I am using HRD with Sat tracker module (thanks, Simon!)
I notice that for the non-FM birds I am working (AO7, FO29, VO52) the Doppler is never quite exact. Or I guess I should say there seems to be a constant *offset* that needs to be included in the Doppler calc. Tonight on AO7 mode B pass, my actual downlink frequency seemed to be about 4 kHz lower than what it should be according to the Doppler. As the bird passed by, I followed the "one true rule" and only adjusted the higher freq (uplink in this case). It was consistently about 4 kHz lower.
On FO29 pass, there seemed to be about a 2 kHz offset. When the bird passed overhead, the Doppler "flipped" quickly and I had to find my signal again.
How can I consistently determine this offset so I can include it in with the Doppler correction? Or is it on a case-by-case basis?
PS Thanks, Frank, K9CIS, for the nice CW rag chew on FO29 tonight.
Mark Lunday WD4ELG wd4elg@arrl.net http://wd4elg.net
On 15 Nov 2008 at 23:57, Mark Lunday, WD4ELG wrote:
I am using HRD with Sat tracker module (thanks, Simon!)
I notice that for the non-FM birds I am working (AO7, FO29, VO52) the Doppler is never quite exact. Or I guess I should say there seems to be a constant *offset* that needs to be included in the Doppler calc. Tonight on AO7 mode B pass, my actual downlink frequency seemed to be about 4 kHz lower than what it should be according to the Doppler. As the bird passed by, I followed the "one true rule" and only adjusted the higher freq (uplink in this case). It was consistently about 4 kHz lower.
On FO29 pass, there seemed to be about a 2 kHz offset. When the bird passed overhead, the Doppler "flipped" quickly and I had to find my signal again.
How can I consistently determine this offset so I can include it in with the Doppler correction? Or is it on a case-by-case basis?
PS Thanks, Frank, K9CIS, for the nice CW rag chew on FO29 tonight.
Mark Lunday WD4ELG wd4elg@arrl.net http://wd4elg.net
I am starting to implement automatic doppler control and i note the same effect. I guess it is related to the step of adjustment. I am using 10 hertz and i think especially at TCA on vertical elevation pass the 10HZ step is not enough i have to manually adjust to compensate for this lag. I will try to use higher step.
"-"
Luc Leblanc VE2DWE Skype VE2DWE www.qsl.net/ve2dwe WAC BASIC CW PHONE SATELLITE
Hi Mark,
Almost all the linear sats have some offset due to variations in frequency of the internal oscillators etc. In the case of AO-07 the offset is temperature related and varies slowly depending on the amount of sunlight etc. (AO-07 at the moment is about 4.5kHz) The technique I use to determine the offset at any time is to set my receiver to the centre of the satellite downlink passband (+/- Doppler) and then adjust my transmitter frequency to hear myself correctly on the downlink. The difference between my actual uplink (+/- Doppler) and the nominal centre of the uplink passband is the offset. I save this value and use it built into my Doppler correction until any change warrants a re-calculation.
Alan ZL2BX
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Mark Lunday, WD4ELG Sent: Sunday, 16 November 2008 04:58 To: amsat-bb@amsat.org Subject: [amsat-bb] Some doppler observations/questions
I am using HRD with Sat tracker module (thanks, Simon!)
I notice that for the non-FM birds I am working (AO7, FO29, VO52) the Doppler is never quite exact. Or I guess I should say there seems to be a constant *offset* that needs to be included in the Doppler calc. Tonight on AO7 mode B pass, my actual downlink frequency seemed to be about 4 kHz lower than what it should be according to the Doppler. As the bird passed by, I followed the "one true rule" and only adjusted the higher freq (uplink in this case). It was consistently about 4 kHz lower.
On FO29 pass, there seemed to be about a 2 kHz offset. When the bird passed overhead, the Doppler "flipped" quickly and I had to find my signal again.
How can I consistently determine this offset so I can include it in with the Doppler correction? Or is it on a case-by-case basis?
PS Thanks, Frank, K9CIS, for the nice CW rag chew on FO29 tonight.
Mark Lunday WD4ELG wd4elg@arrl.net http://wd4elg.net
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A couple of ideas:
1) Make sure your Lat / Lon is correct - very correct. 2) Make sure you use good Kepler data, for example from www.celestrak.com .
I test with the CW beacon on LO-19 - if the pitch does not change during a pass then the Doppler is spot-on.
I also notice offsets with some sats - as other have commented this is the LO on the satellite although it could also be the LO on the radio.
Simon Brown, HB9DRV www.ham-radio-deluxe.com
----- Original Message ----- From: "Mark Lunday, WD4ELG" mlunday@nc.rr.com
I notice that for the non-FM birds I am working (AO7, FO29, VO52) the Doppler is never quite exact. Or I guess I should say there seems to be a constant *offset* that needs to be included in the Doppler calc. Tonight on AO7 mode B pass, my actual downlink frequency seemed to be about 4 kHz lower than what it should be according to the Doppler. As the bird passed by, I followed the "one true rule" and only adjusted the higher freq (uplink in this case). It was consistently about 4 kHz lower.
On FO29 pass, there seemed to be about a 2 kHz offset. When the bird passed overhead, the Doppler "flipped" quickly and I had to find my signal again.
Mark,
As others have noted, the transponder "constant" is really a variable depending on lots of things, including the temperature of the satellite, the stability of the voltage regulation, the temperature in your shack and equipment, the freshness of your Keps, the accuracy of your position and system clock, etc. On AO-7, FO-29, and VO-52 I can quickly net in the uplink and downlink, and either SATPC32 or HRD will follow the One True Rule and allow you to tune around and work someone AOS to LOS almost as if it were a HF contact. That "constant" is good for an extended period of time, again depending on primarily the health of the spacecraft systems. It is just one of the many characteristics of satellite operations.
Alan WA4SCA
participants (5)
-
Alan Cresswell -
Alan P. Biddle -
Luc Leblanc -
Mark Lunday, WD4ELG -
Simon (HB9DRV)