Precise Point Positioning Service: A Blessing and a Curse

With the release of our incredible 965-channel Asteri X4i GNSS receiver was the addition of a no-charge precise point positioning (PPP) correction service included with the purchase of every Asteri X4i. With every great service, there are drawbacks, and PPP is no different. It is both a blessing and a curse. 

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The blessing of PPP is that it utilizes a limited number of precise and accurate stations to offer exceptional accuracy. However, this enhanced signal correction comes at the cost of speed. Simply put, PPP takes much longer to converge than RTK.

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Clients who purchased or tested an Asteri X4i before December 20, 2024, received the device with the PPP service activated. However, after listening to feedback from those of you who have been using the Asteri X4i in the field for longer, we have decided to no longer ship it with the PPP service activated. You’re busy and don’t have time to stand around and wait for the signal to converge. 

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We’ve been referring to this feature of the Asteri X4i as “Satellite Assisted Orbitas Correct” because we recommend using the PPP service with our Orbitas Correct service for the best results. Orbitas Correct is our new RTK service, released in 2024, which provides sub-inch or decimeter RTK corrections across the United States. You can learn more about Orbitas Correct on our website or by contacting us directly at info@triglobal.net. 

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When the PPP service is used with Orbitas Correct, decimeter results are delivered within the industry standard two minutes. Once PPP is converged, or “fixed,” you can completely remove RTK corrections and still maintain nearly decimeter results for the remainder of the PPP correction lock (Figure 1). This powerful combination prompted us to release the Asteri X4i with the PPP service automatically activated.

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What happens when you use the Asteri X4i without Orbitas Correct or another RTK network?

Test 1: PPP Activated without RTK correction services applied.

In October 2024 we completed a test collecting data with the Asteri X4i with PPP activated without using any RTK correction service. The results seen in Figure 2 were gathered over one hour. To better understand our results, refer to our support documentation for Orbitas Version 1.0.23. 

Observations:

• While PPP was converging, represented by a blue teardrop icon, the accuracy started worse than the satellite-based augmentation system (SBAS) only. Typical SBAS accuracy is approximately 3 feet, while in our example the first few seconds of PPP lock was approximately 9 feet. 
• After 10 minutes of PPP lock, the accuracy falls below sub-meter. When not using Orbitas Correct, it’s important to wait at least ten minutes before collecting any data. Otherwise, results may be greater than sub-meter. 
• After about 30 minutes, the PPP converges, or “fixes,” and all data has an accuracy of 6 inches or better. While these results are not decimeter-level, the PPP service is still incredible for sub-foot accuracy. We expect that this accuracy will only improve the longer that it remains fixed. 

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Test 2: PPP De-activated with Orbitas Correct applied.

Next, we wanted to demonstrate the Asteri X4i accuracy when using Orbitas Correct without PPP activated. As you can see in Figure 3, there is no need to observe for an hour as we did with the PPP-only test because Orbitas Correct locks and maintains the decimeter accuracy we have come to expect as long as we have an NTRIP connection.

Observations:

• Orbitas Correct accuracy begins just outside of 1 meter with no need to converge more than the normal 30 seconds to 2 minutes.
• Orbitas Correct can “fix” and provide decimeter accuracy within 30 seconds, as expected. 
• With PPP disabled, it fixes much faster because it’s not attempting to fix PPP readings at the same time.
  
  

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As demonstrated in the three tests above, PPP-assisted Orbitas Correct has value to users in areas with poor RTK coverage. However, because PPP requires more time to converge, PPP users will need to wait before collecting. Additionally, for users operating in conditions with multipath and obstructions where PPP cannot converge, additional issues have been reported.

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What happens when you use the Asteri X4i in heavy canopy conditions?

On December 19, of 2024 we ran a couple of tests in heavy canopy.

The tests below represents collection in areas with heavy tree canopy coverage.
For a known value on this type of test, we cannot analyze a monument or data against a fixed location, so we chose to collect data dynamically along the boundary of a road that is visible on aerial imagery.

The yellow line seen in Figure 4 represents our known value that we will compare our results against. The road edge is shrouded by foliage and passes through a hardwood ravine. The ground image on the right shows the hardwood conditions during the test.

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Test 3: PPP Activated with Orbitas Correct applied.

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Observations:

• The greatest achieved accuracy was less than 1 inch while the worst observed accuracy was 43 feet, with only 60% of the readings below sub-meter. 
• When collecting in areas without obstructions, where Orbitas Correct can apply, accuracy was observed to be sub-inch, as indicated by the green locked icon. However, in the worst areas, the data jumps and we receive the blue icon, indicating that PPP is converging. some text
  1. It’s important to note that the accuracy prediction states 0.00 inches because it cannot be determined in these conditions. 
  2. In these areas, the converging PPP data creates erroneous GNSS results that cannot be trusted. If we were only creating points, we would know not to collect data where the blue converging icon appears, but because we’re collecting data in a dynamic mode, we are going to see large spikes in our results. 
• In areas with moderate tree canopy, the data continues to look great, but the overall collection data is flawed as a result of the convergence spikes. 

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Test 4: PPP De-activated with Orbitas Correct applied.

Observations: 

• The greatest observed accuracy was sub-inch while the worst observed accuracy was 7.3 feet, with 98.6% of the readings below sub-meter and 90% sub-foot. 
• Without PPP enabled, Orbitas Correct provides incredible accuracy even in areas with canopy obstruction.

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Summary

While no existing GNSS receiver can provide 100% of data at a sub-foot level around sources of multipath, the Asteri X4i combined with Orbitas Correct is an incredible option for sub-foot work. 

The PPP service available with the Asteri X4i offers significant value with its limitations. For users around sources of multipath or those that are unable to allow the PPP service to fully converge, it’s best to utilize the Asteri X4i with the PPP service disabled. We would like to improve the firmware to allow Orbitas Correct to converge the PPP readings in a much shorter time, in which case it may then be reasonable to utilize PPP in all regions and conditions. 

Until then, Tri-Global will ship all Asteri X4i after December 20, 2024, with PPP disabled. For those who would like to use the PPP service with their new Asteri X4i, we are working on an Orbitas version that would allow users to enable or disable PPP at their discretion. The expected release of this update is 2025. 

If you would like assistance with disabling the PPP service before then, please contact us at support@triglobal.net.

For those unfamiliar with PPP service, there is an excellent article written by our partners at Point One Navigation about Real-time Kinematic (RTK) positioning and how it differs from PPP.