Trimble Dimensions Provides Focus on Range of Satellite-Based Correction Services

November 5, 2014 - By Eric Gakstatter

The 2014 Trimble Dimensions User Conference is being held in Las Vegas this week. Photo: Trimble

With more than 4,000 attendees, this year’s Trimble Dimensions User Conference was the largest ever and, I must say, a well-organized event chock full of technical content — enough to squelch the most intense geospatial hunger pangs you might have.

One could write a book on all the technology and market segments that Trimble is pursuing and offering solutions for. In addition to a wide range of GNSS, geospatial, construction, control, and data management systems previously offered, Trimble boasted a USB stick full of press releases with new product and service announced at Dimensions. So, the challenge is deciding what to write about without writing a little bit about everything.

After my first day at Dimensions, it became clear to me what I needed to do. Among the many product and service announcements was a new GNSS correction service named Viewpoint RTX. While I’ve tried to stay up to speed on Trimble’s various GNSS real-time correction services, this one was the straw that broke the camel’s back for me. I decided I needed to get a solid grip on the range of real-time GNSS correction services that Trimble offers because the picture was getting fuzzier, at least to me, with each new real-time correction service introduced. It used to be pretty simple to decipher; not so much any longer. So I had a conversation with Patty Boothe, general manager of Positioning Services at Trimble. Patty, a 15-year Trimble veteran, was appointed GM of the newly formed group three years ago. Here’s the low-down on the services.

Remember, Trimble acquired the land portion of OmniSTAR’s business a few years ago. For years, OmniSTAR has been one of the two dominant commercial satellite-based, real-time GNSS correction services (the other being John Deere’s Starfire service, as well as new entrant Terrastar). The OmniSTAR acquisition was Trimble’s entry into the satellite-based, real-time GNSS correction services business. Since then, Trimble has introduced the RTX (not to be confused with RTK) range of GNSS correction services. You might say that OmniSTAR and RTX are competitive services within Trimble. They are, to a certain extent, and I’ll attempt to clarify that below.

Following is a list of Trimble’s real-time GNSS correction services, starting with the OmniSTAR services:

OmniSTAR VBS: Satellite-based, real-time submeter service. The VBS service has been made obsolete largely by free public satellite-based augmentation systems (SBAS) such as WAAS/EGNOS/MSAS/GAGAN/SDCM. It is still used in geographic regions where free public SBAS don’t exist, primarily South America, Central and Southern Africa, and Australia. GPS-only service. Requires single-frequency receiver (L1).

OmniSTAR XP: Satellite-based, real-time 15-cm service based on Jet Propulsion Lab (JPL) technology and delivered to users on the ground via OmniSTAR’s geosynchronous satellite network. GPS-only service. Requires dual frequency (L1 and L2).

OmniSTAR HP: Satellite-based, real-time 10-cm service based on OmniSTAR’s reference station network and delivered to users on the ground via OmniSTAR’s geosynchronous satellite network. GPS-only service. Requires dual frequency (L1 and L2).

OmniSTAR G2: Satellite-based, real-time 10-cm service based on Jet Propulsion Lab (JPL) technology and delivered to users on the ground via OmniSTAR’s geosynchronous satellite network. GPS+GLONASS service. Requires dual frequency, dual constellation (L1 and L2).

To use OmniSTAR services, one must have an OmniSTAR-enabled GNSS receiver. There are a several receiver manufacturers that support OmniSTAR GNSS correction services, such as NovAtel and Hemisphere GNSS, in addition to Trimble.

After, or at nearly the same time, Trimble acquired OmniSTAR, the company launched its RTX GNSS correction service. RTX’s infrastructure consists of ~110 GNSS reference stations around the world working to create high-precision corrections on a near global scale. The first significant differentiator is that Trimble RTX services are only offered on Trimble GNSS receivers, so you’ve got to be “all in” with Trimble to utilize RTX.

Viewpoint RTX: Internet-based (notice I didn’t write satellite-based), real-time submeter service. This is a new service introduced this week at Dimensions for the new Leap GNSS receiver and the Geo7 GNSS handheld. GPS+GLONASS service. Requires single-frequency receiver (L1).

Rangepoint RTX: Satellite-based, real-time 50-cm service. GPS+GLONASS service. Requires dual-frequency receiver (L1 and L2).

Centerpoint RTX: Satellite-based, real-time 4-cm service. GPS+GLONASS service. Requires dual-frequency receiver (L1 and L2).

The above are the three RTX services. There are some options for the above, but let’s talk about satellite-based GNSS correction services for a minute.

The advantage of satellite correction services is that, because GNSS corrections are delivered via satellite, your receiver doesn’t need to be connected to the Internet or have any other sort of terrestrial radio communications to receive data from the GNSS reference station(s). Because delivery is by satellite, you could be in the middle of a desert with no mobile phone coverage within 100 km, and you could still use OmniSTAR or RTX services. The only requirement is that your receiver needs to have direct, continuous line-of-sight to the OmniSTAR/RTX geosynchronous satellite (both services use the same geosynchronous satellites to broadcast the corrections).

The primary disadvantage of OmniStar and RTX services is the “convergence” time required to achieve the stated accuracy service levels. With the exception of OmniSTAR VBS (sub-meter), Viewpoint RTX (sub-meter) and Rangepoint RTX (50-cm) services, the OmniSTAR and RTX centimeter and decimeter services require tens of minutes of initialization time to converge to the stated accuracy. For example, if you want to use the 4-cm Centerpoint RTX service, you may have wait up to 30 minutes for it to converge to 4-cm accuracy.

Now, there are a couple of ways to reduce the convergence time:

Start on a known point. For example, if you’re using Centerpoint RTX on a tractor for planting and you shut down for the evening, you can start it up the next morning (assuming you didn’t move the tractor), and it will converge nearly immediately.
Trimble offers a fast convergence option ($) in some geographic areas where it augments RTX with local RTK reference stations. Currently, Trimble offers this service in five U.S. “corn belt” states.

For OmniStar XP, HP and G2 services, the only way to reduce convergence time is number one above, start on a known point.

It’s important to note that all of the centimeter and decimenter satellite-based services described above are based on real-time Precise Point Positioning (PPP) technology, which is different than RTK technology. The fundamental difference is that real-time PPP technology relies on a global, distributed network of reference stations. For example, Trimble has ~110 reference stations to cover the globe (mostly) with its RTX service. On the other hand, RTK requires a much more dense network of GNSS reference stations. For example, in Washington State there are ~100 GNSS reference stations that comprise the state-wide RTK network.

Lastly, Trimble offers a hybird RTK/RTX service called XFill. The idea is that for RTK users who lose communications to their RTK base or RTK network can use the Centerpoint RTX as a “seamless” back-up, maintaining RTK-level accuracy (1-2cm) for the first five minutes of RTX service, and then degrading to Centerpoint RTX accuracy after 20 minutes. Trimble reports there is no convergence time when transitioning from RTK to RTX, like you would if you were starting RTX right away. Standard XFill is included with certain Trimble RTK receivers and allows up to five minutes of RTX satellite time. Last month at the INTERGEO conference, Trimble introduced Expanded XFill which is a subscription service for those users who want more than five minutes of RTX time. For those users, Patty said that users can buy blocks of RTX time starting at 10 hours.

So, you might ask how Trimble handles the horizontal datum differences between RTK and RTX since they are likely not referenced to the same horizontal datum. For example, in the US, Trimble VRS RTK infrastructure is typically referenced to NAD83/2011 while Trimble RTX is referenced to ITRF08. There’s about 1 meter difference between the two. After finding the correct Trimble person, he said that Trimble does a 3-parameter local shift (dX, dY, dZ) on the fly when in RTK mode so that when there’s a transition from RTK to RTX, the horizontal datum difference is already resolved.

A by-product of Trimble’s ~110 global GNSS reference station network is a real-time, world-wide TEC (Total Electron Content) map. Since real-time PPP GNSS correction services (and public SBAS like WAAS/EGNOS/MSAS/GAGAN) rely on accurate models of the TEC in the ionosphere to account for the GNSS measurement delay, real-time TEC maps give users an indication of how the ionosphere’s TEC is behaving. This sort of map is particularly useful in attempting to predict the understand single frequency receivers using services such as public SBAS, OmniStar VBS, and Viewpoint RTX. The next time you here about an impending solar storm, take a look a the map using this link and see the TEC hotspots around the globe. Notice the more intense activity near the geomagnetic equator.

TEC map from Trimble’s ~110 global GNSS receivers. Photo: Trimble

Shifting gears slightly, at the conference, Trimble also introduced a new mobile phone GNSS add-in product called Leap, which uses the Viewpoint RTX service.