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| Perspectives — October 2007 (Special Product Review Issue!)
ProMark 3 RTK You might recall that in last December's issue of GPS World, I wrote the Survey/Construction section of Directions 2007. The article was my attempt at predicting the GNSS trends in the survey/construction industry for the year. I gave L1 RTK the distinction of being the most interesting survey/construction innovation for 2007. Since that time, several L1 RTK products have been introduced. The interesting part about L1 RTK is that it's not a new technology. The concept has been around for years. In fact, if I recall correctly, there's been at least one L1 RTK product in the past. What is new, and will possibly make the difference needed to succeed, is the ingenious use of SBAS (yes, WAAS/MSAS/EGNOS) to aid L1 RTK. One of the companies that introduced an L1 RTK system this year is Magellan. Magellan has been the market leader for L1 post-processing GPS units for a number of years in the surveying/construction industry. So, it makes a lot of sense that it would be one of the pioneers in L1 RTK. Furthermore, Magellan built its L1 RTK product from the company's proven ProMark 3 handheld. ProMark3 RTK The first thing to understand about a GPS surveying system is that it's not just about the GPS hardware. It takes some pretty complex software to make GPS a useful tool for surveying. Magellan (formerly Ashtech) has a long history and extensive experience in producing such hardware and software for the surveying industry. The ProMark3 RTK (PM3 RTK) I tested shipped with the GNSS Solutions Ver. 2.50.06 software on the computer side of things, and Magellan's Survey Ver. 2.01 on the PM3 RTK unit. Also pre-installed was Magellan's Fast Survey Ver. 2.1.0 (built on Carlson Software's SurvCE) for "Advanced RTK" operations. Magellan recommends you purchase Fast Survey with the PM3 RTK; I concur. Magellan's Survey Ver. 2.01 software is fine for dinking around, but not for serious production work. The ProMark 3 has been around for a while, minus the RTK feature. The PM3 is a proven platform, so it's not as though the company is introducing an entirely new system here. For those of you aren't familiar with the PM3, it's a handheld unit with the receiver built inside. The only item outside of the unit is the antenna—and for RTK, the radio. In addition to RTK, the PM3 RTK also has functions for submeter GIS mapping, as well as a built-in highway database for general navigation. With the Fast Survey option installed, the PM3 RTK can also function as a data collector for supported total stations. Supported languages include English, French, German, Spanish, Italian, Portugese, Finnish, Swedish, Dutch, Russian and Chinese. What's In the Box:
I have to tell you that I was paying particular attention to the documentation. I presumed it was going to be a little light on the PM3 RTK system because I've spent many years in the product development business, and quality documentation has a tendency to lag behind product release. While it was true for the PM3 RTK CD included, I found the Magellan FTP site (ftp://ftp.magellangps.com/) a tremendous resource for downloading the up-to-date documentation, firmware and application notes. There are a gazillion documents that are a goldmine of information for current PM3 RTK users, as well as those considering the unit. One final note on documentation: For those considering the PM3 RTK, I'd suggest downloading the PM3 RTK white paper from Magellan's FTP site. It is located in the Application Notes folder. It presents PM3 RTK test data (albeit published by Magellan) and lays out what the PM3 RTK is capable of—and, more importantly, what it's not capable of. It's an easy read and only 14 pages long, so you can digest the contents in less than an hour. Setting Up The entire system (minus the tripod, tribrach and rangepole) is packed in three small canvas bags. Take it from a guy who takes his RTK system on the road (and through airports) a lot: this system could easily be taken as carry-on baggage. Although most of you won't take it on an airplane, that should give you a good idea of how portable it is. For those who prefer a hard case, all of the equipment could fit in a medium-sized Pelican case. Unpacking the box for the first time was a little intimidating; there seemed to be a lot of parts and pieces to assemble, mostly related to the tripod brackets. But once I began to put together the obvious pieces and cables, the system came together quickly. I had both the rover and base assembled in about fifteen minutes without looking at the cheat-sheets. The antenna threads and tripod brackets are standard size, so I was able to use my tripod, tribrach and rangepole without a problem. Overall, setup wasn't as painful as I initially thought it would be. Ergonomics
The only ergonomic disadvantage is the cables: there's an antenna cable and a radio cable. But, honestly, it didn't bother me because I'm used to cables. Someone who has already adapted to cable-less Bluetooth connectivity, however, may find offense in stepping back to cabled connections. The nice part about the system is that the PM3 battery powers both the PM3 and the radio—but you pay for it in battery life. You'll only get six hours on a charge when using both the PM3 and radio, and then you'll have to go with an external battery. Software In terms of operations, the difference between an L1 RTK system and an L1-L2 RTK system isn't going to lie in the data collection software, so data collection processes for staking, topo, etc. are the same as with an L1-L2 RTK. For those tasks, Magellan has chosen to offer Fast Survey 2.1.0. It includes a full suite of RTK functions for staking and topo using RTK:
Fast Survey also supports standard and local-based map projections, as well as hi-res geoid models. Here are a couple of sample screens from Fast Survey:
Other GPS-related functions supported b the PM3 RTK and GNSS Solutions PC software are:
Initialization and Re-initialization (Fixed Solution) This is where the rubber hits the road for measuring the practical use of PM3 RTK versus traditional L1-L2 RTK for locating and staking. Experienced users of modern L1-L2 RTK gear have come to expect fast OTF (on the fly) initializations and re-initialization—typically just a few seconds. In contrast, the PM3 RTK takes up to several minutes to arrive at a fixed solution before you can start working. It supports four different methods of initialization: 1. Known point. This is one of the fastest methods. You occupy a known point, which can be several kilometers away, and hold the rover stationary over the point for about 15 seconds. 2.Initialization bar. You mount this device on the base station tripod and place the rover antenna on it. You don't need to know the coordinates of the point, because the baseline distance on the initialization bar is precisely known. I didn't try this method and don't recommend it. By the time you mess around with finding and setting up the initialization bar, you could have used the OTF method and be initialized already. The manual states the initialization time for this method is 5 minutes. 3. Static on the fly (OTF). This is my preferred method; I use it on my L1-L2 RTK system. You don't have to think too much about it. Just turn it on and leave it alone while you're getting other stuff ready (prepping stakes, checking supplies, etc.). The rover doesn't have to be on a known point, it just can't be moving around. 4. On the fly (OTF). I also use this method a lot with my L1-L2 RTK system. If I need to walk two or three hundred yards from the base to begin my work, by the time I arrive at my starting point, it's either initialized or very close to initializing. The difference between this and Method #3 is that you can be moving. It will take longer than Method #3, but if the circumstances are right (e.g., you need to walk a distance before starting the project), you won't notice. As I mentioned, historically I prefer methods #3 and #4 with my L1-L2 RTK system. With the PM3 RTK, I think #3 is the best. But it's really a matter of preference. #1 and #2 will work just fine if that's what you're comfortable with. Keep in mind, standard RTK protocol is applicable. The base station needs to be set up with a clear view of the sky, not next to buildings, drip lines, etc. Also, for initialization, the rover should be in a similar "open sky" area. Results I tried #3 at two different times during a typical workday. One was the best time (most satellites in view) and one was the worst (fewest in view). These were around 4:00 pm and 8:00 am PST, respectively. During the best part of the day, I had a PDOP of 1.9 and the two WAAS geostationary satellites were in view. The PM3 RTK initialization times were consistently in the 2.5-minute range with about a 75-foot baseline (distance from the base). During the worst time of the day, the PDOP was in the mid-2s and, of course, the WAAS geostationary satellites were in view. The PM3 RTK initialization times were hovering around 3.5 minutes with the same baseline. Not bad; I expected longer. Again, this is using the Static OTF method. Initialization Using SBAS (WAAS/MSAS/EGNOS) GEO Satellites Magellan has employed a method of using SBAS (WAAS/MSAS/EGNOS) satellites to assist in L1-RTK initialization. Believe it or not, they use measurements (pseudo-range and carrier phase) from the WAAS geostationary satellites. Sounds a little funky at first, but it works. I confirmed this with a research scientist at another GPS company who said he's done research on the same subject and confirmed the concept is solid. Essentially, you get two extra satellites to use, and this helps speed up initialization. At the Institute of Navigation (ION) conference last week, I attended a technical presentation where a Magellan research scientist presented on this subject. The Caveats In order to utilize SBAS GEOs, you must have a clear line of sight to them. The two WAAS geo satellites are located above the equator (0 latitude) at 107 W longitude and 133 W longitude. From my location in the south suburbs of the Portland, Oregon, area, I had a clear view of both of the WAAS GEO satellites (they are at approximately 35 and 37 degrees above the horizon, respectively). So my region and local conditions (clear sky) were ideal for the PM3 RTK. Given that, I wouldn't expect the initialization times to get much better than what I experienced, unless you want to use the initialization bar or start on a known point.
So, this begs the question of users who are outside of the WAAS coverage area (Asia, Europe, Africa).
Lastly, India's GAGAN program, another SBAS system, began broadcasting test data earlier this year. I've heard it was unstable at first, but now it may have stabilized and could be useful to PM3 RTK users. This is very unofficial information on GAGAN, so please take it as such. Either way, the future of SBAS systems is bright, and will benefit PM3 RTK users. Who would have thought that SBAS would ever be used to assist in RTK? It sounds similar to what we use GLONASS for. In fact, I spoke to a Magellan GNSS scientist and he said just that; using SBAS GEOs for aiding RTK is very similar to using GLONASS satellites for aiding RTK. It will be interesting to see if other GPS manufacturers follow suit and adopt this method, even for L1-L2 RTK. Another two or three satellites can only help. Other Observations The PM3 RTK rover can be used in an RTK network just like an L1-L2 RTK rover. It has Bluetooth connectivity, so you can wirelessly link to a mobile phone and receive network RTK corrections. Please note that RTK networks don't support SBAS ranging at this time, so you won't have the benefit of using any SBAS GEOs in an RTK network environment. I did not try the PM3 RTK in this mode so I can't comment on its performance in that configuration, but Magellan's PM3 RTK white paper does address network RTK and presents some performance data when using the PM3 RTK in that environment. If you plan on using the PM3 RTK for topo on a four-wheeler or other vehicle, make sure you have your local dealer demonstrate the PM3 RTK in that environment. I noticed it would lose initialization for a couple of seconds in situations where I jarred the rangepole suddenly. Given the fragile nature of spread-spectrum radios, I really wanted some sort of indicator that showed positive radio communication. There's something about a RX LED flashing at 1Hz (once per second) that is reassuring when you're starting to push the limits of the radios. I didn't see any such indicator on the PM3 RTK. Conclusions The PM3 RTK is a unique product. In a world of $30,000-$50,000 RTK systems, the PM3 RTK is an attractive proposition at around $15,000. Can it do everything an L1-L2 RTK system can? No. It will have slower initialization (and re-initialization) times, as well as a shorter baseline limitation (approximately 5-10km). But, if you understand its limitations and respect them, you can be really productive with the unit. I think this innovative technology will expand the market for RTK systems in general. RTK productivity is addictive, and this price point will attract a whole new group of potential "RTK addicts" who can't get past the sticker shock of today's L1-L2 RTK systems. Get GPS World Alerts When News Breaks Get notified immediately via e-mail when an article is posted to this site about your favorite keywords. |
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The system is surprisingly lightweight for having everything on the rangepole. I've used a variety of RTK systems in the past. Some have all the components (receiver, radio, batteries) in a backpack, with only the antenna on the rangepole. Other systems pack everything on the rangepole, which makes it quite heavy if you're topo'ing for eight hours. The PM3 RTK is somewhere in-between, but definitely leans more towards the lightweight side. You can topo with it all day without wearing your arms out.
