[tccrockets] Re: Altimeters
- From: James Dougherty <jafrado@xxxxxxxxx>
- To: TCC chat <tccrockets@xxxxxxxxxxxxx>
- Date: Sun, 4 Jan 2015 22:35:17 -0800
LOL
Good one.
On Sun, Jan 4, 2015 at 10:29 PM, Gary <gary-walker@xxxxxxxxxxx> wrote:
> Hey, when did the chat switch to speaking Greek? ;-)
>
> ------------------------------
> *From: *"James Dougherty" <jafrado@xxxxxxxxx>
> *To: *"TCC chat" <tccrockets@xxxxxxxxxxxxx>
> *Sent: *Sunday, January 4, 2015 10:18:46 PM
> *Subject: *[tccrockets] Re: Altimeters
>
>
> Thanks Cliff,
>
> Exactly!
>
> And as I mentioned earlier on that thread, FSPL is only one part of the
> link budget equation and you need to do the real link budget
> to come up with ranges - using the same antennas on each setup, not some
> 50dB dish with an aiming system! :-)
>
> And, like you said, ... when you get to 2.4 and 5Ghz band, the antenna
> gain is almost meaningless (you get gains from other features -
> e.g. # of MIMO chains, Modulation type and width, etc).
>
> So, let's consider simple link budget with:
>
> TX PWR (ERP) =Power Amp (PA) /Radio Gain dB + Antenna Gain dB – Cable Loss
> dB
>
> RX PWR (PR) = Rx Antenna Gain dB – Noise Floor dB + Additional Margin
> (e.g. 3 chains) dB
>
> and
>
> ERP – Free space Path Loss (FSPL) + PR – Cable Loss > RX Sensitivity
>
> or
>
> TX Power + TX/RX Gains – FSPL – Cable Loss > RX Sensitivity
>
>
> It's easy to see from the above that the higher the TX power, the greater
> the link budget, but let's work through the numbers.
> We'll use two modems, consider 38400 Baud on both Modems (400Mhz and
> 900Mhz) and not add any additional link margin
> achieved by firmware (gold codes, compression, filtering, etc).
>
> Start with data from the Two modem data-sheets:
>
> 400Mhz RX Sensitivity: TI CC1100 (Telemetrum/BRB) = -110dB
> 900Mhz RX Sensitivity: DNT900 (GPS-1/2, Multitronix) = -108dB
>
>
> Get the FSPL from any good online calculator (
> http://www.afar.net/rf-link-budget-calculator/)
> make sure to set the RX/TX gain to zero so we can observe antenna effects:
>
> 900Mhz
> Free Space Path Loss (with no atmospheric Affects) – 135.67dB for 161km
> (100mi)
>
> 434Mhz
> Free Space Path Loss (with no atmospheric Affects) – 129.33dB for 161km
> (100mi)
>
> Antennas
> Compare the same antennas and setup.
>
> - 6dB Directional RX Ground-Station Antenna
> - 2.5dB Airframe TX antenna
>
>
> 400Mhz
>
> ERP = 16dB + 6dB
> PR = 2.5dB
> 22dB – FSPL + 2.5dB > 110dB
>
> Using FSPL for 434Mhz and 100 Mile Range
> We have 22dB – 129.33dB + 2.5dB > -110dB
>
> -104.83 dB > -110dB at 38400 Kbps – it works, but only 5.2 dB of link
> margin
>
>
> 900Mhz
>
> ERP = 30dB + 6dB
> PR = 2.5dB
> 36dB – FSPL + 2.5dB > 108dB
>
> Using FSPL for 900Mhz and 100 Mile Range
> We have 36dB – 135dB + 2.5dB > -108dB
>
> -96.5 dB > -108dB at 38400 Kbps OK – 11.5dB margin left
>
> Remember that a 3dB difference is double, and we have that and much more
> link margin over
> the 400Mhz modem. In this case, the sender with the most output power
> wins, in all cases - the link
> margin is simply higher (even though the FSPL is a numerically higher
> value for the 900Mhz band).
>
> Now, a 6.5dB antenna is cumbersome since it is slightly directional.
> Remember that the higher the gain
> we have on an antenna, the less omnidirectional the reception becomes
> (consider focusing a flashlight,
> the more focused the beam, the less light dispersion).
>
> Lowering the gain on the receiver antenna will decrease the directionality
> of the receiver antenna
> and offer an even better multi-directional range. Suppose we use a 2.5dB
> antenna on the base and
> the receiver? Here we will be omnidirectional and less subject to
> directional fading.
>
> For 400Mhz, We have 18.5dB – 129.33dB + 2.5dB > -110dB
> -108.33dB > -110dB at 38400Kbps – OK - but only 1.6dB of link margin left
> :(
>
>
> For 900Mhz, We have 32.5dB – 135dB + 2.5dB > -108dB
> -100dB > -108dB at 38400Kbps OK – 8dB margin left!
>
> Again, the higher the output power, the better the LOS link budget will
> be. Using a more omnidirectional
> antenna will also give you better horizon and azimuth coverage.
>
> It basically proves why you need a big aimable antenna to get long range
> with 400Mhz :-)
>
> Anyhow, I'm going to be doing some real research on these modems this
> summer; if you're interested, let me know
> and we should get together sometime to discuss.
>
> best regards
> -James
>
>
> On Fri, Jan 2, 2015 at 8:01 PM, Cliff Sojourner <cls@xxxxxxxxxxxxx> wrote:
>
>> On 2014-12-30 22:07, James Dougherty wrote:
>>
>> By the way.. I did a comparison of Altus Metrum and BRB against GPS-1
>> (radio and modem comparison) earlier this year as part of some big debate
>> over 900 versus 400Mhz. I also got real Numbers for antenna efficiency ...
>> There was no comparison :-)
>>
>> The results support the theory that there are two schools of thought -
>> you can focus on antennas - or use higher power and link layer
>> optimizations to get even better results
>>
>>
>> James you are right on here. given that our rocket TX and RX antennas
>> are always compromised (stubbies and 1/4wave pointing the wrong way, etc),
>> link budget says run more power and FEC and let's use every other
>> reliability trick.
>>
>> that said, it's amazing to me that APRS works at all. X25 is a horrible
>> protocol, AX25 didn't help at all, ISO UI frames WTF? untagged positional
>> notation, no TLVs, mixed binary and human readable, no FEC just crummy
>> checksum. source routing!? totally unspecified, there is no one place
>> that has one document that nails it. it's a protocol designers' nightmare.
>>
>> turns out nearly half of all APRS packets are not delivered. no wonder
>> packet radio died.
>>
>>
>> Sent from my iPhone
>>
>> On Dec 30, 2014, at 9:27 PM, James Dougherty <jafrado@xxxxxxxxx> wrote:
>>
>> If it's 54mm or larger I can help you.
>>
>> Some new radio chips this year have me itching to do something for
>> 29mm :-)
>>
>>
>>
>> Sent from my iPhone
>>
>> On Dec 30, 2014, at 8:55 PM, "Rob Diehl" <rwdiehl@xxxxxxxxxxxxx> wrote:
>>
>> Happy New Year everyone!
>>
>>
>>
>> I am looking for a small altimeter/gps tracker for a small rocket. I
>> know that BRB makes a 2 meter version that is supposed to be longer range.
>> My history with BRB has not been the greatest. Has anyone tried the Altus
>> Metrum version? Altitude will be over $25k.
>>
>>
>>
>> I will be using a Real Flight board in my larger project, but I don’t
>> have the room in this one. I have my HAM license already. My HAM
>> transceiver does not have APRS decoder built in. I have seen some that use
>> the headphone jack on the HAM HT into a notebook for decoding. Not sure I
>> like that idea. What is everyone else using for packet decoders?
>>
>>
>>
>> I have an Ipad and a MS Surface Pro, so platforms and portability are not
>> an issue.
>>
>>
>>
>> Other options?
>>
>>
>>
>> Thanks.
>>
>>
>>
>> Rob
>>
>>
>>
>
>
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