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GNSS satellites (at least GPS ones) are in Medium Earth Orbit (MEO). LEO satellites can use GNSS for positioning. Is this still possible for satellites in elliptical, such as Molniya, orbits? For geostationary satellites, all GNSS satellites will be in a relatively small solid angle (but still much larger than the earth one), but in principle, it should still be possible to use GNSS. How far up has GNSS-based positioning been used, and how does the precision degrade with distance to Earth, theoretically and in practice?

Nathan Tuggy
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gerrit
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2 Answers2

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The furthest satellite that I have seen use GPS is the AO-40, an amateur built satellite. In fact, they did a research paper on the subject. The paper states that it was able to achieve a navigation solution at 60,000 km, which is about 1/6th of the way to the moon, and well beyond the belt of Geosynchronous satellites. I'm sure there are other satellites which have demonstrated this technology even further, but just to receive the signal beyond the GPS constellation is impressive.

It should be stated that this problem is difficult because the GPS antennas are all focused at the Earth, thus, not much radiation extends beyond the GPS constellation. Still, it can be done, if you do it carefully. Furthermore, there are now special receivers for HEO GPS reception, such as can be found here.

There is at least one mission that continues to try to break the record further away, being NASA's MMS mission. They have gone beyond AO-40's record to hit 70,000 km, since I originally wrote this answer.

PearsonArtPhoto
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    Right, I guess GPS satellite antennas are directional, so you'd need to get the signal from the other side of the Earth? – gerrit Jul 21 '13 at 00:32
  • @gerrit: Antennas still broadcast a bit of signal beyond their primary direction, but it is much weaker. It is a difficult problem, however. – PearsonArtPhoto Jul 21 '13 at 00:33
  • @PearsonArtPhoto There is some irony in the background of your answer. The higher the satellite's orbit is, the easier it does become to use GPS. In LEO it is painful, in geosynchronous orbit it works fairly well. You implicitly mention the reason for this, the GPS satellites' antennas are pointing to Earth. So you can actually use the signals of satellites at the other side of the Earth, when the antennas in fact point at you and if you are high enough to 'see' them (with Earth mostly out of the way). There are a few good papers on this, need to dig for them ... – s-m-e Jul 21 '13 at 04:54
  • @ernestopheles: I've found LEO to be fairly easy, as you are still below the GPS constellation. It's when you get to be over the constellation that difficulties arise. Maybe it's easier if you are far enough out... Hmmm... Sounds interesting, however. – PearsonArtPhoto Jul 21 '13 at 11:29
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    @ernestopheles I'd be very interested to see these papers... I've never heard this characterization before. –  Jul 21 '13 at 14:14
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    FYI, the link to the HEO GPS receiver is dead – Bohemian Aug 03 '15 at 16:53
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    Does that mean there is a sort of orbital GPS "Goldilocks zone"? Too close and you're on the wrong side of near satellites, but eclipsed from the far ones by Earth; too far and there is too little signal available, or perhaps angles get too small to derive an adequate solution? – Anthony X Aug 03 '15 at 19:04
  • Note, MMS has since beaten that record at 70,000 km (and may set another record next year when they raise the apogee). – 2012rcampion Jul 31 '16 at 15:50
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    You can update your answer now - the record is now 70,000 km! see http://www.space.com/34640-nasa-satellites-gps-world-record.html and https://www.sciencedaily.com/releases/2016/11/161104190848.htm – uhoh Nov 09 '16 at 13:34
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Since the offer for the answer to be updated wasn't taken up I'll post it as a new answer.

As of August 6, 2021 it's still 70,135 km above Earth from 2015

Sources:

The Science Daily link contains:

NASA's Magnetospheric Multiscale mission, or MMS, is breaking records. MMS now holds the Guinness World Record for highest altitude fix of a GPS signal. Operating in a highly elliptical orbit around Earth, the MMS satellites set the record at 43,500 miles above the surface. The four MMS spacecraft incorporate GPS measurements into their precise tracking systems, which require extremely sensitive position and orbit calculations to guide tight flying formations.

From GuinessWordRecords.com's Highest altitude GPS fix:

  • Who: MMS NAVIGATOR GPS RECEIVER
  • What: 70,135 KILOMETRE(S)
  • Where: UNITED STATES (GREENBELT)
  • When: 13 MARCH 2015

The highest altitude GPS fix is 70,135 km above Earth, detected aboard NASA's (USA) spacecraft MMS Navigator, and relayed to Goddard Space Flight Center, Greenbelt, Maryland, USA, on 19 March 2015.

The GPS signals were captured by a NASA's own GPS receiver, named Navigator.

All records listed on our website are current and up-to-date. For a full list of record titles, please use our Record Application Search. (You will need to register / login for access)

NASA Goddard video MMS Breaks World Record:

LOUD, ANNOYING MUSIC warning!

uhoh
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