GIAA Mailbag: September 2009
Q: What is the connection between solar sunspot activity and total electron content, and why should it concern me as a surveyor who uses GPS?
A: Total electron content (TEC) describes one of the most important aspects of the ionosphere as it relates to the transmission of GNSS signals. TEC is a measure of the number of electrons in the path between two points. The unit is TECU (total electron content unit), where 1 TECU is 1016 electrons per square meter. The accurate determination of this measure of ionization of the ionosphere is important in modeling delays in GNSS signals.
Solar activity has a substantial impact on TEC. Magnetic activity in the sun creates sunspots, and these sunspots generate solar storms that can last from hours to days in length. The creation of sunspots and the magnitude of related disturbances in the Earth’s ionosphere is cyclic, extending over 11 years. We have just seen the end of a solar cycle corresponding to what is known as a solar minimum, i.e., a relatively low period of sunspot activity and thus low ioniospheric disturbance. High TEC can cause increased delays of the carrier phase of GNSS signals. It can also cause unpredictability in how the ionoshphere behaves, which usually causes greater uncertainty in positioning, especially when receivers are used autonomously. While this effect can be attenuated somewhat using differential and dual-frequency techniques, these approaches are by no means a panacea. Studies have shown accuracy reductions of over an order of magnitude during periods of intense activity. During such times, it can be difficult or even impossible for receivers to acquire GNSS signals.
Even though we have just experienced a solar minimum, users are advised to monitor current levels of TEC and get updates on the predictions for the next solar cycle. Typically, the greatest period of ionization occurs after the peak of sunspot activity has occurred. The National Oceanic and Atmospheric Administration (NOAA) generally posts predictions on its Web site. Measured and predicted electron content information can also be obtained through the Jet Propulsion Laboratory, and many other sites providing this information can be found by searching for “space weather” in your favorite search engine.
Space weather and its effect on the transmission of GNSS signals through the ionosphere is an important consideration for any surveyor regardless of whether solar activity is predicted to be high or low with respect to the overall 11-year cycle. When the ionosphere is particularly disturbed, surveyors should first perform evaluative surveys to determine whether to refrain from surveying with GNSS or to supplement their GNSS measurements with other techniques to improve accuracy.
Q: When I was first trained to use GPS, we began by learning to use mission planning software. Now, however, I notice that people rarely access this feature. Is mission planning an activity that is no longer needed now that GPS is mature?
A: When the use of GNSS (which started with GPS) began, the number of usable satellites on orbit was relatively low. Since a minimum of four satellites had to be observed simultaneously by the receivers at each end of each baseline for static observations, rigorous mission planning was needed to minimize effort. As the number of satellites has increased to 31 for GPS and 18 for Glonass (as of this writing), the need to ensure that there will be satellite signals to be received has greatly reduced. However, mission planning remains a good practice for a number of reasons, and it will continue to be beneficial even if all 21 planned satellites for Glonass and the 30 planned satellites for Galileo are eventually operational.
Regardless of whether the activity planned is static, kinematic or differential (real time or post processed), one needs to be certain that the minimum number of satellites for the type of GNSS activity planned will be visible to the station (or stations) considered to be the base and every other receiver, whether fixed or moving (see Figure 1). Note that this minimum-number requirement is for the same satellites to be observed simultaneously at the base and point(s) of interest during the time the observation is made.
A further factor is that we have become spoiled with the 31 GPS satellites currently available. It is inevitable that the number of operational satellites will drop in the future. The service commitment of the United States Department of Defense is to have a 95 percent certainty that 24 satellites will be operational at any given time. If the number of satellites drops,* it is likely that surveyors will see “outages” of service due to factors such as higher than acceptable dilution of precision (DOP) values and fewer than the minimum number of satellites visible.
Aside from limitations due to the number of satellites on orbit, the physical features of the site being surveyed can add to the likelihood of outages. Problems can occur when the terrain (canyons and hills) limits horizon-to-horizon access to the sky or when natural and artificial objects near the receiver antennas (such as buildings and trees) block the view to certain portions of the sky. For the base station and survey points used in static observations, an actual assessment should be conducted at each site. You should also meaure the azimuth and vertical angles to objects such as hills and buildings so that your mission planning software can reflect those “curtains” blocking parts of the sky. Such measurements can be rapidly done with a compass and Abney level.
More problematic are the same issues affecting GPS signal reception when using kinematic techniques. It is usually not possible to evaluate every location on the site where measurements will be made, but a good estimate can still be done by physically visiting the site and making an evaluation of the shadowing that may occur. Additionally, it is always a good idea to have a backup plan (and instrumentation) to capture any locations that cannot be surveyed using GNSS technology.
* For a report on the likelihood of this scenario, see “Global Positioning System: Significant Challenges in Sustaining and Upgrading Widely Used Capabilities,” Government Accountability Office, April 30, 2009, www.gao.gov/new.items/d09325.pdf.
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