During RTK surveying, the solution status is one of the first things surveyors check before measuring a point. A FIXED solution generally indicates that the receiver has successfully resolved the integer ambiguities required for centimeter-level positioning.

However, the solution status is not the only indicator of positioning conditions. Differential Age is another important parameter that should be monitored during fieldwork, as it indicates how recently the receiver received correction data from the base station or CORS network.

In this article, we will take a closer look at Differential Age, why it may increase during RTK surveying, how it can affect positioning, and what you should check when the value becomes unusually high.

What Is Differential Age?

Differential Age, also known as Correction Age, represents the time elapsed since the receiver last received differential correction data.

During normal RTK operation, correction data is continuously transmitted from the base station or CORS service to the rover. When the correction data is received normally, the Differential Age generally remains low and stable.

When correction data is delayed or temporarily interrupted, the Differential Age increases. It therefore provides a useful indication of the timeliness and continuity of the correction data being used by the rover.

For more information about the typical Differential Age ranges for different RTK correction methods, you can refer to our previous article, "Tech Chat | Understanding The Meaning & Accuracy of Common RTK Solutions."

Why Does Differential Age Increase?

The reasons for an increased Differential Age depend largely on how the rover receives correction data.

Radio-Based RTK

When using a radio base, Differential Age may increase when the radio communication becomes weak or unstable. The distance between the base and rover, buildings, terrain, trees, and other physical obstructions can all affect the radio link.

For example, when the rover moves behind an obstruction, correction data may not be received continuously, causing the Differential Age to increase.

CORS / NTRIP RTK

When using a CORS or NTRIP service, Differential Age is more closely related to the network connection. Poor cellular coverage, unstable mobile data, or interruptions in the connection to the CORS service can delay the delivery of correction data to the rover.

In these situations, the receiver may remain connected to the correction service, but the correction data itself may no longer be arriving continuously.

Temporary Interruptions

A temporary increase does not necessarily indicate a persistent problem.

For example, if the correction data is briefly interrupted and then restored, the Differential Age may increase for a short period before returning to its normal range.

The key is to pay attention not only to the current value, but also to whether the value is stable or continuously increasing.

How Does High Differential Age Affect RTK Positioning?

RTK positioning relies on continuously updated correction data. When correction data becomes older, it may no longer represent the current conditions at the rover as accurately, which can affect positioning stability and accuracy.

As the Differential Age increases, the RTK solution may become less stable. If the interruption continues, the receiver may eventually lose its FIXED solution and change to FLOAT or another positioning status.

It is also worth noting that FIXED status and Differential Age represent different aspects of RTK positioning. FIXED indicates that the receiver has achieved integer ambiguity resolution, while Differential Age indicates how recently the latest correction data was received.

Therefore, a receiver may still display FIXED for a period of time even when the Differential Age has increased. This is why checking the solution status alone may not provide a complete picture of the current RTK conditions.

Should You Measure a Point When Differential Age Is High?

When preparing to measure an important point, it is not recommended to rely on the FIXED status alone. You should also check whether the Differential Age is stable and within a reasonable range.

If the receiver is FIXED but the Differential Age suddenly becomes much higher than normal, it is better to wait for the correction data to recover before saving the point.

If the value remains high or continues increasing, check the correction data connection:

• Radio mode: Check the radio signal, baseline distance, and possible physical obstructions.

• CORS/NTRIP mode: Check the mobile network connection and CORS service.

• Both modes: Confirm that the correction data is being received continuously and that the Differential Age returns to a stable level.

This simple check can help prevent points from being collected while the correction data connection is unstable.

A Simple Field Check Before Measure A Point

When checking whether an RTK receiver is ready for measurement, Differential Age should be considered together with other positioning quality indicators.

Before saving an important point, you can quickly check:

• RTK Solution: Confirm that the receiver is in FIXED status.

• Differential Age: Make sure the value is stable and within a reasonable range.

• PDOP: Check the satellite geometry and overall positioning conditions.

• HRMS / VRMS: Check the estimated horizontal and vertical positioning accuracy.

These parameters provide different perspectives on positioning quality and should be considered together rather than relying on a single value.

For a more detailed explanation of these indicators, see our previous article, "Tech Chat | Understanding Key Indicators of GNSS Positioning Quality from an RTK Survey Point."

Conclusion

Differential Age is a small but useful indicator that can provide valuable information about the quality and timeliness of RTK correction data.

A stable FIXED solution is important for high-precision surveying, but keeping an eye on Differential Age can help you identify unstable correction data before it affects your measurements. By monitoring Differential Age together with other positioning indicators such as PDOP, HRMS, and VRMS, surveyors can make better-informed decisions about when to measure and save points.

A few seconds of checking these indicators before measurement can help improve the reliability of RTK survey results and reduce the risk of collecting points under unstable conditions.