AMOSSG/4-SN No. 14

25/8/03

AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

FOURTH MEETING

Montreal, 15 to 18 September 2003

Agenda Item	6:	Capability of automatic observing equipment to meet aeronautical requirements

CAPABILITY OF AUTOMATIC OBSERVING EQUIPMENT TO MEET AERONAUTICAL REQUIREMENTS

(Presented by Saad Benarafa)

SUMMARY
The purpose of this paper is to provide information on WMO activities related to the review of attainable accuracy for the various meteorological elements found in Attachment B to Annex 3/Technical Regulations, Volume II, [C.3.1].

1.	INTRODUCTION

1.1	In July 2000, ICAO brought to the attention of WMO that the Air Navigation Commission, at the ninth meeting of its 153rd session, held on 7 March 2000, agreed that WMO be invited to update the column "attainable accuracy of observations of measurements" indicated in Attachment B to Annex 3/Technical Regulations Volume II [C.3.1], Operationally Desirable and Currently Attainable Accuracy of Measurement or Observation.

1.2	This paper prepared by the WMO Secretariat assisted by Mr. van der Meulen, KNMI, co-chair of the CIMO1 OPAG on the Surface Observation technology, highlights WMO main activities related to this issue.

2.	DISCUSSION

2.1	The attainable accuracy for the various meteorological elements found in Attachment B to Annex 3/Technical Regulations Volume II and the reply to CAeM for updating this table have been addressed, among others, by the meeting of the Commission for Instruments and Methods of Observation (CIMO) Working Group on Surface Measurements, held in Geneva from 27 to 31 August 2001. The issue was also discussed during the meeting of the Commission for Basic systems (CBS) Expert Team on Automatic Weather Stations (ET-AWS), held in Geneva from 2 to 6 September 2002. In this regard, both groups made recommendations relating to attainable accuracy of measurements and observations. However, recommendations of Working Groups or Expert Teams have no operational, guidance and regulatory status within the Organization until the relevant higher constituent bodies have approved these recommendations.

2.2	The Review of the "currently attainable accuracy of observations or measurements" is summarized in Table I. The complete Review is attached as Annex I. It should be noted that any future proposals to update this table should be coordinated before final suggestion is put forward for a decision of the CIMO.

2.3	Special attention should be given to the comments. The attainable accuracy is highly affected by environmental, artificial and maintenance impacts. The measure of such impact differs from case to case. In this respect, the common sense is in many cases the major adviser.

3.	CONCLUSION

3.1	The group is invited to note the information provided in this paper.

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AMOSSG/4-SN No. 14

Appendix A

APPENDIX A

TABLE 1

(1)

Variable	(2) to (7)

(irrelevant)	(8)

Achievable

operational

accuracy2	(9)

Remarks

1.	Temperature

1.1	Air temperature

1.2	Dew point température

0.1 K

(old: 0.2 K)

0.1 K

Achievable accuracy and effective time constant may be affected by the design of sensor and solar radiation screen. Impact due to rain, in combination with wind will affect the readings

2.	Atmospheric pressure

2.1	Pressure3

2.2	Tendency

0.1 hPa
(old: 0.3 hPa)

0.1 hPa
(old: 0.2 hPa)	(Range to sea-level)4.

Accuracy seriously affected by dynamic pressure due to wind (fluctuations)and inadequate temperature compensation

Differences between instantaneous values.
3.	Clouds

3.1	Cloud amount

3.2	Height of cloud base

3.3	Height of cloud top

(to be introduced)

1/8 to 2/8

(old: 1/8)

10 m

repeatability*

Period (30s)5 clustering algorithms may be used to estimate low cloud amount automatically.

* Accuracy difficult to determine since no definitions exists for instrumentally measured cloud base height6.
4.	Wind

4.1	Speed

4.2	Direction

4.3	Gust

0.5 m s-1

5 º

0.5 m s-1

Average over 2 and/or 10 minutes. Non-linear devices. Care needed in design of averaging process.

For wind speed < 10 m/s: 3º

Highest 3s average should be recorded.
5. Visibility

5.1	MOR

5.2	RVR

10 – 20 %

10 – 20 %

Achievable instrumental accuracy depend on the cause of obscuration

Accuracy heavily affected by contamination of the optics

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AMOSSG/4-SN No. 14

Appendix B

APPENDIX B

REVIEW OF "CURRENTLY ATTAINABLE ACCURACY OF MEASUREMENT OR OBSERVATION"

(Attachment B of ICAO Annex 3/WMO No. 49 - Vol. II)

For use as a guidance to Meteorological Observations and Reports.

(23 February 2001)

Summary

The Attainable accuracy of measurement or observation from the table on Operational and Desirable and Currently Attainable Accuracy of Measurement or Observation is reviewed. For this purpose the recommended Operational Accuracy Requirements and Typical Instrument Performances, published in WMO-No. 8 (6th edition, 1996) are taken as baseline, with some alterations based on reports on the performances of currently available instruments.

It is concluded that for a number of variables the currently attainable accuracy is better that stated in the table. However inappropriate siting, poor maintenance or ineffective calibration procedures may have serious impact on the quality and representativeness of observations, which will overrule the high quality of any sophisticated instrument. Therefore, the statement "currently attainable" is only related to appropriate measuring procedures, maintenance and use by skilled personnel.

Apart from the accuracy of measurement, the limited representativeness or the averaged values of a time series of a measurement may have more impact on the uncertainty of a reported meteorological variable. Moreover, natural variations, in time and space, of a variable are of significance when considering representativeness. It is recommended to consider this issue first before discussing attainable or achievable accuracy.

References

When considering achievable or attainable accuracy of measurements it is useful to take notice of its historical background, which can be found in the references below:

[1]	WMO-No. 8, Guide to Meteorological Instruments and Methods of Observation (sixth ed., 1996), Chap. I, Annex 1B. (See remarks below)

[2]	- ICAO Annex 3, par. [c.3.1.]4, [c.3.1.]4.1.12 and Attachment B
- WMO-No. 49, Technical Regulations, Vol. II, par. [c.3.1.]4, [c.3.1.]4.1.12 and Attachment B

[3]	ICAO Doc. 8896 - AN/893/4, Manual of Aeronautical Meteorological Practice, 4th edition (1993) (especially table D-3)

[4]	WMO-No. 188, International Meteorological Tables, with formulas and algorithms to calculate reduced values.

[5]	Final Report of the Meeting of Experts on Operational Accuracy Requirements (1991) (see remarks below)

[6]	WMO-No. 731, Guide on Meteorological Observation and Information Distribution Systems at Aerodromes (1990).

[7]	WMO-No. 727, Abridged Final Report of CIMO-X (1989),

[8]	WMO-No. 807, Abridged Final Report of CIMO-XI (1994).

Notes to these references:

-	"Achievable operational accuracy" entered in the 6th edition of WMO-No. 8 (ref. [1]) after a resolution by CIMO-XI (1994) (see refs. [5, 8]). The values are based on overall experiences of instrument performances. This Guide is considered by WMO as the basic WMO document concerning "operational accuracy requirements and typical instruments performances".

-	"Required accuracy" is published in the same Guide following the recommended Statement of Operational Accuracy Requirements of Level II Data according to WMO Codes SYNOP, SHIP, METAR and SPECI of an expert group (with representation from CAeM, see ref. [5]), which was agreed by the WMO Executive Council (EC-XLIV, see ref. [8], Annex X). This Statement hold for both manual and automatic observations and hold be regarded as a universal table for all disciplines within meteorology.

-	In particular, considerations on observations and measurements are stated in ref. [3], par. 4, Adequate Instrument Exposure and in par. 5, Representative Measurements. Of special interest is the statement in par. 5.2 concerning representativeness: "Acceptable margins are not necessary identical with accuracy requirements of measurements".

-	In ref. [1], Part II, chap. 2, Measurements and Observations at Aeronautical Meteorological Stations, and in ref. [6], chapter 2, Aerodrome Observations considerations on observations are given with guidelines for each variable to be measured together with instrument dependent accuracy considerations.

General recommendation

WMO has officially adopted the table on Operational Accuracy Requirements and Typical Instrument Performances containing achievable operational accuracy (ref. [1]). Since publication of this Guide in 1996 this table is not updated. Any update of this table might be expected at the next CIMO-XIII in 2002 by taking into account improvements of any method of observation.

To maintain the uniformity of guidelines with respect to operational observations for both WMO and ICAO it is highly recommended to include (parts of) this table in ref. [2] (ICAO Annex 3/WMO-No. 49), inclusive of the stated remarks and expanded for variables regarding the specific need of ICAO or CAeM (cf. CIMO-XI, ref. [8]).

Specific considerations

1.	Sensor and instrument development and improvements

The "state off the art" of instrument development is presented in the series "Instrument Development Inquiry", published by WMO (latest edition: No. 6, WMO/TD-No. 878, 1998). It is found that for the last ten years instrument development is mainly focused on automation and unattended operation, i.e.:

-	data acquisition and data communication technology,

-	design of automatic weather observing systems (AWOS),

-	improvement of sensor performances to reduce corrective maintenance to enlarge the MTBF (mean time between failure).

Only little effort was observed worldwide in the field of development of new observing technologies or improvement of sensor techniques. Moreover financial constraints, reducing the budgets of the national meteorological centers have reduced the effort by developers to invent more sophisticated alternative technologies in order to increase the accuracy of measurement. Since the international commercial market for professional meteorological instruments is limited (small volume), the effort by industry is also rather modest and mostly focused on improving the cost-benefit ratio of a system only.

2.	Operational accuracy of measurements

Operational accuracy of measurements is based in the first place on appropriate calibration and adjustment of devices. In the second place, long and short-term stability of instruments should be considered seriously. When measuring the environment, another important issue is the impact of temperature variations, wind, rain, radiation, etc. to the devices of course. With regards to calibration, traceability of reference standards to internationally approved primary standards is an essential requirement. If this requirement is not fulfilled, undefined offsets may occur resulting in unpredicted uncertainty.

Moreover ISO guidelines should be considered (like in the Guide to the Expression of Uncertainty Measurement and to ISO/IEC 17025, General Requirements for the Competence of Testing and Calibration Laboratories", both issued by ISO). Note that the Guide to the Expression of Uncertainty of Measurement has defined the term uncertainty of a result of a measurement instead of (achievable or attainable) accuracy. Furthermore, the use of the "±" (plus-minus) symbol should be avoided, except for expressions of measurements like Y = y ± U.

3.	Human observations

Human observations always introduce subjectivity and as a result additional uncertainty. See for a typical example the results of five skilled observers determining visibility during The first WMO Intercomparison of Visibility Measurements (Final report, WMO-No. 401).

4.	Siting and environmental impacts

In refs. [3] and [6] siting and other environmental impacts are discussed in detail. especially the representativeness of any variable, measured not closely to a runway is a point for consideration. It must be noted that this important issue is not involved in the determination of "achievable accuracy".

5.	Time averaging and point measurements

Except for visual observations, all measurements are point measurements and the measure of representativeness for the surrounding area is determined by the homogeneity of the atmosphere at the measuring height. Moreover most meteorological variables vary significantly in time, especially wind.

However, since the atmosphere is not homogeneous, point measurements can only satisfy the requested observational information up to a limited uncertainty. For practical reasons it is good practice to calculate the average value of a time series of observational data in order to eliminate more or less this time and place dependent behavior. Two and ten minutes averaging intervals are commonly used. If a sufficient large number of data is used, apart from the mean value (AVG) a standard deviation (STD) can be calculated as well. Especially STD is an important parameter when interpreting observational data and gives a bright impression of the representativeness of the AVG value, in terms of value = AVG ± 2×STD.

6.	Measurement uncertainty with respect to the individual variables

In the paragraphs below an overview is presented for the accuracy of variables given in Attachment B of Annex 3. Note that it is international practice to use the SI system for representation of quantities and units.

6.1.	Wind direction

Stated required accuracy (WMO-No.8):	5
Desirable accuracy (Annex 3):	10
Attainable accuracy:	3 - 5
(point of concern: adjustment to true North)

6.2.	Wind speed

Stated required accuracy (WMO-No.8):	0,5 m/s	for £ 5 m/s
10%	for > 5 m/s
Desirable accuracy (Annex 3):	2 km/h	for £ 19 km/h
(1 knot)	(£ 10 knot)
10%	for > 19 km/h
(> 10 knot)
- (note that 2 km/h 1 kt)
Attainable accuracy:	0,5 m/s

6.3.	Variations in wind speed

Stated required accuracy in terms of
wind gusts (WMO-No.8):	10%
Desirable accuracy (Annex 3):	4 km/h
(2 kt)
Attainable accuracy:	0,5 m/s

6.4.	Visibility (MOR: Meteorological Optical Range):

Stated required accuracy (WMO-No.8):	50 m	for £ 500 m
10%	for > 500 m
Desirable accuracy (Annex 3):	50 m	up to 600 m
10%	between 600 m and 1500 m
20%	above 1500 m
Attainable accuracy:	10 - 20 %

6.5.	Visibility (RVR: Runway Visual Range):

Stated required accuracy (WMO-No.8):	25 m	for £150 m
50 m	for >150 - £500m
100 m	for >500 - £1000 m
200 m	for >1000 m
Desirable accuracy (Annex 3):	10 m	up to 400 m
25 m	between 400 m and 800 m
10%	above 800 m
Attainable accuracy:	10 - 20 %	(max. range: 1500 m)

6.6.	Cloud amount

Stated required accuracy (WMO-No.8):	1/8
Desirable accuracy (Annex 3):	1 okta
Attainable accuracy:	1/8

6.7.	Height of cloud base (Accuracy difficult to determine since no definition exists for instrumentally-measured cloud base height)

Stated required accuracy (WMO-No.8):	10 m	for 100 m
10%	for >100 m
Desirable accuracy (Annex 3):	10 m (33 ft)	up to 100 m (330 feet)
10%	above 100 m (330 feet)
Attainable accuracy:	10 m	(approx.)

6.8.	Air temperature

Stated required accuracy (WMO-No.8):	0,1 K
Desirable accuracy (Annex 3):	1 C
Attainable accuracy:	0,1-0,2 K

6.9.	Dew point temperature

Stated required accuracy (WMO-No.8):	0,5 K
Desirable accuracy (Annex 3):	1 C
Attainable accuracy:	0,1-0,2 K

6.10.	Atmospheric pressure (QNH, QFE)

Stated required accuracy (WMO-No.8):	0,1 hPa
Desirable accuracy (Annex 3):	0,5 hPa
Attainable accuracy:	0,1 hPa
(point of concern if no precaution is taken: Affection by wind will enlarge uncertainty)

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