NATIONAL RADIO ASTRONOMY OBSERVATORY

Green Bank, West Virginia

ELECTRONICS DIVISION INTERNAL REPORT NO. 298

GBT WEATHER STATION

David Seaman

March 1995

The Green Bank Telescope (GBT), like all other NRAO telescopes, requires accurate weatherinformation. This information is used for protection of the antenna structure from dangerouslyhigh loads from wind, and to correct for pointing errors caused by atmospheric refraction; seeGBT Memo 112 by Ron Maddalena. Due to size of the GBT structure, and the influential effect ofthe surrounding topography on the semi-local wind, it is desired to have two weather stations.One station, consisting of wind speed and direction, temperature, and barometric pressure, islocated on an existing 150' tower opposite the Interferometer Control Building (ICB). This toweris approximately 1800' SW of the GBT. The second tower is located approximately 500' awayfrom the GBT and 35° West of North. This location was selected to keep the tower at the mostnortherly location possible to reduce interference. The slope of the ground into the ditch, and theGBT power line, prohibited a further north placement. The second tower will have wind speed,direction, and temperature sensors.

At the base of both towers is an electronics enclosure. The vendor supplied electronics for thewind and temperature sensors are located here. NRAO added a VLBA MCB standard interfaceboard to digitize the 0-5 VDC analog outputs. The data is then carried by fiber to a local networkconnection (see attached schematic). Fiber was chose as the carrier to eliminate the possibility oflightning entering the control building via the wire, and to eliminate the losses associated withcopper. From the base of the tower, the first station is connected to a VME crate in the ICBbasement. The second tower will be connected to a similar computer in the local GBT controlbuilding. Located inside one of these two buildings is a chilled-mirror dew-point monitor. Aninternal device, which draws in outside air through a tube, was selected to reduce electronic driftdue to changing atmospheric temperature-calibrations. The following is a brief description of theinstruments selected.

Wind Speedmanufacturer Met One Instruments, Model TG1564B, Model 170-41 cup assy.description The wind speed sensor is a light chopping device which outputs a variable

frequency square wave, with frequency linearly proportion to shaft speed.This unit has an accuracy of +1- 0.15 mph, or 1%, whichever is greater. Thestarting threshold is 0.6 mph with the standard three-cup assembly. A six-cupassembly is also available, which reduces the threshold to 0.5 mph.

Wind Directionmanufacturer Met One Instruments, Model TG1565C, Model 53.1 vane assy.description The wind direction sensor incorporates a high resolution resolver which forms

the measuring circuit. This resolver is a non-contact device which reduceswear-out problems associated with potentiometer driven units. The resolveroutputs a dual sine wave, the phase difference between the two gives the winddirection. This unit is accurate to +1-2°.

Temperature Sensormanufacturer Met One Instruments, Model T200, Model 5980 Radiation Shield.description The temperature sensor is a platinum RTD, with a calibrated accuracy of -F1-

0.10°C. The sensor is mounted in a wind-aspirated radiation shield.

Dew Point Sensormanufacturer General Eastern Instruments, Model D2, Model MI Controller.description This unit is a modular-chilled minor system which has a total system accuracy

of +1-0.2°C. The advantage to a modular unit is the ability of having thecontroller and mirror assembly in an environmentally controlled location.This will reduce electronic drift with temperature and should increase thereliability of the system. Air is drawn through the unit via a 114 tubeconnected to a vacuum pump. The pump operates at 2scfh. The outputs ofthe controller are via a RS232 port, or a 0-5VDC analog signal.

Barometric Pressuremanufacturer Setra, Model 270, with #707 option.description This unit has a ceramic capsule which deforms under pressure, which changes

the electrical capacitance. Accuracy is <+/-0.03% of the full scale 1100 mbar,or 0.33 mbar. It will be located in one of the electronics' enclosures, with ahose running to the outside.

The selection of the various components was mainly dictated by the accuracy requirements of GBTMemo 112. The wind direction sensor selection was partially based on information collected at theVLA/VLBA. They had experienced potentiometer wear out on contact-type direction vanes. Thewind speed sensor accuracy dictated using the optical pulse counter rather than a DC voltagegenerator. The DC generators also required calibrations to be done based on the amount of wireconnected to them, thus, not being interchangeable without re-calibration. The pulse countersselected are interchangeable and require only initial factory calibration. For operational safety,when operating under emergency power, a DC voltage-generating wind speed sensor will belocated in the local control building.

Also attached are calibrated certificates from the manufacturers, and various sketches showing thelayout of the system. Complete manuals are available in the Green Bank Electronics Department.

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Translator Model SO/ 7 2, Serial No. NW .21

Sales Order # 94/- L/369 Customer Nail: kollo .45/Ivi ?on,/ Ohs

Test Date '7- 2/ Tested byq,--Z

Test StandardsDMM 3468E3 Ser 2231A01057 CALIBRATED 4/-1-q271 FREQUENCY B&K 1805 Ser A0-06810- CALIBRATED Z-Z)--.4717z

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TECHNIC IANPART NO 270110,-SP

SPEC. FILE: 270100.SPTRANSFER STD.:

S/N: 434767PROD. ORDER: R35887

LOC.: 3A-61DATE: 3-3-95

MODEL: 270RANGE: 800 TO 1100 MBAR

NOM.OUT: 0 TO 5 VnCEXCIT: 12 (JDC

CALIBRATION DATA

(TEMP. 70.0'F +I- 0.9'F)

APPLIED TRANSDUCER EXTRAPOLATEDPRESSURE OUTPUT NONLINEARITY ERRORS(MBAR) (UDC) (X F S ) (Z FS)

800 +00.0004 00.000 ZERO: 00.008900 +01.6670 -00.0071000 +03.3333 -00.0201100 +05.0012 00.000 FS OUT 0).016

SPECIFICATIONS:1. Nonlinearity: +1- 0.03Z FS, END POINT method. ISAAS-37.12. Zero pressure output: 0 VDC +/- 0.1 Z FS3. Full-Scale output: 5 VDC +/- 0.1 FS

NOTES:•. All errors are expressed as Percent Full-Scale output.2. Consult specification sheet for additional specifications.3. This calibration is certified per N.I.S.T traceable primary standards.

Primary std.: P2-906/C2-1567, NIST# TN-249770-92, Cal. date: 02-01-95

OPTIONS:1)REPLACED SENSOR COMPLETE RECAL

45 Nagog Park, Acton, MA 01720/Telephone 1-800-257-3872, In MA (508) 263-1400SS0513 Rev. 1/90

CERTIFICATE OF CONFORMANCECALIBRATION PERFORMED IN ACCORDANCE WITH MIL-STD 45662A

REPORT OF CALIBRATION/SYSTEM UNDER TEST

, MODEL: M1/D-2 S/N: 0460794

NOTE: NIST has a Maximum Uncertainty of ± 0.04 °C from 25 to -35°C and ± 0.1°C for Frost Points from -35 to -70°C.GEI's Transfer Standard and Manufacturing Standard are Compensated to This Uncertainty Band.

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0.05°C •, 0.0°C 0.05°C . ±0.20°C

9.26°C 0.06°C ±0.20°C

21.80°C 21.7°C 0.10°C ±0.20°C

GEl AIR TEMP STD CU$TOMER AIRPOINTTEMP .

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GENERAL EASTERN INSTRUMENTS MEASUREMENT STANDARD

PO INT.. ..... . . .. ....

MODEL: M-3 S/N: 1990693 MODEL: A1011-XX-XX-R141-RT41 S/N: T1189-9142

SENSOR: 1311DR S/N: 1990693 PROBE: Al2001 S/N: 308458

CURRENT NIST I.D. #: H-4475 CURRENT INSTRUMENT NIST TEST #: 811/249718

CURRENT NIST TEST #: 252396 CURRENT PROBE NISI TEST #: 222173/242309/2477770

CALIBRATION DATE: 7-16-1993 CALIBRATION DATE: 10-27-1993

DVM MODEL: Data Precision 2590 S/N: 2021 CALIBRATION DATE: 10-1 9-1 993

CURRENT NIST TEST NO.: 246764

GENERAL EASTERN INSTRUMENT CALIBRATION PROCEDURE: A40076309

RECOMMENDED CALIBRATION INTERVAL: 1 YEAR FORM#: A40103046E

TESTED BY:APPROVED BY:DATE: July 15, 1994

GENERAL EASTERN INSTRUMENTS20 COMMERCE WAY, WOBURN, MA 01801 Tel.: (617) 938-7070 FAX: (617) 938-1071