3. Detailed information

3-a. 3D electron density option

There are three options: "GNSS tomography," "GAIA model," and "IRI model".
*"GNSS tomography" is the default option.

• GNSS tomography: 3-D real-time ionospheric tomography over Japan based on GNSS observations
  https://www.enri.go.jp/cnspub/tomo3/
• GAIA: Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy
  https://gaia-web.nict.go.jp/index_e.html
• IRI: Widely used International Reference Ionosphere model. IRI-2016 model is used in the HF-START service.
  http://irimodel.org/

3-b. Target date and time (UTC)

You can set the date and time (UTC) for calculation.
*The most recent date and time, which can be used in the calculation, are set as default values. You can also click the "Latest" button to make sure that it is the most recent time for calculation. Date and time can be inputted freely in the available data range.

3-c. Transmitting point for calculation

You can set the transmitting point into fields.
*"Lon: 139.5,  Lat: 35.7" are set as default values of the transmitting point longitude and latitude. 
You can enter the transmitting point by the numerical input of longitude and latitude or by selecting from major cities in Japan and the world. You can switch the format with the button at the top of the input field. **Selectable transmitting points differ between "GNSS tomography," "GAIA model," and "IRI model," as shown in the following table.

	GNSS tomography	GAIA model	IRI model
Tx lon [degree E]	128 to 147	-180 to 180	-180 to 180
Tx lat [degree N]	30 to 46	-90 to 90	-90 to 90

3-d. Transmission information and receiver point for calculation

You can set transmission information and receiver point into fields, and up to 10 sets of these can be registered by clicking "Set ray path" for the 1^st set and "Add ray path" for the 2^nd to the 10^th sets. The registered set can be removed by clicking the "X" mark.
Depending on whether the Ham radio mode is on or off, you can input parameters differently, as written below. At each mode, multiple formats of input are possible. Select the preferred option at the top of each input field.

[When Ham radio mode is OFF]

• Transmission angle: You can enter the zenith angle (degree) or elevation angle (degree) numerically.
• Frequency: You can enter the frequency (MHz) or wavelength (m) numerically.
• Receiver point: You can enter the transmission point by the numerical input of longitude and latitude or by selecting from major cities in Japan and the world.

When you open the web tool, the default settings are "Zenith: 30," "Frequency: 6.055," "Rx lon: 130.6," and "Rx lat: 31.2".

[When Ham radio mode is ON]

• Transmission angle: You can enter the launch angle (degree) numerically.
• Frequency: You can chose from the list of commonly used frequency bands (MHz)/ wavelengths (m).
• Receiver point: You can choose from major cities in Japan or DXCC entities.
• For the "GAIA model" and "IRI model," you can select the long path calculation by clicking the "Long-path" checkbox.

When you open the web tool, the default settings are "Launch angle: 60," "Freq. band/ Wavelength: 3.5 MHz/80 m", and "Hokkaido (@Sapporo)".

**The available ranges for "GNSS tomography," "GAIA model," and "IRI model" are shown in the following table.

	GNSS tomography	GAIA model	IRI model
Zenith angle [degree]	0 to 79.9	0 to 79.9	0 to 79.9
Elevation angle [degree]	10.1 to 90	10.1 to 90	10.1 to 90
Launch angle [degree]	10.1 to 90	10.1 to 90	10.1 to 90
Frequency [MHz]	3 to 30	3 to 30	3 to 30
Wavelength [m]	10 to 100	10 to 100	10 to 100
Rx lon [degree E]	128 to 147	-180 to 180	-180 to 180
Rx lat [degree N]	30 to 46	-90 to 90	-90 to 90

3-e. Error massages

Dialogue title	Dialogue massage	Meaning
Service Unavailable	Please try again later.	Server connection was failed.
No density data	Specified time's density data does not exist.	For some reason, specified time's density data does not exist.
Time-out Error	Your process has timed out.	Calculation request has timed out.
Error on processing program	Process ID: [ID]	Calculation request was failed on [ID].
This server is too busy	Please try again after a while.	Currently, the server capacity is overflown.
Internal Server Error	Unexpected error has occurred.	Unexpected error has occurred.

3-f. Propagation information

The propagation information is available for every Tx-Rx path. Information from each Tx-Rx path is started with "Pathxx", where xx is the path number (i.e., 01-10). The zenith angle (degree) and azimuth angle (degree) of the first hop, Tx latitude (degree) and longitude (degree), Rx latitude (degree) and longitude (degree), and the frequency (MHz) are listed.

Next, the propagation information from each hop is listed. At each hop, "Refraction point" indicates the highest point [latitude (degree), longitude (degree), and altitude (km)], where the radio wave refracts back from the ionosphere. "Propagation time" in milliseconds indicates the time when the radio wave propagates in one hop from one point to another point on the ground. "Next launch" indicates the zenith angle (degree) and azimuth angle (degree) for launching the next hop. "Reach point" indicates the point [latitude (degree), longitude (degree), and altitude (km)], where the radio wave reaches the ground. It is also the position where the next hop will be launched. "Distance to destination" indicates the nearest distance on the ground from the reach point to the Rx.

When no more hops can propagate to the Rx, the message "No more hop is possible from now" will be displayed. "Max hop" indicates the maximum hop number that the radio wave can propagate from Tx to Rx. Only the ground-to-ground propagation is counted as one hop.

4-a. Plot region

You can set the longitude range and latitude range.
The default settings for "GNSS tomography," "GAIA model," and "IRI model" are shown in the following table. You can reset these default values by clicking the "ResetSettings" button.

	GNSS tomography	GAIA model	IRI model
Min.Longitude (deg)	127	0	0
Max Longitude(deg)	147	360	360
Min.Latitude(deg)[degree N]	30	-90	-90
Max Latitude(deg)[degree N]	45	90	90

4-b. Plot rotation

You can set depression and azimuth into fields.
The depression is the angle at which you look down on the image; a larger depression will make the image appear as if it is being viewed from a higher perspective, and a depression of 0 degrees will make the image appear as if it is being viewed from directly on the side.
The azimuth is based on a diagram viewed from the viewpoint on the east side facing west (angle = 0 degrees), and the diagram rotates horizontally and clockwise by an angle; for example, 270 degrees is the diagram viewed from the viewpoint on the south side facing north.
When you open the AdvancedSettings window, the default settings are "Depression: 40" and "Azimuth: 285". You can reset these default values by clicking the "ResetSettings" button.

5. Reference

5-a. HF-START

• K. Hozumi, M. Ishii, S. Saito, T. Maruyama, H. Nakata, and T. Tsugawa, "HF-START: Application in Aid of Radio Communications/Navigation", In: Electronic Navigation Research Institute (eds) Air Traffic Management and Systems III. EIWAC 2017. Lecture Notes in Electrical Engineering, vol 555. Springer, Singapore, https://link.springer.com/chapter/10.1007/978-981-13-7086-1_19.[Paper link]
• K. Hozumi, T. Kondo, S. Saito, H. Nakata, T. Maruyama, T. Tsugawa, and M. Ishii, "Low Cost Development of HF Receiver Prototype for HF-START Field Campaign", 2018 2nd URSI Atlantic Radio Science Meeting (AT-RASC), Meloneras, Spain, pp. 1-4. doi: 10.23919/URSI-AT-RASC.2018.8471477, 2018. (Young Scientist Award)　[Paper link]

5-b. GNSS tomography

• S. Saito, S. Susuki, M. Yamamoto, A. Saito, "Real-time ionosphere monitoring by three- dimensional tomography over Japan", Inst Navig 64:495–504. https://doi.org/10.1002/navi.213, 2017.　[Paper link]

5-c. GAIA

• Tao, C., H. Jin, Y. Miyoshi, H. Shinagawa, H. Fujiwara, M. Nishioka, and M. Ishii, "Numerical forecast of the upper atmosphere and ionosphere using GAIA", Earth Planets Space, 72:178, https://doi.org/10.1186/s40623-020-01307-x, 2020.　[Paper link]

5-d. IRI

• D. Bilitza, D. Altadill, V. Truhlik, V. Shubin, I. Galkin, B. Reinisch, and X. Huang, "International Reference Ionosphere 2016: From ionospheric climate to real-time weather predictions", Space Weather, 15, 418-429, doi:10.1002/2016SW001593, 2017.　[Paper link]