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Compensating for Doppler Shift When Contacting the ISS

One of the questions from newcomers is "Do I have to compensate for the doppler shift when I am making a radio contact with the ISS?". The answer is "It depends.....". Let's take a look.

Enjoy, and feel free to drop me an e-mail if you have any questions.

About the ISS's orbit

Understanding some of the fine points of the orbit of the ISS will help you understand about the inter-relationship between elevation of a pass, the apparent shift in frequency due to doppler effect, the range or distance to the ISS, and the merit of compensating for the doppler shift.

I'll start out the discussion assuming that you're using two meters (144.49 Mhz) for the uplink frequency, and (145.80 Mhz) for the downlink frequency. Using the crossband mode and the UHF uplink frequencies modifies the generalizations below and that will be addressed later on this page.

Once you begin to see the relationships between elevation, doppler shift, the range to the ISS, and the advantage to be gained compensating for doppler, you'll be able to increase your chances of a successful contact with the ISS.

If you're unfamiliar with the relationships, you can look over this set of data from a telebridge conference between the ISS and the WH6PN ground station get an idea of the times of pass time, elevation. azimuth, doppler and range for a 57 degree pass.

Tuning Increment For Your Radio

To compensate for doppler, you'll need to know when the doppler shift reaches the point that is equal to an amount of one-half of the tuning increment. For example, for a radio that can tune in increments of 5 Khz, the point of interest is 2.5 Khz. While the doppler shift is less than 2.5 Khz, there is no value in changing frequencies. When the doppler shift exceeds 2.5 Khz, you change the frequency of your radio to the next frequency above or below the one that you're using.

A Word About the Elevation of the ISS Orbital Passes and Doppler Shift

I find most of the passes of the ISS are below 25 degrees in elevation above the horizon. When the pass is just skimming above the horizon at .33 degrees for a total pass time of about two minutes, the doppler shift at the beginning and end of the pass is only about 500 Hz. For a 1.5 degree pass, the maximum doppler rises to 1,300 Hz.

Here's a chart to use as a point of reference for the discussion below. It looks busy, but as I step through the curves and lines, you'll begin to better understand the relationship of the elevation of an orbit, the effect on doppler, and when you should switch through your various pre-programmed memory channels to compensate for the effects of doppler.

The chart shows the effects of doppler shift during a given pass of the ISS. Along the horizontal axis is the duration of the pass, measured in seconds. The center of the pass is positioned to the center of the horizontal axis. The vertical axis is the amount of doppler shift for a 145.800 MHz downlink, measured in Hertz.

The effects of five orbital passes are plotted as a series of S-shaped curves. The lowest angle pass, a one degree pass, is shown as red, and is practically a straight line. Two other passes, a 10 degree and a 26 degree pass are show in purple and green, respectively. The last two passes, a 63 and an 83 degree pass are shown in aqua and blue, respectively.

Pausing to study the curves, you can see that the 63 and 83 degree pass displays practically identical effects regarding the doppler shift. You can see the relationship that as the elevation of the orbital pass becomes less, the pass times become less, and the effects of doppler becomes less.

We will return to discuss the horizontal red line, and the vertical purple and blue lines. Let us examine the effects of elevation on the doppler shift.

Doppler Summary

Max Elevation (degrees) Max Passtime (mm:ss) Distance at AOS (km) Max Doppler (Hz) Max Advantage (Hz) 2.5 KHz Transition Time (mm:ss) Vertical line Distance at 2.5 KHz Transition Time (km) Usable Time w/o compensation (mm:ss) Comments
1 2:44 2,087 1,302 N/A N/A N/A 2:44 ISS remains over 1,990 km/ 1,236 miles away
10 7:48 2,081 2,779 558 3:02 1,798 6:00
26 9:10 2,077 3,265 1,530 1:40 996 3:18
63 9:26 2,082 3,421 1,579 0:52 527 1:44 Minimum distance is 366 km. Contact time is short without doppler compensation
83 9:26 2,089 3,432 1,568 0:46 471 1:32 Minimum distance is 331 km. Contact time is short without doppler compensation

Doppler on a 1.5 Degree Pass

In this example, for a 1.5 degree pass, the maximum doppler is about 1,300 Hz, and it has a relatively short passtime.

Doppler on a 10 Degree Pass

When the passes are ten degrees or less, the maximum doppler shift at the ends of the pass is about +/- 2.8 kHz. Since most modern two meter radios can tune in 5 Khz increments, its almost not worth the bother of dealing with the doppler.

A ten degree pass lasts about 7 minutes, 48 seconds. The middle of the pass which is within 2.5 kHz is 6 minutes wide. If you use 5 kHz tuning increments, you will need to retune the radio for the first and last 50 seconds of the pass, and the elevation would be three degress or less above the horizon. That's a lot of work and distraction, with not much improvement.

For example, if you shift the frequency of the radio by 5 Khz, and the doppler at the moment is 2.7 kHz, the difference will now be 2.3 kHz and the net improvement overall is the difference in frequency between 2.7 and 2.3 kHz, which is .4 kHz.

As the elevation rises higher than about ten degrees, the shift due to doppler increases. As you'll see below, as the elevation rises, the amount of time that the doppler shift remains within 2.5 kHz steadily decreases. Then it's worth CONSIDERING doppler compensation.

Doppler on a 26 Degree Pass

About a fourth of the passes, the ISS climbs higher than 25 degrees. On a 26 degree pass, the doppler shift gets more pronounced to +/- 3.3 kHz and the pass time increases to 9 minutes 10 seconds. You would improve your situation from being 3.3 kHz off frequency to (5.0 - 3.3) 1.8 Khz off frequency if you shift your frequency 5 kHz up or down to the next channel. The duration of the pass when the doppler shift is less than 2.5 kHz is 3 minutes, 20 seconds. When the doppler is less than 2.5 kHz, the range of the ISS is within 1,020 km.

Doppler on a 63 Degree Pass

When it's above 60 degress, the doppler shift is slightly more than +/- 3.4 kHz. For a 63 degree pass, the passtime is 9 minutes, 26 seconds. To compensate for doppler, about the first "third" of the pass (about 3 minutes, 40 seconds), you transmit 5 kHz low, and receive 5 kHz high. You would improve your situation from being 3.4 kHz off frequency to (5.0 - 3.4) 1.6 kHz off frequency. During the middle "third" of the pass (next 2 minutes), you disregard doppler shift and transmit and receive on frequency. During the last "third" (last three minutes, 40 seconds), one uses doppler compensation again and transmits 5 kHz high, and receives 5 kHz low.

Doppler on an 83 Degree Pass

At an 83 degree pass, the doppler shift is +/- 3.425 kHz and the pass lasts 9 minutes 26 seconds. An 83 degree pass will rise from the horizon, pass practically overhead, then continue in almost a straight line towards towards the opposite horizon. The following information gives you an approximation of how to time the elevation and doppler shift during the pass and is derived from a computer program calculating in two second increments.

The first "third" of the pass lasts about 4 minutes, and the range decreases from 2,089 km to 471 km and the doppler is greater than 2.5 kHz. The ISS has risen from the horizon and is now almost 43 degrees elevation. The middle "third" of the pass when the doppler is less than 2.5 kHz lasts about 1 minute 32 seconds and the range changes from 471 Km down to 331 Km then increases to 479 km. The ISS moves from 43 degree elevation, goes nearly overhead, then drops to about 42 degrees. During this middle "third", the ISS is within a radius of about 342 km from you. The last "third" of the pass lasts about 3 minutes 50 seconds and the range goes from 471 km to 2,057 km and again the doppler is greater than 2.5 kHz.

You can see the effects of doppler on the 63 and 83 degree passes are almost identical.

The Red Horizontal Line

If you use 5 kHz increments for your radio, the red horizontal line is the threshold where you should retune your radio from being 5 kHz off frequency (145.805, or 145.795) to compensate for doppler, to the uncorrected downlink frequency of 145.800. Examine the locations where the red line intersects the doppler curve for the respective orbital passes.

The Purple and Blue Vertical Lines

The vertical lines represent the intersection where the red horizontal lines intersect with the S-curve for the respecive orbits of 10 degrees (purple) and 83 degrees (blue). You can now see the effect that a higher elevation orbital pass has on doppler, and when to retune.

The portion between the vertical lines represents the effective time you have with the ISS if you don't compensate for doppler. Chances are, even if there are no other limitations or obstacles to making a contact, the ISS won't be able to hear your signal because your signal will be shifted by doppler to the edge of the usable bandwidth of the radio onboard the ISS, and the FM receiver onboard the ISS will capture a station that is closer to its receive frequency. That is more likely to be another ground station that is closer to the ISS and has a smaller doppler effect.

While a higher pass means that the ISS is passing closer to you, the effective time that you have to communicate with the ISS, assuming you don't compensate for doppler, drops significantly. The time between the two blue lines is much less than the time available between the two purple lines. That means, on a higher orbital pass, you'll have a much better chance of making a contact due to the closer distance, but the amount of time you have for a usable QSO is quite short. If you can use a handheld beam or other gain antenna that can be pointed towards the ISS, you further increase your chances by increasing your overall radiated power.

The purple and blue lines work with the assumption that the usable bandpass of your radio and the radios onboard the ISS is +/-2.5 kHz wide. In practice, the usable bandwidth may be slightly more, in which case, there would be a few more seconds available than indicated by the lines. For the sake of discussion, +/-2.5 kHz for the usable bandwidth is assumed.

Ground-to-ISS Space Communications, Doppler and RF Power

I find that during most passes when no one else is active, the audio quality is such that one doesn't need doppler compensation. The FM capture by the ISS is pretty much good down to the horizon even if your signal is off freq by more than 3 kHz. I find it more than adequate when working APRS. Distances overhead could be around 270 km when directly overhead and increases to about 2,000 km as the ISS approaches the horizon. Five watts into a vertical antenna is adequate when the ISS is overhead, but one needs at least 10 watts as it approaches the horizon. Bill McArthur, KC5ACR, reported that a 5 watt signal sent to the ISS when it's within six degrees of the horizon was audible, but the voice quality suffered to the point where the person could not be distinguished and identified by the sound of the voice.

The Effects of Neighboring Stations

But, if you expect:

  • others to be operating near the perimeter of the footprint of the ISS -- those stations who would be closer to the ISS and also closer on frequency
  • or if you expect to operate on voice
  • or contact the packet mailbox onboard the ISS
you would gain back some of the disadvantage if you used compensation for doppler. Otherwise, your signal would appear to the ISS to be off frequency by 3.3 kHz and the stations directly below the ISS would be favored and captured by the FM radio onboard the ISS because their signals appear to be on frequency.

Furthermore, your distance could be something like 2,000 km to the ISS, and the other person might be 400 km away from the ISS. That's a five-times ratio in distance, and you'll need to transmit with power equal to the square of that ratio to be heard at the receive end with the same amount of received power. So you would need something like 25 times more power than the other person to be received at near the same signal strength by the ISS -- and you would still have to overcome the handicap of being off frequency by 2.5 kHz.

Remember: FM receivers capture transmit stations that are more on frequency, and stronger than the other stations. The capture ratio can be as low as two-to-one, meaning if your signal as received by the target receiver is twice as strong as your competitor, you'll capture the target receiver and render the competitor's signal substantially inaudible.

In the 83 degree example above, if you do not compensate for doppler, you will be at a disadvantage compared to stations 342 Km away from you, for example, that is under the ISS. That's because while the ISS is approaching you at an elevation of about 43 degrees, you would be observing a doppler shift of 2.5 kHz and decreasing. However, the ISS would be directly overhead of a station 342 Km distant from you at a range of about 490 km from you, yet his distance to the space station would be about 270 km (overhead) and with zero doppler shift. You are at a disadvantage.

So compensating for doppler makes sense if you want to continue working the ISS during the lower thirds of your pass when others are operating at the edge of the footprint of the ISS. Combine the compensation for doppler shift with a directional antenna like the Arrow beam antenna, and you have a much better chance of making/maintaining contact at the lower portions of the pass.

Programming Memories for Doppler, ISS Voice Operations, North America

Here is the information for programming consecutive memory channels in VHF radios to use for contacting the International Space Station. This table is based on the ISS frequency plan for Region 2 (Asia, Oceania) and Region 3 (North and South America).

Programming Memories for 5 kHz Tuning increments, Voice, Regions 2 & 3

Memory Channel RX Doppler Offset (kHz) Transmit Frequency (MHz) Receive Frequency (MHz) Memory Offset (MHz) Comments
M1 +5 144.485 145.805 -1.320
M2 0 144.490 145.800 -1.310
M3 -5 144.495 145.795 -1.300

Programming Memories for 5 kHz Tuning increments, Voice, Region 1

Memory Channel RX Doppler Offset (kHz) Transmit Frequency (MHz) Receive Frequency (MHz) Memory Offset (MHz) Comments
M1 +5 145.195 145.805 -0.610
M2 0 145.200 145.800 -0.600
M3 -5 145.205 145.795 -0.590

Programming Memories for 1 kHz Tuning increments, Voice, North America

Memory Channel RX Doppler Offset (kHz) Transmit Frequency (MHz) Receive Frequency (MHz) Memory Offset (MHz) Comments
M1 +3.4 144.486.6 145.803.4 -1.316.6
M2 +3.0 144.487 145.803 -1.316
M3 +2.0 144.488 145.802 -1.314
M4 +1.0 144.489 145.801 -1.312
M5 0 144.490 145.800 -1.310
M6 -1.0 144.491 145.799 -1.308
M7 -2.0 144.492 145.798 -1.306
M8 -3.0 144.493 145.797 -1.304
M9 -3.4 144.493.4 145.796.6 -1.3032

Programming Memories for 5 kHz Tuning increments, Packet, World-wide

Memory Channel RX Doppler Offset (kHz) Transmit Frequency (MHz) Receive Frequency (MHz) Memory Offset (MHz) Comments
M1 +5 145.820 145.830 -0.010
M2 0 145.825 145.825 0.000
M3 -5 145.830 145.820 +0.010

Doppler and the voice UHF Crossband Repeater

Since the UHF uplink frequency is three times the frequency of the VHF Two Meter downlink frequency, the effects of doppler shift are magnified three times. Therefore, the doppler shift now becomes more than 10 kHz when the ISS is in the lower third of the pass. Therefore, it's almost always advantageous to factor and compensate for doppler shift. Instead of programming three sets of frequencies into memory as you would for VHF, you'll need to program five sets of frequencies for the UHF Crossband repeater, using 5 kHz tuning increments.

Memory Channel TX Doppler Offset (kHz) Transmit Frequency (MHz) Receive Frequency (MHz) Comments
M1 -10 437.790 145.805
M2 -5 437.795 145.800
M3 0 437.800 145.800
M4 +5 437.805 145.800
M5 +10 437.810 145.795

Here are two sample files of doppler calculations for the crossband repeat mode so you can study the effects of doppler. The first file has details on a 61 degree pass.

The second file has details on two lower passes for Honolulu on the island of Oahu, plus Kailua-Kona on the Big Island of Hawaii.

You can see that if you can put your radio into 5 kHz tuning steps, instead of the usual 25 kHz tuning steps, you'll greatly increase the usable passtime and thereby increase your chances of making a cross-band repeater contact.

The Voice UHF REVERSE Crossband Repeater

On January 3, 2009, a new crossband mode was introduced on the ISS. It features a VHF uplink with a PL tone of 67.0, and a UHF downlink. The programmed memories follows.

Memory Channel RX Doppler Offset (kHz) Transmit Frequency (MHz) Receive Frequency (MHz) Comments
M1 +10 145.985, PL 67.0 437.810
M2 +5 145.990, PL 67.0 437.805
M3 0 145.990, PL 67.0 437.800
M4 -5 145.990, PL 67.0 437.795
M5 -10 145.995, PL 67.0 437.790

Here are two sample files of doppler calculations for the crossband repeat mode so you can study the effects of doppler. The first file has details for Honolulu.

The second file has details for the island of Lanai.

Again, you can see that if you can put your radio into 5 kHz tuning steps, instead of the usual 25 kHz tuning steps, you'll greatly increase the usable passtime and thereby increase your chances of making a cross-band repeater contact.

VHF Doppler vs Obstructions

I have more problems due to tall buildings and mountains blocking the path to the ISS so I compensate for that and drive to the part of the island that consistently favors the ISS pass without the obstructions. Most of the times these days, I don't compensate for doppler on VHF transmit or receive when using 5 kHz tuning increments on the low passes. If I'm serious about the pass, I'll use a satellite radio, which can tune in 100 Hz increments.

On high passes, I use a quarter-wave vertical on the rear trunk of my car. If I want good signals down to the horizon, I'll put a quarter-wave (for high passes) or end-fed half-wave (for gain) magnetic mount vertical in the center of the roof of the car to better ensure consistent coverage in all directions.

The trade-off for not having to worry about compensating for doppler shift is that the digipeating and VHF work is limited to Hawaii. As a result, few people in Hawaii monitor and work the ISS on a regular basis.

Find out more by contacting:  rhashiro(remove this part)@hawaiiantel.net
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Updated: January 10, 2009
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