This article is meant as an extension to our Brief Introduction to GPS and GLONASS blogpost. This article will focus on the various types of measurement biases and errors can be encountered; some of which is hardware instrument error and some user.
Hardware Error Types
Various GNSS receivers have their own particulars about how they function and whatever particular proprietary algorithms they may utilize. However, there are certain things users can do and be aware of when using a GNSS receiver to increase the chances they are achieving the maximum accuracy the conditions and equipment will allow.
Satellite Receiver Positional Errors
We will not go into detail about much of the instrument error, but these can include:
- Satellite clock error
- Receiver clock error
- Orbital (ephemeris) error
- Atmospheric error; Ionospheric delay and Tropospheric delay
- Multipath error
Users have no control over clock, orbital, and atmospheric error and no way to see or know these types of errors are taking place. Corrections for these types of errors can be achieved through various calculations and algorithms performed by GNSS receivers or the correction obtained by Differential GPS (a topic for another post). On the other hand, the multipath errors occurs right around the receiver.
Multipath error is an important type for users to understand. It is caused by reflections from nearby objects, ground, or water surfaces that redirect the satellite signal before it reaches the GNSS receiver. Taller objects such as buildings and canyon walls reflect and redirect the signal path, thus changing the distance the signal travels before reaching the GNSS receiver. In other words, multiple paths are taken by the signal from a satellite before it reaches the GNSS receiver. The correct path is the shorter path, but the GNSS receiver may not detect that and correct for that error. Multipath error isn’t a very big problem in a moving vehicle, but it can be when accuracy below a couple meters is necessary. Sometimes users can actually see this error is occurring as it can visualize as your position jumping around in your GIS app view, even though you are stationary.
Another error type that the user can detect is receiver clock error. If you notice that your receiver clock time is off by a few minutes or more, then it is likely there are clock or timing errors being introduced. We have only ever seen this a few times in thousands of field hours, but it can happen, especially with less expensive receivers. We find that cycling the power on the receiver usually resets the device and when you power it back on the time is corrected.
We have come across various datasets showing estimates of error types. All the tables vary some in error values since they are estimates and were created at various times over the years. So, we present the following table as an example of the amount of error each type can create and not meant as an absolute error amount. It is also a good description of how Differential GPS can greatly decrease location error. And of interesting note, across multiple error tables, they usually report the same error amounts for the Differential GPS (DGPS) category.
Summary of GPS Error Sources (per satellites)
|Error Source||Standard Handheld GPS (meters)||Differential GPS (meters)|
User and Software Errors
It’s All in How You Stand
It is important for users to remember the little things when collecting data with GNSS receivers. The receiver’s antennae requires “clear sky view”, which means there should be no obstacles between it and the satellites. Another important factor in this is trying to maximize the number of satellites the receiver has in clear view. If the user’s receiver has a sky view of 10 satellites from a hilltop, and then the user moves to the valley floor and finds the number of satellites has dropped to 6, that is because those 4 “missing” satellites were likely low on the horizon and are now blocked by the surrounding hills. There is nothing the user can do about solid earth blocking the view of satellites, however, simply facing south could add one or two of those missing satellites.
Remember that to a certain extent, our location in North America means there are more satellites to the south of our position, or towards the equator. Although, with the addition of GLONASS satellites, this has become less of an issue. However, the two WAAS satellites are usually south of the user’s position in the U.S., thus user’s can “help” maximize the chances of incorporating DGPS by facing the receiver south.
Why Doesn’t My Position Look Correct on the Aerial in My GIS App?
We have already discussed the problem with multipath errors. Let us use an example of working in a canyon. We want to record accurate positional data with our GNSS receiver, but find the device only has access to 4 satellites from this canyon location. We also see, that our position shown on our iPad’s mobile GIS app is either bouncing around a little or obviously out of place compared to the GIS app’s aerial imagery. This is a tricky situation, but there are a couple ways to “work around” this as long as the necessary accuracy only has to be within a few meters, like for non-build features such as natural resources like bird nests, rare plant species, or paleo and cultural assets.
Potential solutions include;
- Have an aerial imagery layer available on your GIS app, and then hand place your asset point based on your surroundings (i.e. tree, cliff, large rock, fence line, road, stream). However, there are inherent inaccuracies with orthorectified imagery (see HERE for a description). Another solution could be to;
- Have a topographic layer available on our GIS app, and then hand place your asset point based on your surroundings and the topo lines.
- If this canyon situation is a known problem before conducting your field survey, you could also revert to old-school methods and take a printed topo map with you and hand plot the points on paper and then digitize it back in the office.
Summary of GNSS User Best Practices
Use the best GNSS receiver you can afford.
At the start, allow your GNSS device to track the open sky for 15 minutes to adequately update the almanac and download the current DGPS corrections for real-time corrections that eliminates the need for post-processing.
Hold the receiver’s antenna out at arm's length and at head level while facing south, or attach it to a range pole or your hat
When walking to a new position to record a point, stop and hold out the receiver for at least a minute before recording the point so the location information can “catch up” to your new position
When walking around, keep your GNSS on and the receiver pointing at the sky at all times. Remember, if you place the receiver antenna in your pocket then when you take it out to record a new point it will need a few minutes to re-establish a connection with the satellites and acquire your position with any accuracy.
Record GNSS offsets if needed. Many times, “hand-placing” your asset based on the aerial image in your GIS app is better than walking under a large tree to record the point. Obviously, not recommended for work that requires submeter accuracy.
Multipath image credit EngineeringsALL.com