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 (ISS). 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 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 1,400 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 or fifty 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.
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 1,400 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.
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 215 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.
My favorite PC software is STSPLUS. You can get the keplerian elements (fancy name for orbital parameters) from ARRL. You can also subscribe for the bulletins, which are emailed twice a week.
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. 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, North America
| Memory Channel | RX Doppler Offset (KHz) | Transmit Frequency (MHz) | Receive Frequency (MHz) | Memory Offset (MHz) | Comments |
| M1 | +5 | 144.485 | 145.805 | -1.32 | |
| M2 | 0 | 144.490 | 145.800 | -1.31 | |
| M3 | -5 | 144.495 | 145.795 | -1.30 |
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
Click here to play the video file of a QSO with Bill McArthur, KC5ACR. QSO of March 5, 2006 11:09Z.
Cross-band Repeater Mode
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. 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 audio too loud.
Copyright © 1997-2007 Ron Hashiro
Updated: April 2, 2009
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