Q. What's the effect of thunderstorms on the GPS signal? Can GPS be used in the rain or in snow or sleet? -T.R. Muncie, Ind.

A. John C. Bohlke, Sokkia Corp.: The lightning associated with thunderstorms may cause cycle slips or excessive signal noise, and other weather elements existing in the troposphere delay the propagation of GPS signals thereby affecting the accuracy of the measurements. However the temperature, barometric pressure and humidity may be used to model the delay and subsequently nullify the troposphere effects. Rain, snow and sleet cause some multipath effects that may degrade the accuracy but antenna ground planes and longer observation times help to eliminate the multipath errors.

Wendy Corcoran, NovAtel Communications: Electrically, thunderstorms have little effect on a GPS receiver's operation but the thunderstorms could affect the troposphere corrections when measuring GPS baselines. If a thunderstorm is local or isolated to a small area, the weather at the base station GPS receiver can be significantly different than at the remote GPS receiver. It is assumed, when processing differentially, that weather effects are similar at the remote and base and therefore cancel to some degree. If the meteorological conditions are different at both stations, this could cause some error in the data processing. Meteorological data would have to be collected for each GPS receiver and a tropospheric model would have to be applied during the processing in order to compensate for the different conditions. GPS can be used in rain, snow or sleet if the equipment is water resistant and the precipitation does not accumulate on the antenna surface.

Arthur Lange, Trimble Navigation: There are a number of possible effects of thunderstorms on GPS signals and GPS receivers. The largest effect is caused by a direct or near-miss lightning strike, which could permanently damage the GPS receiver. To protect GPS receivers in fixed installations, lightning protection is recommended. A description of how to protect a base station in locations with possible lightning activity is available from the Trimble Faxback system (dial 408-481-7704, and request document number 3034).
      The second possible effect of lightning is from the electromagnetic noise generated by the multitude of lightning strikes in a nearby thunderstorm interfering with the differential GPS radio link. The low frequency U.S. Coast Guard beacon transmissions are subject to interference, unless a magnetic loop antenna is used on the beacon receiver. There are no measurable effects on the GPS receiver or GPS signal of nearby (but not too close!) lightning strikes.
      Rain, sleet or snow in the air between the GPS antenna and a satellite will normally have no measurable effect on the GPS signal. Snow or ice, however, may pile up on a GPS antenna and absorb some of the GPS signal reaching the GPS antenna.

William Martin, Ashtech Inc.: Thunderstorms can have an adverse effect on the GPS signal, not due to lightning but to the increased moisture content in the atmosphere. Atmospheric moisture content adversely affects the speed at which the signal passes from the GPS satellite to your antenna. In theory, the signal travels at the speed of light. In practice, this is only true in a vacuum. As we all know, the tropospheric portion of the Earth's atmosphere (the portion ranging from the surface up to 9-16 km) is not a vacuum. We would all have trouble breathing if it was. Therefore, as the GPS signal passes through the troposphere, it is delayed slightly. One of the elements in the troposphere delaying the GPS signal is moisture.
      In an effort to compensate for the tropospheric delay of the GPS signal, the effect of the troposphere has been modeled. Several models exist and are used by GPS systems (hardware and software) to minimize the error caused by the signal delay. Unfortunately, these models only approximate the effect of the troposphere on the GPS signal since they are based on average conditions.
      On a rainy day, the delay of the GPS signal, as it passes through the troposphere, will be greater than the model can correct for. The residual tropospheric effect will contaminate the data collected. If the rain is consistent throughout the area in which the GPS data is being collected, all GPS receivers will encounter basically the same residual tropospheric error. With the same residual tropospheric effect in all of the data sets, the buld of the error will cancel out during processing of the GPS data resulting in very little if any apparent effect on the accuracy of the survey.
      The problem arises when thunderstorms produce rain in one part of the project area while other parts have blue skies. In this case, some data sets will contain the residual tropospheric error while others will not. Processing this data together may result in degraded accuracies.
      In practice, do not be overly concerned about the weather when using GPS. GPS is designed as an all weather system. One word of caution. If you insist on collecting data during a thunderstorm, stand clear of your equipment. That metal antenna sitting on top of a tripod or pole makes a great lightning rod.

Joan Yau, Leica Canada Inc.: Thunderstorms will affect GPS observation and signal reception due to high electromagnetic activities in the atmosphere during such conditions. GPS receivers intended for outdoor use are built to operate in severe conditions. So rain or snow or sleet will not cause interference to the GPS signal except causing inconvenience to the GPS operator.

Q. What is NMEA? What is it used for? By whom? Why? - M.Y. Boise, Id.

A. Bohlke: The National Marine Electronics Association developed this standard for interfacing with marine electronic devices. Some GPS receivers use the NMEA interface, like the RTCM real-time DGPS interface, for a variety of applications. An NMEA output can be used to externally store the receiver's position, velocity of heading or, the GPS data can be sent to other devices to enhance their capabilities. Some examples include outputting the receiver's position to an RTCM receiver for improving the differential correction or "stamping" a digital photo with an accurate GPS position. Users of real-time DGPS systems benefit from using an NMEA output through increased accuracy. Aerial photographers automatically label their photographs with an exact location for improved accuracy and efficiency. GPS applications that require an operator to take pictures of buildings use an NMEA output to label the photos with a "GPS address" thereby integrating their data colelction procedures.

Corcoran: NMEA stands for National Marine Electronics Association. The NMEA group develops standards for compatibility of equipment and communications in the marine and navigation industry. The 0183 standards define signal requirement, data transmission protocol and timing and ASCII output formats for a 4800 baud serial data bus. Most GPS receivers adopt this protocol so that various equipment used in the marine industry will be compatible.

Lange: NMEA stands for National Marine Electronics Association. The NMEA 0183 Interface Standard defines electrical signal requirements, data transmission protocol and timing, and specific sentence formats for a 4800 serial data bus. The latest version is version 2.02, and can be obtained from the National Marine Electronics Association at 410-263-1742.
      Normally, only marine GPS receivers are fully compliant with the NMEA 0183 Standard, and these receivers provide plug-and-play compatibility with other marine instruments. The NMEA 0183 standard calls for an electrical bus compatible with EIA-422. However, many resource grade GPS receivers use an EIA-232 serial data port, in order to be compatible with personal computer serial ports. While NMEA 0183 calls for 4800 baud timing, many GPS receivers are capable of a wide range of baud rates, both faster and slower, making them more likely to be interfaced with other, non-marine instruments. GPS receivers that mention NMEA compatibility on their data sheets usually refer to the ability to output NMEA sentences. Some of the NMEA sentences most used by GPS receivers are GLL, GGA, RMC, VTG, and ZDA. These sentences contain latitude, longitude, speed, heading, time and other related information. An important piece of information that is missing in these NMEA sentences is the datum used to compute the latitude and longitude, so it is possible to have large errors when using a GPS receiver with NMEA output when the datum that the receiver is set to a different datum than the datum of users map or GIS data set.

Martin: NMEA is an acronym for National Marine Electronics Association. NMEA, the organization, set out to develop a 'standard for interfacing marine electronic equipment.' This standard, in its most recent form, is NMEA 0183 Version 2.01.
      The standard is used primarily to facilitate data communication between electronic marine instruments, navigation equipment, and communications equipment when interconnected via an appropriate system. The GPS industry adopted the standard since it seemed to meet their needs for communication between a GPS receiver and an external electronic device.
      There exist approximately 50 pre-defined NMEA messages designed to supply many different types of information. Each message has a three character identifier. For example, the GGL message will provide the current latitude and longitude, the GSV message will output information on the GPS satellites in view.
      NMEA messages are used when a secondary electronic device needs information from the GPS receiver. This device can be anything from a desktop computer to a aircraft flight navigation system to a road grader.

Yau: NMEA stands for National Marine Electronics Association. NMEA sentences are essential as they provide the link of GPS position with depth readings as in hydrographic surveys. These sentences mainly contain position and time information of the GPS rover unit in ASCII format and are output via RS232 serial interface to other systems, sensors or devices such as echo sounders.

About the participants:
John C. Bohlke serves as GPS support manager at Sokkia Corp. in Overland Park, Kan. He may be reached at 913-492-4900 or 800-4-SOKKIA in the U.S.
Wendy Corcoran is manager, survey and mapping products at NovAtel Communications, Ltd. in Calgary, Alberta, Canada. She may be reached at 403-295-4789.
Arthur Lange is the GIS product manager for Trimble Navigation in Sunnyvale, Calif. He may be reached at 408-481-2994.
William Martin is marketing manager, survey products at Ashtech Inc. in Sunnyvale, Calif. He may be reached at 408-524-1507.
Joan Yau works for Leica Canada Inc., in Richmond, B.C. She may be reached at 604-278-3898.

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