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The localization based on TDoA needs to solve the problem of time synchronization and synchronization accuracy first.
Back to the source - Why is time out of sync?
Each UWB positioning device has a crystal oscillator inside, which is used to generate clock frequency. The UWB signal of the UWB
positioning device is sent and received under the rhythm of the clock, and the sending and receiving time is also measured on the
basis of the clock. This means that each hardware device has its own internal clock, and the base station operates according to its
own time system, which leads to the initial time difference of each base station.
In addition, there is a clock drift phenomenon, which makes it impossible for multiple clocks to stay in sync for long even if they start
at the same standard time. Just like each of our watches, after a period of time will not be allowed, the main reason is that the
accuracy of our clock, that is, the 1s of our watch, is not the standard 1s, but close to 1s, if the table is not correct for a long time,
our time system will be offset.
In addition, there are many factors that affect clock drift, such as: production process, temperature change, environmental change,
aging degree, etc. To sum up, the problem of time asynchronism is unavoidable.
How to solve time asynchronism?
We know that UWB positioning system has extremely high requirements for time measurement, and inaccurate time measurement
will seriously affect the positioning accuracy, and a time difference of 1ns will cause a position error of 30cm. Therefore, the time
synchronization problem must be solved, at present, there are two most commonly used solutions, wired synchronization and
wireless synchronization.
1, wired synchronization
Wired synchronization is equivalent to all UWB positioning base stations using the same table, that is, the synchronization controller.
The synchronous controller transmits the time information to each base station through wired communication, so that each base
station is in the same time system to realize time synchronization.
The advantage of this synchronization method is that the time measurement accuracy is high, the time between each UWB
positioning base station can be highly synchronized, and the technical difficulty is low and easy to achieve. The problem is that the
deployment cost is high, because each base station needs to be connected to the synchronization controller through a wired way,
line planning becomes complicated, and there is a lot of wiring when deploying the UWB positioning system. But the synchronization
accuracy will be relatively high.
2, wireless synchronization
Wireless synchronization is that each UWB positioning base station exchanges its own clock information with other base stations
continuously, and finally achieves and maintains global time coordination. That is to say, the UWB positioning base stations carry
out signal interaction at regular intervals, telling other base stations what time they are now in, and then adjusting their own time
to be consistent with other base stations.
The wireless synchronization method depends on the measurement time accuracy of UWB. The higher the time accuracy, the
smaller the time error between two base stations, and the higher the degree of time synchronization between them. The advantage
of wireless synchronization is that there is no need for a wired connection between base stations, so the deployment of UWB
positioning systems is less difficult and cost-effective.
Whether wired or wireless time synchronization is used, synchronization is only related to the base station, and has nothing to do
with the label. Relatively speaking, wired time synchronization is better than wireless time synchronization
1) With higher reliability, the clock source with wired time synchronization is transmitted through the network and is not affecte
by the surrounding environment;
2) With better accuracy, because the same clock source, even if there are temperature drift, aging problems, but still the same
clock source, will not affect the accuracy;
3) Environmental friendliness, wireless synchronization will be affected by the environment, such as shielding on the synchronous
link, multi-path, etc., changes in the factory environment may lead to the accuracy of the entire system is not expected;
TDoA location
The difficulty of TDoA positioning system lies in the synchronization of the positioning base station. During positioning, the tag only
needs to send a broadcast message. Therefore, the power consumption of the tag is much higher than that of the ToF based
positioning mode.
TDoA positioning features:
1, high positioning accuracy, in the non-occluded mode, the accuracy of TDoA can achieve higher accuracy;
2. The label power consumption is low, mainly because the packet duration is very short. Therefore, the power consumption is
relatively low.
3. The system capacity is large. If each label of UWB packets is calculated at 0.5ms, the number of terminals in a single system can
reach 2000.
4, can not meet the positioning outside the area, because outside the area, the hyperbola is in a divergent state, positioning accuracy
is relatively poor;
5, positioning accuracy is easily affected by interference, such as a base station due to occlusion and other reasons will bring the
positioning results beat;
Considering the characteristics of TDoA positioning, its application scenarios are mainly in:
1, the factory category, the number of located materials is huge, requiring the label to work for a long standby time;
2, the positioning accuracy requirements are relatively high;
The TDoA positioning mentioned above is based on the label uplink, that is, the label sends the message, and the base station
receives the message and provides it to the server for calculation. This is a typical factory mode.
In addition to the ascending TDoA, there is the descending TDoA, and all global positioning systems are based on the descending
TDoA. That is, the base station broadcasts, the label receives, obtains the TDoA value between satellites, and calculates its own
position. The characteristics of downlink TDoA are as follows:
1, the terminal is not limited to capacity, anyone can receive to calculate their position;
2, the system power consumption is high, much higher than the uplink TDoA, because it needs to continue to receive GPS satellite
signals, if the UWB downlink TDoA, the tag also needs to continue for a certain time to receive;
3, the highest positioning frequency is limited by the broadcast frequency, can only be lower than it, can not be higher than the
broadcast frequency;
4, high timeliness, this terminal carries out its own data sampling, can calculate its own position, can obtain MS-level calculation delay,
if the server is calculated in the delivery position, it will be affected by the network.
