Survey Perspectives – Late February 2008

February 21, 2008  - By 0 Comments

With nearly 60 GPS engineers and scientists, the Jet Propulsion Lab (JPL) is one of the biggest GPS R&D centers in the world today. It operates as a division of the California Institute of Technology (Caltech), which manages the lab for the National Aeronautics and Space Administration (NASA).

Among other things, JPL operates the Global Differential GPS (GDGPS) system, which sells technical services and data and licenses software. The GDGPS system within JPL employs a vast, worldwide network of more than 100 L1/L2 GPS reference stations owned by itself and its partners.

Each reference station streams GPS measurements back to the GDGPS Operations Centers once per second. Data is then processed and analyzed in real time. Talk about redundancy — each GPS satellite is always observed by at least ten reference stations, and twenty-five is typical. Read more

It’s easy to get lost in JPL’s wide array of GPS product and service offerings, so I’ll try to stick with the part that’s closest to survey and construction.

Among other activities, JPL has people dedicated to monitoring and modeling the atmosphere — especially the ionosphere, which strongly impacts GPS measurements. They provide real-time, global maps of the Total Electron Content (TEC) used for L1 differential corrections around the world (think SBAS like WAAS and MSAS) and also for predicting ionospheric storms.

Dr. Michael Whitehead of Satloc, Inc. (now a division of Hemisphere GPS, Inc.), lead the first Wide-Area Differential GPS (WADGPS) commercial ventures to license JPL’s clock/orbit correctors and iono modeling services. This was back in the mid-90s, and Satloc’s target market was agriculture. Remember, this was before Selective Availability (SA) was turned off, so without a source of corrections, horizontal GPS accuracy without augmentation would routinely blow out to 100 meters. With its system, Satloc was able to deliver sub-meter L1 corrections to users via communications satellite.

“They (JPL) provided core technology. It worked great. The accuracy was there,” said Whitehead.

Whitehead said Satloc operated its own GPS reference network and internal software for generating corrections, but it also used JPL’s service to provide system redundancy. The Satloc system was set up to use corrections from either system (Satloc or JPL), and could automatically switch between the two systems.

The Satloc network was eventually sold to Fugro/OmniSTAR, another WADGPS service provider that integrated JPL data into its product offering. Hemisphere GPS/Satloc products now rely on WAAS (Wide Area Augmentation System) for their source of corrections. WAAS is built on core JPL technology, a predecessor of the GDGPS software. According to Whitehead, WAAS is very similar to the system that Satloc originally developed.

Fugro/OmniSTAR

Fugro/OmniSTAR operates its own GPS reference station network (over 100 worldwide, with 21 of those in North America) and has offered a WADGPS service in certain regions of the world dating back to the late 80s on a subscription basis. Until the late 90s, OmniSTAR/Fugro was a “one-trick pony,” offering a sub-meter “VBS” service for L1 GPS receivers. This is based on its worldwide network of GPS reference stations. Since then, the company has expanded its services in response to demand for greater accuracy and system redundancy.

Now, Fugro/OmniSTAR offers two additional levels of service: HP and XP. Both require the user to have a dual-frequency receiver (L1/L2). The upside is that the HP service provides +/-10cm horizontal accuracy using carrier phase (a sort of float solution). The HP service is based on Fugro/OmniSTAR’s proprietary GPS reference network and software. HP service is available in various regions throughout the world such as North America, parts of South America, Europe, the Middle East, Central Asia, and Australasia. The HP service is reference-station-dependent, meaning that the performance degrades as the user moves farther away from the nearest reference station (with a 300-mile limit).

Fugro/OmniSTAR’s other precise service, XP, is based on data licensed from JPL. The XP service offers horizontal accuracy of +/-15cm. The HP and XP services are similar in accuracy, but the JPL-based XP service offers global service rather than a regional service like HP. The difference is that while the HP service is baseline-dependent, the JPL-based XP service is not. That enhances Fugro/OmniSTAR’s coverage in remote locations where reference station coverage is sparse.

NavCom Technology

A leading-edge GPS design company licensing data from JPL is NavCom Technology, Inc., from Torrance, CA. Although the company name isn’t well known in the Survey/Construction industry, many of the engineers at NavCom are the same ones that designed the original Leica survey receivers while they were at Magnavox. There is some pretty high-end GPS design talent there — enough that John Deere Company bought NavCom, which now operates as a wholly owned subsidiary of Deere.

NavCom created and operates a GSBAS (Global Satellite-Based Augmentation System) called StarFire. While NavCom operates its own network of 20 worldwide GPS reference stations, it also has license agreements with JPL for reference station data and certain software. NavCom then refines and optimizes the data for NavCom receivers and distribution via the StarFire network. The result is that StarFire can deliver horizontal accuracies in the sub-10cm range after initialization.

NavCom has also created an interesting innovation it calls RTKExtend. Users of traditional RTK systems know that when the data link is interrupted, RTK operations are halted until the data link can be re-established. However, NavCom has combined traditional RTK with its StarFire network to assist RTK users. Users begin work using the traditional base/rover RTK configuration. If the data link is interrupted, the NavCom receiver automatically transitions to use the StarFire network, so the user can continue to operate at the centimeter level for up to 15 minutes.

Satloc, Fugro/OmniSTAR, and NavCom are just a few examples of commercial organizations that have successfully utilized JPL’s leading-edge GPS technology. There are also applications outside of the high-precision industry, such as mobile phone service providers licensing JPL to provide A-GPS data for E-911 anywhere in the world. With its unique global reach, JPL’s technology enables precision GPS applications even in regions of the world that lack infrastructure. It’s truly impressive to realize that decimeter-level positioning is available in most places in the world today; it’s just a matter of how to deliver the corrections. With the proliferation of wireless communications, even this problem will eventually be solved.

This article is tagged with and posted in Newsletter Editorials, Opinions, Survey, Survey Scene

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