The U. Please report them to the responsible parties using the links in our Address, Route, and Map Problems section. Go there.
View document. The accuracy commitments do not apply to GPS devices, but rather to the signals transmitted in space. Actual performance is typically much better. To be clear, URE is not user accuracy. The ongoing GPS modernization program will further improve accuracy for civilian and military users. Learn more. The government distributes UTC as maintained by the U.
If you want to wait a year or two, this will find its way into the worldwide ecosystem and the Android fused location provider API, but we want to give you a chance for a one- to two-year lead by taking accurate measurements and turning them into accurate location. We want to work with you to accelerate development and bring the present closer to the future.
You might wonder, why do I need better location accuracy anyway? Even for the most loved outdoor applications such as map directions and finding alternate routes in traffic, we could benefit from higher accuracy than we have now. For example, when you came here this morning in a car, you probably had your arrival time estimated using the average speed of the traffic. Wi-Fi RTT ranging and indoor position estimation is based on making measurements of the time of flight of RF signals, and can be used to estimate your indoor position to an accuracy of one to two meters.
Before we get into the details of Wi-Fi RTT, we want to tell you how we currently calculate an indoor location. Basically, we can calculate distance as a function of signal strength.
Figure 1, with the access point in the center, shows a heat map of the signal strength around a Wi-Fi access point AP. Figure 1. Figure 2. Wi-Fi RTT principles, basic concept. Figure 3. Wi-Fi RTT principles in practice. Image: authors. The green is the strongest signal, near the AP and the red is the weakest, measured toward the edges.
Notice that the phone on the right is further away from the access point than the phone on the left. The signal strength can therefore vary at the same distance, which unfortunately makes it very hard to make accurate range measurements based on this type of measurement.
There are lots of algorithms and tricks that can be used to improve this, but the greatest improvement can be achieved using a new Wi-Fi technology.
It uses time-of-flight instead of signal strength. It measures the time it takes to send a Wi-Fi RF packet from an access point to a phone and back again. Because radio signals travel at the same speed as visible light, if we multiply the total round-trip time of a Wi-Fi packet by the speed of light and divide by two, we get distance, and therefore the range from the phone to the access point. If you want to use several ranges to nearby access points to calculate your position, we have to use a process called multi-lateration.
The key thing to think about here is that the more ranges you have, the more accurate the position you can estimate. If you can use at least four ranges, then we think you can achieve a location accuracy of about one to two meters in most buildings. Why not last year or before? So, in the near future, any time there are RTT access points in the vicinity of a phone, the estimated position accuracy will be greater.
The By the fall of this year, we will release the public API so that you can all have access to this capability and can build your own applications around the technology. The ranging process starts with a standard Wi-Fi scan. The phone discovers the access points that are nearby, and, based on certain bits in information elements IEs contained in the Wi-Fi beacons and the probe responses, we can figure out which of those access points are RTT-capable, and the phone can choose one of them to range to.
It starts by making a request to the access point; as a result, the access point will start a ping-pong protocol in response. The ping sent to the phone is called a fine timing measurement FTM packet, and the pong sent back to the access point is an acknowledgment of that packet.
The arrival and departure time stamps are recorded at each end of the transaction, but for the phone to calculate the total round-trip time, it needs to have all four of those times. So the access point sends one more packet to the phone, and this third message contains the missing times. The phone then simply calculates the round-trip time by subtracting the time stamps from the AP, and subtracting its own packet turnaround timestamps.
The difference between these times leaves just the packet time-of-flight. Now, imagine drawing a circle or ellipse that encompasses about 95 percent of the points.
What would the radius of that circle or ellipse be? In other words, what is your receiver's positioning error? The answer depends in part on your receiver. If you used a very low cost GPS receiver, the radius of the circle you drew might be as much as ten meters to capture 95 percent of the points. But, if you were to invest several thousand dollars in a dual frequency, survey-grade receiver, your error circle radius might be as small as a centimeter or less. In general, GPS users get what they pay for.
As the market for GPS positioning grows, receivers are becoming cheaper. Still, there are lots of mapping applications for which it's not practical to use a survey-grade unit.
For example, if your assignment was to GPS 1, manholes for your municipality, you probably wouldn't want to set up and calibrate a survey-grade receiver 1, times. How, then, can you minimize errors associated with mapping-grade receivers? Factor 1. The Position of Satellites Satellites are the basic elements of positioning. Table 1. Factor 3. The Signal Effect from Surrounding Environment Since the signals from satellites to GPS receivers need to travel a long way, the propagation environment affects the signal strength and positioning correctness a lot.
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