International Space Station in cross-band repeater mode!!
mcfadyenusa at yahoo.com
Fri Jul 22 08:29:05 CDT 2005
I want to report to the list that the cross-band repeater on board the International Space Station has been re-activated, as of Wednesday afternoon. It has been a long time since this mode was active. For the majority of the last year, the equipment has been in a 'packet' mode.
The cross-band repeater uplink (from ground to ISS) is 437.800 MHz, FM.
The downlink is on 145.800 MHz. FM
No PL tones required. Instead, the one critical issue which has to be addressed is doppler.
For a typical pass, you will have to adjust your transmit frequency on the uplink, to take care of the +/- 9kHz doppler effect. Initially, as the ISS comes over the horizon into view, you must transmit around 9kHz below the nominal frequency: i.e. 437.791 MHz
As the ISS flies overhead, or as it reaches its closest point to you, your uplink frequency must be adjusted back to nominal (437.800). And as the ISS disappears over the horizon, moving away from you, you must adjust your uplink to around 437.809 MHz.
On the downlink, the doppler is less noticeable, and generally, your FM receiver will successfully receive the downlink signal for the whole pass, even if you leave your receiver tuned to 145.800 for the whole pass. But tuning up to 145.803 at "acquisition of satellite" , and slowly tuning down to 145.797, may give you a slightly better receiver signal.
A typical pass lasts around 8-9 minutes. The ISS is 250 miles above earth.
To track the ISS, you can use any of the available satellite tracking programs. I use Satscape (free, available from http://www.satscape.co.uk) , but there are plenty of others to choose from. If you do not want to download such a program, there are other sites which will give you information: www.issfanclub.com has a tracking page, (but it is offline at the moment for some reason. Go to the issfanclub.com homepage, click 'tracking', then enter your city name. I use "USA" and "McLean" (Don't put the state in the search window)).
Another site which gives good orbital pass predictions is www.heavens-above.com . You either register, or use the anonymous mode. If you drill down through the ISS pages, it will give you the NEXT pass of the ISS, or the next 'visible-to-the-naked eye' pass. Not quite so helpful. It is really aimed at visual observations of planets and spacecraft.
Today's passes for the Northern Virginia area will commence at 11:15, 12:48, 14:24, 16:01, 17:37, 19:13, and 20:48 EDT.
Regrettably, the ISS receiver squelch is set rather fierce, so a fair amount of power is needed to open the squelch. I have been unsuccessful using 5watts on UHF. But I can hear the ISS clearly on a HT with a rubber duck on every pass. So, blow the cobwebs off those 70cm amplifiers, and give it a try!
If anyone needs help setting up Satscape, just contact me and I'll help out. It really just needs you to set up your own Lat/Long and accurate time.
The rate of change of the observed signals from the initial frequency to the final frequency, is highly dependent on the elevation of the pass: If the pass goes directly overhead your location, then your uplink frequency should remain almost stable at 437.791 till almost half way through the pass, then during a period of about 1 minute, you will have to move your transmit frequency down up to 437.809, then for the remainder of the pass, your transmit frequency will remain at 437.809. If however the pass is very low on the horizon, then the rate of change of doppler remains almost constant all the way through the pass.
Iain KI4HLV McLean VA
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