I have friends who never wear a seat belt. None of them have died yet, so I’ve decided seat belts are BS.
I’ve gotten fairly consistent signal at 15,000 ft in a Lear 45.
Definitely not impossible. Should be able to get signal at even higher altitudes as well.
Not exactly a happy example, but cell calls were made from Flight 93 on 9/11 at altitudes in excess of 30000ft. It was probably easier to make high altitude calls on those older networks, but there’s no fundamental reason calls can’t go through at that height even today, at least in some cases.
While altitude is an issue, rapidly switching between cell towers is also a real problem. Most mobile networks aren’t really designed to handle it very well. For the occasional phone, it probably isn’t much of a problem, but hundreds or thousands can be.
Were the calls from Flight 93 made on cell phones or on the airplane phone that was built into the middle seatback of every row & cost something like $5.99/min to use?
Also, back then, they were analog cell phones, analog would get better reception at the fringes of service than digital does.
Well, one thing I don’t know if anyone mentioned, but the spectrum used has changed. The old analogue cellular phones used, IIRC, 800 Mhz, while today they use higher and narrower bands (900/1800/1900 Mhz). The old analogue cell phones had significant RF leakage. But my WAG as to the origin of this probably had more to do with those big ass batteries and the potential for a short in a pure oxygen environment. THAT could be and issue (today as well), and one I’d go with the ‘better safe than sorry’ saying.
I haven’t really worked much in hospitals, so this is guess work. I have put in cellular boosters, microwave uplinks as well as WiFi access point systems near hospitals, and I know that hospitals have, or used to have extensive RF bubbles of their own, usually having some sort of internal radio systems from the looks of things as well as gawd awful RF leakage (my WAG is things like their MRI machines, maybe X-Ray and the like, but like I said, never did any projects for or in actual hospitals personally). I remember looking at the waterfall graphs from near a hospital where we were putting in systems and it was pretty substantial.
Almost all from the air phones in the seat backs. 2 or 3 were made using personal mobiles.
Indeed so, AMPS, IIRC. I think one of the mobile calls was iffy and the other(s) were lucky to get through and stay connected as long as they did. It helped connectivity they were at lower altitude towards the end.
Early cell phones back then put out a lot more RF interference, and at a more used frequency. Also, the hospital equipment of the time was less shielded against such interference. Still, most equipment (like 905) would have been un-affected. But the 10% that were affected might have been something critical. So out of ‘better safe than sorry’ thinking, hospitals adopted a no cell phone use policy.
Since then, cell phones have been redesigned, and moved to new frequencies, and the manufacturers of hospital equipment have recognized that their equipment will constantly be subject to cell phone signals, and have designed in shielding to neutralize this.
Cell phones did have a dangerous in hospitals back then, though – on the bottom line! Hospitals used to make a lot of money on charging extra fees for beds with a telephone. High enough fees that a few days of a hospital phone would cost as much as a month’s phone bill for a home phone. That was a nice money-maker for hospitals, that was nearly completely killed off by cell phones.
It is a lot more messy than this now.
Spectrum is used wherever it is freed up. 700MHz is a big thing here in Oz, as it has much better coverage in some terrain. If you look across the planet there are bits of spectrum allocated in all sorts of corners, and as other services are retired, there is intense competition to grab even more.
One of the issues that came up with the first GSM phones was that the peak radiated power was higher than the analog (AMPS and earlier) systems. The average power was less, but the nature of the digital modulation is such that you could see much more intrusive peaks. It don’t think it has improved all that much. Spread spectrum doesn’t really help, the spread is across a sufficiently narrow range that for the purposes of shielding it is all the same frequency.
One of the difficulties with aircraft was that it was very difficult to test and predict interference - because the conditions in flight were sufficiently different to on the ground that you could never be sure that the craft’s systems were really immune. There were a number of occasions where in-flight interference from a passenger device was found, but it was not possible to reproduce in the ground. That tends to make people nervous.
Most of the key points have already been covered, but for the few who want a bit more technical detail:
(credentials/experience in the topic: 1998-2003 in the ECG telemetry business; part of the design team that came up with the first (and still in use) frequency hopping/TDMA ECG telemetry system; patents in the field; all too much time spent in an FCC/FDA working group on medical telemetry regulation; blah blah blah)
(this is pretty US-centric, but then so have been a lot of the followups: for a variety of complex reasons the US and Japan are the only countries - or at least **were **the only countries - with large scale medical telemetry setups)
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Medical equipment designed in the 80s and 90s frequently had a disheartening level of susceptibility to EMI. I’m not really sure why that was, but true as evidenced by X-ray vision’s first link. I entered the medical electronics business from aerospace/military, and while my military industrial complex employers’ products had many deficiencies, EMI susceptibility was not among them.
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90’s vintage cell phones (AMPS and D-AMPS, aka IS-54 and IS-136) were, err, not so good about ramping the (RF) power amp turnon, which created broad spectrum burst noise (the sub-1GHz FCC test with the 120 kHz CISPR quasi-peak detector would let a great deal of impulse noise through the device authorization process: the Fourier transform of the emission was spread out, but the Laplace transform - not so much). That has gotten a lot better, although in truth I’m not really sure why, given that the FCC authorization requirements for sub-GHz haven’t really changed.
2a) The rectification of the envelope of AMPS and D-AMPS transmissions (remember, it doesn’t take that much to create a -20 dBc rectifier) would all too often fall into the passband of a physiologically important signal (e.g. ECG in the 60-100 Hz range, thoracic impedance respiration in the high tens of kHz). Those physiological signals are pretty low amplitude at the sensor (<< 10mV peak to peak) , so it does not take that much energy to overwhelm them. Hard PA turnon/turnoff (see 2) produces big components at the inverse of the D-AMPS slot time or 150 Hz - enough to bleed through low pass filters on the ECG acquisition front end (of course, **we **did better with fancy Sallen-Key bandpass filters).
- Older ECG telemetry systems (not like my team’s system: one of our sales demos was to hold a cellphone with the speaker on right next to the [ECG] electrodes of a pretend patient) were fixed frequency, narrowband (< 25 kHz) and running near their sensitivity limit - the typical 90s vintage ECG telemetry system with its turnstile antennas and weak band select filtering would intermodulate like mad into the ECG system carrier if a cellphone hundreds of MHz away was transmitting underneath one of those antennas. And given that the carrier power at the first LNA was all too often far below -100 dBm in 25 kHz (lab sensitivity limit -114 dBm or so: Real World more like -104 to -106), with dubious first downconverter linearity and a too-slow AGC loop feeding a limiter/discriminator, it was not so hard for an absurdly loud IS-53/136 handset (with transmit power above 1W EIRP) to stomp on the ECG telemetry system.
Link should be to an iPad with 2 bars of 3G signal at 31000 feet. The altimeter and iPad signal are both visible.
actually, the no phone rule these days is due to privacy laws … same reason you cant use a phone at the pharmacy counter
Where is it that customers can’t use the phone at the pharmacy counter? A "no cell phone use’ in a waiting room is clearly directed at patients not staff.
My hospital has free wi-fi. I went there for a gallbladder scan (with radioactive dye) and brought a book just in case, but I was allowed to use my phone, so I listened to a podcast.
The big cell phone sign at my CVS say, in so many words, “Hey, while you’re waiting in line why not download our phone app?”
CMC fnord!
I once flew without my seatbelt being buckled (I’d forgotten). At some point I realized it wasn’t fastened, and was surprised there wasn’t a ding ding ding in the cockpit to alert the pilot.
Because, in days gone by, not all seats in airplanes were sold on every flight, and planes often flew with non-zero empty seats. A system like you were thinking of would mean a lot of dinging, or would require all empty seats to be manually buckled. Instead airlines go with visual checks by cabin personnel of seat-belt compliance, which is obviously not perfect. End of tangent.
A lot more glass in the cockpit than in the cabin…
ISTR that (installed) car phones had better, more powerful antennas that that first bag phone you carried. I’d assume this was true on airplanes, too???
I’ve been at 5000’ in an open cockpit & not had service. Some of it depends on terrain, antenna angle, & tower density.
Put your phone by a cabin window and it’s got plenty of “glass” to see through.
Look, you don’t get great results at altitude but it is far from “impossible”.
Or they could do like almost every car manufacturer these days and also include a weight sensor in the seat …
I think it’s more about the fact that seat belts in cars buckle at the side, while the airplane belt buckles in the middle. Aaand that you’d have to rig that whole system in a very cheap way while not changing how the manufacturer and airline deals with installing seats in various configurations and replacing and repairing seats.