UNCLASSIFIED
TM-CAL-002
GPS 1PPS TIMING REFERENCE
GPS Receiver 1 Pulse-per-Second Output as a UTC-Traceable Timing Standard
Prepared by: Mervyn Martin, KO6NNH
Merced, California  •  26 May 2026
Amateur Radio / Electronics — Not for commercial calibration use

Table of Contents

  1. Chapter 1 — General Information
  2. Chapter 2 — Theory of Operation
  3. Chapter 3 — Materials and Construction
  4. Chapter 4 — Assembly Procedures
  5. Chapter 5 — Calibration Procedure
  6. Chapter 6 — Tuning and Adjustment
  7. Chapter 7 — Verification
  8. Appendix A — Calculations and Formulas
  9. Appendix B — Example Results

CHAPTER 1 — GENERAL INFORMATION

1-1. SCOPE

This manual covers use of a GPS receiver's 1 pulse-per-second (1PPS) output as a timing reference for calibrating frequency counters, oscilloscopes, and time-interval measurement equipment. GPS 1PPS is traceable to UTC within ±100 ns under normal operating conditions.

1-2. REFERENCES

  • IS-GPS-200 (GPS Interface Specification)
  • u-blox NEO-M8N Product Summary
  • NIST SP-432: Time and Frequency Services

1-3. SAFETY

NOTEGPS receiver modules operate at 3.3 V or 5 V DC. No shock hazard under normal operating conditions.
NOTEOutdoor GPS antenna installation: maintain safe clearance from power lines.

CHAPTER 2 — THEORY OF OPERATION

2-1. GPS TIMING PRINCIPLE

GPS satellites carry cesium and rubidium atomic clocks synchronized to the GPS master control segment, which in turn is steered to UTC(USNO). The GPS receiver continuously solves for the time offset between the receiver clock and GPS time. Once a navigation solution is obtained (position locked), the 1PPS output is synchronized to UTC(GPS) within the stated accuracy.

Typical 1PPS accuracy specifications:

GPS Receiver 1PPS Accuracy
Module1PPS Accuracy (RMS)1PPS Accuracy (Peak)
u-blox NEO-M8N (standard)±30 ns RMS±60 ns peak
u-blox NEO-M8N (timing mode)±20 ns RMS±30 ns peak
Generic SiRF IV module±100 ns RMS±300 ns peak
Generic NEO-6M±50 ns RMS±100 ns peak

2-2. 1PPS SIGNAL CHARACTERISTICS

The 1PPS pulse is a TTL-level (3.3 V or 5 V, depending on module) positive pulse, typically 100 ms wide, with the rising edge coinciding with the UTC second boundary. The pulse width is configurable on most modules.

NOTEThe 1PPS output is valid only after the receiver achieves a 3D position lock (typically 60–120 seconds outdoors, longer with indoor antenna). Always verify lock status before using 1PPS for calibration.

CHAPTER 3 — MATERIALS AND CONSTRUCTION

3-1. BILL OF MATERIALS

Required Components
QtyItemSpecificationNotes
1GPS receiver moduleNEO-M8N preferred; NEO-6M acceptableMust have 1PPS output pin
1GPS patch antennaActive, 25×25 mm or 35×35 mmMagnetic mount for mobility
1SMA or IPEX cable50 Ω, length as neededGPS antenna to module
15 V power supply100 mA minimum, clean DCUSB supply acceptable
1SMA/BNC adapterFor 1PPS outputOr BNC connector direct
150 Ω BNC cableRG-58, 0.5–1 m1PPS to counter/scope

3-2. LEVEL SHIFTING

If the GPS module outputs 3.3 V logic and the counter requires 5 V TTL, a simple level-shift circuit is needed.

Voltage divider for 5V to 3.3V (GPS antenna power only)Not required for 1PPS: most counters accept 3.3 V on TTL input
NOTEThe NanoVNA, TinySA Ultra, and most modern frequency counters accept 3.3 V logic on TTL inputs. No level shifting is required for these instruments.

CHAPTER 4 — ASSEMBLY PROCEDURES

  1. Solder or connect the GPS module to 5 V power supply. Observe polarity. Module draws approximately 30–50 mA during acquisition.
  2. Connect GPS patch antenna to module antenna port (IPEX or SMA connector).
  3. Identify the 1PPS output pin. On NEO-M8N breakout boards, this is labeled TIMEPULSE or 1PPS. On bare modules, pin 3 (TIMEPULSE).
  4. Connect a 100 Ω series resistor in-line with the 1PPS output to protect the module output stage from capacitive loading.
  5. Connect 1PPS output to BNC connector on enclosure. Use short lead lengths (<10 cm) to minimize EMI pickup.
  6. Place module in enclosure with antenna cable routed to exterior.
  7. Apply power. Confirm LED blink on most breakout boards; 1 Hz blink indicates lock. Acquisition takes 60–120 seconds outdoors.
  8. Verify 1PPS output with oscilloscope: 3.3 V (or 5 V) positive pulse, 100 ms wide, 1.000000 Hz repetition rate.

CHAPTER 5 — CALIBRATION PROCEDURE

CAUTIONDo not use 1PPS for calibration until GPS lock is confirmed. Many modules continue to output 1PPS without lock; unlocked 1PPS can be off by milliseconds.

5-1. FREQUENCY COUNTER GATE CALIBRATION

  1. Set frequency counter to 1 Hz external gate input or 1 s internal gate.
  2. Apply 1PPS to the counter's external trigger or gate input.
  3. Measure a 10 MHz oscillator with 10 s gate time. The count should be exactly 100,000,000 ± 1 count for a perfect oscillator.
  4. Record: counts = fosc / f1PPS × gate time.
  5. Error in ppm = (actual counts − expected counts) / (expected counts) × 106.
  6. Example: gate = 10 s, expected 100,000,000 counts, actual 100,000,210 counts → +2.1 ppm error.

5-2. OSCILLOSCOPE TIMEBASE CALIBRATION

  1. Connect 1PPS to oscilloscope input. Set trigger to rising edge.
  2. Set timebase to 200 ms/div (2 seconds full screen for a 10-div display).
  3. The 1PPS pulse should appear at exactly 1.000000 s intervals.
  4. Measure the displayed period using cursors or automated measurement.
  5. Compare displayed period to 1.000000 s. Error >0.5% indicates timebase drift.
  6. For 2–div cursor placement: set cursors at 0 ms and 1000 ms displayed. Count actual division spacing. Error = (measured − 1000 ms) / 1000 ms × 100%.

CHAPTER 6 — TUNING AND ADJUSTMENT

6-1. GPS RECEIVER CONFIGURATION

The u-blox NEO-M8N timing mode can be enabled via UBX configuration commands to reduce 1PPS jitter from ~30 ns to ~20 ns. This requires UART or USB connection to the module and u-center software or custom serial commands.

NOTEFor most amateur calibration work, the default timing mode is entirely adequate. Enabling timing mode is optional.

6-2. PULSE WIDTH ADJUSTMENT

Default 1PPS pulse width is 100 ms. For triggering older counters or oscilloscopes with high trigger thresholds, a one-shot circuit can reshape the pulse width. The pulse timing (rising edge position) is not affected by pulse width adjustments.

CHAPTER 7 — VERIFICATION

  1. Confirm GPS lock: check lock indicator LED (1 Hz blink on most modules) or parse NMEA GPRMC sentence for validity character 'A' (active).
  2. Measure 1PPS frequency with calibrated frequency counter. Should read 1.000000 Hz ±0.000001 Hz (1 μHz).
  3. Compare 1PPS timing to WWV second tick (audio): within 0.5 s over 10 s is normal for ionospheric delay; phase difference should be constant.
  4. Log: date, time, GPS module model, satellite count, 1PPS frequency measured, ambient temperature.

APPENDIX A — CALCULATIONS AND FORMULAS

Frequency error from 1PPS gate counterrorppm = (Nactual − Nexpected) / Nexpected × 106
Expected counts (10 MHz oscillator, 10 s gate)Nexpected = fosc × tgate = 10×106 × 10 = 100,000,000
1PPS accuracy in seconds per daydrifts/day = accuracyns × 10−9 (this is per-pulse jitter, not accumulation)

APPENDIX B — EXAMPLE RESULTS

GPS 1PPS Calibration Log
DateGPS ModuleSatsCounter Ref (10 MHz)ErrorNotes
2026-05-26NEO-M8N910.000000 MHz0 ppmTiming mode enabled
2026-05-26NEO-6M79.999978 MHz−2.2 ppmTCXO drift
2026-05-26NEO-M8N11FNIRSI scope 1 ms/div+0.3%Acceptable
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