GNSS (GPS) accuracy explained

10 Apr 2020

Some updated content from 10 Apr 2020, original article from 9 Sep 2011:

GNSS (Global Navigation Satellite Systems) such as GPS and GLONASS allow for fairly simple location-based data collection. But because of the way these systems work, the statistical accuracy of the location-based data collected through such systems can vary significantly.

For the United States GPS (Global Positioning System) in particular, some information in this regard is provided at the following web page.

http://www.gps.gov/systems/gps/performance/accuracy/

Accuracy of GNSS (GPS) data at the time of collection can vary significantly due to several reasons, including but not limited to the following.

The specifications (data sheet) for GNSS receivers usually state accuracy using terms based on statistical probability or confidence, somewhat similar to as listed in the following basic table.

Term

Definition

Accuracy / Probability / Confidence

CEP

Circular Error Probable

50%

RMS

Root Mean Square

63–68%

2DRMS

Two times the distance of RMS

95–98%

R95

Radius 95%

95%


To explain further, if the specifications on the data sheet of a GPS receiver state a horizontal (distance) accuracy of 3 - 5 meters CEP, this means that during testing the location coordinates of data points collected with the receiver were found to be within a 3 - 5 meters radius 50% of the time. Though in such testing the manufacturer may have compared data points to the averaged coordinates of several previously collected data points, to the certified surveyed coordinates of a physical marker, or to some other reference. It depends on the test criteria of the manufacturer. When comparing location information from different sources (such as comparing a GPS receiver to a survey marker), you must account for the datum, coordinate system, projection, and other factors like is mentioned at these web pages.

For example, the test criteria for the Hemisphere GNSS receivers (such as the OEM P206 receiver in the Geode, or the XF101 receiver for the original Archer) is provided on page 5 of the following technical manual. Evaluating performance of the receiver is described starting on page 39.

https://www.hemispheregnss.com/wp-content/uploads/2020/02/hemispheregnss_technicalreferencemanual_v3.0_12302019.pdf

So it needs to be understood that the stated accuracy of a GNSS (GPS) receiver is in relative terms. When comparing the accuracy of different receivers from different manufacturers to each other (particularly those that state to provide higher accuracy such as sub-meter or better), it is important to understand the test criteria of the stated accuracy for each receiver. It is also always beneficial to simply perform your own testing with each receiver in your own applicable operating conditions.

The stated horizontal accuracy (again not necessarily directly comparable) of several common receivers is listed in the table below.

Receiver

Stated Horizontal Accuracy

Juniper Geode (Hemisphere OEM P206) 0.6 meters 2DRMS (DGPS)
0.3 meters RMS (DGPS)
1.2 meters RMS (autonomous)
u-Blox NEO M8 (in Mesa 3 Windows, Mesa 2, Allegro 3, Archer 3, Allegro 2, Archer 2)

Standard M8N and optional M8T
   2 meters CEP (GPS with SBAS)
   4 meters CEP (GLONASS only)
Optional M8T only
   1 meter RMS (differential mode (RTCM))
   0.6-0.9 meters RMS (RTCM with Tallysman
   TW2710 external antenna)
   

Hemisphere GPS XF101 (Crescent OEM P102, for original Archer)

<0.4 meters RMS (DGPS)
<1.8 meters RMS (autonomous)
<0.3 meters RMS (DGPS with external antenna)

NVS NV08C-MCM (built-in to previous Archer 2 and Allegro 2 units)

2.5 meters RMS (autonomous)
1 meter RMS (differential mode (RTCM))

GlobalSAT BC-337 (for original Archer)

10 meters 2D RMS (autonomous)
5 meters 2D RMS (DGPS using WAAS)

uBlox NEO 5Q and 6Q (built-in to Mesa and AMXU pod)

2.5 meters CEP (autonomous)
2.0 meters CEP (DGPS)

IkeGPS

<0.6 meters RMS (DGPS)
<1.5 meters RMS (autonomous)
Older units, <2.5 meters CEP (autonomous)

Most receiver manufacturers do not provide a stated vertical (altitude) accuracy specification. But as an observed measure, vertical accuracy of GNSS (GPS) receivers seems to typically be 1.7 times that of the stated horizontal accuracy. So, using the previously stated example, a receiver with specs that state 3 - 5 m CEP horizontal accuracy would likely provide 5.1 - 8.5 m CEP vertical accuracy. But again, this is just a general observation in brief encounters with many different receivers, and is not based on thorough testing of any one particular receiver make and model.