Differential GPS (DGPS) performance: which statement best represents typical position accuracies as a function of rover distance from the reference station?
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APositional accuracies ~ 1 – 2 m if rover is less than 1–2 km from the reference station
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BPositional accuracies ~ 2 – 5 m if rover is less than 2–5 km from the reference station
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CPositional accuracies ~ 5 – 10 m if rover is less than 5–10 km from reference station
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DPositional accuracies ~ 5 – 10 m if rover is less than 25 km
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EBetter than 0.1 m at any distance without carrier-phase processing
Answer
Correct Answer: Positional accuracies ~ 1 – 2 m if rover is less than 1–2 km from the reference station
Explanation
Introduction / Context:Differential GPS (DGPS) corrects many common-mode errors by referencing a fixed base station. Accuracy depends on baseline length (distance from rover to reference), ionospheric conditions, and whether code-based or carrier-phase techniques are used. This question targets typical code-based DGPS performance over short baselines.
Given Data / Assumptions:
- Standard real-time code-based DGPS corrections.
- Short baselines in the 1–2 km range with good geometry.
- No advanced RTK carrier-phase ambiguity resolution (which can reach centimetre level).
Concept / Approach:As the rover moves farther from the reference, spatial decorrelation of errors (ionosphere, troposphere) reduces the effectiveness of differential corrections, degrading accuracy. Over very short baselines, DGPS commonly achieves around 1–2 m horizontal accuracy. Larger baselines (tens of kilometres) show several metres of error, unless more sophisticated techniques (e.g., RTK, network RTK) are used.
Step-by-Step Solution:Identify the code-based DGPS accuracy regime (metre-level).Note best performance at short baselines ~1–2 km: ~1–2 m accuracy.Recognize accuracy degrades with longer baselines to several metres.Therefore, select the statement describing ~1–2 m at ~1–2 km.
Verification / Alternative check:Manufacturer datasheets and field results commonly cite ~1–3 m DGPS accuracy under good conditions for short baselines, improving to sub-metre only with augmentation or carrier-phase techniques.
Why Other Options Are Wrong:
- 2–5 m at 2–5 km or 5–10 m at 5–10 km/25 km represent degraded performance relative to best-case short baselines.
- 0.1 m without carrier-phase is unrealistic for code-based DGPS.
Common Pitfalls:Conflating DGPS with RTK; ignoring baseline dependence; assuming advertised best-case values apply at all distances.
Final Answer:Positional accuracies ~ 1 – 2 m if rover is less than 1–2 km from the reference station