RECENT WORLDWIDE GPS RADIOSONDE PERFORMANCE Incorporating the review of WMO GPS questionnaire, 2001

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1 1. INTRODUCTION RECENT WORLDWIDE GPS RADIOSONDE PERFORMANCE Incorporating the review of WMO GPS questionnaire, 2001 J.B. Elms, MetOffice, UK Rapporteur on Radiosonde Compatibility Monitoring within the WMO/CIMO Working Group on Ground-based Upper-air Observing Systems The closure of the Omega navigation chain in 1997 created a significant change in upper air windfinding systems at about 200 radiosonde stations throughout the world. The majority of the stations replaced their Omega windfinding using the Vaisala RS80 GPS system, but some also changed to Loran-C windfinding. The map in Figure 1(a) shows that in October 2001 about 95% of GPS windfinding systems providing data to the Global Telecommunications System (GTS) were still using Vaisala RS80 GPS radiosondes. All Vaisala GPS systems were using the RS80G radiosonde, with the Vaisala RS90 GPS radiosonde yet to be used in operations. Germany (3 systems) and Australia (9) use autosonde automatic launcher systems to deploy GPS radiosondes. Fig. 1(a) Distribution of operational GPS radiosonde observations. Other refers to operational stations using a windfinding method other than GPS. Fig. 1(b) Distribution of operational Loran-C radiosonde observations. Other refers to operational stations using windfinding method other than Loran-C.

2 - 2 - Fig. 1(b) shows the distribution of operational Loran-C radiosondes in October Whilst the Loran-C navigation chain is being upgraded in North America and will continue operations for at least another ten years, the European Loran-C chain may close at the end of 2005 unless a new source of funding is found for the system. 2. RESULTS OF QUESTIONNAIRES Many WMO Members considered the operational performance of Vaisala and SIPPICAN GPS radiosonde wind systems in 1998/9 unsatisfactory. A questionnaire was therefore sent to all WMO member countries in 1999 asking for information on the performance of operational GPS-based upper air systems. Since the initial questionnaire, the manufacturers have made various design modifications to the ground station software, radiosonde hardware and radiosonde manufacturing methods. To check whether these changes have produced improvements in wind measurement reliability, a second questionnaire was distributed by WMO for the 6-month period from July to December More than 60 countries replied including most of the main users of GPS systems. In both questionnaires, statistics received were for Vaisala RS80 GPS systems except for the returns from the Philippines, Cape Verde and U.S.A. in the second questionnaire relating to the Sippican MkII sounding system. The results from the two sample periods are summarised in Table 1. The total number of measurements attempted was similar in both sample periods. In most failure categories apart from ground system failure there was a significant decrease in the failure rates in 2000 compared to CATEGORY 1999 (JAN- JUNE) 2000 (JULY DEC) Number % Number % Total number of launches Failure to achieve GPS satellite constellation lock before launch. 2 Failure to produce any winds during ascent. 3 Unacceptable gaps in wind measurements in the troposphere. 4 Unacceptable gaps in wind measurements in the stratosphere. 5 Large numbers of spurious (bogus winds) 6 Ground System Failure Table 1 Summary of GPS radiosonde wind failures from the samples in 1999 and 2001 Member Price $ Member Price $ TUNISIA 90 CHILE 150 CAPE VERDE IS. 95 ARGENTINA 159 SENEGAL 97 OMAN 164 FRANCE 99 SPAIN 168 AUSTRIA 100 VIETNAM 179 NEW ZEALAND 106 GERMANY 180 CYPRUS 106 MONGOLIA 185 DENMARK 119 BANGLADESH 190 UK 120 QATAR 192 USA 120 PHILIPPINES 200 SOUTH AFRICA 139 IRAN 200 BRUNEI 145 CZECH REPUBLIC 206 CANADA 149 MEXICO 220 SEYCHELLES 150 JAPAN 250 Table 2 Summary of the cost of a GPS radiosonde in 2001 Table 2 shows that there was a very wide range in the cost of an individual GPS radiosonde between the countries. The price charged for a GPS radiosonde has dropped with time to some extent, and there is also a reduction in price for larger orders, but this does not really explain all the variations between countries

3 - 3 - seen in Table 2. WMO members should realise that the radiosonde price has fallen with time. They need to ensure that they benefit from these reductions. They should also ensure that in contractual arrangements they receive some compensation if excessive numbers of radiosondes fail in flight. Variation in failure rates from country to country Figure 2 shows that the percentage failure rates reported within the survey varied greatly from country to country. These percentages have been calculated from the Total number of Failures in line 5.6 of the recent questionnaire and the Total number of Launches in line 5.1. Note that the highest failure rates are generally found in countries launching less than one radiosonde per day through the survey period (white or grey columns) in Figure 2. It is suggested that the operations managers in the countries with high failure rates, above 20 per cent, contact the manufacturer to identify the reason, or alternatively discuss operational procedures with the Rapporteur. Fig. 2 Worldwide Percentage Failure Rate of GPS Radiosonde Flights. (All radiosondes Vaisala RS80GPS except where labelled Sippican) The average overall failure rate reported in the questionnaire (i.e. percentage of missing or inadequate GPS winds produced by the system) for this 6 month period in 2000 was about 12 per cent of the attempted launches (compared with about 26 per cent total failure rate in the 1999). This represents a significant improvement in codeless GPS windfinding, but this failure rate is still too high for the codeless GPS system to be regarded as an ideal operational system. (Failure rates of radiosondes using other windfinding methods (e.g. Loran) are expected to be less than 5 per cent of flights attempted). Some of the variation is also due to differences in the criteria representatives may have applied to evaluate amounts of unacceptable data. The most accurate way of determining the amounts of unmeasured GPS wind data during a period is to determine the amount of winds flagged as interpolated in some versions of Vaisala. Hence combining wind gaps (usually periods longer than the default maximum of 4 minutes) and interpolated winds gives the total time during any ascent for which GPS wind measurements were unavailable. Comparing this (as a percentage) with the total flight time (time to burst or cessation of PTU data) gives the most accurate percentage of available wind data. As an example of this method of analysis, the Australian Bureau of Meteorology produce diagnostics of the GPS wind performance for each of their 7 manned GPS stations and 9 autosonde stations. Figure 3(a) shows an example of autosonde performance from an individual station spanning the two periods of the questionnaires. Note: In this Australian summary, the cases where a radiosonde failed to work and second radiosondes had to be launched are not taken into account.

4 - 4 - Fig. 3(a) Wind Finding Performance at Mt. Isa Autosonde Station, Australia. Fig. 3(b) Performance of Australian Manned and Unmanned GPS Radiosonde Stations Figure 3(b) shows the monthly variation in GPS windfinding performance in Australia for all stations during a similar period. This shows that the performance of the automated GPS windfinding systems in Australia is not significantly different to that from their manned sites. When Vaisala RS80 GPS radiosondes are launched from ships, navigation radar interference, inadequate launch facilities and less well-trained operators may result in less than optimum results. This can be seen in the poor relative performance of the ASAP system supported by the UK when compared with performance from three U.K. land stations, see Figure 4.

5 - 5 - Fig. 4 Comparison of missing winds from the UK ASAP and the three UK land stations using similar radiosonde systems. 3. REASONS FOR FAILURE Windfinding measurements from Vaisala codeless GPS radiosondes may be disrupted due to reasons such as: - (i) Electromagnetic interference from radar or other transmissions near the launch site. This can disrupt synchronisation of the GPS before launch. (ii) Instability in the GPS ASIC on the radiosonde, which was susceptible to changes in ambient relative humidity. This led to IF drift on the ASIC outside of the tolerances allowed by the ground system processing. Manufacture of the ASIC s was changed and also software was modified to reduce the number of failures. (iii) Sudden translation in the position of the radiosonde leading to loss of GPS synchronisation. This can be expected to occur more often if the radiosonde is not deployed on the full 60m suspension supplied by the manufacturer. This long suspension causes problem for launch in windy environments and is not acceptable as a long term solution for an operational radiosonde system (iv) Inability to synchronise reliably in regions where WAFS satellite transmissions are introduced. Here, it is essential to obtain the latest version of Vaisala software available and upgrade software every year. (v) Condensation on the GPS receiver board, if the radiosonde is prepared in air conditioned conditions and then taken out into hot and humid tropical conditions. The following sections are comments on some of the failure categories in the questionnaire: Failure to achieve GPS satellite constellation lock before launch The main cause of this problem with Vaisala radiosondes was probably the radiosonde IF drifting outside acceptable limits. Vaisala have implemented improvements in the production environment of GPS receivers to prevent IF drift caused by humidity changes. The 2000 statistics show a very significant decrease in the number of codeless GPS ascents failing to achieve GPS lock before launch. The reply to the questionnaire from the Philippines showed that the 24% failure rate of the Sippican system shown in Figure 2 was due to radiosondes rejected prior to launch. Sippican need to improve the reliability and ease of operation of their radiosondes prior to launch. 3.2 Failure To Produce Any Winds in an Ascent The statistics from the questionnaire relating to the Vaisala RS80 GPS radiosondes show that on 3.7 % of occasions, GPS radiosondes failed to produce any wind measurements after launch. There are several possible reasons for these total failures of which the most likely are: -

6 (i) (ii) Interference from radar or other local transmissions immediately before or during launch. Radar interference has been responsible for interfering with Vaisala RS80 GPS synchronisation at several recent tests in the UK and in Brazil. Condensation on the radiosonde GPS receiver board. During the recent WMO GPS Radiosonde Trial in Brazil (June 2001) RS80 GPS radiosondes failed to measure any GPS winds on several flights. Condensation onto the circuit board within the GPS receiver helix may become trapped within the polystyrene cover and cause failure in windfinding, when the radiosonde is moved from an air-conditioned environment into hot humid conditions to launch. (iii) Using a very old radiosonde or outdated software. The date of calibration of the RS80G radiosonde may be obtained from the first 3 numbers of the calibration tape. The first digit is the last digit of the year and the next two digits refer to the week in which the radiosonde was calibrated. (E.g was calibrated in the second week of 1999). GPS receivers on radiosondes calibrated before the third week of January 2000 were not produced in a humidity-controlled environment and this sometimes caused IF drift. Software versions prior to (July 1999) did not include expanded IF oscillator range to accommodate larger drifts. 3.3 Unacceptable Gaps in Wind Measurements in the Troposphere Changes have also been made to the Vaisala system to reduce the size of wind gaps so the system reacquires satellite data quicker after an outage. GPS data loss often occurs immediately after launch when the suspension unwinds and the radiosonde oscillates rapidly on the suspension. The availability of RS80 GPS wind measurements directly after launch has been monitored at UK stations for several years. Figure 5 shows that in mid-1999 there was a significant decrease in the time elapsing from launch to acquisition of the first measured wind. Fig 5. Time series of the time between launch and the first reported wind at two UK stations 3.4 Failure of Ground system There was no evidence to suggest that the (mainly) Digicora ground systems used by countries answering the questionnaire fail more often because they are being used for GPS rather than another form of windfinding. 4. CONCLUSIONS The failure rate of the Vaisala RS80 GPS radiosonde wind measurements has decreased significantly since the first survey was made in However, the globally observed 12% overall failure rate is still higher than desirable.

7 5. RECOMMENDATIONS i) Manufacturers need to continue to improve their GPS windfinding capabilities. Use of GPS ASICs capable of decoding the GPS signals, will allow rapid resynchronisation and provide reliable GPS winds, as was demonstrated by several manufacturers in the recent WMO GPS radiosonde test in Brazil. Several radiosonde manufacturers need to improve the quality of their production engineering to meet the challenge of supplying reliable GPS radiosondes. ii) Members need to consult the relevant manufacturer to identify when GPS radiosondes decoding the GPS signals will become available and make plans to change to the new systems. This should be soon. iii) Countries using the current Vaisala RS80 GPS radiosondes should ensure that ground system software is as up to date as possible. The manufacturer normally supplies software updates free of charge. Software versions prior to version 8303 especially should be upgraded. iv) Countries where interference from other transmissions is causing problems to the Vaisala RS80 GPS radiosonde (for example on ships) might also consider the possibility of upgrading to the Vaisala RS90 GPS radiosonde, which is less susceptible to this type of RF interference. 6. ACKNOWLEDGEMENTS The Rapporteur gratefully acknowledges the input from the representatives of the countries returning the answered questionnaires and further correspondence from Mr. Johan Van Der Merwe (South African Weather Service) and Mr Michael Joyce (Australian Bureau of Meteorology). The help of Mr Klaus Schulze and Ms Jennifer Best (WMO, Geneva) is also very much appreciated. *****