Comparison of Signal Levels from GPS and WAAS

Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
jimh
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Comparison of Signal Levels from GPS and WAAS

Postby jimh » Sat Feb 06, 2016 10:57 pm

The signal levels provided to users' receivers from the Global Positioning System (GPS) and the Wide Area Augmentation System (WAAS) are specified in formal documents from each system provider, the U.S. Air Force (USAF) and the U.S. Federal Aeronautics Administration (FAA), respectively

For GPS, the interface specification document lists the minimum signal levels delivered as -158.5dBW for the L1 Coarse/Acquisition signal from Group II R/M/F satellites. See Chart 3-Va in http://www.gps.gov/technical/icwg/IS-GPS-200H.pdf

For WAAS, the FAA specifies the user signal level to be the same, -158.5dBW. See section 3.3.1.6 User-Received Signal Levels in http://www.gps.gov/technical/ps/2008-WAAS-performance-standard.pdf

This suggests that reception of the WAAS space-based augmentation signal (SBAS) will be at a level equivalent to the GPS L1 C/A signal. This only makes sense. For purposes of obtaining an augmented precision position fix, the WAAS signal is of little value if the GPS signal cannot be received, and vice-versa. This also suggests that if your GPS with WAAS receiver can receive signals from GPS satellites, it should also be able to receive signals from WAAS satellites.

There is one very significant difference in coverage: GPS coverage is global; WAAS coverage is only for North America (including Alaska and Hawaii).

jimh
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Re: Comparison of Signal Levels from GPS and WAAS

Postby jimh » Wed Feb 17, 2016 1:01 pm

As a corollary to signal strength, the look-angle or elevation from your location to a GPS and WAAS satellite is also a parameter to be compared. In general, the higher the elevation to the signal source, the shorter the path and the stronger the signal. However, it is common in GNSS receivers that the antenna has a gain pattern that compensates for this somewhat, that is, the GNSS receiver antenna may have more gain at lower elevation angles than at 90-degrees. Higher elevation angles generally avoid terrestrial obstructions or other local obstructions in the antenna's view of the sky.

For GPS satellites, their orbital inclination is always 55-degrees. As long as your location is in latitude between 55-degrees-N and 55-degrees-S there is a possibility that a GPS satellite could pass overhead of your location. Whether or not this will occur depends entirely on your location and the time. There might be a satellite orbit that just happens to pass overhead at some particular combination of place and time where you are located. So one could say that the elevation to a GPS satellite can vary from as high as overhead or 90-degrees elevation to as low as on the horizon, or 0-degrees elevation. As a general rule there should be at least four satellites in view at any time from any location. I don't know of any way to predict what the elevation would be to those satellites. There is certainly some probability that at a specific place and time there might be no GPS satellites in view with elevations higher than 30-degrees.

For the three WAAS satellites, their orbits are geo-stationary and are at the equator. Unless you are at a specific location on the equator directly under a WAAS satellite, it will never be overhead. The elevation to a WAAS satellite will depend on your location, with the elevation angle decreasing as latitude increases. Elevation also will vary with longitude, with the highest elevations occurring when you are on the satellite's meridian. For the contiguous states of the USA, there is always a WAAS satellite in view with an elevation of at least 30-degrees, except in New England. Also, no WAAS satellite will have an elevation of more than about 55-degrees in the 48-states. This is shown below in two graphics:

Image
Elevation to WAAS: Solid line is PRN138; dashed line is PRN135

Image
Elevation to WAAS PRN133

Since there are generally always three WAAS satellites in view over most of North America, and generally the elevation to them is always higher than 30-degrees, there is no reason to think that WAAS satellites will be available less often than GPS satellites. Further, it takes four GPS satellites to get a three-dimension position fix, but it only takes one WAAS satellite to get the correction information to enhance the precision of the position solution.

If you have curiosity about the creation of those graphics, I explain the process in more detail in the article they originally appeared in.

jimh
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Re: Comparison of Signal Levels from GPS and WAAS

Postby jimh » Fri Feb 19, 2016 10:28 am

It is quite possible that someone could offer anecdotal evidence that WAAS reception was not as good as GPS reception. I can. In the instance I recall from my own experience, I noted that the orientation of my chart plotter with its internal GPS receiver happened to be such that the metal frame of the boat windshield was obstructing the view toward the south at the precise angle the WAAS signals would be arriving. A short time later, when the boat heading changed, the WAAS signals were received. I have to note that the metal frame of the windshield of my boat will block any satellite whose signal's path happens to intersect that obstruction, not just the WAAS signals, that is, all satellite signals are susceptible to being blocked by an obstruction, either a terrain obstruction or something close to the GPS receiver's antenna, and not just WAAS signals are affected by obstructions.

It is also possible that a particular make or model or generation of GPS receiver might have some limitations on WAAS reception or a bias toward concentrating on GPS ranging signals over WAAS correction signals. The amount of electronics and mathematical manipulation occurring in a GPS receiver is really amazing, and it is entirely possible that due to limitations of processor capability that a particular GPS receiver might be designed so that it dedicates most of its resources to tracking GPS satellite ranging signals and calculating a position fix, and only looks for WAAS signals and tries to apply their corrections as a secondary process. I suspect that is actually a good strategy for a GPS receiver to follow, as a WAAS correction signal is of little value until a GPS position fix has been achieved. This may be reflected in a situation where the GPS receiver is trying to track marginal signals from the GPS constellation, and achieving a position solution is consuming all of the resources in the receiver. The receiver may be trying to get good tracking an a number of marginal signals besides the initial three or four it used to solve for a 2D or 3D position. This is also a good strategy for a GNSS receiver because the signals in view are constantly changing. Once a receiver tracks enough satellites to get a fix, it usually tries to acquire more satellites so that it can maintain a position fix when one or more of the initial satellites it acquired moves out of view.

The computational power of modern GPS receivers is often marketed by noting the number of "channels" they have. That is a confusing term, but I believe it is used as a way to describe the number of simultaneous tracking loop processes the GPS receiver can apply to the signals it is receiving. Typically for each individual PRN there will be at least three "channels" in use in the tracking process. Since four satellites must be tracked to get a three-dimension position, the minimum number of "channels" is 3 x 4 = 12. Many earlier generation marine chart plotters were sold with GPS receivers that were only 16-channel, which suggests that the maximum number of satellites that could be tracked simultaneously was about five. More modern GPS receivers are now available with greater than 60 channels and can simultaneously track ten or more satellites. An older GPS receiver with barely enough channel capacity to get a GPS fix might not have the same resources for acquiring WAAS signals as a modern receiver with much greater channel capacity. The use of older receivers might also cause some anecdotal reporting of lack of WAAS reception. These limitations of older receivers should not be confused as an indicator that WAAS signals are weaker or more difficult to receive or more prone to being blocked by obstructions than the signals from GPS satellites. If any of these were really true, they would represent substantial flaws in the design and implementation of the WAAS program. Inasmuch as WAAS was intended to provide assistance to commercial passenger-carrying aircraft in making their navigation more precise, it would seem unlikely that flaws in the design of this GPS augmentation system could be overlooked and the system implemented without awareness of such flaws, and that such flaws (if they actually existed) would remain undetected by the entire GPS and WAAS community, only to be discovered by a couple of recreational boaters.