RTK Status Explained: Single, DGNSS, Float & Fixed

The solution status displayed by your geodetic receiver represents the current mathematical state of its positioning engine. Real-Time Kinematic (RTK) systems function by comparing raw satellite observations from a mobile rover against known coordinates at a reference station. As the telemetry link streams data, the inner algorithms move through specific signal processing stages based on the type of correction data received and the statistical confidence of the positioning engine.

The mathematical progression always moves from Single to DGNSS, then Float, and finally Fixed. Each consecutive step narrows down the spatial uncertainty as more observation epochs accumulate and the internal processing core resolves carrier-phase tracking ambiguities. Under normal clear-sky field conditions with a dependable network connection or a local base station within a 20km radius, this initialization pipeline finishes within 10 to 60 seconds.

This active status status is continuously parsed from standard NMEA data strings and displayed on the main mapping interface of the field data controller. To ensure data integrity, operators can configure the software to reject non-fixed observations completely, blocking sub-metre variations from corrupting the core database during automated field logging workflows.

The Single status, often termed Autonomous or Single Point Positioning (SPP) across engineering literature, indicates that the GNSS hardware is tracking satellites and computing a standalone coordinate position without receiving any external differential correction data. The positioning engine relies exclusively on the broadcast ephemeris messages directly transmitted by the satellite constellations.

When It Appears in the Field

• Immediately after system boot-up before the hardware establishes a cellular network handshake or a radio link connection.
• When an internal SIM card is missing, disabled, or fails to latch onto a local mobile communications tower.
• When NTRIP server credentials, IP configurations, port entries, or specific source mountpoints are incorrectly typed.
• When a local reference transmitter is powered down, suffers an electronic failure, or is separated by distances that exceed the range of the internal UHF or LoRa radio data link.
• During complete cellular blackouts caused by heavy topographical features, remote work zones, or regional service infrastructure failures.

Accuracy Profile and Recording Safety

In Single mode, spatial accuracy drops to the $\pm3\text{ m}$ to $\pm5\text{ m}$ horizontal margin. This tier matches basic consumer smartphones or automotive satellite navigation hardware. Because the receiver cannot compensate for ionospheric propagation delays, tropospheric scattering, or satellite clock bias deviations, coordinate readings fluctuate significantly across the screen, tracing an uncorrected cloud of points over a several-metre radius.

Is it safe to record data? Absolutely not. Single mode is unacceptable for engineering, cadastral, or construction tasks. Coordinates logged under this status will display massive discrepancies relative to established site control benchmarks, which will cause serious alignment problems when the spatial data layers are integrated into CAD or GIS environments.

Corrective Field Steps

When Single status continues, open the software communications terminal to verify your data link configuration. Check the cellular data signal level, ensure the correction modem shows active diagnostic loops, and verify that the base transmitter is broadcasting if using a physical base-and-rover kit. If settings match but the status remains unchanged, refer to our detailed technical troubleshooting reference on RTK not getting Fixed.

Differential GNSS (DGNSS) indicates that the mobile rover has successfully connected to a correction provider and is actively parsing correction streams, but is restricted to processing code-phase pseudorange measurements. It does not use the phase characteristics of the underlying carrier waves.

When It Appears in the Field

• As a transient step lasting only a few seconds while the positioning algorithm switches from Single mode into carrier-phase tracking logic.
• When hooked into legacy reference networks or utilizing regional Satellite-Based Augmentation Systems (SBAS) like WAAS, EGNOS, or MSAS that only transmit code-level range corrections.
• When an NTRIP provider streams a correction format that lacks the necessary RTCM carrier-phase message types, keeping the rover engine from initializing higher-precision tracking modes.

Accuracy Profile and Recording Safety

DGNSS solutions generally deliver a horizontal precision range of $\pm0.3\text{ m}$ to $\pm1\text{ m}$. While this represents a significant improvement over autonomous single-point positioning by removing major common-mode atmospheric errors, it cannot deliver the millimetric precision required for professional engineering tasks.

Is it safe to record data? No, DGNSS is inadequate for property boundary definitions, road design stakeouts, or dense topographic grid collections. It is only suitable for general GIS asset inventories, asset location mapping, environmental sample positioning, or general forestry workflows where sub-metre variations meet project tolerances. Always confirm accuracy parameters before saving coordinates under this status.

Corrective Field Steps

Pause field work and monitor the solution diagnostic panel. Under normal operational conditions, DGNSS should transition to Float status within seconds. If the receiver remains locked in DGNSS continuously, examine the active mountpoint configuration. Ensure the network reference source streams an RTCM 3.x or MSM format that includes carrier-phase observations, rather than an unguided DGPS-only correction message.

Float is frequently misunderstood by field operators. Because the position readouts stop fluctuating and seem stable on the hand-held controller screen, users often assume the system has stabilized and begin logging points. This misunderstanding is a primary cause of coordinate corruption in field datasets.

The Underlying Physical and Geodetic Reality

When an instrument switches to Float status, it has successfully isolated and started tracking the individual carrier phase profiles of the satellite signals. This process involves utilizing the precise wave-based structure of L1, L2, and L5 frequencies, where wavelengths are relatively short (e.g., approximately $19\text{ cm}$ for L1). However, the internal positioning engine has not yet resolved the carrier-phase integer ambiguity. This parameter represents the exact, integer number of full radio wave cycles between the satellite transmitter and the rover antenna.

Instead of locking this value down to an absolute whole number, a statistical engine (typically an advanced Kalman filter) estimates the ambiguity as a floating-point decimal value within a probability distribution. Because this integer matrix is unresolved, the resulting coordinates carry an underlying positional uncertainty of $\pm0.3\text{ m}$ to $\pm1\text{ m}$. This variation persists even though the screen layout may appear stable and deceptively precise.

Field Appearance and Recording Risks

On the hand-held controller display, the coordinate readings stop jumping and shift slowly. This visual stability often tricks operators into thinking the position is correct. Under tight project schedules or demanding field conditions, field personnel often record stakeout benchmarks or topographic features while in Float mode because the numbers look steady.

Is it safe to record data? Absolutely not. Recording measurements in Float status introduces hidden errors that can cause structural issues on engineering projects, leading to failed inspections during subsequent grading or validation checks. To safeguard data quality, access the configuration parameters inside your software suite and set the storage criteria to reject Float recordings across all primary survey scripts.

Expected Processing Durations

Under clear sky conditions with a short baseline distance to the reference station (under 20km), Float status resolves into a Fixed solution within 5 to 30 seconds. In more challenging operational settings — such as working beneath partial forest canopies, encountering heavy urban multipath reflections, or running extended baselines — this integer ambiguity resolution loop can take between 1 and 3 minutes to settle down.

A Fixed status indicates that the positioning engine has fully resolved the integer ambiguities. The internal search algorithms have processed the carrier-phase data, eliminated the statistical decimal spread, and locked onto the exact number of full wavelengths between the receiver antenna and every tracked satellite.

The Achieved Performance Tier

Once a Fixed solution is achieved, true centimetre-level positioning becomes active. Under clear-sky conditions with clean satellite line-of-sight, the system achieves a baseline precision threshold of $\pm8\text{ mm}$ horizontally and $\pm15\text{ mm}$ vertically. This provides the accuracy required for high-precision engineering and geodetic applications.

This accuracy degrades gradually as the baseline distance to the reference station increases, due to spatial decorrelation of atmospheric conditions. This effect typically adds roughly $1\text{ mm}$ of additional uncertainty per kilometre of baseline distance, resulting in a total horizontal variance of $\pm10\text{--}20\text{ mm}$ when operating 30km away from the reference node.

The Critical Metric: Correction Data Age

The field controller displays a real-time diagnostic value labeled "Age" or "Latency" alongside the active Fixed status icon. This parameter records the exact time elapsed since the rover parsed its last valid differential correction packet. When linked to an NTRIP server network, this latency value should remain under 2 seconds. When operating via local UHF or LoRa base station transmissions, it should stay under 4 seconds. If the age value rises above 5 seconds, it indicates data throughput restrictions or signal degradation on the wireless link, which can cause the underlying precision of the Fixed solution to break down.

Operational Clearance

Is it safe to record data? Yes, Fixed is the only status level that provides survey-grade precision. Field operators must verify that the receiver holds a stable Fixed status before capturing topographic features, establishing site benchmarks, or executing structural stakeouts.

Differential age measures the latency of the inbound correction stream, tracking the time elapsed since the rover last received a valid update from its reference station. This metric is usually displayed prominently on the data controller as "Age: 1s" or "Latency: 0.8s".

The Underlying Physics of Signal Degradation

A receiver showing a Fixed indicator with an internal age value of 8 seconds does not deliver $\pm8\text{ mm}$ accuracy. Because satellite clock states, orbital trajectories, and ionospheric profiles drift continuously, correction packets represent an atmospheric snapshot that validly models the spatial errors only at that specific epoch. When correction data fragments are delayed over multiple seconds, the rover must extrapolate these orbital and atmospheric corrections forward in time.

This time gap causes a rapid expansion of the underlying error envelope. By the time the differential age reaches 8 to 10 seconds, the true horizontal positioning error can degrade to $\pm50\text{--}200\text{ mm}$, even if the status flag on the hand-held controller still shows Fixed. This discrepancy can easily cause errors during precise engineering alignments or tight grading checks.

Target Latency Parameters

• CORS and NTRIP Links: Latency should remain $\le 2\text{ seconds}$ for optimal network error modeling.
• Base-to-Rover UHF/LoRa Configurations: Latency should stay $\le 4\text{ seconds}$ to handle standard radio modulation delays.
• 5 to 10 Seconds: Indicates a degraded correction state; stop recording critical points and inspect the wireless data link.
• Greater than 10 Seconds: The Fixed status flag is no longer reliable; stop all field measurements immediately.

Common Causes of Latency Issues

High age values are typically caused by weak cellular signals that disrupt NTRIP TCP/IP streams, local radio frequency interference on the site channel, approaching the maximum range of the base radio, or data packet loss at the base station transmitter.

Operational Field Rule

To safeguard your project data against accidental coordinate corruption, configure your field software settings to restrict data storage before starting your survey work. Navigate to Settings → Survey → Minimum Solution Status inside the ApekSurv environment and select Fixed. This software rule prevents the system from logging coordinates unless the positioning engine has resolved all integer ambiguities. Make it a standard practice to verify this software rule at the start of every field project.