Recently, float solution has appeared frequently while using a same CORS account for the same RTK in the same area, especially in the afternoon, which was rare to happen before. The main of reason can be regarded as that the ionosphere is active. 

After learning from a lot of relative information, following 6 problems maybe are the most concerned:

1. What is ionosphere?

2. How does ionosphere affect the "Fixed Solutions"?

3. Which factors will make ionosphere active?

4. Why did float solutionsrarely happen in the past, but did frequently recently?

5. Are there any methods to solve the float solution while using CORS accountsfor RTK?

6. Is the effect of active ionosphere short-time or long-term?

1. What is ionosphere?

1.1 Ionosphere

Ionosphere is the upper atmosphere ionized by high-energy radiation from the sun and cosmic rays. Above 60 kilometers, the entire atmosphere of the Earth is partially or completely ionized. Ionosphere shielded us from a lot of cosmic rays, keeping everything on Earth alive.

1.2 Ionosphere’s vertical stratification

In the direction of height, the distribution of ionospheric electron density is uneven. Generally, it is divided into ionosphere D, E, and F from low to high. At the same time, ionosphere is easily affected by solar activities.

Ionospheric Stratification Diagram

Ionosphere D: The variation of electron density is mainly related to solar radiation. In every afternoon, it generally occurs the maximum ionospheric density distribution. However, the electron density gradually decreases with sunset, and it can be basically negligible at night.

Attention: The above is the reason that why float solution often happens in the afternoon, but rarely in the morning and at night.

Ionosphere E: The ionospheric electron density changes regularly during the day, while it changes greatly at night.

Ionosphere F: It has the highest electron density. During the daytime in summer, it can also be subdivided into ionosphere F1 and F2. Ionosphere F1 mainly appears in the middle latitude in summer, but disappears at night. On the contrary, ionosphere F2 will always be there.

1.3 Ionosphere distribution in the South and North

Under the influence of geomagnetic activity, the content of ionospheric electron is almost distributed along magnetic line of force around the world.  According to the distribution characteristics, it generally can be divided into several areas, such as at low latitude, middle latitude and high latitude and so on.

At low latitude areas, the content of ionospheric electron changes more actively, and the maximum content of ionospheric electron occurs at the north and south sides of the equator rather than at the equator(Equatorial Anomaly).

In the middle latitude areas, the ionospheric activity is less active. But it is affected by the solar radiation and weather change, so there are obvious periodic phenomena for the variation of ionospheric electron content. 

In the polar areas, it has less variation in ionospheric electron content between day and night because of obvious differences in solar irradiation, compared with other areas. Besides, the seasonal change is also very slow.

The ionospheric intensity corresponds to longitude and latitude

2. How does ionosphere affect the "Fixed Solutions"?

There are many factors affecting GNSS positioning, and ionosphere is the one that make the most serious influence and difficult to control.

2.1 Influence of ionosphere on satellite signals

GNSS satellite signals are transmitted from space through the atmosphere to RTK receivers. Due to a large number of electrons and ions, the transmission speed and direction of signals will change when through ionosphere. Besides, the transmission path will be slightly curved, causing the delay and offset of the satellite signals. However, RTK receivers themselves can't reflect how much the satellite signals are delayed and offset.

Influence of ionosphere on GNSS signals

2.2 How does CORS service reduce and eliminate the influence of ionosphere?

There are a variety of modeling methods for CORS ionosphere, which can be simply understood as follows:

In the following diagram, ionosphere status of the rover at point b is allowed to be calculated through interpolation modeling based on the ionosphere status at point a and c observed at CORS base 1 and 2, and then the ionosphere status will be sent to the rover to calculate the "Fixed Solutions".

Diagram of CORS Ionosphere Interpolation

This figure is just for demonstration only, and 3 bases are required at least for CORS net.

2.3 Influence of active ionosphere on CORS service

If the ionosphere is active, the accuracy of ionosphere at point b calculated by the surrounding CORS base 1 and 2 will be reduced, and if send such kind of data to RTK receivers, it is not allowed to confirm integer ambiguity. As a result, "Float Solutions" will appear in handheld  controllers for a long time, or even if integer ambiguity can be confirmed to get fixed solutions, the STD will be reduced, compared with the surveying results when ionosphere is not active, difficult to reach nominal accuracy of RTK receivers: plane±8mm+1ppm, elevation±15mm+1ppm.

3. Which factors will make ionosphere active?

It is mainly caused by geomagnetic activity, solar activity.

4. Why did float solutionsrarely happen in the past, but did frequently recently?

The solar cycle affects the ionosphere on an 11-year cycle invisibly. According to the information in NASA website, the 25th solar cycle has begun in Dec., 2019, and now it is in an upward cycle of frequent solar activity, which is expected to peak in July, 2025, and then decline, ending around 2030.

The 25th Solar Cycle

Attention: In the peak year of solar cycle, the electron density of ionosphere D is several times that in the off-peak year, for which it is observed that it is easier to get float solutions than that were in last year, the year before that or even earlier time when using CORS accounts.

5. Are there any methods to solve the float solution while using CORS accountsfor RTK?

It is only can be suggested to survey by avoiding as far as possible the active period of ionosphere because the float solutions are caused by active ionosphere. If it is going to solve the issue of float solutions completely, it can only rely on breakthrough on industrial technology to deal with.

6. Is the effect of active ionosphere short-time or long-term?

It is foreseeable that the ionosphere will become more active from now to 2025, but this does not mean that CORS accounts will float frequently in the future, which is a problem faced by the whole industry. It is very happy to see the industry's continuous investment in technology research and development, optimization of CORS service algorithm and terminal RTK algorithm, to reduce the probability of float solutions resulting from ionosphere activity, improving the efficiency of first-line surveying and mapping personnel.

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