--- title: "Sensors & instruments, explained | Nexus Observatory" description: "Interactive explainer: sensors & instruments in 3D, chapter by chapter: the site sets the ceiling on quality, every value leaves the logger with a flag…" url: https://observatory.global/sensors/explainer/ publisher: Nexus Observatory Inc. updated: 2026-10-28 --- Class 1 · 100 m Class 2 · 30 m Class 3 · 10 m Class 4 · 3 m 24 m · **class 3** 0 min 60 min 120 min 180 min -10 °C 0 °C 10 °C 20 °C 30 °C **Logger** · 1-min air temperature **76** flagged **SI kelvin · ITS-90** **National metrology institute** **Accredited laboratory** **Field reference** **Station sensor** U 0.040 °C U 0.057 °C U 0.101 °C 1 · Observed variable 2 · Purpose of observation 3 · Station/platform 4 · Environment 5 · Instruments and methods 6 · Sampling 7 · Data processing and reporting 8 · Data quality 9 · Ownership and data policy 10 · Contact 0 km 50 km 100 km 150 km 200 km 5 km 10 km 15 km 0.5° · **1.5 km** **P** **S** 30 s · 100 sps 0 dbar 1,000 dbar 2,000 dbar Ascent · **5.5 h** Drift · 10 day cycle 01 02 03 04 05 06 07 Class 1 · 100 m; Class 2 · 30 m; Class 3 · 10 m; Class 4 · 3 m Sensors · Explainer Interactive # A number is only as good as its chain. An observation carries where it was made, how it was checked, what it traces back to and who can vouch for it. Seven steps from a weather station's mast to a float two kilometres down. **Standards**: WMO-No. 8 · No. 1192 · GUM **Networks**: Weather · radar · seismic · ocean **Chapters**: 7 **01** Siting **02** Quality control **03** Traceability **04** WIGOS metadata **05** Radar volume scan **06** Seismic vault **07** Ocean profile **01** · Siting WMO-No. 8 ## The site sets the ceiling on quality. WMO-No. 8 grades a site from class 1 to 5 for each variable. For air temperature it is the distance to heat sources such as buildings, car parks and water: class 1 beyond 100 m, then 30, 10 and 3 m. For wind it is distance in obstacle heights: 30, 10, 5 and 2.5 times. Classes 3 to 5 add up to 1, 2 and 5 °C of uncertainty to temperature. Variable **Siting class**: Class 3 **Distance for the next class up**: 30 m for class 2 **Added uncertainty, temperature**: up to 1 °C **02** · Quality control Flags ## Every value leaves the logger with a flag. The sensor feeds a logger that averages one-minute values; automatic checks flag each one before it travels. The range check rejects air temperatures outside −80 to +60 °C, the step check doubts a jump of more than 3 °C in a minute, and the persistence check doubts an hour that varies by less than 0.1 °C. Inject a fault and watch the flags. Faults **Values in three hours**: 180 **Flagged erroneous, range**: 0 **Flagged suspect, step**: 1 **Flagged suspect, persistence**: 75 **Passed as good**: 104 · 57.8 % **03** · Traceability GUM ## An unbroken chain back to the kelvin. Each calibration compares an instrument with a better one and adds its own uncertainty. Following the GUM, independent standard uncertainties combine as uc = √Σu², and the expanded uncertainty U = 2uc covers about 95 %. A display that reads in 0.1 °C steps alone contributes 0.1/√12. WMO-No. 8 asks for 0.1 K on air temperature. Display resolution Expanded uncertainty down the chain, k = 2 Lab reference 0.040 °C + bath 0.057 °C Station sensor 0.101 °C WMO-No. 8 target 0.100 °C **04** · WIGOS metadata WMO-No. 1192 ## Metadata makes a number reusable. The WIGOS Metadata Standard describes an observation in ten categories, and a WIGOS station identifier names the station worldwide: series, issuer, issue number and local identifier, as 0-20000-0-71xxx for a station with a WMO number. Fill the categories this station reports. Categories reported Issuer of identifier **Categories complete**: 6 of 10 · 60 % **WIGOS station identifier**: 0-20000-0-… **Missing**: 4, 6, 8, 9 **05** · Radar volume scan VCP ## Fourteen tilts, and the Earth curves away. A weather radar sweeps a stack of elevation angles every few minutes. Under standard refraction the beam rides an Earth of 4/3 its radius: h = √(r² + (kea)² + 2rkea sin θ) − kea. Far from the radar, the lowest tilt overshoots the weather near the ground and the beam widens as r·θ. Heights are stretched tenfold. Radar **Beam centre above the radar**: 1.46 km **Beam bottom to top**: 0.00 – 3.03 km **Beam width across**: 3,142 m **Lowest tilt's centre**: 1.46 km at 0.5° **06** · Seismic vault GNSS time ## Two arrivals give the distance; the clock keeps it honest. The P wave outruns the S wave, about 6 against 3.5 km/s in the crust, so the gap between them grows with distance: S − P = R(1/Vs − 1/Vp), about 8.4 km per second of gap. A GNSS receiver on the vault stamps every sample; any clock error moves the event by Vp × error. Clock error Samples per second **P · S arrival**: 10.14 s · 17.38 s **S − P**: 7.24 s → 60.8 km **Sample interval · as distance**: 10 ms · 60 m **Clock error as distance**: 6 mm **07** · Ocean profile Argo ## A float that profiles two kilometres of ocean. A profiling float changes its buoyancy with an oil bladder: it sinks to drift at 1,000 dbar, drops to its profile depth, then rises sampling temperature, salinity and pressure, and sends the profile by satellite before it sinks again. A decibar of pressure is close to a metre of depth. Sample every Cycle **Ascent time**: 5.5 h **Samples per profile**: 1,001 **Profiles per year**: 36.5 **Years at 200 cycles**: 5.5 [Nexus Observatory](https://observatory.global/)**Sensors & instruments** . 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