Comparison of L-Band Correction Services for Survey and Land-Based Applications

At Canal Geomatics, we’re committed to helping customers find the best solution for their unique applications. The available L-Band correction services vary by land, marine and air. This article discusses the available L-Band correction services for land-based applications, including:

Positioning Solutions

There are four primary solutions available: private Real Time Kinematic (RTK), subscription-based Real Time Kinematic (RTK), Differential GNSS (DGNSS) and Precise Point Positioning (PPP).

Generally speaking, RTK, both provided by a privately owned reference station or obtained from a third party service provider, is the most accurate, because it resolves in real time the complex geometry of a tridimensional vector between the known and accurate position of a base station and the desired accurate position of a rover static or mobile station. Often, in the best conditions, RTK provides centimeter-level accuracies.

The privately owned reference station RTK mode requires a user to setup two GNSS receivers, the first one on a survey mark for which the position is accurately known and the second that measures new positions. The secret resides into a permanent communication between the two, whether it comes from a digital radio apparatus or by the means of Internet and a cellular data link. The information transmitted from the reference station provide the rover station what it needs to obtain the orientation and distance separating both instruments, resulting into an accurate new position for the rover in relationship with the already known position of the reference station.

The third party provided RTK mode uses the same geometry logic and algorithms to find accurate positions of a rover GNSS receiver but instead of using a privately owned GNSS receiver as a base and communication apparatus, it uses Internet and a network of reference stations operated by a service provider. Many of such services offer some sort of improvements by interpolating corrections from the nearest stations to reduce the uncertainties related to the distances between them and the client’s rover. This type of service makes it such that a user no longer needs to work with two GNSS receivers, only one being required, most often including a cellular data link to access the service provider’s correction services. Recently implemented high-end such services are now using satellites on what is called the “L-Band”, requiring a special capability at the GNSS receiver end to receive correction data on this frequency from other geostationary satellites. The accuracy varies but is generally still at centimeter level if an open sky is available and a sufficient number of satellites can be received.

Differential GNSS (DGNSS) can be provided by a few sources. Initially, such services, requiring a separate digital radio receiver, were provided by many country’s Coast Guards to increase the accuracy of maritime navigation but these services have been mostly replaced by a few geostationary satellites that act at the same time as GPS satellites, but also providing corrections. This service is called SBAS (Satellite Based Augmentation System). In all cases, DGPS provides only pseudo-distances corrections on each satellite and positioning accuracies that can only reach one meter in the best conditions.

Precise Point Positioning (PPP) is the latest and most complete corrections method. It uses a better knowledge of satellites ephemeris (their accurate orbits), of ionospheric and tropospheric delays in the user’s region and finally, wherever possible the resolution of frequency carrier’s phase ambiguities in comparison with fixed ground stations. PPP services are provided by “L-Band” satellite-based corrections with convergence times required to obtain higher accuracies that can vary with many factors, notably the distance separating the user from permanent pursuit stations. The PPP mode advantage is the possibility to obtain centimetric levels of accuracy in remote territories where referencing to geodetic networks is impossible.

Survey and Mapping

RTK is the first choice for many surveyors. RTK can quickly provide accuracies up to a few centimeters, however, alternatives are increasing in popularity.

PPP is another solution that is growing in use. PPP requires no base stations and has no range restrictions, eliminating site selection, setup and potential station problems, without paying additional RTN fees and is available everywhere. PPP is now also able to achieve centimeter-level accuracy, but is slower at high accuracy levels and involves higher monthly costs.

Agriculture

For many agricultural applications, RTK is often the preferred choice. While there is a higher initial cost, it provides unmatched speed and accuracy. For most agricultural applications, including heavy equipment operation, range restrictions are not a problem.

RTK based L-Band correction services are ideal for farm mapping, soil sampling, seeding and planning, automated steering, yield monitoring and mapping, spraying and harvesting.

Mining, Oil & Gas

In applications for mining, oil, and gas, there are several considerations that will factor into your decision.

• Level of Horizontal and Vertical Positioning Accuracy
• Stability and Reliability of Signal Reception
• Vehicles and Vessels
  • How many are there? How fast are they going? How are they operated?

• Type and Nature of Mobile Equipment Installation
• Type of Installation
  • Is it permanent or short-term? This will influence the placement of cables and location of antennas.

Applying RTK, PPP, and DGNSS solutions in the mining, oil, & gas industries all have their own sets of advantages and disadvantages. Given the characteristics of these solutions, the preferred one will depend on the activity. The optimal solution may differ among activities such as drill guidance, asset tracking (of lighting plant and mobile generators for example), access and zone control for visiting vehicles, collision avoidance, fleet management, etc. because the considerations for each one varies.

Rail

For the rail industry, DGNSS is the preferred L-band correction service for its applications.

While GNSS has been widely utilized in marine (link to blog post), aviation (link to blog post), and vehicle navigation, it has only begun to be implemented in rail transport systems around the world. This is due to the fact that this industry operates under stringent safety regulations and possesses unique characteristics that aren’t present in other applications. For example, the unavailability of satellite signals when a train enters a tunnel or passes through a cutting or densely built-up area has prevented the use of GNSS in safety-critical applications (e.g. train control and signaling).

When used in conjunction with other sensors, computers, and communication systems, GNSS can greatly enhance safety, accuracy, and effectiveness.

Examples of railway applications:

• Management of Rolling Stock
• Movement Tracking of Locomotives
• Collection of Passenger Information
• Level Crossing Approach
• Cargo Tracking Signalling
• Asset Tracking

Correction Services

OmniSTAR

With four different DGNSS solutions, OmniSTAR offers plenty of options for coverage:

• OmniSTAR HP
  • Requires a dual frequency receiver
  • 10 cm at 2-sigma (95%)
  • No need for local base stations

• OmniSTAR G2
  • Short-term accuracy of 3 cm to 5 cm
  • Long-term repeatability of 10 cm or better, 95% CEP
  • Includes GLONASS satellites and correction data, useful in situations with limited satellite visibility (terrain, buildings, and vegetation)

• OmniSTAR XP
  • Requires a dual frequency receiver
  • Short-term accuracy of 3 cm to 5 cm
  • Long-term repeatability of 10 cm or better, 95% CEP
  • Slightly less accurate than OmniSTAR HP but has global availability

• OmniSTAR VBS
  • L1 only, code phase pseudo-range solution
  • Typical 24-hour sample will show a 2-sigma (95%) of significantly less than 1m horizontal position error
  • 3-sigma (99%) horizontal error will be close to 1 m
  • Ideal in situations where repeatability and accuracy are not the primary concerns

NovAtel CORRECT

NovAtel has also specialized in all three services.

• NovAtel’s RTK solution provides accuracy of 1 cm + 1 ppm horizontal and 1 cm vertical (baseline range ≤ 40 km).
• Novatel’s DGNSS service provides 40 cm accuracy at ≤ 100 km of range.
• Two Novatel PPP solutions:
  • TerraStar-C has an accuracy of 4 cm RMS horizontal and 6.5 cm RMS vertical with a 20 to 40-minute convergence time
  • TerraStar-L has an accuracy to 40 cm RMS horizontal and 50 cm RMS vertical with a convergence time of < 5 minutes.

Hemisphere Atlas

Atlas provides three PPP solutions:

• H10: 8cm 95%, 4 cm RMS horizontal accuracy, 12 to 20-minute convergence time
• H30: 30 cm 95%, 15 cm RMS horizontal accuracy, 4 to 5-minute convergence times
• H100: 1m 95%, 50 cm RMS horizontal accuracy, close to instantaneous convergence

For land and survey applications that are able to work within a limited range but require a very high level of accuracy and no convergence times (e.g. safety-critical applications), RTK solutions would be ideal if you are able to set up a base station.

DGNSS is similar to RTK in that you need a base station and it doesn’t take any time for its convergence. But it is ideal in applications where a larger range is necessary and decimeter-level accuracy is sufficient.

PPP is the most economical solution and provides high accuracy (centimeter to decimeter-level). However, it requires a longer period of initialization and convergence than RTK and DGNSS solutions. Thanks to its global availability, PPP L-Band correction services would provide tremendous benefit to applications that operate within a large range (e.g. geological mapping).