Principle
RTK (Real-Time Kinematic) uses the known coordinates of a fixed reference station and carrier phase observations to send differential corrections to the rover station, eliminating atmospheric and orbital errors to achieve centimeter-level relative positioning. It is widely used in drones, autonomous agricultural machinery, and surveying instruments. The rover station needs to be within the baseline length limit from the reference station and maintain continuous satellite lock.
Communication
Differential data can be transmitted via radio, network NTRIP, or cellular link; 4G/5G modules often handle both service and differential data channels, requiring low latency and low packet loss.
Limitations
Under trees or in urban canyons, signal lock is easily lost; when multipath is severe, float solution accuracy decreases. Fusion with INS can improve short-term occlusion performance.
Engineering aspects for reference stations, links, and modules
For self-built single-base RTK services, the reference station coordinates must be known and stable, and the antenna phase center and benchmark height must be precisely measured; for multi-base networks, network adjustment and joint atmospheric model solving are required. When differential data is delivered via NTRIP or private network, 4G/5G modules must control jitter and packet loss; otherwise, the rover station will frequently switch between fixed and float solutions. Module firmware should expose GGA sentence quality flags and base station distance, allowing the application layer to decide whether to permit high-safety actions such as automatic steering of agricultural machinery. Regarding fees, continuous high-volume differential data upload and download for several hours requires assessing IoT SIM cards' data plans and edge caching strategies.
Deployment acceptance and maintenance
In addition to static accuracy, project acceptance should also include dynamic path and repeat line tests, recording convergence time and re-capture time. Aging of antenna cables and water ingress at joints can cause phase center drift, requiring annual calibration. Legally, surveying and geographic information results provided externally must comply with qualification and confidentiality requirements. When combined with drones, the frequency separation of the remote control link and RTK data link must also be considered to avoid interference that could cause positioning jumps.
Engineering Practice Supplement
Regarding 'RTK positioning,' it is recommended to distinguish between real-time control links and non-real-time telemetry links in the architecture: control and interlocks should be closed-loop locally as much as possible, with cellular used for monitoring, configuration, and OTA. On the cloud platform side, observability metrics (connection duration, message latency distribution, failure codes, reconnection reasons) should drive optimization, rather than just checking 'connectivity.' Documentation delivery includes interface specifications, error code tables, extreme working conditions, and known limitations; test delivery includes test cases, data, and conclusions, facilitating third-party retesting or auditing.
Integer Ambiguity and Differential Age
Real-time kinematic positioning uses carrier phase, not pseudorange alone. The base station's coordinates are known. It sends phase observations to the rover as differential data. The rover uses that to remove orbit and atmosphere errors common to both sides, then solves the integer ambiguity. After the ambiguity is fixed, relative position can reach centimeters. Before it is fixed there is only a float solution, with a larger error that jumps. The age of the differential data is how old this correction is. As age grows, changes in the atmosphere are no longer seen on both sides at once, and a fixed solution falls back to float. The cellular module here is the pipe for differential data, and often the pipe for service data as well. It does not produce coordinates. Jitter in the pipe becomes age.
The conditions of use are a baseline length inside what this service can support, continuous tracking of enough satellites at the rover, and a measured antenna phase center and pole height. When corrections are broadcast over the network, the account, the mount point, and the coordinate frame have to match the base. After a fixed solution is lost, agricultural machinery or a drone must fall back to manual control or stop the automatic action. It must not keep using the centimeter accuracy of the previous second. In urban canyons and under canopy, expect loss of lock, and define behavior during reacquisition in advance.
A Float Solution Treated as Fixed
The most dangerous misuse is to look only at whether a latitude and longitude exist, and not at solution status and quality factor, so a float solution is written into automatic steering. The second is to keep reporting fixed on an expired correction as soon as the differential link stalls, after the age has already left the assumptions of the algorithm. The third is a wrong base coordinate or antenna height. The network solution looks consistent internally, and the absolute position is shifted as a whole. Water in a connector lets the phase center drift slowly. A static retest finds it. During dynamic work it shows only as an occasional jump.
Whether link delay comes from registration, an idle rebuild, or a hairpinned user plane has to be separated with connectivity and the roaming path; see connectivity diagnostics and IoT roaming. If a coordinate jump appears exactly when the device crosses into a visited network, suspect a break in the differential data before suspecting the satellite count.
When the rover output goes to the control logic, it should carry at least solution status, differential age, whether satellite tracking is continuous, and distance to the base. If any of those is missing, automatic steering can keep moving on a float solution or an expired correction. Age beyond the time the algorithm allows should degrade immediately, even if the status flag has not yet moved from fixed to float. After the base coordinates or antenna height change, the old differential service has to be taken out of use. A rover must not connect to a station whose coordinates have moved and still display fixed. Water in a connector or cable shows up as a slow phase drift. Repeated static measurements find it sooner than watching the live decimals. A cellular outage and a loss of satellite lock should be two alarms. The first can fix again once differential data returns. The second has to wait until the sky is visible. Merging them into "positioning failed" sends people to check the SIM under trees and the antenna where there is no network. Once the two alarms are separate, the field can decide whether to restore the correction or wait for satellites.