In Real Time: Conditions Influencing RT Positioning
February 25, 2011
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.
1. Satellite Count and Geometry. The number of GNSS satellites available and their position overhead may affect the speed of ambiguity resolution (initialization to a fixed solution) and the statistical spread of the solution, as shown in the root mean square (RMS) value displayed in the data collector. Most GNSS manufacturers provide a planning tool (many are free stand-alone programs) to let the RT user take a look at the number of satellites (GPS and/or GLONASS) available during a user-selected time frame at a specified location.
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).
2. Atmosphere. Two major layers of the atmosphere affect RT positioning. The ionosphere, found approximately between 80 km and 300 km, has a quantity of free electrons bouncing around that cause a group signal delay and a phase advance. This electron count is monitored to produce a model called total electron content (TEC) and shows the total number of free electrons along the path between the satellite and GNSS receiver--usually at zenith.
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
The troposphere is the layer of our “weather” found up to around 80 km. The dry or hydrostatic component comprises 90 percent of the troposphere and can be well modeled (approximately 1 percent error). The wet component, as water vapor, is the other 10 percent, but cannot be easily modeled (10 percent to 20 percent error). Furthermore, the wet delay component variances are measured in the magnitude of tens of meters and in seconds, and it is extremely hard to isolate the errors associated with this component in adjustments.
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.
3. Multipath. Multipath is an error source that causes noise in the solution and sometimes incorrect ambiguity resolution. Because multipath is cyclical, it can be modeled with static GNSS sessions of even 20 minutes, but it remains unquantified due to the shorter occupations of a few seconds up to a few minutes for RT positioning. It is therefore incumbent upon the field technician to be aware of conditions that will lead to data of less precision and confidence. Multipath is simply the reception of reflected signal at the rover antenna. It can be specular--i.e., discretely reflected from a sign, building, pole, vehicle or other nearby object--or it can be diffuse--i.e., reflected from broad features, such as a lake, tree canopy or even road surface.
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.