# My DVB-T and DVB sat reception scrpts

• posted
Fair enough - I fly at Gransden lodge (254 ft AMSL) and was told that it was about the highest point in Cambs by an old boy who flew Mosquitos from there during WW2. His point was that it was high enough to have fairly frequent radiation fogs about the time when they launched the night's mission and hence were often on instruments during the take-off run.
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Martin    | martin at
Gregorie  | gregorie dot org```
• posted
involve
There are some, e.g. there are wind sensors using travel time for ultrasound signals (see Vaisala WXT 532). The real airspeed is of little interest to the pilot, he is more interested in the dynamic air pressure, as the airfoils react to it. The current pitot/static system is head-on to that purpose.
The relation between pressure and altitude is not a plain exponential, as the air temperature decreases 6.5 degrees (C or K) per kilometer rise, and this changes the density from the straight exponential model.
The barometric formula relates the relative absolute temperature (in Kelvin) to the relative pressure at altitude. The altitude enters the formula via the temperature relation.
p / p0 = (T / T0) to power 5.255883
p = pressure at altitude p0 = standard pressure, 1013.2 millibar T = temperature at altitude, T0 - 6.5 K / km T0 = standard temperature at surface, 288 K (15 C).
It depends on the weather. GPS measures the geometric altitude, but the aircraft instruments are using the pressure formula, and it will show less than the real airspeed as soon as the density of the air is less than that in standard conditions (1.29 kg/m3). The predictions from the barometric formula will be off as soon as the vertical temperature profile does not follow the -6.5 degrees / km ratio. An example is a quite common phenomenon with weather changes called an inversion, where the temperature temporarily rises with altitude.
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-TV```
• posted
So you don't know what port knocking is then!
Port knocking is when you access an apparent random set of ports in a specific order on the WAN side and the router software opens up an inbound port to the LAN side or gains root access to the firmware. Typically used by ISPs et al. to adjust user router configurations. Again, after the port knock sequence the magic access will close itself off if you don't access the opened port in time or when you disconnect. It allows completely invisible access to those who know the knock sequence.
The result is people run a GRC test (which is useful) see all green stealthy ports and think they are safe. It is the primary reason never to use a router provided by your ISP. For those of us on things like Virgin Cable where the modem and router are provided, you always run the Virgin box in modem mode only and hang another cable router on the output. Virgin can play to their heart's content with the modem but they can't do anything to the LAN side due to another firewall/NAT box etc.
The point being, if you don't know and control the software in the thing connects you to the internet then you are nowhere near as secure as you may think you are irrespective of what you have done to the user accessible features.
• posted
My router wasn't provided by an ISP: I bought it from Amazon or eBuyer - I forget which, so it won't have been set up for port knocking by an ISP. In addition I've crawled all over its web and SSH interfaces to see what it can do and have never seen anything remotely like port knocking configuration tools.
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Martin    | martin at
Gregorie  | gregorie dot org```
• posted
So you have only the manufacturer to worry about - viz concerns over Huwaeii equipment in the UK 5G infrastructure.
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Steve O'Hara-Smith                          |   Directable Mirror Arrays
C:\>WIN                                     | A better way to focus the sun ```
• posted
Seems like a reasonable trade-off seeing that I haven't seen any security notices for the make and model of ADSL router I'm using *and* that all machines on my lan have firewalls running with only the ports open. The only exception is my RPi, which is off for over 95% of the time.
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Martin    | martin at
Gregorie  | gregorie dot org```
• posted
I think GCHQ (and the other Five Eyes) will do their best to ensure the details of the backdoors they have had added remain secret.
To be serious, if you don't know what the router is running, you don't know what the router is doing. At some point you end up trusting someone else and as Snowden et al. have shown, you can't trust anybody!
• posted
You're paranoid, but are you paranoid enough ?
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Steve O'Hara-Smith                          |   Directable Mirror Arrays
C:\>WIN                                     | A better way to focus the sun ```
• posted
On Thu, 13 Dec 2018 19:12:53 -0000, "NY" declaimed the following:
At the time of my books (over a decade old now -- primarily GPS Satellite Surveying 2nd Ed [Alfred Leick, 1995 John Wiley & Sons]), GPS signals are sent on L1 and L2 (frequencies: 1575.42MHz, 1227.60MHz). L1 carries the C/A (coarse/acquisition) signal -- it is a ground control option to also have C/A on L2.. L1 and L2 carry the precise position signal. PPS is encrypted and requires receivers with government furnished decryption keys. While the keys do contain indicators for service type, allowing the government to disable, say, commercial air receivers without affecting military air receivers, it still requires the receivers to be periodically updated by someone with crypto clearance (I can just see the hassle that would be for, say, a Chinese airline using a US-made FMS and related equipment -- which is not that odd; GE Aviation provides the FMS boxes for Boeing, Airbus, and I think is on contract for China's big airliner)..
The "chip rate" for PPS is 10 times that of the C/A signal (putting it at ~10MHz; you can envision the processor needed to receive [and separate out the various satellites -- they are identified by the pattern of pseudorandom noise impressed on the data stream], decode [for PPS], and apply computed corrections. C/A is ~1MHz data stream.
I have vague memories that they were adding L3 and L4 to new generation NavStar. If they also put C/A on L2, it would allow consumer gear to use L1 and L2 time differences to correct for ionospheric delays, while still not requiring decryption equipment. I suspect commercial aircraft may be running gear that can detect/synchronize the PPS signal without decoding it -- which would be sufficient to perform the ionospheric correction while still using the C/A signal for navigation. Unless they are relying upon fully automated landings, a 5-10m error circle is still sufficient to navigate to a visual runway approach.
Ever watch a weather barometer? If a major storm front comes through, the pressure can drop by an inch or more. Even for consumer (hiking) GPS with barometric altitude the recommendation is to calibrate to surveyed altitude (you do have proper map showing altitude for your starting point) at the start of the trip, and periodically (like, stopping for lunch at an identifiable point on the map).
It also varies with temperature. To compute altitude you need: sea level pressure, pressure at altitude, and temperature.

It used to be standard procedure to adjust the aircraft barometric altimeter to use a standard pressure setting before making final approach, either mean sea level pressure or a setting provided by the ATC for a given runway. This ensured that all approaching aircraft were on the same (relative) altitude readings, rather than having them using altitudes based upon the pressures present at their departure runways. Some aircraft may have radar altimeters, but those are going to be more erratic the closer to the ground one becomes (at cruise altitude, the reflection can be averaged over the ground, and landing altitudes, just crossing a warehouse would jump the altitude by 20-30 feet)
What needle
Modern flight decks are all digital. The sensor inputs would be massaged by PID algorithms to produce controller outputs

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Wulfraed                 Dennis Lee Bieber         AF6VN
wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/```
• posted
So on a gusty day (ie air pressure fluctuates, even at a fixed position) how much variation in displayed airspeed and height do you see? Presumably it's small enough that you can still see the average value. That's why I suggested that the speed/height meters may need long-time-constant damping to perform any averaging - ie with a period longer than the typical cycle of variation due to gusting.
• posted
None that I've noticed in altitude and very little or no rapid fluctuation on the ASI at altitude. Yes, on a windy day we fly finals faster than on a calm day (rule of thumb is 50kts plus half windspeed) but you're busy on finals in those conditions and monitoring the ASI amongst several items. I'll see changes that need reacting to NOW but AFAIK the ASI doesn't flicker. I don't watch it closely enough to see if it does so because I also need to be paying attention to a lot of other stuff such as approach angle, drift, sink, other gliders and anything moving on the airfield.
There is no visible 'actual value', i.e. the ASI doesn't move appreciably during a glance though of course 2 seconds later it may be different as a gust passes. Hence no extra damping is needed: the ASI is displaying pressure in a tube facing into the airflow and gusts will not tend to change that much in relation to their effect on the glider's airspeed.
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Martin    | martin at
Gregorie  | gregorie dot org```
• posted
The best way to see whether damping is needed or not is to visit your local gliding club and take a trial flight - preferably on a relatively turbulent day. Then you'll see for yourself how the instruments behave and, who knows, you may get hooked and get yourself a new sport to enjoy. I've been flying gliders for 20 years and still think there's nothing to match the sensation of a winch launch or the view from 5000ft, just under the base of a nice cumulus cloud, while flying cross country on a fine summer day.
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Martin    | martin at
Gregorie  | gregorie dot org```
• posted
It's good that you added the "extra". Every instrument has some internal damping due to inertia of some sort or another. In this case that's enough although we don't seem able to quantify it.
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• posted
Fascinating.
My experience is flying aircraft at much lower heoghs and wingspans < 2 meters.
Gusts are quite slow things. Once clear of the boundary layer, windspeeds do vary but over seconds. Not tenths of a second, That would be heard as infrasound. Like thunder etc.
Most full size pilots seem to not use ASI for final appriach as far as I can tell - they adopt the techniques I discovered myself with models. Keep the speed UP and fly into the ground. Dont glide in. That way there is a bit of excess speed to allow for downdraughts or a wind lull. Once on the ground, well, use the controls to STAY there and dump airspeed as fast as possible...
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• posted
Yes, I took it as read that the mechanical nature of the dial gives its own inertial damping. As you say, it seems from what people say that this is enough to iron out any jitter caused by gusts. If the gauges were digital, with no mechanical inertia, it would no doubt be a different story!
For airspeed, I can see that it is the speed of the air over the wings, rather than the speed relative to the ground, which is the critical thing for flying, so pitot tubes have a BIG advantage over GPS, even if GPS had less error, which it sounds is not always the case.
But (dragging this thread kicking and screaming back on-topic!) for height above ground, GPS or radar have the big advantage over air pressure that they are measuring the actual height - as long as the readings are not subject to error - and don't need constant adjustment for variations in sea-level pressure as you move from one place to another. Obviously in the case of GPS, you need a good terrain map to convert height above sea level into height above ground at the place where you are, which you don't need with radar.
It's interesting to hear people say that variations in pressure caused by gusts are sufficiently small that they do not swamp the variation of pressure with altitude. Without doing the sums, I don't know how much the pressure varies with altitude, but my gut feeling (which is evidently wrong!) would have been that it was smaller than the variations every time there is a gust of wind. Good thing that it's not the case, otherwise pressure-driven altimeters would be unusable.
• posted
First of all altimeters are damped by virtue of having relatively small apertures to the 'outside world'
Secondly gust overpressure is relatively small compared with altitude. you dont have to swallow every time a gust comes along. Or every time you pull up from 70mph.
the far bigger changes in pressure are cyclonic lows that drive the wind.
Those operate over hundreds of miles and many hour timescales. And are factored into altimeters which are set to local air pressure at (adjusted to) mean sea level.
Wind itself does not imply pressure. Only to accelerate air to make wind.
Once made by e.g. a cyclonic low, the pattern persists until it meets some kind of resistance - with ground or sea.
There is of course an overall pressure differential across a cyclonic low, but it's tens of millibars over hundreds of miles. So a few percent at most. In a hurricane maybe 10-15%.
IIRC 60% of the air pressure is gone at 15,000 feet or so.

So no, altimeters are not affected by gusts really. Only pitot tubes facing into wind are. And that is because the pitot tube translates airspeed into pressure by HALTING the airflow with respect to the pitot head - the resultant momentum change is the force that is measured as pressure.
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Religion is regarded by the common people as true, by the wise as
foolish, and by the rulers as useful. ```
• posted
But the readings are subject to error: GPS is +/- 5m at best unless you're using a DGPS service at an airport, and air pressure can change quite fast - I've zeroed the altimeter to airfield level before spending 3 hours soaring the Pennines and, after landing, found the altimeter was reading 100 ft - the result of pressure drop as a low pressure area came in.
However, judging an approach is what a pilot's eyes are for and why so much training time is allocated to approach and landing practise. In a glider you get the landing right first time, every time because you can't open a throttle and go round for another try.
As to radar and similar electronics - whether you find that useful depends on what you're flying and whether it has an engine to derive a generator. Without that you pay a lot of attention to the power consumption of whatever electronic instruments you carry, what your radio and electronic conspicuity systems draw. FLARM is great: it works very well and only uses 50mA at 12v. Transponders have been really hungry in the past but new systems are much more power efficient.
The only instrument that's really sensitive to gusts is an electronic variometer - this is a pressure sensing instrument thats optimised for measuring altitude change. Glider pilots use them to find thermals and other lift sources and to avoid sink.
Turn one on sitting on the bench. Raise it reasonably slowly above your head, i.e. take 1-2 seconds to lift it a metre, and it will show around a 0.5 knot (50 fpm) climb rate and you'll hear 'I'm climbing' beeps from its speaker.
Varios also notice gusts and turbulence round the edge of a thermal: this in conjunction with 'the surge' felt in your gluteus maximus shows you're entering a thermal and a rising wing tip show which way to turn to get centred in it.
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Martin    | martin at
Gregorie  | gregorie dot org```
• posted
Disagree - full size technique is rather different from RC. All fixed- wing aircraft, both powered and glider, trim for approach speed and then use the throttle (or airbrakes in a glider) to regulate sink speed and hence the touchdown point. Fully held-off landings are the the norm - flare at 2-3 feet up and keep the stick coming back to hold that altitude until the plane stops flying and sinks onto the runway.
I've only known one RC pilot who used that technique for his models - and he had 20,000 hours on Bristol Freighters and saw no reason to fly a model any differently to his normal full-size practise. BTW, he was an above average RC aerobatic pilot too.
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Martin    | martin at
Gregorie  | gregorie dot org```
• posted
when i landed in NY in 2005 the co-pilot most definitely drove the plane into the ground. mid you we had just experienced a side-wind gust & lift on final approach (less than rooftop level) so he was making sure that when he put it down it stayed down.
a firm landing but the right call
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There's nothing wrong with teenagers that reasoning with them won't
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• posted
The altitude change with pressure in standard conditions near sea level is -27 ft/millibar (or -8.1 m/mb).
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-TV```

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