The ISS is moving very fast relative to your location. That motion causes the received radio frequency to shift slightly higher as the ISS approaches you and slightly lower as it moves away. This is called the Doppler effect.
The Doppler helper uses the ISS position and your GPS location to estimate the frequency where the 437.800 MHz downlink should be heard at that moment. As the ISS passes overhead, the displayed frequency will continuously change. Use it as a tuning guide while listening; actual reception can vary slightly with your radio, location, and current orbital data.
When using ham radio to communicate through the International Space Station, you may notice that the ISS voice downlink does not stay exactly on 437.800 MHz throughout the entire pass. The reason is Doppler shift.
The ISS is traveling at roughly 17,000 mph (27,000 km/h). As it approaches your location, its motion causes the UHF radio signal you receive to appear slightly higher in frequency than 437.800 MHz.
As the ISS reaches the closest portion of its pass, the received frequency moves toward the published 437.800 MHz downlink frequency. After the ISS passes your location and begins moving away, the received signal appears slightly lower in frequency.
ISS approaching → receive slightly ABOVE 437.800 MHz
ISS near closest approach → receive near 437.800 MHz
ISS moving away → receive slightly BELOW 437.800 MHz
During an ISS pass, you may need to gradually tune your receiver downward as the spacecraft travels across the sky. Doppler shift is particularly noticeable on the 437.800 MHz UHF voice downlink.
The Doppler Helper on this page uses the ISS's current orbital position and your GPS location to estimate where the 437.800 MHz voice downlink should appear on your radio at that moment.
For example, if the tracker displays Doppler Helper: 437.80753 MHz, the ISS's movement relative to your location currently makes the 437.800 MHz downlink appear at approximately 437.80753 MHz.
As the ISS crosses the sky, watch the Doppler Helper. The displayed frequency will continuously move downward. Adjust your receiver along with it to help keep the ISS voice signal clear.
The calculated frequency is a tuning aid, not an absolute measurement. Radio tuning steps, receiver characteristics, orbital-data age, and other factors can cause the best actual receive frequency to differ slightly.