In Real Time: Conditions Influencing RT Positioning
To date, we’ve briefly discussed two of the “seven C’s” of best methods for real-time (RT) GNSS positioning from NOAA’s NGS in previous issues of POB. We’ll now move to the third “C”--Conditions.
The capable RT technician has a good knowledge of three sectors of the RT positioning environment. These areas are: satellite count and geometry; atmosphere; and multipath. Let’s discuss them in order.
To initialize, RT GNSS positioning requires at least five common satellites between the base and rover to account for the unknowns (X, Y, Z, time, and redundancy check of the solution). However, as shown from empirical evidence, it is recommended to have seven GPS satellites used in the initialization and in the location of important points--i.e., those used to establish additional data, such as project control densification or photo ID points. At this point, it should be noted that having the ability to use the current 23 GLONASS (GLN) satellites can yield the benefit of keeping you working where GPS alone might not provide the requisite five common satellites at a couple times during a 24-hour cycle or in locations of satellite obstruction at the rover. Currently, GLN will allow you to work where GPS alone might not, but it will not natively provide a better combined RT solution than GPS alone.
In addition to showing the number of satellites, a planning tool will allow you to view the dilution of precision (DOP) relating to the geometry of the satellites overhead. DOP is a unitless number that is an error multiplier of the user equivalent range error (UERE) to indicate how the solution may be affected by satellite location (see Figure 1). A DOP of “1” would mean there is ideal satellite geometry, and there is no error multiplier (a very rare condition indeed!). In addition to horizontal DOP, vertical DOP, time DOP, geometrical DOP and relative DOP, most rover data collectors display PDOP, which is position dilution of precision and is given by PDOP2 = HDOP2 + VDOP2. For highest precision RT work at the 95 percent confidence level, surveying with a PDOP of 2.0 or less is recommended (see Figure 2).
TEC varies with the changes of solar and geomagnetic conditions during the day, geographic location and season. The number of sunspots monitored directly relate to geomagnetic and other solar radiation storms, which may affect the ionosphere and thus the GNSS signal. Because these sunspot numbers are very cyclical and reach a maximum every 11 years, we can expect adverse conditions to increase as we move toward the cycle’s peak around 2013. The impact on GNSS signals will increase, resulting in more problems even at midlatitudes, which are typically not present during the benign times of the cycle. Because of this, we can expect communication problems between the base and rover or to the real-time network (RTN) and also an increase in solution noise, shown in the RMS values. In some cases, we may expect an inability to initialize at all for several hours.1
In single-base RT, the conditions are assumed to be the same at rover and base; thus, the rover will use the base corrections for both the ionospheric delay/advance and for the tropospheric delay at the base location. In a RTN, these corrections are interpolated to the actual site of the rover, which enables longer distances away from any actual base station. With dual-frequency GNSS receivers, frequency modeling can eliminate first order ionospheric error because the ionosphere is a dispersive medium, which means it affects the signals differently--in an inverse ratio to their frequency. Tropospheric (and orbit) errors are geometrical in that they are site specific, but have the same effect on all the frequencies.
Because even one nanosecond (one-billionth of a second) delay means 30 cm in range error, the rover must deal with this noise to resolve the correct ambiguity count, even with the signal being transmitted near the speed of light. Redundant observations of important points at staggered times (having different satellite geometry--typically after four hours) will provide different multipath conditions. This staggering of redundant observations is one of the NGS guidelines for achieving 95 percent confidence levels with your RT work.2
With proper planning and knowledge of how these conditions affect precision at the rover, the field technician can produce excellent results with a high degree of confidence using RT for her or his work. In the next column, I will discuss whether or not we should constrain our RT survey to passive marks around the project site.
Notes1. See NOAA’s Space Weather Prediction Center (SWPC) for extensive details on this subject at www.swpc.noaa.gov.
2. See the entire single-base guideline at www.ngs.noaa.gov/PUBS_LIB/NGSRealTimeUserGuidelines.v1.1.pdf.