[aprssig] weak signal ISS packet

pfbram at comcast.net pfbram at comcast.net
Mon Jan 12 12:27:20 EST 2015

I've noticed highly varying signal qualities on passes over the past couple years I've regularly digipeated the ISS (Kenwood D710, vertical Comet GP-3). To the point where I wonder if they switch radios between the Ericcson and Kenwood occasionally, reduce the power sometimes, or other variables are at play. I'm guilty of re-TX'ing because I don't see a report come through on ariss.net, though I suspect it's not that the packets failed to get through but that there aren't enough igate stations, especially to the west or north of my QTH. Whereas some days are audibly clean & remarkably crisp for TX/RX generally speaking, you can even tell by ear. 

Is the Doppler effect at play with ISS+APRS? When I worked AO-27 and SO-50 a few times (FM voice, TH-D72, Arrow II), Doppler was a big deal. 

I should start taking note of variables for packets which my rig cannot decode. A simple chart with a few columns: ascending vs. descending pass, the azimuth and elevation, etc. I'm interested in seeing if there's any pattern with weak non-decodeable packets at my QTH related to my setup or the nature of the pass. My D710's Comet is mounted to about a 2m high mast, anchored to footings on a large concrete patio slab. I know it has some nulls. 

I worked NO-44 for the first time yesterday. Early too -- a bit before 9 am local time for me. 

73, KD0KZE / Paul 

----- Original Message -----

From: "Bill" <wa7nwp at gmail.com> 
To: "TAPR APRS Mailing List" <aprssig at tapr.org> 
Sent: Monday, January 12, 2015 9:33:42 AM 
Subject: Re: [aprssig] weak signal ISS packet 

I bet the folks on the ISS could make an audio recording of the packets they hear on a busy pass across the continental US. That would make for some interesting analysis. 


> On Jan 9, 2015, at 8:06 AM, Robert Bruninga <bruninga at usna.edu> wrote: 
> Yes, again, "best packet" all depends on the settings of the transmitting 
> station (and matching it on receive). 
> When the ISS used a kenwood, then another kenwood would always work the 
> best. Because both are built at the factory for optimum performance (to each 
> other) and they use direct modulation and demodulation to the same levels. 
> My kenwoods can decode another kenwood (direct) with only 3 bars. BUT 
> similarly, I can see many signals that appear to have full scale signals and 
> then barely deocde because so many signals on the air were never adjusted 
> properly. AND there are two standards. 
> Do the sending and receiving radios use pre- and de-emphasis (speaker and 
> Mic connections)? Or do they operate flat with direct modulation and 
> demodulation. Is the receiver an FM demodulator or Phase demodulator? 
> BUT on ISS, I believe they are currently using another radio, not a kenwood. 
> SO now you need a receiving system best matched to this radios transmitted 
> waveform. 
> Maybe someone on the AMSAT-BB has captured the existing ISS waveform and we 
> can see what kind of receiver is best. But then again, the captured 
> waveform MUST indicate exactly what kind of receiver (with or without 
> de-emphasis) was used in capturing the waveform! 
> Two manufactured APRS radios work BEST between them because they were 
> factory adjusted for BALANCED tone levels and NO preemphasis or de-emphasis. 
> But other stations may just use the Mic connections and speaker connections 
> and get over 6 dB of performance loss if they are not accounted for. 
> The use of speaker/mic connections(with pre- and de-emphasis) and direct 
> modulation and discriminator demodulation can each have optimum performance. 
> But there are always problems when crossing between the two. BUT, it IS 
> possible to adjust on the transmit side for a compromise between the two and 
> then receivers of each type can decode pretty good. 
> Though maybe 1% of hams go to this trouble. Packet radio is NOT 
> Plug-N-play. The transmitter must be adjusted for optimum waveform! 
> I should be able to tell everyone what the RIGHT way to do it is, but I 
> never trust my memory anymore, and don’t have time today to go do all the 
> research again... But I'm going to recall from memory, that I think the 
> WORST situation of most TNC's is if the LOW tone is higher than the HIGH 
> tone, that decoding always suffers. And suffers FAR WORSE than the other 
> way around. So that is why the compromise is to make the high tone about 3 
> dB higher (30% on an OO scope) so that it doesn’t get completely lost when 
> it goes through a speaker connected receiver and gets de-emphasized. 
> OR, If I remember wrong and it is the other way around! 
> Good luck. 
> Bob 
> -----Original Message----- 
> I was hoping for some ideas to better my station and am very pleased you 
> have responded. I want to better utilize my station and increase my 
> throughput with the ISS in particular. I have available 10 element crossed 
> Yagi antennas and Yaesu az el rotor and several radios with TNC's built in 
> including Yaesu FTM-350 and FTM-400 and Kenwood 
> TM-D700 and TS-2000. MFJ and Kantronics TNC's as well as products from 
> Byonics and Argent Data are found on my bench. I am currently running a 
> Kenwood TM-D700 to a M2 Eggbeater as a 24/7 ISS station with UISS software 
> and modules to upload to the internet. 
> I want to increase my capabilities with respect to the ISS and 
> APRS. I have found that the Kenwood TM-D700 requires that you get about 
> almost a half scale reading to effect good packet reception. I don't want to 
> use the fully automated station (FT-847) that I have when I'm not home 
> because in the past a failure caused my rotor to jam and I had to lower my 
> tower to remove and repair the az-el rotor. I suspect that the best way 
> might be to use a sound card software and computer to give the best 
> decoding. Thanks for the response Steve KB1CHU 
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