June 24 Webinar QA and ESRI Survey Summit/User Conference

July 8, 2010 By: Eric Gakstatter

Survey Scene, July 2010

On June 24, I conducted a webinar for Geospatial Solutions (GSS), a sister publication that I’m editor of. Many of the questions from the audience were GPS-specific and I thought they would be relevant to address in this newsletter. If you want to review the 60-minute webinar, you can view the archived webinar by clicking here.

Following are the GPS-related audience questions from the webinar:

Question #1: In your opinion, would a dual-frequency handheld device be useful for GIS applications?

Gakstatter: Yes. There are several dual-frequency handheld units available today. In many cases, they are used for GIS applications. However, they are relatively expensive. But, looking out several years, that is going to change. Dual-frequency receivers for GIS are going to become much less expensive than they are today. There is an old saying that accuracy is addictive. That stands true in GIS.

I suggest you might want to read the article I wrote last month entitled What’s Going to Happen When High-Accuracy GPS is Cheap? where I discuss this in more detail.

Question #2: For GPS, what are ideal conditions and what are the consequences of each non-ideal situation?

Gakstatter: Ideal conditions are when the GPS antenna has an unobstructed view (line-of-sight) to every GPS satellite visible at the particular time. Secondly, an ideal condition would be if there were no interruptions in satellite tracking — for example, if you placed the GPS antenna in your vest or in such a location that it couldn’t maintain continuous tracking of the GPS satellites.

For centimeter-level surveying GPS receivers, the above conditions are a must because they rely on constantly tracking the carrier phase. For code phase receivers (sub-meter accuracy and above), optimal conditions will produce the best accuracy. As conditions deteriorate, accurate will deteriorate.

There are generally three scenarios that will degrade GPS accuracy of code phase receivers.

1. Operating under tree canopy.

This is the most common scenario. The amount of accuracy degradation depends on the density of the tree canopy. It’s difficult to express because “heavy canopy” in one geographic region is not the same as “heavy canopy” in another geographic region.

Tree canopy has two primary effects on GPS signals. First, it can block one or more signals from being used. This will generally degrade satellite geometry (such as increase PDOP), which adversely affects accuracy. Secondly, it can create a multipath environment which degrades accuracy.

How much will accuracy degrade under tree canopy?

This is a tough question to answer. It depends on the quality of the receiver you are using and the severity of the tree canopy.

Even the highest quality code phase (submeter) GPS receivers can produce accuracy results in the 8+ meter range under heavy, dense tree canopy. Poorer quality GPS receivers can produce accuracy results in the 20-30 meter range under heavy tree canopy.

If the tree canopy is severe enough, dense and moist, it’s possible a GPS receiver won’t be able to track enough GPS satellites to calculate a position.

Finally, accuracy specifications from the GPS receiver manufacturer aren’t valid in non-ideal GPS conditions. Make sure you adjust your expectations accordingly. Test your GPS receiver in a typical tree canopy environment you’ll be working in.

2. Operating in rugged terrain.

The consequence of operating a GPS receiver in rugged terrain (eg. hills, mountains) is that a number of GPS satellite signals will be blocked by the terrain. This will likely reduce the quality of the satellite geometry and increase PDOP, which will degrade accuracy.

3. Operating next to tall buildings.

This will have the same effect as operating in rugged terrain, but is more severe because buildings will block a greater part of the sky and also have the potential to create a multipath environment.

Question #3: Will the future L5 signal availability still be impacted by heavy canopy multipath?

Gakstatter: Yes, but how much is still the question. The broadcast signal power of L5 is four times greater than L2C. This could help but we won't know until there are enough GPS satellites broadcasting L5, which is still several years in the future.

The most helpful would be more satellite signals. With GPS, GLONASS and Galileo all in full operation, that would be approximately 90 satellites which would mean about 25 satellites in view at any one time during the day as compared to an average of 6-10 GPS satellites in view.

Question #4: What about GNSS for mapping grade work?

Gakstatter: By GNSS, I think the person is referring to using satellites other than GPS (eg. GLONASS) for mapping. It's a good idea to used GLONASS satellites to augment GPS. The problem is that not much of the GPS infrastructure supports GLONASS. For example, SBAS (WAAS/EGNOS/MSAS) doesn't support GLONASS and neither do the DGPS systems (radiobeacon) around the world. Some RTK Networks support DGPS using GLONASS, but many don't. This means that if you want a real-time correction and you're not in range of a RTK Network that supports GLONASS, you'll have to setup your own base station to broadcast corrections to yourself.

On another note, I’ll be attending the ESRI Surveying and Engineering GIS Summit this weekend in San Diego as well as the ESRI User Conference next week at the same venue. I’ll be blogging, shooting video interviews and pictures that will be published on the Geospatial Solutions and GPS World websites.

Thanks and see you next week.

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