66 GPS Accuracy: Enhancement Decisions

66.1 Start With the Situation

A basic receiver cannot meet the required accuracy on its own. The team must decide whether corrections, RTK, or a different product promise is justified by the site, safety need, and operating cost.

66.2 Overview

This route compares enhancement methods and carries the choice into cost, safety, and deployment evidence.

This is part 2 of 2. Review GPS Accuracy: Error Budgets when you need the first route.

66.3 Learning Objectives

By the end of this chapter, you will be able to:

• explain differential GPS corrections
• compare DGPS and RTK deployment trade-offs
• justify accuracy technology from cost and safety evidence

66.4 Chapter Roadmap

Follow the original sections below in order. They begin at the reviewed split boundary and keep every worked example, figure, check, and supporting banner with the section that owns it.

66.5 Differential GPS (DGPS)

The first enhancement tier is differential correction: use a known reference to cancel shared outdoor GNSS errors. Read Figure 66.1 from the common satellite observations to the surveyed base, then follow the calculated correction over the data link to the rover. The useful question is which errors are sufficiently correlated across that baseline to be cancelled.

Figure 66.1 shows the base using its true coordinates to compare against its GNSS solution and distribute a local correction that the rover applies. Satellite clock, orbit, and broad atmospheric effects can be shared; rover-local multipath and receiver noise are not magically removed. The system therefore improves the error budget while still requiring correction age, link state, and local quality to remain visible.

DGPS Principle:

1. Base Stations at Known Locations:

   • Precisely surveyed reference positions
   • Distributed around the world

2. Base Stations Calculate Local Errors:

   • Position themselves using GPS
   • Compare to known true position
   • Model local GNSS errors

3. Transmit Corrections:

   • Broadcast corrections out-of-band (radio, internet)
   • Local receivers apply same corrections
   • Accuracy: 1-3 meters (vs 5-10m for standard GPS)

Historical Note:

• Came about due to US “Selective Availability” (SA)
• SA: Intentional GPS errors for security (civilians got degraded service)
• SA turned off in May 2000
• DGPS still valuable for removing natural errors

The historical change in Figure 66.2 separates intentional degradation from the natural errors that remain. Read left to right across the 4:00 AM marker: the wide, noisy band before the change collapses quickly, then settles around a smaller but non-zero error floor. That residual is why differential and later carrier-phase techniques still have a role.

Figure 66.2 shows a stylized reconstruction, so use it to understand the step change rather than to infer a particular receiver trace. The remaining band represents the continuing accuracy problem: atmosphere, orbit knowledge, clock residuals, geometry, multipath, and receiver noise. This closes the DGPS section by connecting correction architecture to an honest product state rather than a promise of perfect coordinates.

66.6 Advanced GPS Techniques

The three enhancements solve different positioning errors, so accuracy alone is not enough to select one. The comparison diagram in Figure 66.3 separates faster acquisition, centimetre precision, and ionospheric correction before the chapter applies those choices to a field system.

The A-GPS panel in Figure 66.3 starts with an ephemeris download of about 20 seconds per satellite and uses cellular or Wi-Fi assistance to improve Time To First Fix, while remaining in the 3–5 m class. RTK (Carrier Phase) instead combines a Base station + rover pair and continuous satellite lock to reach 1–5 cm. Multi-Frequency compares L1, L2, and L5 so ionospheric delay can be removed, producing metre-level improvement without making the RTK infrastructure claim. This comparison keeps speed, precision, and correction mechanism distinct.

1. Assisted-GPS (A-GPS):

Problem: Downloading ephemeris (satellite orbital data) takes ~20s per satellite. Solution: Get ephemeris via cellular or Wi-Fi data connection. Benefit: Faster Time To First Fix (TTFF), works indoors better.

2. Carrier Phase Positioning (RTK):

GPS signal wavelength: ~19 cm (L1 band at 1575 MHz). Measure phase of carrier wave (not just code). Accuracy: Centimeter-level (2-5 cm). Requirements:. Very accurate initial location. Observe satellites for extended period. Maintain continuous lock (no “cycle slips”). Applications: Surveying, precision agriculture, autonomous vehicles.

3. Signal Structure:

Multiple frequency bands: L1 (1575 MHz), L2 (1227 MHz), L5 (1176 MHz). Data rate: Only 50 bits per second (very slow!). Uses spread-spectrum (CDSS) for signal strength and interference resistance.

66.7 RTK GPS in Precision Agriculture

RTK is the chapter’s clearest example of accuracy as a business and safety requirement rather than a luxury specification.

Understanding when augmentation (DGPS, RTK) is needed is critical for IoT system design. Let’s examine a real-world precision agriculture case where centimeter-level accuracy is mandatory.

The crop-row requirement now needs an end-to-end correction path, not just an RTK accuracy number. The system diagram in Figure 66.4 shows the infrastructure that must stay intact for an autonomous tractor to hold its line.

In the system diagram Figure 66.4, the Base Station occupies a surveyed known location and observes the same GPS Sat 1, GPS Sat 2, and GPS Sat 3 as the tractor. It computes error corrections and sends Carrier Phase Corrections over the 900 MHz Radio Link. The Rover Receiver on the autonomous tractor applies them to produce a 1–2 cm fix between the marked 50 cm rows. That dependency chain explains why loss of satellite lock, base survey error, or a blocked correction link must trigger the fallback behavior discussed next.

Checkpoint: Enhancement Selection

You now know:

• DGPS improves nearby outdoor receivers by correcting shared errors.
• RTK needs continuous carrier-phase lock, correction delivery, and clear enough sky to keep centimeter confidence.
• The right selection record states the accuracy tier, cost, power, fallback mode, and failure message.

66.8 Knowledge Check

66.9 What’s Next

Continue to Indoor Positioning Technologies to learn about Wi-Fi fingerprinting, BLE beacons, UWB, and sensor fusion techniques for positioning where GPS signals cannot reach.