6 ms·

> "Well, currently GPS can estimate your location to an area of about 20 feet." With DGPS we get about +-10cm for our non-military robotics project. So this is

> "Well, currently GPS can estimate your location to an area of about 20 feet." With DGPS we get about +-10cm for our non-military robotics project. So this isn't really true. We use a paid subscription but the free WAAS signal brings the accuracy to about +- 1m which is already pretty friggin good. I wonder what justified this (probably expensive) upgrade?

jws 16y ago
The existing satellites have been in service too long. It is a "replace it or lose it" question. It took me a while to find out why they are more accurate, I eventually found this quote: Greater accuracy is a product of advanced atomic clocks in the new satellites, Torok said. I think it's safe to safe that tiny atomic clock technology advanced in the decade between the existing satellites and this round. They added a new military signal, presumably jam resistant. (It has a "spotlight" antenna that can provide a 100 times more power to a fighting theater. I'll see your alibab.com gps jammer and raise you 20dB…) They also added another civilian signal (L5 - safety of life) which presumably has a purpose, but darned if I can find it. Probably related to different frequency, more power, different coding, something about the ionosphere… my guess is that it won't crap out at the same time the other one does from interference.
- superdude 16y ago
  The L5 moniker "safety of life" is an accurate name. It will assist GPS aircraft navigation, especially during the safety-critical landing approaches. Aircraft rely heavily on both GPS and WAAS satellites for corrections. Actually, L5 has been broadcasting on the two WAAS satellites for several years, but most end-users would not have used it. AFAIK, L5 decreases the error from multipathing and the ionsphere, among other improvements.
jwilberding 16y ago
I am not sure where you get your numbers, but DGPS can get within half a cm about. GPS is used by many land surveyors for this reason.
- tesseract 16y ago
  To get the sub-cm accuracy you need an RTK GPS system which as I recall is not quite the same thing as DGPS (as DGPS is traditionally thought of).
yellowbkpk 16y ago
You're using ground-based assistance to get that extra accuracy. This upgrade gives the satellite signal better accuracy without the need for ground based assistance stations (and expensive receivers).
- makmanalp 16y ago
  I know, that's the point of DGPS. What I meant was, we already have that infrastructure and accuracy, so why are they putting new satellites? I guess the ones up there are getting quite old now.
  - yellowbkpk 16y ago
    So you can achieve the better accuracy without the expensive DGPS equipment and subscriptions to the DGPS services?
  - jwilberding 16y ago
    DGPS requires a fixed base in order to calculation the differentials I believe. Some states, like Iowa (http://www.iowadot.gov/rtn/ http://www.iowadot.gov/rtn/), also have real time information you can access to reduce the margin of error.
bodhi 16y ago
Are your robots moving around? I'm just curious because the positioning research I've been exposed to couldn't get better than ~50cm accuracy for objects in motion (for example, a person walking around at a normal pace).
- georgecmu 16y ago
  He's subscribing to Omnistar HP service. Basically, it's a float RTK solution relative to a synthetic base station. They boast 10 cm error 95% of the time. It's expensive (IMHO) and breaks whenever sun flares up. True RTK + IMU positioning systems will get you 1-2 cm 95% of the time (see Applanix, Novatel).
  - makmanalp 16y ago
    Exactly what you said. But OmniStar is giving it away to us for free so why the heck not! :)
    - georgecmu 16y ago
      Just wait till your robot drives under a tree canopy )
  - bodhi 16y ago
    Interesting! I don't use this stuff in anger, so my questions are merely idle curiosity. Omnistar refers to the HP accuracy as "real-time"[1] and: > [it] will usually have a 2-sigma (95%) horizontal error of about 6 centimeters and a 99% horizontal error of less than 10 centimeters Do you happen to know what kind of movement this is based on? For example, if it was attached to an RC car (assuming that the equipment is this compact, it may not be?) would you still be able to get this level of precision? Or is it designed for more consistent movement patterns? [1] http://www.omnistar.com/about.html http://www.omnistar.com/about.html
    - georgecmu 16y ago
      My guess is that it doesn't assume anything about your motion model. The real question you should ask is what's the frequency of the positioning solution. Your generic GPS unit computes solutions at 1Hz, more sophisticated ones do it at 5Hz, surveyor quality units can do it at 10-20Hz depending on firmware, but I think they're pushing the limit and probably assume something about the motion model. Applanix and Novatel solutions that use inertial sensors definitely make pretty strong assumptions of the motion model (see Kalman filter). OmniSTAR in particular and DGPS in general will operate under assumption that errors due to ionospheric and tropospheric effects change slowly in time and space. If this assumption is correct, then dynamics of the vehicle won't affect the corrections themselves. My point with all this is that 99% of the corrected GPS solution should theoretically be within x cm of some point on the true path of the vehicle no matter how it moves in a plane. The question is, if you're interested in real-time solution, how far will the vehicle be from that point by the time you get this fix, and if you're interested in capturing vehicle's travel, how well will the curve fit through these points match the true vehicle path.

jws 16y ago

The existing satellites have been in service too long. It is a "replace it or lose it" question. It took me a while to find out why they are more accurate, I eventually found this quote: Greater accuracy is a product of advanced atomic clocks in the new satellites, Torok said. I think it's safe to safe that tiny atomic clock technology advanced in the decade between the existing satellites and this round. They added a new military signal, presumably jam resistant. (It has a "spotlight" antenna that can provide a 100 times more power to a fighting theater. I'll see your alibab.com gps jammer and raise you 20dB…) They also added another civilian signal (L5 - safety of life) which presumably has a purpose, but darned if I can find it. Probably related to different frequency, more power, different coding, something about the ionosphere… my guess is that it won't crap out at the same time the other one does from interference.

superdude 16y ago
The L5 moniker "safety of life" is an accurate name. It will assist GPS aircraft navigation, especially during the safety-critical landing approaches. Aircraft rely heavily on both GPS and WAAS satellites for corrections. Actually, L5 has been broadcasting on the two WAAS satellites for several years, but most end-users would not have used it. AFAIK, L5 decreases the error from multipathing and the ionsphere, among other improvements.

jwilberding 16y ago

I am not sure where you get your numbers, but DGPS can get within half a cm about. GPS is used by many land surveyors for this reason.

tesseract 16y ago
To get the sub-cm accuracy you need an RTK GPS system which as I recall is not quite the same thing as DGPS (as DGPS is traditionally thought of).

yellowbkpk 16y ago

You're using ground-based assistance to get that extra accuracy. This upgrade gives the satellite signal better accuracy without the need for ground based assistance stations (and expensive receivers).

makmanalp 16y ago
I know, that's the point of DGPS. What I meant was, we already have that infrastructure and accuracy, so why are they putting new satellites? I guess the ones up there are getting quite old now.
- yellowbkpk 16y ago
  So you can achieve the better accuracy without the expensive DGPS equipment and subscriptions to the DGPS services?
- jwilberding 16y ago
  DGPS requires a fixed base in order to calculation the differentials I believe. Some states, like Iowa (http://www.iowadot.gov/rtn/ http://www.iowadot.gov/rtn/), also have real time information you can access to reduce the margin of error.

bodhi 16y ago

Are your robots moving around? I'm just curious because the positioning research I've been exposed to couldn't get better than ~50cm accuracy for objects in motion (for example, a person walking around at a normal pace).

georgecmu 16y ago
He's subscribing to Omnistar HP service. Basically, it's a float RTK solution relative to a synthetic base station. They boast 10 cm error 95% of the time. It's expensive (IMHO) and breaks whenever sun flares up. True RTK + IMU positioning systems will get you 1-2 cm 95% of the time (see Applanix, Novatel).
- makmanalp 16y ago
  Exactly what you said. But OmniStar is giving it away to us for free so why the heck not! :)
  - georgecmu 16y ago
    Just wait till your robot drives under a tree canopy )
- bodhi 16y ago
  Interesting! I don't use this stuff in anger, so my questions are merely idle curiosity. Omnistar refers to the HP accuracy as "real-time"[1] and: > [it] will usually have a 2-sigma (95%) horizontal error of about 6 centimeters and a 99% horizontal error of less than 10 centimeters Do you happen to know what kind of movement this is based on? For example, if it was attached to an RC car (assuming that the equipment is this compact, it may not be?) would you still be able to get this level of precision? Or is it designed for more consistent movement patterns? [1] http://www.omnistar.com/about.html http://www.omnistar.com/about.html
  - georgecmu 16y ago
    My guess is that it doesn't assume anything about your motion model. The real question you should ask is what's the frequency of the positioning solution. Your generic GPS unit computes solutions at 1Hz, more sophisticated ones do it at 5Hz, surveyor quality units can do it at 10-20Hz depending on firmware, but I think they're pushing the limit and probably assume something about the motion model. Applanix and Novatel solutions that use inertial sensors definitely make pretty strong assumptions of the motion model (see Kalman filter). OmniSTAR in particular and DGPS in general will operate under assumption that errors due to ionospheric and tropospheric effects change slowly in time and space. If this assumption is correct, then dynamics of the vehicle won't affect the corrections themselves. My point with all this is that 99% of the corrected GPS solution should theoretically be within x cm of some point on the true path of the vehicle no matter how it moves in a plane. The question is, if you're interested in real-time solution, how far will the vehicle be from that point by the time you get this fix, and if you're interested in capturing vehicle's travel, how well will the curve fit through these points match the true vehicle path.