contrary to assertions by JREF and Unexplained Mysteries bloggers, radio waves do not just "keep on propagating" outwards at their original radiated power levels, without being attenuated over distance. When the path losses and other factors exceed the transmitter power and effective radiated power, as well as minimum discernible signal sensitivity of the receiver system, on any radial from the aircraft, and any angle of incidence, there is no possible communication then. Does this mean that the signals just stop moving thru space? Absolutely not. But what this does mean is that as the distance increases to a point whereupon the signal power level decays below any possibility of detection by even triple conversion receivers with very selective front end circuitry that have incredibly high (-148 dBm or thereabouts) minimum discernible signal capture capability (which ACARS does not, nominally have, at it’s fairly robust –107 dBm), then beyond that range, detection and demodulation by any terrestrial, high gain, very directional and perhaps non interferometry based receive system, is highly unlikely to happen. In any case, radio signals do not keep on radiating outwards without attenuation occurring due to signal density issues resultant from the fact that the greater distance from a transmitter from any antenna, the power distribution in fixed space, by laws of physics, will diminish with distance. Due to the totally unpredictable nature of the antenna radiation patterns of VHF blade antennae used by ACARS, it is a false assertion to state that there are no limitations between the RGS stations and the aircraft transmitter antenna for COM3, usually used on most Boeing airplanes, circa 1980’s and onwards, for ACARS.
In any case, what I would like to start with is that the aircraft and the antenna jointly form the transmit and receive radiation pattern in ways that are radically different from these same antenna systems being mounted on flat ground planes that offer at the very least a 90 degree counterpoise to the plane wave or electrical field leaving the antenna. The reason I cite this is that for any isotropic or non-directional antenna system, which these are not, due to their shapes being swept to minimize aerodynamic drag factors and prevent ice pickup or vibration resonances, these antennae used on commercial and military aircraft for VHF COMMUNICATIONS are what are referred to as ‘blade’ antennae of the type used on most commercial aircraft, including B-757/B-767’s.

For such antennae, there are some considerations that apply to them due to their shorter than quarter wave physical radiating length offset from the aircraft’s very curved fuselage or empennage. This results in significantly higher V.S.W.R. or ‘reflected’ energy back to the transmitter and not into free space. In addition to this, the shorter physical radiating element length versus the curved fuselage will radically change the radiation pattern and add directivity to these antennae, typically along the fuselage plane, fore and aft, as well as pushing the radiation pattern upwards or away (assuming the antenna is top mounted) so that the beam shape is not cardioid in nature but scalloped and with significant major pattern lobes that tend to create significant null zones that effectively reduce radiated power in those areas to the sides or in the ‘X’ plane (horizontal) by several ‘dB’ of gain, while significantly favoring the fore and aft orientation along the fuselage. In the ‘Y’ axis (Vertical), looking at the beam pattern, the field lobes from the antenna depart the skin of the aircraft and contour upwards, once again, assuming the antenna is top mounted, and conversely, ‘downwards’ if the antenna is belly mounted. To characterize the full radiation or beam pattern, it would appear heavily scalloped in the ‘X’ axis, and quite finger or balloon lobed in the ‘Y’ axis, (vertical) plane. In effect, the antenna that was supposed to be fairly omni-directional now is quite directional both fore and aft and has markedly higher gain factors in those directions, as well as pushing the pattern upwards into free space at any number of angles less than 90 degrees off the horizontal. The resultant 3-dimensional plot begins to look like a twisted doggie balloon shape and no longer resembles a spherical omni-directional beam plot.

The reason I mention this is that this directivity is nothing that can be readily compensated for without exotic counterpoise shapes around the base of the antenna, or multiple ‘Yagi’ type of elements in other directions to flatten or change the beam pattern. To compensate for these exotic and quite scalloped and lobed patterns that become reality when these relatively inefficient due to shorter radiating element length antennae are joined to a curving cylindrical tube which airplane fuselages tend to be, relative to the radiated field. And I would like to state that though there is a great deal of reciprocity between TRANSMIT and RECEIVE patterns, they do not necessarily fully reciprocate or remain the same for both functions due to any number of factors we will not discuss herein.

In any case, what I have just described is the highly irregular and moderately unpredictable and undesirable directivity of an otherwise fairly omni-directional antenna system. It is important to note this because a 3 dBm power differential in any of these planes, radiation pattern wise, effectively is a half power addition or reduction, with significant null points deeper than this that occur quite naturally, rendering the pattern even more complex and difficult to factor using system loss calculations, without actually making measurements in every axis with the aircraft either raised up and outside of what is called ‘near field’ distances of the ground below the aircraft. Simply put, real world antenna radiation pattern measurements are possible and actually done, but in most cases scaled models with scaled frequencies, are typically used on non full-sized models inside of anechoic chambers or OTS (outdoor test) areas. By large, most aircraft manufacturers do not attempt to make these measurements but make assumptions about radiation patterns that are not necessarily founded in known factual testing data, as usually is derived by MILITARY aircraft certifications laboratories who have a much more vested interest in characterizing these radiation patterns to maximize system effectiveness and head off or prevent unwanted ‘dead zones’ around the fighter or bomber or jamming platform these antennae are mounted on. On many tactical jamming aircraft, the blade antenna arrays are more or less in ‘Yagi’ configurations to give directional gain that the mission capability of the aircraft needs to make maximum effective use of a communications jammer mounted on the aircraft. This is, however, never done on commercial airliner or Part 23 aircraft, to minimize cost and reduce weight.

As we have now more or less just barely touched on the radiation pattern, we have not yet begun to talk about signal polarization issues, and why they go hand in hand with more fully understanding how a theoretical two-way path loss calculation and predicted range of a system can be quite off in comparison to real world or ‘true’ and valid path losses and system effective radiated power determination, or receive sensitivity calculations.

What I am getting here is that a perfect isotropic vertical radiator element that rises 90 degrees off of a ground plane, or is oriented in the pure ‘Y’ axis pointed straight up, effectively has a VERTICAL ELECTRICAL FIELD POLARIZATION OF THE SIGNAL. In lay-person’s terms, ‘polarity’ of the electrical field, ideally, should be the same on both the transmit and receive end, to maximize signal transfer. In real life, this is difficult to achieve with swept blade antennae and curving aircraft fuselages which are the ground plane or ‘counterpoise’ for the antenna itself. The resultant polarization skewing hence causes a coupling factor inefficiency to occur, relative to the very polarization fixed antenna on the ground, intended to receive these signals.
IF the electrical field polarization shifts fully 90 degrees so it is not vertical at all but more ‘horizontal’ in relation to the very VERTICAL ground antennae respective polarity, the coupling mismatch is now at the very least ‘6’ dB less than optimal, of not more due to the curving fuselage, which is not a flat ground plane at all. Every ‘3’ dB is a half power loss when that number is a negative change. Why is this important?

Let’s theorize that from the transmitter rack, of the 50 watts transmitted off the jack in the rack itself, after ‘xx’ dB of loss in the transmission line, which in some cases is several meters in length, we are not going to see 50 watts of R.F. energy radiated into free space off of this antenna. Instead, if we are lucky, we’re more likely to see far less than 20 watts or thereabouts, leaving in all directions, not just going out in a single plane wave directly at the receive system on the ground, miles below. To better visualize this, imagine a light bulb on a lamp without a shade radiating light energy in all directions more or less evenly. Though light energy is significantly higher in frequency than is 130 Mhz. ACARS data link signal data, imagine the light bulb now has an array of fiber optics tubes radiating in all directions, affixed to the light bulb. In effect, the result is not anywhere near an omni-directional pattern at all, but a highly irregular, porcupine spoked shaped pattern of peaks and nulls radiating outwards, resulting in shadows in all directions on the walls of the room, and bright spots. This is more or less what the irregular radiation pattern of the ACARS blade antenna would look like if this were optical energy at a higher frequency. This analogy, as coarse as it may seem, is more or less descriptive of R.F. power density variations from a very spiked and valley ridden pattern radiating off the fuselage from the ACARS blade antenna, typically mounted on the aft belly of the fuselage on some Boeing aircraft, such as is the case with the B-737NG.

Why is this important to understand, relative to the ACARS two-way range issue? Well, simply put, the moderately less than optimal V.S.W.R. of the antenna and transmission line and airplane fuselage now has reflected or ‘lost’ significant amounts of outbound R.F. energy that would have gone into free space in any number of ‘X’ and ‘Y’ and in fact, ‘Z’ plane directions from the radiating antenna or ‘blade’ on the fuselage. So we’re not dealing with our original 50 watts any longer, we are now pushing less than half of that into the free space around the airplane, in virtually all directions, in a very far from omni-directional radiation pattern, with deep peaks and nulls radiating outwards in all directions but overall favoring the fore and aft longitudinal axis of the aircraft. Translated, it means the myriad of ground stations around the aircraft will not be seeing anything close to the optimal range nomograph numbers at all, based on free space losses, as well as the very irregular antenna radiation pattern of the aircraft’s VHF blade antennas that are used by ACARS.

So now we are going to bring up the 200 mile claimed range number here. And the reason why, is that yes, in theory, due to the minimum discernible signal or sensitivity level of the ACARS receiver system of both the plane and the ground station is more or less known or can be approximated, what cannot be known is the coupling factor perturbations due to beam shaping and irregular radiation patterns that occur when blade antennas used in VHF communications are mounted on tubular aircraft fuselages. This results in significantly ‘lower’ than the expected or theoretical best 200 mile range supposition, by as much as 80 percent in real life.

Any experienced aviator who has used VOR navigation and who understands how the F.A.A. determined SERVICE VOLUME models for the each class of VOR facility, knows that even at FL-450, and above, a realm that few airliners routinely will fly at, by the way, due to cabin pressurization issues and aerodynamics, the effective range of the slightly lower in frequency of 108 to 118 Mhz., is reduced from that altitude to FL-600 or sixty thousand feet. Why is this? Well, ground obstructions, and earth curvature now come into play here, as well as other similar signal degrading changes, and therefore the F.A.A.’s own data available to look at for these VOR facilities is less above FL-450. The F.A.A. didn’t arbitrarily pick these numbers out of some hat somewhere. They were derived by in-flight testing over literally all terrain types and at all altitudes, inclusive of FL-600 in some cases. The service volume diagrams from the F.A.A. website are relatively transferable to ACARS due to similarity of known and demonstrated effective ranges derived thru years of collective experience testing VOR reception service volumes, as well as R.C.O (remote communications outlet) communications ranges.

So we have more or less touched on irregular and highly unpredictable antenna radiation patterns in both the transmit and receive capability of 130 Mhz. communications and navigation systems in use today, and for all practical purposes really destroyed the rather inconsistent with real world performance, claims of ACARS working out to 200 nautical miles from any RGS station, due to these factors. In real life, no experienced line pilot could ever with a straight face claim that he’s tracked VOR’s at 200 nautical miles without flag tripping and sporadic reception, but also, it would be quite a stretch of the imagination to expect the performance for ACARS to routinely, every time, ever truly meet the outrageous claims of functionality at 200 miles from any aircraft, due to these factors cited here. Power output is not the determining factor here. Earth curvature is the predominant range limiter, as is terrain irregularity and obstructions around the ground station inside both ‘near field’ and ‘far field’ distances from the RGS stations.

As a commercially rated pilot with experience flying at virtually all of these altitudes up thru at least FL-350 in civil and military aircraft both as pilot and test engineer, I have to stipulate here that it is both ‘laughable’ and quite ‘uncommon’ to expect VOR reception much beyond 135 nautical miles even at the higher altitude regimes, and for that matter, to expect ACARS to work much further than these ranges over hilly and irregular terrain with mountains and ridges everywhere to throw a major ‘curve’ into any range calculation for these ACARS path losses.

Unfortunately many people who are claiming expertise in these areas are neither qualified to make these judgments due to a lack of R.F. engineering experience or real world testing experience and backgrounds, nor do they have any real world aircraft flight testing experience or flight experience using these systems in the real world. To assert that ACARS can communicate at 200 miles is not factual nor is it realistic. And it is wholly unfounded by any known factual testing data or practical experience of these outrageously optimistic ranges. Furthermore, I’ll even go one step further here. If ANY experienced and non-anonymous airman with a valid pilot certificate at COMMERCIAL level and decades and thousands of hours of flight experience can validate VOR receptions beyond 150 miles under any circumstances, I would like to see their data and where this took place, in which aircraft, at what altitude. I do not expect many challenges to this more or less hard and fast rule here for VHF being pretty line-of-sight limited with very little effective terrain contouring or tropospheric ducting taking place. Any reasonably experienced line captain who has extensive ACARS experience in the Positive Controlled Airspace is welcome to controvert these claims with hard factual data.

In summary, the radiation patterns of real VHF communications blade antennas on heavy transport category aircraft, are both complex and highly unpredictable except to state that nominally there is some beam pattern lobing that prefers the longitudinal axis of the aircraft, to the fore and aft directions, while also shifting the electrical field far from optimal vertical polarization into any number of less optimal, and very inefficient signal polarizations that result in reduced ranges in most directions, and predictable and even more complex changes when these aircraft roll and bank in turns, further altering their radiation and reception patterns significantly from straight and level flight. Furthermore, as the aircraft’s crew is quite unaware this is in fact occurring, no action can be taken by either the crew of the aircraft’s system that can compensate for this. It is out of their hands entirely.

For the most part, it is unclear why some would surmise that ACARS should always be able to realistically and effectively work out to 200 nautical miles, and furthermore, that stations significantly closer to the aircraft would be totally unseen and not communicating under any circumstances, given what has been discussed here. Yes, the radiation pattern is lobed in all ‘3’ axis off of the aircraft, and yes this means the signal will follow the ground plane of the fuselage and wing structures somewhat, but to make the assertion that a station more than 100 nautical miles away would be favored over one that is less than 20 miles away, is not genuine nor credible.

We did not discuss a whole slew of other factors that adversely impact the effective ranges, such are atmospheric noise sources, co-channel interference, and things such as multi-path and reflections from both near the RGS station as well as the aircraft’s structures itself. Even something such as flap extension or landing gear deployment will significantly change the radiation patterns of most aircraft, whether the aircraft design or ARINC radio engineers like that or not.

ACARS is not likely to work much further than 150 miles in most cases, under any and all circumstances, due to very extensive and recognized use of VOR changeover points on LOW ALTITUDE enroute IFR charts, showing that even at low altitudes up to and inclusive of FL-180, various enroute or Victor Airway reception ranges have been know for years and are published in both U.S. Government and Jeppesen/Sanderson IFR enroute charts and terminal charts for very very good reasons. These changeover points are the very best illustration of actual line-of-sight VHF communications ranges being adversely impacted by hilly terrain and other issues much more so than altitude, because if altitude above the ground were the sole arbiter of this, all changeover points for these low altitude airways or Victor Airways would be the same every single time, but they are not.

They are individual as hairs on your head. And so are the radial distances and ranges from any ACARS ground station using VHF line-of-sight frequencies. By necessity the technical assessment offered here is significantly less technical for a reason. You don’t need to be a pilot or a radio engineer to grasp most of these concepts explained within this moderately brief attempt to illustrate the point that several things are always going to be in play with radio ranges. Those are, to sum them up:

  1. Radio signal strength decay over distance due to atmospheric attenuation and power density diminishment due to radial distances that dictate that with ‘X’ watts of energy radiated outwards from any radio source, that the power density window decreases on any given point far from the origin, even with highly directional and focused beams of radio energy. 

  2. Aircraft radiation patterns are not by their nature remotely close to omni-directional at all, even in the best cases such are transponder antennae at 1030 and 1090 Mhz, perhaps one of the shorter stub blades in use on any airplane, requiring far less ground plane uniformity to properly form an omni-directional radiation pattern.

  3. Curvature of the earth and ground obstructions and line-of-sight VHF propagation characteristics are also significant range limiters at 130 MHz.

  4. RGS stations an order of magnitude CLOSER to distant ones are far more likely to be the best LINE OF SIGHT paths under any and all but the most unusual cases where a mountain or a building between the RGS and the plane may in fact really make a much bigger attenuation factor between a closer station perhaps and a more distant one with a very clean, very direct, non-obstructed path, e.g.: if the more distant station is 100 or more miles away, it’s a real fantasy to state that one a mere 7 to 10 miles away, almost directly under the plane itself, would not be the preferred station, based on path losses alone, and virtually zero earth curvature to deal with there.

And added here as a side note.

The one thing that most lay-persons do not experience in real flight regimes is loss of both communications signals and navigation signals, with something as simple as a small heading change. Something that typically is not a factor on most civilian air carrier aircraft but has been a major problem with general aviation airplanes, is antenna position due to other structures like landing gear, exhaust systems, and other small protuberances on the airplane that really skew the signal so much that it necessitates the pilot change course or heading to blindly fish around and find a better angle offset to allow both ATC to hear you, or for your transponder's signal to be seen by a distant radar station, because your exhaust system or landing gear (in some cases always down because it is not retractable) gets in the way and changes things.

High altitude planes don't typically drop gear and flaps, but they do have many other shape perturbations in both the near field and the far field regions of these antennae that make it unlikely that the aircrew is going to alter course or turn to better line up an antenna pattern 'lobe' so the ground can now see their signal and communicate.

After years of flying both fixed gear and retractable gear planes with all sorts of shapes, I have found that antenna radiation patterns are very very spooky and variable and are not published in any flight manual. But any pilot who knows about near field and far field interference and blocking of his signal would, like me, change course to try to get the signal reception needed for a little bit longer at the extreme outer effective ranges of radar, transponder, and communications / navigation equipment. Airline crews don't do this. But over three decades of flying, I have done it routinely to regain IFF to RADAR functionality, or DME functionality, and for that matter, even COMM/NAV functionality. In one case due to the location of a pair of GPS antennae on the skin of a plane, even GPS was affected somewhat, and it changed RAIM factor on approaches...due to the ground based WAAS augmentation being less optimal on some IFR approaches.

In any case if I find a good VHF radiation pattern plot to ship, I'll send it. But these patterns are very complex. Very very complex, and in real life, as airplane makers don't get real data about this, very highly unpredictable and way less than optimal per engineering calculations for them.

Dennis Cimino
  • Electrical Engineer
  • Commercial Pilot Rating, since 1981 , IFR, MEL ratings since 1980. 
  • Navy Combat Systems Specialist: RADAR, ECM, cryptographic communications   Navy EMI troubleshooter for COMNAVSURFPAC via MOTU-5, San Diego, CA., and under NAVELEX contract.  AN/SLQ-32(V)3 Countermeasures System support specialist.
  • Flight Data Recorder Engineer Smiths Aerospace (Now G.E. Aero)
  • BA-609, IDARS, Military and Commercial
  • Millimeter wave RADAR and countermeasures expert since 1973
  • Two patents held for high powered modulator Triton HP, C-Band, 250kw LONG RANGE Doppler RADAR ( Kavouras ):
  • long pulsewidth RADAR droop compensation network that improved radar output power thru long pulse transmissions, effectively imrproving weather phenomena detection ranges.
  • and wave guide arc detection for high powered RADAR system

Saturday, January 28, 2012

Analysis of United 93 U-turn over Cleveland based on the ARINC logs

Posted by: Sergio

As explained in the introductory post, Warren Stutt and other people on Unexplained Mysteries claim, based on the ARINC logs publicly available, that the RGS (remote ground station) which actually transmits an uplink sent by the airline is indicated in the "Stn=" field within the first ULBLK block, while the RGS shown in the ULMSG block (which precedes the ULBLK block and represents the ACARS message as sent by the airline) only indicates the "predicted RGS" based on the original flight plan. One of the several problems with this claim is the fact that, if we accept this theory, then we must necessarily conclude that United 93 constantly flew within the Pittsburgh's RGS coverage area with no discontinuity from 9:23 EDT to 10:12 EDT without being detected by any other RGS with stronger signal for almost one hour. As anyone can easily notice reading through the 211 pages PDF of the ARINC logs, the station reported in the "Stn=" field in all the ULBLK blocks from 9:23 EDT until the end of the ACARS communications is always "PITC6", with the sole exception of a couple of occurrences for "Stn= IADA6" at 10:12 EDT in the final instants of the aircraft's ACARS log history. This raises the question as to how could United 93 possibly make the famous U-turn above Cleveland before disappearing from the radar screens as secondary track at 9:41 EDT in the vicinity of Canton, as reported by the 9/11 Commission (1), and yet never enter either in the Cleveland (CLE) or in the Akron/Canton (CAK) RGS coverage area.

In order to determine the distance of United 93 from the closest ground stations during the U-turn over Cleveland, I used the official radar positions for United 93 from 9:32 to 9:40 EDT (which corresponds to the time when United 93 allegedly made the U-turn over Cleveland), as provided by the 84 RADES Radar Data Spreadsheet released through FOIA. Then, I used GPS Visualizer to calculate the distance from the aircraft's position and the PIT, CLE and CAK remote ground stations respectively. Finally, I reported the resulting data into a Google Earth map to provide a visual representation of the Lat/Long position of the aircraft when messages #7, #8, #10, #11 and #12 were uplinked from the United Airlines dispatchers (2) and its distance from the closest ground stations.

Distance of United 93 from CLE, CAK and PIT remote ground stations at 9:35 EDT

Any blue aircraft icon on the map above represents the relevant position of United 93 as detected by the DTW radar site. The timestamp (i.e. 9:32) indicates the time of the radar hit. As anyone can notice at first glance, between 9:32 EDT and 9:36 EDT United 93 flew literally on the top of the Cleveland's RGS (CLE). After 9:37 EDT the aircraft began the second part of its U-turn heading SE to the direction of the Akron/Canton's RGS (CAK) while remaining at moderate distance from the CLE RGS. During this timeframe several messages were sent to the aircraft from UAL dispatchers, yet not one single occurrence for "CLEXX" or "CAKXX" is reported as "Stn=" in any ULBLK block, nor such transceivers appear as "BepStnName =" within any datalink from the aircraft (DLBLK), which is at the very least unexpected. Translated, this means that during its U-turn over Cleveland United 93 was not detected at any time by any Cleveland or Akron/Canton transceiver, but continued to communicate with the ARINC CPS through the Pittsburgh's station (PITC6), who was at more than 90 nm. Of course, it would be very interesting to analyze the radar hits after 9:40 EDT. However the last radar position available for United 93 before the hijackers allegedly turned off the transponder was detected at 9:40:03 EDT and was 41°00'38.662"N  081°51'46.830"W.

Position of United 93 at 9:35 EDT

Message #10 (according to the Winter's numbering) was sent by the UAL dispatcher Chad McCurdy exactly at 9:35 EDT. Therefore this message offers an excellent case study. As we can see from the map above, at 9:35 EDT the aircraft was literally flying over the CLE RGS. This is what Winter declared in 2002 in his interview to the FBI about this message:

Message #10 was from UAL flight Dispatcher CHAD McCURDY to the aircraft.
Message #10 was sent to the aircraft from CHIDD and was sent to the ACARS screen only. The RGS in this instance was near Cleveland, OH CLE from the line "AN N591UA/GL CLE..."

This is the log of this uplink as it appears from the printout of ACARS logs provided by Ed Ballinger to the 9/11 Commission:


;09111336 108575 0625

Finally, this is how the complete ARINC log for this message appears in the ARINC logs:

ARINC logs, p. 118

According to Warren Stutt and other posters on the Unexplained Mysteries' forum, the RGS shown after "AN N591UA/GL" field in the first log above (taken from Ballinger's logs) and the "Target Stn=" field within the ULMSG block (in the ARINC log screenshot) only indicate the predicted RGS, that is the RGS provided by the airline when the message is originally sent. In other words, it is the RGS expected after a certain time of flight according to the original flight plan. According to this theory, the RGS who actually transmits the ACARS is indicated in the first ULBLK block in the "Stn =" field. If this is true, then PITC6 and not any closer RGS transmitted the message sent by CHAD MCCURDY at 9:35 (as well as any other uplink until 10:12 EDT).

Using GPS Visualizer, I calculated the distance from the position of the aircraft at 9:35 EDT and the PIT, CLE and CAK remote ground stations. These are the Lat/Long positions shown on the map:

Aircraft/RGS Lat/Long position
United 93 at 9:35 EDT 41°20'17.89"N   81°58'55.62"W
PIT RGS: 40°29'29.40"N   80°13'57.72"W
CLE RGS: 41°24'38.88"N   81°50'58.18"W
CAK RGS: 40°54'53.70"N   81°26'10.69"W

And these are the distances between the above coordinates as determined by the GPS Visualizer's calculator:

United 93 at 9:35 EDT Distance (in nm)
PIT RGS (see red pointer A): 94.394
CLE RGS (see red pointer B): 7.4020
CAK RGS (see red pointer C): 35.455

If we believe to the claim supported by Stutt and other people on Unexplained Mysteries, the Pittsburgh's RGS had at 9:35 EDT the strongest signal although its distance from United 93 was 94.394 nm. For some obscure reason, the PIT station overrode both the Cleveland's RGS (CLE), which was at less than one tenth of its distance (7.4020 nm) and the Akron/Canton's RGS (CAK), who was at only 35.455 nm (i.e. well less than half the distance from PIT). If one has a minimal knowledge about how ACARS really work in the real world, the only possible conclusion after reading this data is that something definitely does not add up, either in the theory or in the data themselves. Giving priority to a station at more than 94 nm while having a station as close as 7 nm available and, in the event this were inoperative or temporarily unavailable due to congestion or any other interference, an alternative ground station at only 35 nm as a second option, is something we would definitely not expect from the CPS (the ARINC's Central Processing System that processes each and every message to and from any aircraft). At the very least, this situation is to be considered as highly abnormal, if not completely impossible. True is that there are sometimes instances where the closest station is not always the best station, for example in case of message collisions, terrain blockages, metropolitan interferences, temporary obstructions. Whenever the transmission through the best station is not successful, the message delivery occurs through a logical sequence of "next best" choices of stations. However, we know that the CLE and CAK ground stations were operative on the morning of 9/11, as testified by several occurrences for CLEA2, CLEB2 and CAKA2 in the "BEPStnName =" field both in the UA93 and in the AA77 log history. Actually what happens in the real world (and can be confirmed by any ACARS expert) is that the closest station is in almost 100% of cases the chosen station because of its signal strength.

Another claim raised by Warren Stutt and his followers on Unexplained Mysteries' forum is that the stations reported in the "BepStnName =" fields within the DLBLK blocks designate the ground stations "within range" at a particular time:

The DSP (ARINC) makes use of the location downlinks referred to in previous documents to keep its internal routing tables updated, but the airline does not. The DPSs use of this is evident by the ARINC 620-4 documentation and from the PDF supplied by Warren Stutt. In fact, Warren's PDF shows us a perfect example of the Category A network protocol in action with each DLBLK that you see following each ULMSG and ULBLK reference. The aircraft's omnidirectional transmissions are picked up by every RGS within range and it appears as though the routing table sequences a priority based on signal strength of those transmissions; the RGS with the strongest signal received is assigned as the initial station for future uplinks. This routing table is dynamically updated with each downlink from the aircraft, just as the previously uncovered reference material has described, and this is distinct from the static information (i.e. the GL text element which I outlined in my previous post) which is supplied by the airline dispatch in each message sent to the DSP for delivery.

I marked the most significant parts of the above claim in bold. The first one is "but the aircraft does not", which means that the targeted station reported in the ULMSG blocks is never dynamically updated by the CPS based on the actual positional data and always designates the expected RGS based on the flight plan. This claim is flatly contradicted by several cases that I will present in separate posts in the next future. However, for the purposes of the current analysis, it is more important to focus on the second claim marked in bold: "The aircraft's omnidirectional transmissions are picked up by every RGS within range". If this were true, then we would expect to see some references to Cleveland's and/or Akron/Canton's transceivers (such as "CLEXX", "CAKXX") which were clearly "within range" at 9:35 EDT as shown in the map. However, this is not what is actually reported in the ARINC logs. As we can see from the above screenshot, the alleged transceivers reported in the "BEPStnName =" fields refer to stations and locations which are very distant from the known position of the aircraft at 9:35 EDT, some of them being at nearly 200 nm, which is the maximum technical distance guaranteed by ARINC at FL290. These are the five transceivers reported in the DLBLK blocks:

BepStnName = YYZA6
BepStnName = YYZB6
BepStnName = YYZC6
BepStnName = PITA6
BepStnName = HTSA6

I reported these (alleged) ground stations on a Google Earth map including other possible alternative RGS that were closer to United 93 at 9:35 EDT but, for some reasons, were not detected by the aircraft and calculated their distance from the aircraft at 9:35 EDT using GPS Visualizer. This is the result:

Distance of United 93 from the transceivers shown in the "BepStnName=" field at 9:35 EDT

United 93 at 9:35 EDT Distance (in nm)
YYZ RGS (see red pointer D): 175.50 nm
HTS RGS (see red pointer E):  180.08 nm
TOL RGS (see red pointer F): 83.696 nm
FWA RGS (see red pointer G): 147.18 nm
DTW RGS (see red pointer H): 81.191 nm
CMH RGS (see red pointer I): 90.302 nm

So, let's take a look again at the screenshot from the ARINC logs and let's try to draw some conclusions after looking at the distances calculated by GPS Visualizer. The first three DLBLK blocks contain references to the Toronto ground station (YYZ). At 9:35 EDT the distance between the radar position of United 93 and the Toronto RGS was 175.50 nm. The fourth DLBLK block contains a reference to PITC6. As we have seen, at that time the distance from the Pittsburgh's RGS was 94.394 nm. Finally, the fifth DLBLK block reports a reference to Huntington (HTS), which at 9:35 EDT was at a distance of 180.08 nm.

Although all distances indicated above are technically within the 200 nm range, we should believe that the United 93's onboard ACARS MU not only completely ignored very close stations such as CLE (see pointer B), who was at only 7.4020 nm, and CAK, who was at a distance of only 35.455 nm (see pointer C), but also other relatively close RGS such as DTW (Detroit) (see pointer H), which was at 81.191 nm, TOL (Toledo) (see pointer F) which was at 83.696 nm, CMH (Columbus) (see pointer I) which was at 90.302 nm and finally FWA (Fort Wayne) (see pointer G) which was at 147.18 nm. Instead, United 93 apparently detected YYZ at 175.50 nm and HTS at 180.08 nm. Look at the same screenshot shown above with a zoom on the aircraft position:

Distance of United 93 from the transceivers shown in the "BepStnName=" field at 9:35 EDT. Zoomed view

It is obvious that something does not add up here. Claiming that "the aircraft's omnidirectional transmissions are picked up by every RGS within range" when several close stations are completely ignored and others as distant as 180 nm are detected means that either the theory is an unsubstantiated speculation or the data are not genuine and, therefore, nor reliable. 

One last example should conclusively clarify this issue. There is only one reference to the Cleveland's RGS in the DLBLK blocks related to United 93 log history:

ARINC logs, p. 93

As we can see, it is referred as "CLEA2" and is contained within a DLBLK block showing 13:03:06 UTC as timestamp, that is 9:03:06 EDT. Coincidentally there is a radar hit from the DAN site in the 84 RADES Radar Data Spreadsheet file which shows exactly the same Zulu time, 13:03:06,600. The radar position reported is 40°45'09.713"N   076°49'05.680"W. One more time this allows us to plot the relevant position of the aircraft on a Google Earth map and calculate its distance from the target using GPS Visualizer. This is the result:

Distance of United 93 from CLEA2 at 9:03 EDT

Again, the graphic and the distance data are self-explaining. According to the theory claimed by Stutt and others on Unexplained Mysteries, United 93 was able to detect the Cleveland ground station in a downlink at 9:03 EDT at a distance of 231.23 nm, which is well above the 200 nautical miles limit indicated by ARINC as maximum technical threshold for a successful ACARS transmission. But, for some incredible unknown reason, the CLE RGS was not close enough or its signal was not strong enough at 9:35 EDT to be reported in the "Stn=" field as the transmitting RGS while the aircraft was only 7 miles away and had literally flown over the Hopkins International airport one or two minutes before.

Position of United 93 at 9:40 EDT

Now let's take a look at the situation of United 93 at 9:40 EDT when Message #11 (according to Winter's numbering) was sent from United Airlines' dispatcher Ed Ballinger to United 93.

As mentioned above, the last officially known radar Lat/Long position for United 93 was tracked at 13:40:03,000 UTC (9:40:03 EDT) and was 41°00'38.662"N   081°51'46.830"W. This is when the alleged hijackers turned the transponder off. The aircraft was eventually monitored as primary track only. Now, Message #11 was sent by Ballinger at 9:40 EDT, 13:40 UTC. However, the message was probably delivered at 9:41 EDT, as confirmed by the line "BEPts=20010911 13:41:10" in the ARINC logs (FLoc=345838614, p. 119), which indicates then time when an uplink is received and processed by the ARINC CPS in Annapolis (the actual delivery to the aircraft normally occurs about ten seconds later). All the DLBLK blocks downlinked from the aircraft I will refer to below have a timestamp comprised between 13:41:12 UTC and 13:41:20 UTC. Therefore, in this case, the aircraft position represented on the map does not match exactly the actual position of the aircraft when the DLBLK blocks were downlinked. In other words, there is a little discrepancy of about one minute. Taking into account that the aircraft was headed Southwest at that time and that an aircraft expected to make a ground speed of 480 knots would cover 8 miles per minute or 1 mile every 7.5 seconds (240 NM in 30 minutes), we can assume that the position of United 93 at 9:41 EDT was up to 8 miles SW compared to the position shown in the map. This discrepancy is absolutely irrelevant given the magnitude of the distances shown below and has no significant impact on the conclusions of the resulting claim, however I thought it was correct to point out this issue before presenting the conclusion of this analysis.

As usual, Mr. Winter's statement will not be deemed as coming from an expert by Stutt and his friends on Unexplained Mysteries, although Winter worked as Manager of Flight Dispatch at United Airlines for 14 years having previously worked as Flight Dispatcher at Midwest Airlines from 1987 to 1990. Anyway, this is what he declared to the FBI about Message #11:

Messages #11 and #12 were sent to the aircraft from CHIDD using the RGS near Cleveland, OH. These messages also activated the audible signal in the aircraft.…

Giving for granted that "using the RGS near Cleveland, OH" will be one more time interpreted as "CLE was only the predicted RGS", let's take a look at the relevant printout coming from Ballinger's desk:

.CHIAKUA 111340/ED


;09111341 108575 0638

Finally this is the screenshot of the corresponding log in the ARINC log PDF. Please note that only the first part of this log is visible in the screenshot. This message is quite long, the header begins at the end of p. 118 and extends until the end of p. 120. Therefore please refer to the relevant pages in the document to have a full view of all the blocks contained in this message:

ARINC logs, p. 118

I reported the aircraft position at 9:40 EDT with the closer RGS' on a Google Earth map:

Distance of United 93 from CLE, CAK and PIT remote ground stations at 9:40 EDT

If we believe that "Stn=" indicates the actual transmitting station, then we must conclude that something very unusual happened one more time, just as we have seen at 9:35 EDT. At 9:40 EDT United 93 was very close to the CAK's remote ground station (20.229 nm), while CLE was an excellent second option at only 23.998 nm. Yet, we should believe that the CPS ignored two very close RGS' and arbitrarily routed Message #11 through PIT, which was at a distance of 80.596 nm, i.e. four times the distance compared to CAK and CLE. This is absolutely implausible and has no technical nor logical explanation. Moreover, since the position of the aircraft at 9:41 EDT was slightly more SW than shown on the map, as mentioned above, then we can conclude that United 93 was even closer to CAK. Therefore we would definitely expect to see a reference to the Akron/Canton RGS such as "CAKXX" in the "Stn=" field instead of "PITC6". But this is not what is reported in the ARINC logs.

If someone speculates that the Cleveland airspace was congested on the morning on 9/11 as explanation for the odd situation we have seen at 9:35 EDT, then he should also explain why the CAK's RGS was clogged or unavailable at that time as well. It is obvious that these "explanations" have no real technical basis. They are pure speculations which aim to support a theory which is simply wrong and contradicted by the same document it claims to be founded on.

However the bad news for the supporters of this theory do not end here. An analysis of the stations shown in the "BepStnName=" fields within the DLBLK blocks conclusively proves how poorly substantiated this theory is.

Distance of United 93 from the transceivers shown in the "BepStnName=" field at 9:40 EDT

These are the RGS' shown in the DLBLK blocks for this message and their distance from United 93 at 9:40 EDT:

Distance from United 93 at 9:40 EDT (in nm)
PITC6 80.596
IADC6 (pointer R) 238.05
YYZA6 (pointer T) 188.88
YYZB6 (pointer T) 188.88
HTSA6 (pointer S) 161.70
CRWA6 (pointer Q) 158.77
ORDA6 (pointer P) 278.46
DTWB6 (pointer U) 98.871
CVGA6 (pointer V) 174.29
YYZA6 (pointer T) 188.88
PITA6 80.596

One more time the aircraft appears to have been selectively "blind". While it was unable to see very close stations at only 20 nm such as CAK or CLE or other stations at a fairly distance such as TOL, incredibly he could see ORD (Chicago) at 278.46 nm and IAD (Washington, Dulles) at 238.05 nm, whose distance at 9:40 EDT was widely beyond the technical maximum threshold of 200 nm indicated by ARINC, and other stations such as YYZ (Toronto) at 188.88 nm, CVG (Cincinnati) at 174.29 nm, HTS (Huntington Tri-State) at 161.70 nm, CRW (Charleston) at 158.77 nm and DTW (Detroit) at 98.871 nm.

The only one possible conclusion after reviewing the above data is that either this theory is false or one of the following conditions applies:
  1. the radar data for United 93 were manipulated and the aircraft was not where the radar data claim it was (in which case, we must conclude that United 93 never made its famous U-turn over Cleveland, but kept on flying somewhere over the Pittsburgh, maybe in circles, for almost one hour) OR
  2. the ARINC logs, as publicly released, were manipulated and are not genuine

It is worth reminding that neither ACARS nor VHF transmissions are a guessing game. One cannot defeat laws of physics (electronics) with wishful thinking. How United 93 was able to "see" Chicago at 9:40 EDT at almost 280 nm while remaining blind to stations at less than 10 or 20 nm is honestly beyond me.

* republished with some edits from post #156 and post #165 on Pilots for 911 Truth  

(1) At 9:41, Cleveland Center lost United 93's transponder signal. The controller located it on primary radar, matched its position with visual sightings from other aircraft, and tracked the flight as it turned east, then south. Source: The 9/11 Commission Report, p. 46 (PDF version).
(2) The numbering of the ACARS messages follows Michael J Winter's interview in FBI302, p. 55-57.

Short introduction to the 9/11 ACARS debate

Posted by: Sergio

As known to some 9/11 researchers, the last months of 2011 and the first few weeks of 2012 have been characterized by an harsh debate over the ACARS messages to and from the four aircraft involved in 9/11. The ACARS issue was originally raised by Woody Box in 2009 after the release of the printout of Ed Ballinger's ACARS logs (1) released under FOIA. However, it was only at the end of the last year that the most significant implications raised by Woody Box claims have been publicly analyzed and debated, mainly on Unexplained Mysteries' and Pilots for 911 Truth's forum sections. Pilots for 911 Truth eventually published two articles (2). Warren Stutt and other posters on Unexplained Mysteries raised a counterclaim supporting the official story (3).

The theory raised by Warren Stutt and other people on Unexplained Mysteries is entirely based on the printout of ARINC logs (4), a document apparently created in 2004 and released years later under FOIA. However, no information about the submitter and the release date of such FOIA request are known as today. Also, to the best of my knowledge, the ARINC logs were never publicly accessible until made public by Warren Stutt on his own server with a link in the Pilots for 9/11 Truth's forum on December 14, 2011 and eventually uploaded by him on Scribd on December 19, 2011. All this, along with the fact that the data logs for United 175 are completely missing from this document, the fact that the data related to United 93 appear to be clearly in conflict with the statements made by several UAL dispatchers to the FBI as early as 2002 and other incongruences, make the authenticity of this document at least questionable. That said, Stutt's claim is essentially based on the assumption that the "Stn=" field within the first ULBLK block in the ARINC logs would indicate the actual ground station transmitting an uplink sent by the airline dispatchers, while the RGS (remote ground station) shown in "Target Stn=" field within the ULMSG block (that matches the RGS shown in the dispatcher's logs) would merely indicate a "predicted station" based on the original flight plan. Not one single section from any ARINC specification, not one single statement from any ARINC employee or ACARS expert corroborates this assumption. Also, during our research we found several cases that appear to flatly contradict this claim. All these issues will be treated in detail in the next articles. Some of them have been originally posted by me on Pilots for 9/11 Truth and will be republished here with some edits.