[amsat-bb] Re: [amsat-edu] Link Budget for Cornell University's student built satellite Violet
quadpugh at bellsouth.net
Sat Apr 18 12:59:14 PDT 2009
The major factor on how small a signal you can receive is controlled by the noise coming into the antenna. There are many articles written about thermal noise. But all objects above absolute 0 deg k produce noise. See http://vk1od.net/rx/noise/noise.htm. It turns out the temperature of the antenna is the temperature of the object it is pointing to. If you are pointing to cold sky at 400 MHz the temperature is about 30 deg k. If your antenna points to the horizon the temperature is 290 deg k. To overcome the effect of the thermal noise you will have to raise the transmitter power by several db to obtain the same signal to noise ratio. Read the article and go to the spread sheet an increase the antenna temperatures and look at the change in the link margin. Remember ever 3 db in link margin you improve you could reduce your transmitter power by 50%. Notice the spread sheet convert all elements in the receiver into the equivalent noise temperatures and the system temperatures is the sum of all the noise temperatures. The power at the antenna in watts = Pn=kb*T*B where kb is Boltzmann's constant (kb=1.3806504×10−23 joule/Kelvin) and T is the absolute temperature of the resistor in K B is the bandwidth in Hz. Your satellite must produce a signal that is higher than the thermal plus 9.6 db because you chose GMSK modulation.
Hope this helps
From: Yoon Ho Lee [mailto:yl482 at cornell.edu]
Sent: Saturday, April 18, 2009 1:16 PM
Cc: amsat-bb at amsat.org; Chris Mancuso; Lee Landry; PaulDarby at aol.com; Prathamesh Chawan; Ryan Leblanc
Subject: Re: [amsat-edu] Link Budget for Cornell University's student built satellite Violet
Our satellite is actually a nanosat, with 50cm by 50cm by 60cm (W x L x
H), so we do have enough space to use two patch antennas.
Would you like to explain more about the receiver noise temperature ?
I'm having a hard time estimating how much effect the noise temperature
has on a link.
> Hi Danny
> You did a good job on the link budget. Jan King is the author and he will
> comment if you send him a copy. Here are my comments.
> 1. Patch antennas are hard to put on a cubesat because of their
> 2. If you fly dipoles they have very deep lobes of the end of the
> ends and will cause deep
> fades when the antenna alimented off the ends.
> 3. The ground antenna when at 5 degrees elevation look into the
> warm earth i.e. 290 deg K
> For a reality check go copy AO 51. Copy the telemetry beacon. It uses FSK
> modulation and if you chose GMSK it is several db improvement over FSK. The
> antenna are turnstile with 2 db gain circular polarized. AO-51 keeps the
> antennas nadir pointing so there is very little fading due to polarization
> misalignments and this is a challenge on 10 cm cube.
> If you make it to the cube sat conference at San Louis Obispo look me up I
> am 6 ft tall with red hair and 64 years old.
> Good luck
> -----Original Message-----
> From: Yoon Ho Lee [mailto:yl482 at cornell.edu]
> Sent: Wednesday, April 15, 2009 9:18 AM
> To: Nick
> Subject: Re: [amsat-edu] Link Budget for Cornell University's student built
> satellite Violet
> Hello Nick,
> It seems like the listserv denied my attachment. Here are the files.
> Thank you for your interest and help!
> Nick wrote:
>> Hi Danny
>> I will review your link budget but it was not attached to your email.
>> Please reply directly to this email off list
>> -----Original Message-----
>> From: amsat-edu-bounces at amsat.org [mailto:amsat-edu-bounces at amsat.org]
>> On Behalf Of Yoon Ho Lee
>> Sent: Tuesday, April 14, 2009 7:12 PM
>> To: amsat-edu at amsat.org
>> Subject: [amsat-edu] Link Budget for Cornell University's student
>> built satellite Violet
>> Dear whom it may concern,
>> I'm Danny Lee from Cornell University's student built satellite team
>> Violet, working on Electrical Subsystem, specifically Telemetry and
>> For those of you familiar with CUSat, our team is under the same UNP
>> program, on the 6th competition.
>> I have worked out a preliminary link budget from limited resources and
>> knowledge, and I would greatly appreciate any input from experts out
>> Please read the note before proceeding to the spreadsheet.
>> Thank you very much in advance, and I look forward to your input.
>> Danny Lee
>> Electrical Subsystem,
>> Team Violet,
>> Cornell University
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