---
title: The measurements
description: Every number on this site, with the methodology that produced it. Check our work.
sidebar:
  order: 2
---

All numbers from **one** Raspberry Pi 4 + BerryGPS-IMU v4 (u-blox CAM-M8C), PPS on
GPIO18. n = 1. Check them against your own board.

## Headline progression

| Change | RMS offset | Root dispersion |
|---|---|---|
| Baseline (stock kernel, stock chrony) | 823 ns | 16.8 µs |
| chrony `filter 10` + `prefer` on PPS refclock | 440 ns | 5 µs |
| PREEMPT_RT, unpatched | **2468 ns** ← *worse* | 11.6 µs |
| **PREEMPT_RT + [`IRQF_NO_THREAD`](/reference/the-patch/)** | **199 ns** | 6.3 µs |

## Raw PPS jitter (kernel-timestamped)

| Kernel | jitter (σ) | peak-to-peak |
|---|---|---|
| Stock | 2134 ns | 11 µs |
| PREEMPT_RT (threaded handler) | 6947 ns | 38 µs |
| PREEMPT_RT + patch (hard-irq handler) | 2568 ns | 18 µs |

## The dashboard's tax

A/B/A, 62 s per round. [Why this matters](/explanation/the-observer-effect/).

| | Dashboard off | Dashboard on |
|---|---|---|
| Before fix | 1304 ns | 1912 / 2179 ns (**+36%**) |
| After fix | 1437 ns | 1169 / 1450 ns (**no penalty**) |

## Things that did nothing

| Change | Result |
|---|---|
| Baud 9600 → 115200 | PPS offset **−1 ns** either way. Identical. |
| SBAS disabled | No measurable change to PPS. |
| `isolcpus` alone | Inconclusive-to-harmful (concentrates load onto the PPS core). |

## Methodology

**Don't trust chrony's own stats for this.** `chronyc sourcestats` reports a
windowed, median-filtered figure that lags reality and hides what you're trying to
see. Measure the kernel's PPS timestamps directly:

```bash
sudo timeout 62 ppstest /dev/pps0 | awk '
/assert/ {
  split($0, a, "assert "); split(a[2], b, ","); t = b[1] + 0;
  if (prev > 0) {
    d = (t - prev - 1.0) * 1e9;          # deviation from exactly 1.000000000 s, in ns
    n++; sum += d; sumsq += d*d;
    if (d > max || n == 1) max = d;
    if (d < min || n == 1) min = d;
  }
  prev = t
}
END {
  mean = sum/n; sd = sqrt(sumsq/n - mean*mean);
  printf "n=%d  jitter_sd=%.0f ns  p2p=%.0f ns\n", n, sd, max-min
}'
```

Each pulse should be exactly 1.000000000 s after the last. The deviation *is* the
jitter.

**Always run A/B/A**, never A/B. Clock behaviour drifts on the scale of minutes;
if you measure on-then-off you cannot tell a real effect from thermal drift or a
satellite geometry change. Go on → off → on, and require the two "on" rounds to
agree before you believe the middle one.