Getting Started Contacting the ISS
If you have a two meter walkie or mobile radio, it's easy to make contact with the International Space Station. Take a look at the notes below.
If you're in Hawaii, you're especially well positioned to work the ISS. It's one of the most isolated locations in the world, such that only Hawaii amateurs can work the ISS when it orbits over Hawaii. (There is an exception when the ISS passes over Hawaii and heads towards Seattle, where there is about a 30 second overlap with San Francisco and California.)
So, if you've made contacts via a two meter repeater, you have quite a bit of experience under your belt to make contacts with the International Space Station (ISS). Just a few adaptations in how you use your equipment, and you're set.
Enjoy, and feel free to drop me an e-mail if you have any questions.
Basic radio setup
Contacting the International Space Station (ISS) resembles making an FM repeater contact over greater distances than you've experienced on earth. You can think of it as making a long, long range two meter FM simplex contact to a station that's moving real fast.
Distances from the ground to the ISS can vary from about 215 miles when the ISS is overhead, to about 1,350 miles when it approaches the horizon. Ten watts of radio output fed into a unity gain vertical antenna such as a ground plane is adequate to about 1,000 miles, provided you're the only station operating within sight of the ISS. Five watts of radio output fed into a unity gain vertical antenna is adequate to 900 miles with some noise as heard by the astronauts.
You can see it is possible to talk with the ISS using a 5 watt walkie when it is overhead, and the crew is awake and at the ARISS radio station. When the ISS is that close, you can make contact using a rubber duck antenna. As the ISS moves overhead, do not forget to rotate your handheld so that the antenna is oriented more horizontal, to send out a better signal in the vertical direction.
As the angle lowers towards the horizon, a handheld beam antenna is useful for increasing the antenna gain when transmitting and receiving.
A better setup is to use a mobile or base VHF transceiver radio. One that puts out ten or more watts is ideal. Most modern VHF radios put out a mix of low, medium and high power typically covering five, fifteen and fifty watts (as an example).
As an in-between compromise, you may use an external power amplifier hooked up to a two meter VHF walkie. It is not as flexible as the VHF mobile transceiver but it will get the job done.
Basic antenna setup
For VHF voice communications, the astronauts are usually transmitting ten watts into a "vertical" antenna. Because the ISS can be oriented in a number of directions at a given moment, the "vertical" orientation may or may not match a vertical orientation on earth.
The simplest antenna to use to contact the space station is a quarter-wave vertical antenna. A 1/4 wave magnetic mount antenna mounted in the middle of the roof of a car is very adequate for most situations. For orbital passes that are sixty degrees and higher, it will be adequate to drive the antenna with a five watt radio. As the ISS gets closer to the horizon, its distance increases and so the power output should be increased. Adequate communication can be made with ten watts, but twenty five, fifty or more watts is recommended for passes lower than fifteen degrees in elevation.
If the pass is lower than twenty degrees, a vertical antenna with additional gain (3 dB or more) may be advantageous. But, if the pass is higher than about forty five degrees, do not use such a gain vertical antenna. Otherwise, as the ISS passes overhead, the signal to and from the ISS will actual drop rather than increase in strength because the gain is focused towards the horizon rather than overhead.
My favorite handheld beam antenna is the Arrow Antenna Model 146/437-10. I find the roll up bag very valuable for keeping all the parts together and readily accessible for use. I also use the antenna for radio direction finding (RDF) activities.
It is possible to hand-hold the antenna and point it as the ISS moves across the sky. I find it easier to place the antenna on a good photo or video tripod and use the handle on the tripod head to point the antenna. That allows me to concentrate on tracking the ISS, or work the laptop to make contact using the computer to work the APRS or the packet radio mailbox modes. The Arrow is ideal in that sense, as it has a pre-drilled and pre-threaded hole in the base that easily screws onto a photo tripod. I use a Manfrotto Bogen 3221 tripod which I use at other times for photos and videos. I recommend using a good, solid tripod so it doesn't blow down at the most inopportune time.
About the ISS's orbit
Understanding some of the fine points of the orbit of the ISS will give you the edge in forecasting and making contacts with the ISS.
First off, assuming you are using a vertical antenna, you'll need a location or situation that you can transmit unobstructed from the horizon, up through to the maximum elevation, and back down to the horizon "behind you", for at least as much of the pass as possible. The exact line in the sky that the ISS traces varies based on these parameters.
- If it's a south-to-north pass (ascending), or north-to-south (descending)
- Whether the pass is towards the east or west. It's rarely directly overhead, and we tend to get two consecutive passes (one to the east, one to the west) timed about 90 minutes apart.
- The maximum elevation of the pass, or maximum height above the horizon the ISS appears
Elevation and Range
The first item of interest is the elevation, or the angle above the horizon of a particular pass. As the elevation increases, the distance between you and the ISS decreases. When the ISS is at the horizon, the ISS is typically about 1,200 miles away. At an elevation of 20 degress, the distance (or "range") of the ISS is about 510 miles away. When it is about 45 degrees above the horizon, the distance drops to a little less than 300 miles away. When it is overhead, it is something like 215 miles overhead. So, you would find high elevation passes to be of interest...especially if it is near twilight and the ISS is visible!
The elevation also influences the amount of time that the ISS is visible above the horizon. That amount of time is called the "pass time". A one degree pass might have a total "visible" pass time of say three minutes. A fifty five degree pass has more than nine minutes of pass time.
The orbital altitude of the ISS varies from day-to-day due to the effects of atmospheric friction on the ISS, which causes the orbit to decay. About once a month, ISS fires a rocket booster that lifts the ISS into a higher orbit by about 20 miles. For typical estimation, one could use a figure of 220 miles altitude as a starting point.
While you may be able to work the ISS with five watts while it's overhead, at distances approaching 275 miles or so, you'll need more transmit power, a better receive antenna and a good receiver.
Generally speaking, for a given elevation above the horizon, the distance to the ISS is about the same and doesn't change with the maximum elevation for the pass. We can use this information to accumulate knowledge about our station's performance in terms of what elevation, and therefore, what distance or range we typically can expect to hear or contact the ISS. We typically would expect to hear the ISS at a given elevation, and expect to lose contact at about the same elevation. The elevation/range combination may change for different modes. You can use this PDF file as a rough guide for correlating the elevation and range to the ISS. Notice that above 30 degrees, the horizontal distance to the ISS quickly becomes less of a factor and the range approaches and approximates the altitude of the ISS.
Visualizing the pattern of orbits in your neighborhood
Since the orbit of the ISS is inclined 51 degrees relative to the equator, on a directly overhead 90 degree pass, for an ascending pass it'll trace a line from a bearing of 231 degrees, to directly overhead, to 51 degrees on the other side. Anything to the East or West will appear to be coming off the side of that line, going up to a maximum elevation, and going down -- again, off to the side of the line just painted above by the 90 degree pass.
Similarly, for a descending pass, the trace in the sky is 309 degrees down to 129 degrees. And, being east or west means being off to one side or another of this line.
If you want to see the effect by getting a list of azimuth (compass bearing) and elevation for a given pass, use the pass predictor on the website magnetic declination, if you're using a compass instead of a map. You can use this magnetic declination calculator.
For example, I know for the location I'm using that looking out square into the street in front of me is bearing 211 degrees. So, 90 degrees left and right are 121 and 301 degrees (which is the bearing of the street in front of me), and behind me is 31 degrees. With that basic orientation noted, I estimate where 45 degree increments (1/8 of a circle), 22.5 degree increments (1/16), and 10 and 20 degrees variances of these points are. Even with an Arrow beam antenna, if you're within 10 degrees, it's good enough. The satellite is moving anyways, so you'll need to periodically move the beam antenna.
Similarly, I know how high 90 degrees is, and half of that is 45 degrees, and half of that is 22.5 degrees. It gives you an idea where to look.
If you look at various series of azimuths and elevations for a given ascending or descending pass, you'll begin to recognize where these points are in the sky above you. You will see that these points and paths remain pretty consistent for a given type of path (in other words, 45 degree ascending passes to the East tend to trace pretty much the same line in the sky).
If you're using a vertical antenna, of course, you can't steer it (unless you tilt it). If it's a low pass, you want to use a gain antenna to get as much signal out towards the horizon. The ISS is about 1,200 miles away when it appears over the horizon, and about 220 miles when directly overhead.
If it's a high pass and you're not going to steer it by changing the orientation during the pass, use a unity gain (quarter wave) vertical antenna -- because it sends and receives a lot of it's signal going upwards as well as towards the horizon. You'll need as much signal going upwards as you can for a high pass.
If it's a high pass and you use a Diamond NR73 which is a vertical gain antenna, you can hand tilt the antenna so that the maximum radiation which comes off perpendicular to the antenna -- but be careful you don't transmit more than 7 watts, or you'll get an RF safety exposure problem. If you attempt this, remember to hold the magnetic mount base, and not the loading coil of the antenna. Not only will you cut off the transmit and receive signal, you'll expose your hand to excessive RF. It would be better if you put it on a surface and tilt the surface -- or even temporarily tape the antenna to non-conductive pole and mount the pole in such a way as to make it tilt.
Tilt the antenna, so that the side of the antenna is pointing to where the maximum elevation (and corresponding azimuth) would occur in the sky. (The direction of the tip of the antenna should make a right angle to that location in the sky.) In other words, if the maximum elevation is 30 degrees, and the azimuth is 125 degrees (which is the southwest direction), tilt the antenna 30 degrees (the tip should be pointing at a 60 degree angle) and point it in the northeast direction. It won't exactly match the path traced in the sky for a 30 degree path, but it'll be good enough. BTW, I saw Honolulu Electronics had these in stock for about $70 (mag mount extra for about another $40).
Orbital Period - The Time it takes for the ISS to orbit the Earth
The ISS orbits the earth once about every 90 minutes. The ISS orbits about 220 miles above the earth. The actual altitude varies as time goes on, and the onboard rocket engines are started occasionally to boost the altitude.
The location of the orbit of the ISS over the Earth
As the ISS makes an orbit around the earth, the earth moves underneath it. So, when the ISS returns to the original spot about 90 minutes later, the earth has rotated approximately 22.5 degrees in longitude. This regular, successive movement with each orbit is known as "procession". Due to the procession, the ISS makes about 16 orbits during each 24 hour day.
As a result, a given location can see the ISS perhaps once, twice, or even three orbits in a row as the ISS processes through each orbit. After approximately ten hours or so, the earth rotates enough that you come in contact with the "other side" of the ISS's orbit. Another couple of orbits, and about another ten hours later, and you're back near the original starting point.
So, in a given day, you have two windows of opportunity to make contact with the ISS, spaced about twelve hours apart and each window may be one, two or three consecutive orbits.
There are patterns to the series of elevations and successive orbits. This gives you a feel that there are patterns to the successive orbits, just as there are patterns with the tides of the ocean and the phase of the moon.
Generally, the first orbit passes off to the east of the Hawaiian Islands, and the second orbit is to the west of the islands. Often, the elevation (the angle above the horizon that the ISS seems to fly over) of the first pass is in the range of say five to 25 degrees as it passes off to the east. The second pass will have a similar elevation to the west.
On occasion, the elevation on a particular orbit increases to something above 45 degrees. When this happens, the other passes will be quite low to the horizon, say less than five degrees. When the elevation is high enough, you might get only one pass.
For example, you might see a 35 degree pass to the east, followed on the next orbit by a 7 degree pass to the west. Continuing with the above example, the next pass about eight hours later is a single orbital pass of 61 degrees to the east. About nine hours later, you see a 7 degree pass to the east followed by a 32 degree pass to the west. So you see that high and low elevation passes tend to alternate somewhat. The number of passes, the elevation of the passes, and the relative location to the east or west gives you a feel on how to approach setting up for a contact.
Inclination and Ascending/Descending Passes
The orbit of the ISS is angled about 51 degrees relative to the equator. This angle is called inclination. That means that the ISS moves in two general directions: (a) either from the Southwest to the Northeast (called an ascending pass), or (b) from the Northwest to the Southeast (called a descending pass).
The term ascending or descending refers to the satellite's movement relative to the equator. Ascending means that the satellite is heading north relative to the equator. Descending means that the satellite is heading south relative to the equator.
So, if it's a low elevation pass, in a descending direction, East of you, you should generally be prepared to point your antenna to the North and East.
If it's a low elevation pass, in a descending direction, West of you, you should generally be prepared to point your antenna to the West and South.
If it's a low elevation pass, in a ascending direction, East of you, you should generally be prepared to point your antenna to the South and East.
If it's a low elevation pass, in a ascending direction, West of you, you should generally be prepared to point your antenna to the West and North.
After a while, you'll learn to recognize whether a pass is ascending or descending, to the East or West of you, and the elevation. Passes of the ISS that have these three parameters match will tend to trace the same path in the sky. You can use this knowledge to work around obstacles such as mountains and tall buildings and find suitable operating locations. If you can note how well you did at a given location with a given orbital pass described by these three parameters, you can pretty much anticipate and forecast how well you'll do with a pass next time it has similar matching parameters.
Ascending Passes Over Hawaii
Ascending passes in Hawaii are valuable, because if the astronauts are talking on voice mode when the ISS passes over Hawaii, they'll likely keep talking as the ISS flies overhead towards the continental US. So, if the astronaut is as enthusiastic about ham radio and space communications as Bill McArthur, KC5ACR, it just might pay to be on the radio and ready during ascending passes.
If you live in metro Honolulu, knowing the ascending or descending passes is useful information. The compass bearing of Nuuanu, Manoa and Palolo Valleys is approximately 45 degrees, which is similar to the inclination of the ISS's orbit of 51 degrees. That means if you have an ascending orbital pass (southwest to northeast), of sufficient elevation (say above 20 or 25 degrees), you can operate the ISS from within the valley for a good amount of the pass. You would have a much easier time compared to a similar descending pass, because the ISS would be visible and accessible for a much longer period.
Special Applications for the ISS Over Hawaii
One of the special applications of the ISS is the ability for the ISS to monitor Hawaii during the peak of a hurricane. Depending on the timing of the orbits, the ISS may be in an advantageous position to make contacts with those taking shelter and able to operate two meters while portable, pointing the antenna out of the side of the shelter not exposed to the high winds and rains and passing status information to the crew aboard the ISS.
Tools for Forecasting the Orbit of the ISS
There is a rule in ham radio: "You can't hear 'em, you can't work 'em." Similarly, you can't work (or contact) the ISS if it's not "visible" above the horizon. So, you need to forecast when the ISS will be visible, and at which point in the sky it will be present. My favorite site for getting current news and a rough forecast of the ISS location is www.issfanclub.com. Not only does it calculate usable ISS passes for the next 24 hours, it allows amateurs around the world to file activity reports for the four common modes: packet radio, voice contacts, the cross-band repeater and slow-scan TV transmissions.
I also use the website heavens-above.com to get an indication of all the interesting space activity overhead.
One thing about space communications and the ISS. Orbital dynamics is a very mathematical and precise phenomenon. There's hardly anything that will vary and affect the orbit of the ISS in an unscheduled fashion. For that reason, with good keplerian elements and a good computer program (or web page), you can be assured that the ISS will be there, rain or shine, sunspots or no sunspots.
ISS Crew Hours
If you intend to contact the ISS astronauts, you need to be aware of their hours of operations. Generally, they are awake and about from about 6:00Z to 22:00Z UTC time. That translates to about 8:00 pm to 12 noon Hawaii Standard Time. Their start and end times for each day may change, especially when the arrival of the Space Shuttle is expected. The astronauts have a regimen of daily exercise, eating and other routines, so the opportunity to work the ARISS amateur radio space station is less than the full work day. As you can see, the possibility of working the astronauts from 12 noon to about 8:00 pm daily Hawaii Standard Time is just about nil. So, if you want to talk with the astronauts, you need to be flexible and adjust your waking hours.
The crew may have some free time on Saturday (Friday evening through Saturday noon Hawaii time) and on Sunday (Saturday evening through Sunday noon Hawaii time). These are the best times to listen and be ready to talk with the astronauts.
Additionally, the crew will turn off the ARISS radios for dockings and undockings, EVAs (space walks) and other special events. As a result, you may not hear or contact the automated packet radio station during these times.
Overview of Operating Modes of the ISS
Packet Radio modes
Coming soon. In the meantime, check out this site for information on APRS.
The equipment onboard the ISS supports two modes of automated operation using packet radio. The first is enables the ISS to be an APRS (Automated Position Reporting System) digipeater located over 200 miles above the Earth. It can repeat packets of data transmissions formatted into APRS packets detected within a diameter of more than 1,000 miles. The second mode is as an automated packet bulletin board system, enabling short email messages to be uploaded and viewed by other stations.
- Uplink and downlink: 145.825 Mhz
Two Meter VHF Voice mode
A favorite mode is direct conversation with the astronauts onboard the ISS. Ocassionally during their free time, they will appear on the VHF radio. While over Hawaii, they receive on 144.490 Mhz (the uplink frequency) and transmit on 145.800 Mhz (the downlink frequency). You will need to adjust your radio to transmit with a minus 1.31 Mhz offset. (See the manual for your radio for details.) For future use, its a good idea to store the transmit/receive pair in a memory channel in your radio.
- Memory 1, Freq=145.80 Mhz, Offset =-1.31 Mhz.
In order to gain maximum advantage in the contact, it is advantageous to compensate for the effects of doppler shifting in the transmit and received frequencies. As the space station approaches you, the frequencies will appear to increase due to the oncoming motion. It is like the increase in pitch of a car horn as it approach you. Similarly, the frequency appears to drop after the space station passes overhead and heads to the horizon. In Hawaii however, due to the relatively limited number of radio stations and operators present (compared to North America or Europe), programming additional memory with the different frequencies for doppler compensation is not necessary. But, if you want to include it, here are the memory channels.
Programming Memories for 5 Khz Tuning increments, VHF, North America
|RX Doppler Offset (KHz)
|Transmit Frequency (MHz)
|Receive Frequency (MHz)
|Memory Offset (MHz)
As the ISS approaches, use Memory 1. As it goes higher than forty five degrees high, use Memory 2 until it goes overhead and below forty five degrees. Then, switch to Memory 3.
For a more indepth discussion on doppler compensation and ISS operations, see this site
Cross-band Repeater Mode (Normal)
The PCSAT2 station and Material Experiment Package was removed from the exterior of the ISS on September 15, 2006. We may get more opportunities to use the cross-band repeater in the near future. Keep your fingers crossed.
- Uplink: 437.800 Mhz
- Downlink: 145.800 Mhz
In Hawaii, you're more likely to work another station from Hawaii. But it's still a thrill. You should use a VHF/UHF dual-band mobile transceiver with two separate receivers. For the UHF side, set your tuning steps to 5 kHz tuning, instead of the default 25 kHz tuning.
Cross-band Repeater Mode (Reverse)
Starting on Saturday, January 3, 2009, a new mode was introduced to the ISS. The mode was verified first over Hawaii at 12:15 pm Hawaii time. It features the use of VHF for the uplink, and UHF for the downlink. The downlink is 5 watts. The advantages are:
- You do not have to tune as much for doppler on the uplink, since doppler for VHF is less than UHF
- But, you do have to tune for doppler on the downlink if you want to hear more than 30 seconds of contact
- On the other hand, YOU are in control of the downlink doppler, and you can tune for best reception
- It is real easy for the astronaut to pick up the microphone and talk with you, if he's nearby. No further changes on his part is needed.
- Uplink: 145.990 Mhz, PL 67.0. You must set the PL subaudible tone for transmit, or the ISS won't hear your signal.
- Downlink: 437.800 Mhz
Next, you should, if possible compensate for doppler on both the transmit and receive. If you don't adjust for doppler on the VHF uplink, it isn't so bad as the maximum doppler shift is 3.3 kHz and you won't lose out on the middle half of the pass. But, if you didn't compensate for doppler shift on the UHF downlink, your usable time in the middle is only 30 seconds. You wouldn't hear anything before, or afterwards, as your receiver would probably pick up a 5 watt UHF signal, 280 miles away that's off frequency by about 3 kHz or less. Anything more off frequency, and you won't hear it. If you compensate for doppler on the downlink to keep it where your receiver tuned can hear it, you would add another 90 seconds. So, this is not an item to ignore.
On receive, set your UHF radio to 5 kHz tuning increments. It's best to leave it in VFO mode, if you can. You start with 437.810, then change to 437.805. In the middle of the pass, change to 437.800, then 437.795, 437.790. It moves quite quickly in the middle. If you choose to just leave the 437.800 frequency for the middle of the pass, you'll hear only the middle 30 seconds.
On transmit, you start low at 145.987, 145.988, 145.989, 145.990 -- and proceed to 145.991, 145.992, 145.993. If you only have 5 kHz tuning steps, then you transmit on 145.985, 145.990, 145,995 respectively.
|RX Doppler Offset (kHz)
|Transmit Frequency (MHz)
|Receive Frequency (MHz)
|145.985, PL 67.0
|145.990, PL 67.0
|145.990, PL 67.0
|145.990, PL 67.0
|145.995, PL 67.0
In terms of when to switch the receive channels for the doppler effect, split the difference between each of the memory channels (ie, 2.5 kHz between each pair of frequencies), and change memory channels when the ISS doppler approaches that point.
Once again, the program STSPLUS is a very valuable PC program for calculating ahead of time the times, azimuth, elevation and doppler shifts to prepare for the pass. From the main menu, press F3, F8, n, F, 1, 8. STSPLUS then displays a list of upcoming passes, in UTC time. Type the one or two digit number representing which row in the list you're interested in. Select which frequency pair to use for the pass, let it sit for ten minutes. It will display the upcoming pass second-by-second, calculate the azimuth, elevation and doppler, and log it into the file STSPLUS.LOG.
|Look for (MHz)
|Switch to (MHz)
|Start of the pass
Lastly, you should be using as much power as you have to make the contact. You COULD do it with 5 watts, but 50 watts (for example) would overcome a lot of misgivings of not doing doppler compensation on the VHF uplink transmit. Remember, the ISS repeater needs to hear your signal well and clearly if it's going to repeat it. If it hears mush, it will repeat mush -- which we don't want. If you use a beam antenna such as the Arrow crossband satellite antenna, you'll get a double advantage of improving your transmit and receive.
It would be best if you could do 1 kHz increments on the VHF uplink for doppler compensation, with good transmit power and an antenna, as then you'll get the cleanest signal into the ISS crossband repeat, and you'll get clean audio coming down on the downlink. If you don't get into the ISS repeater very well (not enough signal strength, poor audio from being off frequency), then you'll get a poor audio signal coming down.
And, if you don't tune for doppler, you could possibly be heard by others, but you won't hear yourself or others calling you except for the middle 30 seconds.
Then, the golden rule of amateur radio then takes place. If you can't hear 'em, you can't work 'em.
Keep in mind that if you leave the volume on the receiver too loud, the audio from the downlink frequency will get into your microphone and go back on the uplink -- causing audio feedback and squeals. So, don't turn up your receive audio too loud.
There is a chance that the astronauts will come on during on Saturday and Sunday during their waking hours, as they tend to come on during the weekends and during their lunch time which is between 3 and 4 in the morning our time.
Even is you have the most basic of equipment, it's worth the try. Eric, NH6TY and Rick, KH6OM have mastered these techniques using vertical antennas -- I believe the antennas are outdoor. I believe they're using 5 kHz doppler correction on both the VHF uplink and UHF downlink. They're consistently on earlier and later in the pass. I believe they're running 50 watts.
Chuck, N6NCT, achieved spectacular performance on his first try by following these instructions carefully. He was running a Kenwood TM-D700 indoors, 50 watts, and a single vertical antenna that was not tilted.
For the high-end station, I'm running an Icom IC-910H, 100 watts, into an Arrow Satellite Crossband antenna mounted on a video tripod indoors. The radio tuning is computer controlled by a Macbook and MacDoppler. I am transmitting through a wooden roof and walls to make the contact, and manually moving the beam about twice a minute. I can easily work the ISS down to about ten degree elevation -- the exact time of acquisition of the ISS varies due to the orientation of the ISS.
Although this may seem like a lot of material to absorb and operate, making a contact through the reverse crossband mode of the ISS is the most thrilling means of making a space contact to happen in several years. Think of it as the amateur radio version of making a perfect golf swing. You work on understanding each component separately, then bring it all together to make that perfect swing and score the hole-in-one!
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Updated: January 12, 2009
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