29th ANNUAL CONVENTION & TECHNICAL SYMPOSIUM OF THE INTERNATIONAL LORAN ASSOCIATION (ILA), NOVEMBER 13-15, 2000, WASHINGTON, DC

LORAN-C BASED WINDFINDING IN

Juhana Jaatinen*, Sakari Kajosaari Vaisala Oyj, Helsinki, Finland

ABSTRACT

There has been growing uncertainty in the meteorological community about the future of upper air soundings using Loran-C radionavigation system. This paper summarizes latest information and covers technical and financial issues in order to give a concise view of the impact of Loran-C on meteorology.

1. WINDFINDING IN because of its accuracy, reliability and METEOROLOGY inexpensive . In the Southern Hemisphere GPS and Radiotheodolite are the There are two main classes of windfinding in most cost effective solutions. meteorology: surface and upper air wind. Surface are commonly measured with a wind vane, cup or an ultrasonic wind sensor. All these sensors are ground based. Upper air winds are measured by tracking the movement of a a radiosonde or reflector that is hanging from a rising weather . A weather balloon with radiosonde provides wind profiles typically upto a height of 30 km. is a radiosonde with a parachute and it is dropped from an airplane from an altitude of 5 to 10 km. An acoustic or frequency radar can also be used to obtain wind profiles from the surface upto 500 m or upto 5 km, respectively. Experimental wind profilers provide even higher altitude winds in good conditions. Radiosonde is the most common and cost- effective of these windfinding methods and it also provides valuable pressure, temperature and (PTU) profiles. Radiosonde data is the basis for medium range (up to 7 days) weather forecasts. A radiosonde in a weather balloon can be tracked using navigation aid (Navaid) networks such as Loran-C or GPS, by a radiotheodolite, or by a transponder (secondary radar). In areas where Loran-C is available, it should Figure 1 Weather balloon be considered the primary windfinding option

* Corresponding author address: Juhana Jaatinen, Vaisala Oyj, Upper Air Division, P.O. Box 26, FIN-00421 Helsinki, Finland [email protected] 2. LORAN-C BASED WINDFINDING 2.2 Automatic chain selection

Loran-C based windfinding is an inexpensive Before each radiosonde launch, the receiver method to meet the World Meteorological synchronizes itself up to four chains and selects Organization (WMO) requirements for upper-air the best chain pair for the sounding. The selection wind: wind speed standard error ±1 m/s from is based on sounding geometry and the number of surface to 100 hPa and ±2 m/s from 100 hPa to 5 available Loran-C transmitters. hPa, WMO (1996). The automatic chain selection feature Loran-C based windfinding does not require enhances operational capability and simplifies free horizon for accurate wind measurement, not operator's work during Loran-C service breaks even in mountainous areas. Windfinding accuracy and when using chains outside their coverage is independent of the distance to the radiosonde, area. and is not deteriorated at elevation angles below 10 degrees, which is a practical limit for 2.3 Windfinding and radiotheodolites. Determination of wind velocity (speed and 2.1 Sounding system direction) is based on Loran-C signals relayed by the radiosonde to the Loran-C receiver at the Vaisala Loran-C receiver uses a cross-chain ground station. A local antenna receives signals approach utilizing transmitters from two Loran-C directly from transmitting stations and these chains simultaneously. The receiver uses both the signals are used for monitoring and ground wave and skywave signal, which makes synchronizing the Loran-C receiver. windfinding outside navigation coverage area possible. • 9007-1

• 9007-2

• 9007-M

• 9007-3 • 8000-1

• 6731-3 • 8000-4 • 8000-M • 8000-2

• 6731-M

• 8000-3 • 6731-1

Figure 2 Loran-C sounding system LoranEurope_01.wmf, 2000-10-17 / JJa Figure 3 Radiosonde relays Loran-C signals Another feature that enables windfinding outside coverage areas is the ability to • 9007-1 synchronize on the basis of secondary stations only, when the master is too far to be received. • 9007-2 The master-station-sync is also available. Digital signal processing (DSP) is used in • 9007-M nearly all signal conditioning and signal • 9007-3 • 8000-1 processing stages which leads to a flexible and upgradable software implementation. Compared to conventional analog implementation • 6731-3 • 8000-4 • 8000-M advantages are achieved in precision, high • 8000-2 performance and long-term stability. Self- diagnostics are performed automatically after • 6731-M power-up. • 8000-3 • 6731-1

LoranVelocityComponent_01.wmf, 2000-10-17 / JJa Figure 4 Velocity components The receiver calculates relative motion of the These results are in line with other research radiosonde for all transmitting stations. The on the same subject, Jaatinen and Elms (2000), velocity components are entered into a set of Elms et al. (1996) and Nash (1994). equations from which a wind vector can be solved. Consecutive wind vectors form a wind 3. IMPORTANCE OF LORAN-C profile as the radiosonde ascends. Loran-C is a very important part of the ..\99021617b\LORANC.EDT meteorological upper air windfinding mix in

25 25 Europe, North-America and Asia both today and in the future. It provides excellent windfinding 20 20 accuracy with a low-cost radiosonde using existing installations. 15 15 Figure 6 shows the reported windfinding

HEIGHT [ km ] method (WMO TEMP B) in July 2000 for all 10 10 synoptic sounding stations. Weather forecasts are based on the TEMP messages. 5 5 July 2000 TEMP Part B ( 678 fixed stations )

0 0 100 0 10 20 30 40 50 60 0 100 200 300 100 WIND SPEED [ m / s ] DIRECTION [ Degree ] 30366 messages per month 90 6531 Loran-C 80 Figure 5 Wind profile 70 60 Loran-C 23% The wind profile together with pressure, 50 40 temperature and humidity profiles form the 26 30 23 19 18 20 contents of a meteorological TEMP message 11 10 3 discussed in section 3. The TEMP message is the 0 0 0 0 0 main output of a synoptic sounding system. Tot NR Opt RDF RDR NU VLF LOR PRF GPS Res Windfinding method 2.4 Windfinding accuracy Figure 6 Windfinding method globally The accuracy of a windfinding system is The labels in Figure 6 and the following affected by a variety of disturbances. These figures follow the TEMP message classification: disturbances can depend on the atmosphere, climate, time of the day, time of the year, signal Tot Total number of messages propagation direction, characteristics of the NR No windfinding, PTU only propagation path, system electronics, system Opt Optical direction finding software and the geometry of the system RDF Radio direction finding (radiotheodolite) transmission stations and receivers to name a RDR Ranging (radar) few. NU Not used Most of these disturbances can not be VLF Omega, Alpha, Communications VLF avoided but their influence to the windfinding LOR Cross-chain Loran-C solution can be estimated, Jaatinen and Pälä PRF Wind profiler (1998). GPS GPS satellite navigation In the light of the results from completed Res Reserved measurements, the most accurate windfinding system is GPS, offering a 0.1 m/s windfinding accuracy. The second one is a Loran-C based system which offers windfinding accuracy between 0.5 - 1.0 m/s. The third accuracy class is achieved with radiotheodolite (Vaisala model RT20), providing 1.0 m/s windfinding accuracy. Finally, a combined windfinding system for the late Omega, Alpha and ComVLF is the most inaccurate one of the examined systems, offering 2.0 - 2.5 m/s windfinding accuracy. Figure 7 shows the distribution of the 678 3.2 North America fixed synoptic stations of which 129 use Loran-C windfinding. There are also mobile ship-based Loran-C is an important part of the sounding stations and airplane-based dropsonde windfinding mix in North America. 21% of the stations. This information is also used for meteorological TEMP messages are measured . with Loran-C windfinding.

July 2000 / All / TEMP B / Sounding stations ( 678 fixed stations ) July 2000 TEMP Part B / Region IV North and Central America ( 161 stations ) 90 100 100 60 8634 messages per month 90 1799 Loran-C 80 30 70 61 60 Loran-C 21% 0 50 -180 -130 -80 -30 20 70 120 170 40 -30 30 21 20 7 6 5 -60 10 0 0 0 0 0 0 Total NR Opt RDF RDR NU VLF LOR PRF GPS Res -90 Longitude (deg) Windfinding method

Figure 7 Fixed synoptic sounding stations Figure 9 Windfinding method in North and Since Loran-C is a regional radionavigation Central America system with coverage in Europe, North America and Asia, these areas are discussed in detail in the 3.3 Asia following sections. A significant number of messages are obtained using Loran-C Loran-C is used on areas where it is available windfinding in all three areas, eventhough by the Far East Radio Navigation System percentages vary. (FERNS). 9% of the meteorological TEMP messages are measured with Loran-C 3.1 Europe windfinding. Loran-C is the dominant windfinding method July 2000 / Region II Asia / TEMP Part B ( 179 stations ) 100 100 in Europe. 55% of the meteorological TEMP 7035 messages per month 90 650 Loran-C messages are measured with Loran-C 80 windfinding. 70 60 July 2000 / Region VI Europe / TEMP Part B ( 152 stations ) 50 46 Loran-C 9% 100 40 100 7964 messages per month 30 23 90 18 4401 Loran-C Loran-C 55% 20 80 9 10 4 70 0 0 0 0 0 0 60 55 Tot NR Opt RDF RDR NU VLF LOR PRF GPS Res 50 Windfinding method 40 30 23 20 12 Figure 10 Windfinding method in Asia 8 10 1 0 0 1 0 0 0 Tot NR Opt RDF RDR NU VLF LOR PRF GPS Res Windfinding method 3.4 Research and defence forces

Figure 8 Windfinding method in Europe The synoptic use covers only half of the yearly radiosonde consumption. Other users of are research facilities and defence forces. Also part of these wind profiles are measured with Loran-C. 4. SUMMARY Nash, J. 1994: Upper wind observing systems used for meteorological operations. Annales Availability and continuity of service are the Geophysicae 12, 691 - 710. two most important factors in meteorological use WMO 1996: Guide to Meteorological of Loran-C. In areas where Loran-C is available, Instruments and Methods of Observation. it should be considered the primary upper-air WMO-No.8, sixth edition. World windfinding method because of the inexpensive Meteorological Organization. radiosonde, good accuracy and reliability. This paper shows clearly that Loran-C is very important in Europe, North-America and Asia. If Loran-C were to be closed, the financial consequences to the meteorological community 6. BIOGRAPHIES would be drastic. Juhana Jaatinen received the M.Sc in Electrical Engineering degree from the Helsinki University of Technology, 5. REFERENCES Helsinki, Finland, in 1991. Since 1992, he has been with Elms, J. B., Nash, J., Stancombe, J. 1996: Second Vaisala Oyj, Helsinki, Finland. Evaluation of Vaisala GPS Radiosonde Currently he is a Development Manager for System, Camborne (not published). Bracknell, digital signal processing applications in the Berks, UK Meteorological Office. 50 p. Upper Air Division. His previous projects have Jaatinen J., 1996: VLF and Loran-C based wind involved in software receivers for several finding. Proceedings of the twenty-first terrestrial Navaids: Loran-C, Omega, Alpha and annual meeting, International Navigation Communications VLF. Association, 233-239. From 1991 to 1995 he taught electrical engineering and digital signal processing in the Jaatinen, J. Saarnimo, T. 1997: Reliable Helsinki University of Technology and two windfinding after Omega phase-out. Preprints, technical institutes. 13th International conference on interactive information and processing systems (IIPS) for Sakari Kajosaari received the meteorology, oceanography, and hydrology, M.Sc. in Meteorology degree from American Meteorological Society, 366-372. the University of Helsinki, Jaatinen, J., Pälä, E., 1998: On the windfinding Helsinki, Finland, in 1977. accuracy of terrestrial Navaids. Preprints, Since 1984, he has been with 10th Symposium on meteorological Vaisala Oyj, Helsinki, Finland. observations and instrumentation, American Currently he is a Development Meteorological Society, 45-50. Manager for radiosounding system applications in the Upper Air Division. Jaatinen, J., Åkerberg, J., 1999: Automatic Upto 1984 he was with the Finnish Loran-C chain selection. Preprints, 15th Meteorological Institute. International conference on interactive information and processing systems (IIPS) for meteorology, oceanography, and hydrology, American Meteorological Society, 267-270. Jaatinen, J., Elms, J. B., 2000: On the Windfinding Accuracy of Loran-C, GPS and Radar. Preprints, 16th International conference on interactive information and processing systems (IIPS) for meteorology, oceanography, and hydrology, American Meteorological Society, 406-409.

Ken Goss presents a paper on behalf of Vaisala