Five receive memories
The ARISS mentor's Doppler sheet gives an approximate swing of +10 to -10 kHz and several radio step-size options. The 5 kHz option supplies this compact sequence for a 437.550 MHz downlink:
| Memory | Receive frequency | Offset | Use during the pass |
|---|---|---|---|
| 1 | 437.560 MHz | +10 kHz | Strongly approaching |
| 2 | 437.555 MHz | +5 kHz | Approaching |
| 3 | 437.550 MHz | 0 kHz | Near zero range-rate |
| 4 | 437.545 MHz | -5 kHz | Receding |
| 5 | 437.540 MHz | -10 kHz | Strongly receding |
The progression depends on the orbit and your station, not on equal time intervals. A low pass may use only part of the sequence. Use a pass prediction or the signal's observed frequency to select the next memory. Check the radio's supported step size and its actual programmed frequency; a displayed rounded value can hide a different tuning step.
Why three times the correction?
For speeds much lower than the speed of light, the magnitude of the shift is approximately |delta f| = f0 × |radial velocity| / c. Here c = 299,792,458 m/s. At an illustrative radial speed of 7.0 km/s, the shift is about 10.22 kHz at 437.550 MHz and 3.40 kHz at 145.800 MHz. Those are calculated examples, not a prediction for a particular pass.
A spacecraft approaching the station is heard above its nominal transmit frequency; a receding one is heard below it. A tracking program can follow this continuously. Receiver frequency calibration remains a separate correction: an oscillator offset can displace the whole pass even when the Doppler trend is right.
FM bandwidth and levels
Check the announced radio mode as well as the image mode. AMSAT-UK recommends the wider FM deviation setting where a receiver offers wide and narrow choices. A filter that clips the signal at the edge of the pass cannot be repaired by raising the speaker volume. With an SDR, watch the carrier stay inside the selected channel while tuning follows it.
Set audio gain below clipping. For a speaker-to-microphone path, hold the phone and radio still and reduce room noise. For a wired path, confirm the selected input before AOS and test the complete route with a known recording. Note any squelch cuts, AGC changes or frequency steps that occur during a 36-second image.
Record a recoverable signal
A PCM WAV is a useful archive of demodulated SSTV audio. Preserve the original sample rate and leave time before the image header. A lossy voice recording may modify narrow tones, and a cropped file can discard the VIS sequence needed for automatic mode selection.
A complex I/Q recording retains the radio signal before FM demodulation. Store its sample rate, centre frequency, sample encoding and I,Q order with the file. Choose a capture bandwidth that includes the signal's modulation, its expected Doppler excursion and calibration uncertainty. A later replay can then select the FM channel again; demodulated speaker audio cannot restore spectrum removed by the original receiver filter.
The input reference explains the distinction for local files, rtl_tcp, SoapyRemote and SDR++ sources. The recording workflow covers the station notes to retain alongside a WAV.
Read the next notice afresh
The October Series 33 campaign used Robot 36 at this UHF frequency. That does not fix the parameters of the next ISS event. Recheck frequency, mode, UTC window and possible school-contact interruptions on the ARISS event page.