Trying to understand downlink corrections
Greetings, I have been using a SDR dongle to receive the downlinks of SSB satellites. The combination of SatPC32's Downlink Correction feature and the SDR's visual display helps to zero beat the CW beacon exceptionally well. The Lat/Lon is good to 6 decimals. However, I find that the downlink correction needed is quite different for different satellites. Could it be due to rounding off errors or due to temperature variations at the satellite itself ? Maybe this has been explained earlier, but I wonder why ? Obviously, the more I learn, shows how less I know... Thanks for any pointers ! 73! Umesh k6vug
Umesh,
You're right, some satellites i.e AO-91, AO-73 do show temperature and solar illumination afects frequency, other possible cause is older TLE keplers data. ( For often updated keps try http://amsat.org.ar/keps.txt )
Learn is a never ending activity :)
73, LU7ABF, Pedro
On 7/10/18, k6vug@sbcglobal.net k6vug@sbcglobal.net wrote:
Greetings, I have been using a SDR dongle to receive the downlinks of SSB satellites. The combination of SatPC32's Downlink Correction feature and the SDR's visual display helps to zero beat the CW beacon exceptionally well. The Lat/Lon is good to 6 decimals. However, I find that the downlink correction needed is quite different for different satellites. Could it be due to rounding off errors or due to temperature variations at the satellite itself ? Maybe this has been explained earlier, but I wonder why ?
Obviously, the more I learn, shows how less I know... Thanks for any pointers !
73! Umesh k6vug
Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
Umesh,
There are two primary satellite issues. First, the published frequencies are usually nominal values obtained during ground checkout. Over time due to normal component aging, radiation, loss of voltage regulation, etc they can drift.
The second is the temperature effect you mentioned. You will find that the beacon and transponder calibration will change slightly with the spacecraft temperature as it goes through periods of eclipse and full illumination. There is also a slight seasonal shift which you can see. This is most prominent in CubeSats which have very little thermal inertia. Also, the are tradeoffs to be made in terms of power consumption, radiation resistance, and performance. AO-7 also drifts due to the lack of a battery to stabilize the voltage.
Another issue is your station computer clock. They can drift rather badly, and the operating system may not do a very good job of syncing them with a reliable time standard. There are some free programs which do an excellent job available.
73,
Alan WA4SCA
<-----Original Message----- <From: AMSAT-BB [mailto:amsat-bb-bounces@amsat.org] On Behalf Of <k6vug@sbcglobal.net <Sent: Tuesday, July 10, 2018 12:28 PM <To: AMSAT BB amsat-bb@amsat.org <Subject: [amsat-bb] Trying to understand downlink corrections < <Greetings, <I have been using a SDR dongle to receive the downlinks of SSB satellites. The <combination of SatPC32's Downlink Correction feature and the SDR's visual <display helps to zero beat the CW beacon exceptionally well. The Lat/Lon is <good to 6 decimals. However, I find that the downlink correction needed is <quite different for different satellites. Could it be due to rounding off errors or <due to temperature variations at the satellite itself ? Maybe this has been <explained earlier, but I wonder why ? < <Obviously, the more I learn, shows how less I know... Thanks for any pointers <! < <73! <Umesh <k6vug < <_______________________________________________ <Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available <to all interested persons worldwide without requiring membership. Opinions <expressed <are solely those of the author, and do not reflect the official views of AMSAT- <NA. <Not an AMSAT-NA member? Join now to support the amateur satellite <program! <Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
Don't forget your SDR could be off by a few parts per million in frequency if you haven't calibrated it to a known frequency source. At 435 MHz, being off by 3 parts per million would translate to a 3*435 =1305 Hz or 1.305 KHz offset from the actual intended frequency of reception. Jay F. CoxKG5BZW "Knowledge can only exist if doubt is possible." -- stolen from an amazon.com review of "On Certainty:" by by Ludwig Wittgenstein.
On Tuesday, July 10, 2018, 1:43:09 PM CDT, Alan wa4sca@gmail.com wrote:
Umesh,
There are two primary satellite issues. First, the published frequencies are usually nominal values obtained during ground checkout. Over time due to normal component aging, radiation, loss of voltage regulation, etc they can drift.
The second is the temperature effect you mentioned. You will find that the beacon and transponder calibration will change slightly with the spacecraft temperature as it goes through periods of eclipse and full illumination. There is also a slight seasonal shift which you can see. This is most prominent in CubeSats which have very little thermal inertia. Also, the are tradeoffs to be made in terms of power consumption, radiation resistance, and performance. AO-7 also drifts due to the lack of a battery to stabilize the voltage.
Another issue is your station computer clock. They can drift rather badly, and the operating system may not do a very good job of syncing them with a reliable time standard. There are some free programs which do an excellent job available.
73,
Alan WA4SCA
<-----Original Message----- <From: AMSAT-BB [mailto:amsat-bb-bounces@amsat.org] On Behalf Of <k6vug@sbcglobal.net <Sent: Tuesday, July 10, 2018 12:28 PM <To: AMSAT BB amsat-bb@amsat.org <Subject: [amsat-bb] Trying to understand downlink corrections < <Greetings, <I have been using a SDR dongle to receive the downlinks of SSB satellites. The <combination of SatPC32's Downlink Correction feature and the SDR's visual <display helps to zero beat the CW beacon exceptionally well. The Lat/Lon is <good to 6 decimals. However, I find that the downlink correction needed is <quite different for different satellites. Could it be due to rounding off errors or <due to temperature variations at the satellite itself ? Maybe this has been <explained earlier, but I wonder why ? < <Obviously, the more I learn, shows how less I know... Thanks for any pointers <! < <73! <Umesh <k6vug < <_______________________________________________ <Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available <to all interested persons worldwide without requiring membership. Opinions <expressed <are solely those of the author, and do not reflect the official views of AMSAT- <NA. <Not an AMSAT-NA member? Join now to support the amateur satellite <program! <Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
participants (4)
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Alan -
Jay Cox -
k6vug＠sbcglobal.net -
Pedro Converso