GPSDO 10 MHz and PPS Outputs Explained

[Pavel]

Short answer: A GPSDO's 10 MHz output is a disciplined frequency reference; PPS is a once-per-second timing edge aligned to the reference time. Use 10 MHz for oscillator accuracy and PPS to set or compare device time, while checking level, impedance and lock state for each output.

Before you start​

• Keep GNSS and drone work passive unless the experiment has explicit authorization and an RF-controlled environment; do not jam, spoof or interfere with navigation or aircraft.
• Record receiver clock, antenna, gain, sample rate, observation location precision and ground truth before interpreting anomalies.

Step-by-step method​

1. Step 1: Read the GPSDO output waveform, amplitude, impedance and warm-up/lock indicators, then compare them with every SDR input specification.
2. Step 2: Connect 10 MHz through a proper distribution amplifier when feeding several devices. Configure each SDR's clock source to external and verify lock sensors.
3. Step 3: Connect PPS to time inputs, configure the correct edge and set device time on a future PPS. Check that the last-PPS timestamp advances.
4. Step 4: Measure frequency and timestamp alignment after warm-up and after restart. Record holdover behavior if satellite lock is removed.

Concrete example​

Two SDRs can share 10 MHz and still begin sampling at different device times. Adding shared PPS and initializing both on the same edge aligns time, but deterministic RF phase may require additional calibration.

How to judge the result​

The GPSDO integration is correct when devices report external-reference lock, PPS events are detected and measured frequency/time errors meet the experiment requirement.

What to record​

• Separate frequency-reference accuracy from absolute time alignment: 10 MHz disciplines frequency while PPS marks a time boundary.
• Use multiple independent observables for GNSS interference or spoofing claims, such as spectrum, AGC behavior, correlation, navigation consistency and trusted reference data.
• For direction finding or RF classification, report uncertainty and test across channels, distances, devices and temperatures.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• Splitting a reference with passive tees can violate amplitude and termination requirements, causing apparent intermittent lock.

Final check​

Repeat the passive observation against an independent trusted reference and state uncertainty. Do not make spoofing, interference or aircraft claims from a single receiver screenshot.