Accuracy
Accuracy can be a confusing term in
the agricultural vehicle guidance industry. This is partly due to the
lacks of standards or independent evaluation of vehicle guidance
performance. The sales hype rarely refers to the on ground performance
required for cropping operations. Most performance data provides GPS
accuracy which is not the on-ground guidance performance.
Factors
Factors that compromise the application of GPS test results to vehicle
guidance include:
- GPS accuracy is not work accuracy due to the effect of roll, yaw and implement-hitch dynamics.
- GPS measurements taken at different times or time periods are not comparable.
- GPS performance results from static tests do not take into account the dynamics of the GPS antenna in a cropping environment.
- Different statistical measures are use to further confuse the results. Statistical measures commonly used include:
- One sigma (one standard deviation): 68%
of the readings will be within the stated accuracy.
- Two sigma (two standard deviations): 95% of the readings will be within the stated
accuracy.
- CEP R50: the circular error probability of 50% is the radius
within which 50% of the reading will occur.
Precision
For many agricultural applications we only require precision from one
pass to the next. This means that lower accuracy receivers
(lower costs) can compare favorable if the time interval between passes is
sufficiently short.
Over a longer period of time the comparison is less
favorable.
The above graphic illustrates how many relatively low cost GPS receivers have good precision from one pass to the next as long as the time interval between passes is small. This is possible as the source of GPS errors tends to fluctuate gradually, so if the interval is short the error between passes is small. This does not make these receivers accurate, as they cannot reliably return to the same track after a longer period of time, as illustrated by the pass to pass perfomance below:
High accuracy, high precision GPS is required for
operations that require the implement/wheel tracks/sprayer/.. to return
to exactly the same location year after year. With regular improvements
to the technologies, the division between the low accuracy low cost GPS
receivers and the high accuracy high cost survey grade receivers is
gradually blurring.
Assessment
When evaluating products, make sure that you exercise the full speed
range that you will require.
With hands free steering products make
sure that you exercise the range of load conditions as the control
performance under load may differ from the unloaded performance.
Measure from implement marking to implement mark for runs in opposite
directions. Using opposing directions for measurements effectively
doubles offset errors for easier assessment.
GPS
This section aims to clarify the GPS technology and
terminology used for agricultural vehicle guidance.
GPS/GLONASS/Galileo
The Global Positioning System (GPS) was developed for military use by the US Department of Defense. GPS poitioning is based on triangulation from a constellation of 24 satellites. GPS technology continues to evolve at breathtaking speed as demonstrated by new receiver technology and the recent launch of the Galileo (European GPS system) and Japenese Multi-functional Transport Satellite (MTSAT, Japenese WAAS equivalent).
Aside from the original GPS constellation, there are two competing constellations:
- GLONASS (Global Navigation Satellite System or Global'naya Navigatsionnaya Sputnikovaya Sistema) which is maintained by the Russian Federation Ministry of Defense. Not all the 21 satellites of the GLONASS system are currently active which degrades the potential fix from this constellation. The constellation is being upgraded with the recent launch of three new GLONASS satellites. GLONASS is used to augment GPS positioning fixes when the constellation is limiting such as for open cast mining operations.
- The recent launch of the first Galileo test satellite heralds the start of the deployment of the third GPS constellation.
DGPS
Differential GPS (DGPS) is a term used to describe a range of GPS technologies that use a correction signal to enhance the accuracy of GPS.
The Federal Aviation Administration and the Department of Transportation have developed the Wide Area Augmentation System (WAAS)
to augment GPS accuracy to assist flight approaches. WAAS correction
signals are broadcast via geostationary satellites above North
America. Receivers using WAAS can achieve 10 feet/3 meter accuracy
(95%). Similar DGPS signals are being tested in Europe EGNOS (European Geostationary Navigation Overlay Service) and Japan (MSAS, Multi-Functional Satellite Augmentation System). WAAS, EGNOS and MSAS are free to air signals.
Local DGPS correction signals can be transmitted by ground stations. The best know of these is the Nationwide DGPS Service
that is being developed by the US Coast Guard. Similar correction
signals are transmitted for maritime navigation in coastal regions
around the world.
OmniSTAR and John Deere have global DGPS networks offering submeter and decimetre accuracy. The folowing correction signals are available:
- OmniSTAR VBS - A subscription service with a horizontal accuracy of less than 1 meter (95%).
- StarFire 1 is a subscription free service to all using John Deere StarFire position receivers. This service provides a horizontal accuracy of 75 cm (confidence level?).
- OmniSTAR-HP service will usually have a horizontal error under 10 cm/4 inches (95%).
-
StarFire 2 delivers pass-to-pass accuracy of +/- 4 inches, 95% of the time, in a 15-minute period of time.
Canadian DGPS (CDGPS) is another DGPS correction option. The broadcast CDGPS corrections can be accessed anywhere in Canada and in portions of interior Alaska using a custom-built CDGPS receiver. The service is free.
The High Accuracy-Nationwide Differential Global Positioning System (HA-NDGPS) program provides the capability to broadcast corrections to the Global Positioning System (GPS) over long ranges to achieve a better than 10 centimeter (cm) (95 percent) accuracy throughout the coverage area. HA-NDGPS is currently undergoing a research and development phase. Similar programs are under development around the world.
RTK
Real-Time Kinematic (RTK) GPS uses a local base station to provide sub 2 cm/1 inch accuracy. The limitation of this technology is the requirement for a radio link with the base station and the fact that the distance from the base station is limited by the RTK solution.
More on GPS
There are a wide range of GPS receivers. The performance of GPS recievers from different manufacturers tracking the same signal can differ significantly. Waht makes assessment difficult is that there is no independant certification of performance. This is further complicated by the fast evolution of the technology.
Accuracy Links
US Army Accuracy Manual
GPS Horizontal Position Accuracy
gpsinformation.net