Variables
Values for the data_vars parameter of Time Series and Nearest Time.
| Variable | Description | Unit | JSON value |
|---|---|---|---|
ztd |
Zenith Total Delay | mm | Number |
ztd_sigma |
ZTD uncertainty (standard deviation of the estimate) | mm | Number |
pwv |
Precipitable Water Vapour | mm | Number |
snr |
Carrier-to-noise density ratio (C/N0) per signal band | dB-Hz | Comma-separated string of 9 band slots: GPS L1, L2, L5; Galileo L1, L2, L5; BeiDou L1, L2, L5. A slot is empty when the band is not tracked |
el |
Satellite elevation | degrees | Comma-separated string, one value per satellite in the order of prn |
az |
Satellite azimuth | degrees | Comma-separated string, one value per satellite in the order of prn |
prn |
Satellite pseudo-random noise (PRN) identifiers, e.g. G08, E03 |
- | Comma-separated string |
The API adds prn automatically when you request el, az, or snr.
Satellite identifiers (prn)
Each GNSS satellite broadcasts its own pseudo-random noise (PRN) code, and the PRN number identifies the satellite within its constellation. prn lists the satellites the station tracked at that epoch, as a constellation letter followed by a two-digit number:
| Letter | Constellation | Example |
|---|---|---|
G |
GPS | G08 |
E |
Galileo | E03 |
el and az list one value per satellite in the same order as prn, so the third elevation belongs to the third satellite. A station typically tracks 10 to 25 satellites per epoch, so the list length changes from epoch to epoch.
{ "prn": "E03,E05,G08", "el": "22.3,74.2,9.8", "az": "278.2,255.4,94.4" }
Here G08 is GPS satellite 8, at 9.8 degrees elevation and 94.4 degrees azimuth.
snr is not per satellite: it has one value per signal band (see the table above). Its BeiDou slots are reserved and currently empty.
How PWV is derived
Skyfora calculates PWV from ZTD. The calculation also uses surface pressure, air temperature at 2 m, and relative humidity. These inputs come from the nearest suitable observation. When no observation is available, they come from a numerical weather prediction (NWP) model. Skyfora adjusts them for the height of the GNSS site.
The response does not include these inputs. They are not always measured at the GNSS station.