Unfortunately there is any way to put the antennas on the center of the spacecraft. There may be some spin modulation no matter what we do.

On Sep 27, 2006, at 1:41 PM, John B. Stephensen wrote:

The designer of the code to demodulate class 1 uplinks in the SDX needs to determine whether spin modulation can be compensated for. The carrier tracking loop bandwidth may be close enough to the satellite spin rate that it can't be eliminated. I'm not sure how well open loop compensation would work as the phase pertubation will vary with the arrival angle of the signal and this could vary by as much as 30 degrees at MA32 and MA224.

73,

John KD6OZH ----- Original Message ----- From: Jim Sanford To: Louis McFadin Cc: AMSAT Eagle Sent: Wednesday, September 27, 2006 01:30 UTC Subject: [eagle] Re: Proposed Eagle Antenna Arrays

Team: I've enjoyed monitoring this stream of conciousness discussion.

Question: Is it time to take a design and start formally vetting it?

My sense is that we ARE close to time to develop a formal proposal and do at least a preliminary peer review. If you strongly object, please say so now. I will listen.

I propose the following:

1. Lou and Stan resolve the unresolved issues Lou mentions in his

note. 2. Lou and Stan publish final draft design and design report stating specifications and any notes, design assumptions, etc. This design report to contain enough details that peer reviewers can load into the model of their choosing and model the design as part of the peer review. 3. Peer review committee start reviewing. Committee consists of: John Stephenson, Tom Clark, Bob McGwier, Rick Hambly, and myself. Open to suggestion on additional reviewers. (Paul Wade??) Lou & Stan are designers, not reviewers, so they will participate in the discussions in that role.

Thoughts??

My first comment: I note that none of the arrays, except 2m, are symmetrical (in themselves) about the spin axis. I request that the design report discuss spin modulation and peer reviewers assess same.

Time frame: I'd expect it to take a month for Lou & Stan to finalize design. With Symposium in the middle, make it six weeks. Given potential modelling involved, I'd expect the first round of peer review comments to take 4 to 6 weeks to develop, and 2 to 3 weeks to develop responses or corrections.

It is fair game for a reviewer to suggest a completely different design (with rationale) but let's start here.

Your thoughts??

Thanks & 73, Jim wb4gcs@amsat.org

Louis McFadin wrote:

...

Here is a proposal for arranging the Eagle antennas that Stan Wood and I worked out today. By moving the 2M dipoles to the corners we have sufficient room for full sized elements over the ground plane. The 2M antennas should get about 8dB.

Three L band patches in cavities should give us greater than 12 dB gain. By putting the S1,S2 and C band patches in hex shaped cups we maximize the available space. The cup arrays can be NC machined out of a solid blank and therefore attain precise control of the spacing. It may also serve as heat a radiating device. Each patch will be circular polarized and fed with a single feed from it's matching amplifier.

The 70 CM Circular polarized patch should have around 6 dB gain. We will look at getting more gain on 70 cm. Seven S1 patches will be mounted on top the 70CM patch. They should get about 15dB gain.

Tom's S2 interferometric patches are included.

Omni antennas similar to that used on P3D will be required on the opposite side of the spacecraft.

There should also be room for the Sun Sensors on both sides of the spacecraft.

Lou McFadin W5DID w5did@mac.com

---

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Aren't we glad we do open review of concepts? This may in fact be a deal killer for our very lowest power desires for the SMS concept. In fact, the spin modulation may queer the entire deal irrespective of the antenna being on the spin axis or not. It will certainly be worse off the spin axis.

I will work out this out with Frank to get the numbers. DRAT. I knew there was something we were forgetting.

I am off to tea but the tea and scones will not taste nearly so good now.

Bob

John B. Stephensen wrote:

What prevents use of the center? A patch for 70 cm won't work, but it seems to me that 2 crossed dipoes for 70 cm could be placed very close to the center.

73,

John KD6OZH

-

There would definitely be a structural problem with an antenna such as a 70 cm crossed dipole or turnstile. I think it would also be a problem with the envelope limits.

On Sep 27, 2006, at 2:48 PM, John B. Stephensen wrote:

What prevents use of the center? A patch for 70 cm won't work, but it seems to me that 2 crossed dipoes for 70 cm could be placed very close to the center.

73,

John KD6OZH ----- Original Message ----- From: Louis McFadin To: John B. Stephensen Cc: Jim Sanford ; AMSAT Eagle Sent: Wednesday, September 27, 2006 18:31 UTC Subject: Re: [eagle] Re: Proposed Eagle Antenna Arrays

Unfortunately there is any way to put the antennas on the center of the spacecraft. There may be some spin modulation no matter what we do.

On Sep 27, 2006, at 1:41 PM, John B. Stephensen wrote:

...

The designer of the code to demodulate class 1 uplinks in the SDX needs to determine whether spin modulation can be compensated for. The carrier tracking loop bandwidth may be close enough to the satellite spin rate that it can't be eliminated. I'm not sure how well open loop compensation would work as the phase pertubation will vary with the arrival angle of the signal and this could vary by as much as 30 degrees at MA32 and MA224.

73,

John KD6OZH ----- Original Message ----- From: Jim Sanford To: Louis McFadin Cc: AMSAT Eagle Sent: Wednesday, September 27, 2006 01:30 UTC Subject: [eagle] Re: Proposed Eagle Antenna Arrays

Team: I've enjoyed monitoring this stream of conciousness discussion.

Question: Is it time to take a design and start formally vetting it?

My sense is that we ARE close to time to develop a formal proposal and do at least a preliminary peer review. If you strongly object, please say so now. I will listen.

I propose the following:

1. Lou and Stan resolve the unresolved issues Lou mentions in his

note. 2. Lou and Stan publish final draft design and design report stating specifications and any notes, design assumptions, etc. This design report to contain enough details that peer reviewers can load into the model of their choosing and model the design as part of the peer review. 3. Peer review committee start reviewing. Committee consists of: John Stephenson, Tom Clark, Bob McGwier, Rick Hambly, and myself. Open to suggestion on additional reviewers. (Paul Wade??) Lou & Stan are designers, not reviewers, so they will participate in the discussions in that role.

Thoughts??

My first comment: I note that none of the arrays, except 2m, are symmetrical (in themselves) about the spin axis. I request that the design report discuss spin modulation and peer reviewers assess same.

Time frame: I'd expect it to take a month for Lou & Stan to finalize design. With Symposium in the middle, make it six weeks. Given potential modelling involved, I'd expect the first round of peer review comments to take 4 to 6 weeks to develop, and 2 to 3 weeks to develop responses or corrections.

It is fair game for a reviewer to suggest a completely different design (with rationale) but let's start here.

Your thoughts??

Thanks & 73, Jim wb4gcs@amsat.org

Louis McFadin wrote:

...

Here is a proposal for arranging the Eagle antennas that Stan Wood and I worked out today. By moving the 2M dipoles to the corners we have sufficient room for full sized elements over the ground plane. The 2M antennas should get about 8dB.

Three L band patches in cavities should give us greater than 12 dB gain. By putting the S1,S2 and C band patches in hex shaped cups we maximize the available space. The cup arrays can be NC machined out of a solid blank and therefore attain precise control of the spacing. It may also serve as heat a radiating device. Each patch will be circular polarized and fed with a single feed from it's matching amplifier.

The 70 CM Circular polarized patch should have around 6 dB gain. We will look at getting more gain on 70 cm. Seven S1 patches will be mounted on top the 70CM patch. They should get about 15dB gain.

Tom's S2 interferometric patches are included.

Omni antennas similar to that used on P3D will be required on the opposite side of the spacecraft.

There should also be room for the Sun Sensors on both sides of the spacecraft.

Lou McFadin W5DID w5did@mac.com

---

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---

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The class 1 demodulator can be aided by increasing the carrier leakage to aid in this tracking. Even if this means decreasing the bit rate. The spin rate will probably be lower than this. The high spin rates, used for motor burns, will not be in use when the Class 1 facility is in use (after we release the spacecraft for use). I think Stacey wants "about 10 rpm" or so for sensors. I recall him saying this number.

I simply cannot believe we missed possibly the single biggest marketing tool we can have for SDX: the end of spin modulation on the SSB bandwidth signals. If the computational burden is actually low enough for this to be done to reduce the impact of the spin by a factor of 10-100 (power), it will be a big time win and a huge marketing tool for us to exploit.

Bob

n1al@cds1.net wrote:

What is the carrier tracking loop bandwidth? I'm kind of sruprised it can't compensate a maximum 1/4 Hz sinusoidal phase modulation. (The maximum spin rate is still spec'd at 15 rpm, right?)

Al N1AL

...

On Sep 27, 2006, at 1:41 PM, John B. Stephensen wrote:

The designer of the code to demodulate class 1 uplinks in the SDX

needs to determine whether spin modulation can be compensated for. The carrier tracking loop bandwidth may be close enough to the satellite spin rate that it can't be eliminated. I'm not sure how well open loop compensation would work as the phase pertubation will vary with the arrival angle of the signal and this could vary by as much as 30 degrees at MA32 and MA224.

73,

John
KD6OZH

---

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The most sensitive uplinks are the class 1 uplinks on 70 cm and they are not received on a phased array. If the SDX can be generate a phase-modulated 10.7 MHz LO then some correction can be made. However, in the extreme, signals could be arriving over a 30-60 degree elevation depending on the ground station's location on earth. Won't the amount of phase shift vary consderably?

73,

John KD6OZH ----- Original Message ----- From: Tom Clark, K3IO To: Louis McFadin Cc: John B. Stephensen ; AMSAT Eagle Sent: Wednesday, September 27, 2006 18:55 UTC Subject: Re: [eagle] Re: Proposed Eagle Antenna Arrays

Louis McFadin wrote: Unfortunately there is any way to put the antennas on the center of the spacecraft. There may be some spin modulation no matter what we do.

On Sep 27, 2006, at 1:41 PM, John B. Stephensen wrote:

The designer of the code to demodulate class 1 uplinks in the SDX needs to determine whether spin modulation can be compensated for. The carrier tracking loop bandwidth may be close enough to the satellite spin rate that it can't be eliminated. I'm not sure how well open loop compensation would work as the phase pertubation will vary with the arrival angle of the signal and this could vary by as much as 30 degrees at MA32 and MA224.

Not quite true. For the phased arrays, if the required element phases are computed relative to the spin axis, then spin-induced Doppler is removed. You will get the spin Doppler only if you set the phase reference to be the center of the patch array.

One way this can be done is for some LO (common to all the elements of the phased array) needs to phase modulated with a rate equal to the spin rate and with a magnitude/phase calculated on the basis of the s/c-to-earth geometry. The only residual modulation to be compensated in the PLLs is then determined by the offset of the OBSERVER from the center of the earth.

Last nite I think I convinced Bob that the added S2 interferometric antennas (working against individual S2 array elements) will be able to determine the spin axis orientation (and rate) w.r.t. the line(s) connecting the s/c to dedicated terrestrial beacon(s) with accuracies ~1º or better.

73, Tom

Tom Clark, K3IO wrote:

Louis McFadin wrote:

...

Unfortunately there is any way to put the antennas on the center of the spacecraft. There may be some spin modulation no matter what we do.

On Sep 27, 2006, at 1:41 PM, John B. Stephensen wrote:

...

The designer of the code to demodulate class 1 uplinks in the SDX needs to determine whether spin modulation can be compensated for. The carrier tracking loop bandwidth may be close enough to the satellite spin rate that it can't be eliminated. I'm not sure how well open loop compensation would work as the phase pertubation will vary with the arrival angle of the signal and this could vary by as much as 30 degrees at MA32 and MA224.

Not quite true. For the phased arrays, if the required element phases are computed relative to the spin axis, then spin-induced Doppler is removed. You will get the spin Doppler only if you set the phase reference to be the center of the patch array.

But the discussion here is not about the Microwave phased arrays. Given the rate at which we are signaling and the demodulator requirements that will entail, the spin modulation is a minor factor on the ACP. For the very low bit rate SMS text messaging, and the 70cm uplink, where we were planning absolutely no phasing whatsoever, it becomes a major factor and might even be a deal killer without greatly increased complexity in the 70cm and 2m system. Essentially, you would have to compute and predict a phase offset from the (say) 70 cm patch and apply the correct again before we HELAPS or while we HELAPS on 2m, given information from the phased array for other bands and multiply each sample by the time varying phasor correction in the SDX to compensate. I had not even considered doing this. We need to cost HELAPS and STELLA before we know if this is achievable since I do not think we should increase the power consumed by the DSP by increasing the clock or adding other computational elements. But I do believe if it is achievable computationally, and we can "close the loop" with parametric determination of the correction in band or from the other communications package, we can essentially eliminate the phase offset from the spin axis.

One way this can be done is for some LO (common to all the elements of the phased array) needs to phase modulated with a rate equal to the spin rate and with a magnitude/phase calculated on the basis of the s/c-to-earth geometry. The only residual modulation to be compensated in the PLLs is then determined by the offset of the OBSERVER from the center of the earth.

Last nite I think I convinced Bob that the added S2 interferometric antennas (working against individual S2 array elements) will be able to determine the spin axis orientation (and rate) w.r.t. the line(s) connecting the s/c to dedicated terrestrial beacon(s) with accuracies ~1º or better.

I compute it is actually < 1°

73, Tom

---

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Tom Clark, K3IO wrote:

Robert McGwier wrote:

...

But the discussion here is not about the Microwave phased arrays. Given the rate at which we are signaling and the demodulator requirements that will entail, the spin modulation is a minor factor on the ACP. For the very low bit rate SMS text messaging, and the 70cm uplink, where we were planning absolutely no phasing whatsoever, it becomes a major factor and might even be a deal killer without greatly increased complexity in the 70cm and 2m system. Essentially, you would have to compute and predict a phase offset from the (say) 70 cm patch and apply the correct again before we HELAPS or while we HELAPS on 2m, given information from the phased array for other bands and multiply each sample by the time varying phasor correction in the SDX to compensate.

I think you are over estimating the complexity of the computation. For a non-phased antenna, the amplitude of the phase correction needed is (he pk-to-pk value is twice this):

2pi * sin(angle between boresite and earth) * (distance phase center is offset from spin axis/lambda)

The path that the element makes w.r.t. fixed observer is an ellipse. I seem to recall that any ellipse can be expressed as the sum of two circles of radius R and r, where R*(1+e) = the semi-major axis and r is the semi-minor axis r=R*(1-e) (where e is eccentricity). Let the center of the smaller circle "ride" the larger circle, let the larger R circle spin one turn per orbit prograde (the direction the satellite moves) and the t circle spin retrograde (in opposite direction). Pick the spot on the small circle (x,y) so that R+r is the apogee and R-r is perigee. Then the equation of motion of the spot will be

           x = R*cos(Wt ) + r*cos(-Wt)    = (R+r) cos (Wt)     and
           y = R*sin (Wt) + r*sin  (-Wt)    = (R-r) sin (Wt)   ,
              where W (really omega) = 2*pi*(spin rate)

I am indeed thinking about the cost of computing the phasor and doing a complex multiply on every sample coming in and going out in addition to all of the other tasks we know we must do. The angle with respect to the center of the hemispheric mass changes continuously throughout the orbit so the eccentricity is a function of the position in the orbit. The antenna paints a circle at "apogee" if we are perfectly nadir pointing then and not a circle elsewhere in the orbit segments we care about. At the nadir pointing place in the orbit, the phase distance to the center of the visible earth hemisphere is constant so no correction is needed and the correction needed changes before and after this does it not? I hope we can parameterize a model and not have to solve for the state vector but this will still not be even as simple as this calculation. I think as you say, we can compute what we need from the interferometer but please do not overestimate the computational power of the DSP chip. At 100 kHz doing STELLA and HELAPS predistortion, we are going to have to be careful with this computation. I would like to do this de-spin before we apply any processing and then we must figure out the appropriate place in the HELAPS construction going out the door.

Off to teach class, later!

This is sometimes called the equation of a central ellipse and the method dates back to Eudoxus of Cnidus (c. 400-347 B.C.) and then to Hipparchus of Rhodes (c. 190-120 B.C.) (http://astro.isi.edu/games/kepler.html -- see Fig 1) although it is sometimes attributed to Ptolemy (c. 85-165).

So what the ancient Greeks tell you is that you can tweak one LO in the system (either TX or RX) to have a phase offset that is the sum of two contra-rotating phasors running at the spin rate, with the phase, amplitude and eccentricity determined by geometry determined from the S2 interferometer. No math higher than trig is needed.

73, Tom