[amsat-bb] Re: Path to HEO
schrieb Ken Ernandes on 2013-04-30 09:49: > I note the disclaimer at the bottom, so I'll help with the incorrect > assumptions. > > 1. g = 9.81 m/sec only applies to one Earth radius (i.e., the Earth's > surface). Gravitational acceleration drops of as an inverse square of the > radius. > 2. GEO altitude is close to 36000 km, but GEO radius is approximately 42164 > km (you must add the Earth's radius to altitude to get orbital radius) Gotcha! > Gravity can be simplified by using a constant MU = 398600.4418 km^3/sec^2 > > For any radius you can compute g by: > > g = MU / r^2 > > But make sure you use radius and not altitude. Mean Earth radius at > mid-latitudes is approximately 6371 km and is 6378 at the equator. > > The speed (v) for an elliptical orbit can be computed from the current radius > (r) and semi-major axis (a): > > v = sqrt(MU*(2/r - 1/a)) > > This can be simplified for a circular orbit (r = a): > > v = sqrt(MU/a) These are the formulas they did not tell us at school, thank you! > The more important thing is what Dan Schultz pointed out. At 300 km > altitude, atmospheric drag is a significant factor in a continuous drain of > orbital energy. This is less at 500 km and almost insignificant starting > around 800 km. The drop off is because drag is: > > 1. proportional to atmospheric density which drops off quickly with increased > altitude. > 2. also proportional to the square of the speed relative to the atmosphere. > > If you can get above 800 km without using up a lot of your fuel, you have a > chance to make something workable. >From the last few cubesat rides I reckon that going there instead of only to 300 km is not that impossible. Regards Patrick -- Engineers motto: cheap, good, fast: choose any two Patrick Strasser ___ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
[amsat-bb] Re: Path to HEO
schrieb Stefan Wagener on 2013-04-29 01:29: > LEO (low earth orbit) from 400 to 2000km or something like that > HEO (high earth orbit) >2km > > Going from 310 to 700km in a year is not doing us anything. We need a > highly elliptical orbit (Apogee ~6km, Perigee 900km) similar to AO-40 > to allow for cross continental communication. Let's check with some maths (*): Energy at 300 km: - Total Energy = kinetic energy + potential energy E_kin + E_pot = (m*v^2)/2 + m*g*h For simplicity, we choose mass as 1 kg, h = 300km, v = 1st cosmic velocity =~ 7100m/s 1*7200² + 1*9.81*300x10^3 = 2.8148x10^7 [Joule] Potential Energy is some 10% of the total energy. As this is for one kg of mass, and m goes linear in the above equations, you can scale with the mass of your satellite. Energy at 36000 km: --- Speed from radius and time for one orbit (1 day=84600 sec) v = 2*r*pi/t = 2*36x10^6*3.14159265/84600 = 2673.7 m/s E_kin + E_pot = (m*v^2)/2 + m*g*h = 3.57x10^6 + 3.53^8 =~ 3.56×10^8 [Joule] Now kinetic energy is only about 1% of the total energy! A LEO has about 8% the energy of an GEO. The satellite needs 3.29x10^8 J/Kg Energy to get from LEO to GEO. Lets say it's 10kg and has 50 W of power for thrust. 3.3x10^8 * 10 = 3.3x10^9 J thrust 1 Joule is 1 Watt / 1 second, 1 Watt second = 1 Joule 1 Watt day = 84600 Joule = 8.46x10^4 Joule Our 50 Watt ion drive can increase the energy by 4.23x10^6 Joule a day. How much days will LEO to GEO take: We have some 10^8 divided by some 10^6, it's a matter of months, the calculation says 77.8. But we should be satisfied to get an order of magnitude after the rough assumtions and estimations made before. If I made a mistake above, maybe this is off by an factor of 10, then it's 2 year. Still fine! Of course you have to count in the gas you want to ionize, which reduces the weight over time (but I was really bad at differential equations and would not get that right), and maybe the weight and power estimations are not very realistic, and using steady thrust instead of short impulses decreases efficiency in orbit changes, and changing from polar to equatorial orbit takes extra energy, and maybe an elliptic orbit takes less energy, and maybe some inaccuracies more. But this does not matter: In the end, it seems that changing from LEO to GEO or HEO is possible in sensible time. Regards Patrick (*) Disclaimer: This is High School maths, please double check and correct my calculations -- Engineers motto: cheap, good, fast: choose any two Patrick Strasser ___ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
[amsat-bb] Re: AMSAT at Chaos Communication Camp 2011 - with ARISSAT-1
Am 2011-08-14 00:20, schrieb Trevor .: > Today Mario Lorenz DL5MLO from AMSAT-DL gave a presentation on Amateur Radio > Space Communications as part of the Hacker Space Program track at the Chaos > Communication Camp 2011. > > Mario's talk titled 'From OSCAR 1 to Mars and beyond - Amateur Space > Exploration – The last 50 years, now, and the future' > Hacker Space Program track > http://events.ccc.de/camp/2011/Fahrplan/track/Hacker%20Space%20Program/index.en.html > > Chaos Communication Camp 2011 > http://events.ccc.de/camp/2011/ The video of the talk can be viewed at http://media.ccc.de/browse/conferences/camp2011/cccamp11-4594-from_oscar_1_to_mars_and_beyond-en.html The day before I was tracking with him ARISSAT-1, Mario with his Funcube Dongle, and we were able to track a nice non-eclipsed pass. Mario could successfully decode telemetry, but I do not have his data. 73 Patrick OE6PSE -- Engineers motto: cheap, good, fast: choose any two QTH: JN77rb http://sat.mur.at/ Patrick Strasser OE6PSE ___ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
[amsat-bb] Re: Decoding wideband recordings
Am 2011-08-10 00:54, schrieb Phil Karn: > It's been suggested that I modify my ARISSat-1 BPSK-1000 telemetry > demodulator/decoder to accept wideband quadrature (I & Q) recordings > like those produced by most of the software defined radios out there. > > This is fundamentally not that hard, but first I need some information. > > How many people could actually use this? I guess all people that use SDRs in the style of FCD, like SDR Widget, or generally all DDC receivers that work with soundcard interfaces. > What is the format of the > recorded files? Is there a standard, or does each make of SDR produce > its own? Often the I and Q channel are recorded via Left and Right of a sound interface, either built-in in the computer or via USB. FCD for example is not more than a very powerful DDC chain, an USB audio device with 96kHz bandwidth and a USB HID interface to control all the settings, like filters and gains. So for a recording you'd probaly end up in whatever your recoding program produces, likely WAV, but any format with 16bit and 2 channels would do. For decoding please be aware that I/Q via sound interfaces has a weak spot at the centre frequency. The interfaces all have a high pass characteristic below like 30Hz, which means the resulting spectrum has a notch in the middle. You usually would not like to exactly tune the center frequency to the frequency of your interest, but a little below or above. You can see that notch for example at http://www.oz9aec.net/index.php/funcube-dongle/423-funcube-dongle-on-the-air-tests-at-oz7sat With all oscillators being not perfect this can probably be ignored, as you wouldn't end up at the center frequency anyway. For converting audio files I can recommend sox http://sox.sourceforge.net which is quite common in Linux distributions, but as cross platform tool also available for Windows and Mac OS X. It's also available as library. What I wonder is how good BPSK1000 survives speech encoders, like MP3. Is there any experience already? 73 Patrick -- Engineers motto: cheap, good, fast: choose any two QTH: JN77rb http://sat.mur.at/ Patrick Strasser OE6PSE ___ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
[amsat-bb] Re: ARISSAT-1: No signal
schrieb Greg D. am 2011-04-13 07:18: > I did hear a frequency drifting carrier around 145.920 that disappeared about > the time the Station set, but the carrier was RISING in frequency, which is > inconsistent with a satellite. We had a reasonable pass at 22:59 UTC here at OE6XUG, Austria JN77, and we heard something similar: Rising carrier, and something that could have been BPSK. The only recording device was a digital voice recorder, I hope the compression leaves something for identification or decoding. Cheers Patrick OE6PSE -- Engineers motto: cheap, good, fast: choose any two Patrick Strasser OE6PSE ___ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
