GNSS Needs Assessment

When to choose a GNSS receiver?

There are a wide variety of GNSS devices available today and choosing whether to use a smartphone with built in GNSS capabilities, a handheld consumer GNSS, or a dedicated high accuracy GNSS receiver can be challenging. This document is intended to provide advice on what factors to consider and a high level overview of each factor we suggest.

We have chosen to focus this document on the following factors, GNSS device type, advertised accuracy, GNSS coverage, availability of augmentation systems, and the location where you intend to use a GNSS device.

First begin by pulling together the following project related information.

• What are your project’s minimum accuracy requirements, 8 mm, 50 cm, 1 m, 5 m, 10 m? The more accurate a device is, the more expensive it tends to be and the more complicated it is to operate.
  • This information may be included in project design documents
  • This information may be found in a project’s legal documentation like a contract or MoU.
  • If you are unable to determine the project’s accuracy requirements, it may be wise to determine if the local or national authorities have set any spatial accuracy standards for land surveying or laying out lot lines.
• What has been budgeted for GNSS devices? GNSS devices can easily cost 3 to 4 times what it costs to purchase a GNSS enabled smartphone or handheld consumer GPS device.
  • You can usually find this information in your project budget.
• Where is the area of operations (AO) and what are its terrain and vegetation characteristics?
  • What country, state, city/town is the AO in? Even though a GNSS is by definition a globally available service, coverage is not always uniform everywhere around the globe (especially in latitudes above 60°). Satellite and ground based augmentation systems are only available in specific regions so it is important to determine if the AO your operating in is covered by one of the augmentation systems.
  • Is the AO in a forested area? Dense vegetation can degrade GNSS accuracy.
  • Is the AO in an area of high relief? (High relief = difference between its highest and lowest elevations). Tall buildings, deep valleys, and high mountains can all degrade GNSS accuracy.

Additional factors to note and consider when evaluating GNSS devices.

• Do the software applications you intend to use should support using a GNSS receiver?
• Does the GNSS receiver have the feature set you want?
• Does the GNSS unit neet our performance requirements?
  • Do we need a dual frequency device or a single frequency device?
• How much power does the GNSS device require?
  • Is the device rugged enough to be used for field work?
  • Will the device run on batteries or does it require a mains power supply?
  • How often do the device’s batteries need to be recharged/replaced?

The next step is to review the accuracy, coverage, and augmentation sections below and compare your project’s requirements with the specifications mentioned in each of the sections below.

Accuracy

Consumer Device Accuracy

Handheld GPS receivers are generally accurate to within 15 meters 95% of the time. Under normal conditions, users will see accuracy within 5 to 10 meters.

GPS enabled smart devices like phones and tablets are typically accurate to within a 4.9 m radius under open sky. However, the accuracy of a device is degraded when working in areas that are near buildings, bridges, trees, mountains, or in valleys.

Professional Survey Device Accuracy

GNSS accuracy with single or dual-frequency receivers and/or augmentation systems can capture real time positioning data accurately to within 50 cm, and they can produce long-term corrected measurements down to within about 8 mm of the actual location on the ground. In terms of cost, the more accurate a GNSS device is, the more expensive it is, and the steeper the learning curve is to become proficient in its use.PAGE_BREAK: PageBreak

GNSS Coverage

Augmentation Systems

If you need to determine a position with greater accuracy than the GNSS listed above, GNSS augmentation can be used. Augmentation is a method of improving the navigation system’s accuracy, reliability, and availability by using external correction information to calculate more accurate position fixes. Augmentation information may be offered through a subscription service or it may be available at no cost, only requiring a GNSS receiver that can read the augmentation signal. There are several augmentation systems to choose from but the choice really amounts to choosing the augmentation system that provides coverage in the geographic area that you intend on working in.

Satellite-based augmentation systems (SBAS)

SBAS provide wide-area or regional augmentation through the use of additional satellite-broadcast messages. Using measurements from precisely located ground stations, correction messages are sent to one or more satellites for broadcast to end users as differential signal. The SBAS coverage map shown below a specific geographic region so be sure to verify that the SBAS provides coverage in your area.

Image Source: http://manuals.spectracom.com/GSG/Content/GSG/Topics/Conc/SBAS_Sats.htm

Ground-based augmentation systems (GBAS)

GBAS uses the same measurements from precisely located ground stations but GBAS sends those differential correction messages directly to end users. GBAS are generally associated with a specific geographic region so be sure to verify that the GBAS provides coverage in your area. There are too many GBAS to list here however, with a bit of research you should be able to locate a local GBAS provider that can meet your needs.