logo GNSS satellites available for testing timing of astronomical images

Overview of tools for asteroid observers on this site

Click here for an explanation of what this page is and why it exists, and links to related tools.

Use the form below to find out what GNSS satellites (GPS, Galileo, GLONASS, BeiDou, JXSS) are observable from your MPC code at a given time.

Time: UTC

MPC observatory code :

Minimum altitude:

Sort order:
Elongation from sun
Altitude above horizon
GNSS designation (such as 'G20' or 'R02')
International designation (such as '2000-025A' or '2006-062A')

RA/dec in base 60 (Babylonian)
RA/dec in decimal degrees

Show TLE-based objects (you probably don't need to tick this box)

There are several other tools for asteroid observers on this site. In particular, you can create an ephemeris for a particular GNSS satellite.


Time can be in almost any human-recognizable form, such as

2017 feb 27 03:14:15.9   feb 27 2017 03:14:15.9   27.314159/2    (assumed to be current year)
2017 02 27.314159        27.314159/02/2017        now+3h         (three hours from 'right now')
27.314159fe2017          JD 2457811.814159        Fri 03:14 p.m.

Click here for a full list of possible time formats that can be used. Feel free to experiment, but getting too clever is not advised. I recommend a four-digit year, named month, and day followed by HH:MM:SS or decimal days (the first two examples shown above), leaving no ambiguity in your input.

Time span: Ephemerides are available from 1992 June 20 to four or five days from now. Note that unfortunately, ephemerides for Galileo, QZSS, and BeiDou only run up to about a day ago; from that point on, all we've got are ephems for GPS and GLONASS. I'm working on this.

Satellite IDs with 'G' followed by two digits are GPS satellites. Those with 'R' followed by two digits are (Russian) GLONASS satellites. 'E' is for the (European) Galileo satellites, 'J' for the (Japanese) QZSS, and 'C' the (Chinese) BeiDou satellites.

MPC observatory code: This should be a three-character code from the MPC's list of observatory codes. If you don't have such a code, send me your latitude, longitude, and altitude (see contact details at bottom) and I'll add a code for you.

Satellite magnitudes: Jaime Nomen told me the magnitudes for GPS satellites range from 10 to 14, depending in part on the phase angle. (I'm hoping the GLONASS satellites are brighter, but haven't heard of any observations yet.) By default, the satellites are ordered by elongation; I'd suggest getting satellites with high elongation that aren't marked 'Sha' (in the earth's shadow).

Jaime recommends getting images of multiple satellites at multiple times, to help detect random timing errors.

Accuracy appears to be at least down to 0.2 arcsecond, or about 10 milliseconds of motion. (Which is to say, you could correct your timing to that level.) In theory, it's better than that, but I don't really have a way to test it except by matching it to observations. The best observations thus far have led to timing and positions consistent to that level.

The satellites have positions known to within a few centimeters. Of course, that's the position of the antenna; if the center of light measured on a CCD is, say, a meter away, that would lead to about half a millisecond of error (the satellites move at about 2000 meters a second). We may be at that level of accuracy, but I can't really say for sure.

Source code (C/C++) is available at GitHub.

'GNSS' (Global Navigation Satellite System) includes GPS, GLONASS, Galileo, QZSS, and BeiDou satellites. We have precise positions (within centimeters) for 'right now', and a few days into the future, for the GPS, GLONASS, and Galileo constellations. You can make plans to observe those objects, get data for them, and immediately check your data against the precise values.

For Galileo, QZSS, and BeiDou, we only get that precision a few days after the fact. For 'right now' and the future, we have less precise positions computed using Two-Line Elements (TLEs). Those are good to a few kilometers, enough to let you find the objects with a telescope. Then, after a few days, full-precision positions good to centimeters will be posted, and you can actually check your data against them.

Usually, there are plenty of GPS, GLONASS, and Galileo objects available, and those will be the only one listed for 'right now' and the next few days. If you tick the 'Show TLE-based objects' box above, you'll get a lot more satellites that can be observed... but you you won't be able to actually process your observations against precise positions until a few days after you've made them. This is annoying enough that by default, that box is left unticked. (And given how many objects are in the first three groups, I can't really come up with a good reason to tick it.)


My thanks go to Jaime Nomen and Noelia Sánchez Ortiz for their thoughts on using navigation satellites for timing astrometry, and for providing some example astrometry for testing that I was getting the right answers; and to Dave Tholen for doing some really amazing tests of the sort that require a 2.2-m telescope, the extreme conditions on Mauna Kea, and really knowing how to get the last bit of precision out of astrometry.

Change log

(2018 Jan 1) Galileo, BeiDou, and JXSS navsats are now also listed, along with the GPS and GLONASS satellites, even for "current" times. My hope is that this will provide alternative, possibly brighter targets. Be warned, though, that there's a slight element of inconvenience to them. I can tell you where to look for these objects, with an accuracy of an arcminute or so. But it takes a few days before really precise positions are available for them. Basically, you can look here, find out where to get these satellites, and then check back in a few days to learn where they "really" were.

Contact info

I can be reached at p‮оç.ötŭlpťсéјôřp@otúl‬m. If you're a human instead of a spambot, you can probably figure out how to remove the diacritical marks...