Why Your Ag Guidance System "Pulls" the Tractor Off Line by 1–1.5 m - and How to Fix It

A common issue with budget ag guidance systems

This situation is very common: a farmer installs an affordable ag guidance system with a GPS/GNSS receiver, and the first pass across the field looks perfectly acceptable. The tractor drives straight, overlap is minimal, and everything seems fine.

But by the second or third pass, the problem shows up: the guidance line starts to drift, and the tractor can be pulled off the intended path by 1–1.5 meters (or more).

It’s especially noticeable in this scenario:

• In one direction, the tractor drives more or less straight.

• After turning around and driving back, the navigator shows the wrong heading.

• As a result of unstable heading calculation, the guidance system suggests an incorrect direction. The tractor ends up driving at a slight angle instead of parallel to the previous pass—and that angle can keep changing. Over distance, the error accumulates and leads to a 1–1.5 m (or greater) lateral deviation, even on a flat, level field.

Meanwhile:

• the field is flat,

• there are no slopes,

• and the operator is driving carefully.

So why does this happen?

Main causes of inconsistent accuracy

1. A low-quality GPS/GNSS receiver

Many entry-level systems use GNSS receivers that:

• struggle to determine a stable heading and direction of travel;

• estimate direction only from coordinate changes, without properly accounting for vehicle dynamics;

• behave inconsistently when direction changes (especially after a turn).

As a result, the system “thinks” the tractor is moving in a different direction than it actually is.

2. Limited SBAS performance in Ukraine

You’ll often hear:

“We have SBAS / EGNOS — that should be enough.”

In practice:

• SBAS can be inconsistent in Ukraine;

• in many regions it provides little to no real improvement;

• errors can still remain at the 1–2 meter level, especially while moving.

So even with SBAS enabled, the “pass-to-pass drift” issue often doesn’t go away.

Practical solutions

Solution #1. RTK — maximum accuracy

RTK (Real-Time Kinematic) is a technology that uses:

• paid GNSS correction services (via the Internet or a local base station),

• carrier-phase measurements from satellites.

RTK advantages:

• centimeter-level accuracy (typically 2–3 cm);

• excellent repeatability from pass to pass;

• ideal for autosteer and row-crop operations.

Limitations:

• requires paid corrections;

• depends on Internet coverage or a base station;

• higher total system cost.

Solution #2. GM SMART and GM PRO S GNSS receivers with a DP filter

For many farmers, RTK can be overkill—or simply too expensive for the job.

That’s why we developed the GM SMART GNSS receiver, and later the GM PRO S, specifically for precision farming.

The key feature of GM SMART / GM PRO S is a built-in DP filter.

What a DP filter is — and why it works

DP (Differential Positioning) filtering is a software algorithm that:

• does not treat GNSS positions as isolated “points,”

• but instead analyzes the tractor’s motion as a continuous trajectory.

It takes into account:

• speed,

• acceleration,

• heading,

• previous position,

• changes in the GNSS carrier phase (without requiring a full RTK solution).

What this looks like in the field

A typical GPS receiver

• each position is calculated independently;

• heading errors appear when direction changes;

• parallel passes start to spread apart or drift.

GM SMART / GM PRO S with DP filtering

1. Smooths the position over time, reducing short-term “jumps”.

2. Uses a realistic motion model—a tractor cannot instantly shift by a meter.

3. Stabilizes carrier-phase behavior across satellites.

4. Analyzes the velocity vector to determine direction and heading more reliably.

The result is that parallel passes remain consistent, even if the absolute position of the entire map drifts slightly (for example, by ~1 m over a day).

What farmers gain with GM SMART / GM PRO S

For most farming tasks:

• spraying,

• fertilizer spreading,

• cultivation,

• pre-seeding tillage,

centimeter-level absolute accuracy is not required.

What matters most is that passes stay consistent relative to each other.

Real-world advantages of GM SMART / GM PRO S

• 10–15 cm pass-to-pass repeatability;

• no RTK, no base station, no NTRIP required;

• a more stable guidance line after headland turns;

• simplicity and a lower total cost.

Table 1. Accuracy comparison of guidance technologies

Table 2. Which solution fits which job

The 1–1.5 meter drift you see in the field is not a "crooked field" and it’s usually not the operator’s fault.

In most cases, the cause is the limitations of entry-level GNSS hardware and unstable heading determination.

So if:

• you need maximum accuracy, choose RTK;

• you want stable passes without unnecessary cost, GM SMART / GM PRO S with DP filtering is an excellent choice.