24
\$\begingroup\$

Suppose I have a GPS unit attached to an antenna through a 50 meter coax cable.

How would the location as calculated by the GPS unit be affected by the cable length? As a bonus question, how would the time accuracy of the GPS be affected by the cable?

\$\endgroup\$
  • 1
    \$\begingroup\$ I'm doubtful there'd be much signal left at the end of 50m of coax at 1.5GHz, unless it's really good coax. \$\endgroup\$ – gbarry Jul 1 '15 at 6:55
  • 1
    \$\begingroup\$ @gbarry: It depends on the antenna, most datacenter grade gps disciplined oscillators have antennas that mix down the signal so you can run up to 300m of cable. \$\endgroup\$ – PlasmaHH Jul 1 '15 at 8:35
  • \$\begingroup\$ We use 100 m cable with active amplifiers for GPS \$\endgroup\$ – User323693 Jul 1 '15 at 8:49
26
\$\begingroup\$

The exact position is the phase center of the antenna independent of the length of the cable and location of the chip.

The time delay has to be calibrated by measuring the delay of the cable for the band. (L1 band). Many GPS receivers provide option to key in the delay parameter.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Cable delay could be calibrated, calculated or ignored depending on timing requirements. I'd say most commodity receivers jitter enough not to go beyond a simple cable delay calculation. \$\endgroup\$ – Eugene Ryabtsev Jul 1 '15 at 8:52
  • \$\begingroup\$ @Eugene Yeah. That is subjective. \$\endgroup\$ – User323693 Jul 1 '15 at 8:55
  • \$\begingroup\$ No one stated this was a commodity receiver. This could be a static installation which would be for a differential reference station of a time-source. That would usual mean a high quality receiver is in use and the delays for the cable would be noticeable in the calculations. Additional delays could suggest you are ~cable length lower. GPS is a generic term could mean NAVSTAR, GLONASS or Galileo systems (usually NAVSTAR in the engish speaking world). GLONASS uses different frequencies so phase repose of cable should have significant effects too. \$\endgroup\$ – TafT Jul 1 '15 at 9:25
  • 2
    \$\begingroup\$ @TafT GPS is GPS from US. (en.wikipedia.org/wiki/Global_Positioning_System) GNSS refers to all in general. \$\endgroup\$ – User323693 Jul 1 '15 at 9:33
  • \$\begingroup\$ @Umar as hard as the industry tries for that to be the case I am not sure most end users realise the distinction. That did not matter in the past but as GLONASS receivers are now present in many mobile phones (& presumably other consumer grade products) it could be significant. \$\endgroup\$ – TafT Jul 1 '15 at 10:05
13
\$\begingroup\$

Cable delay adds an equal offset to the pseudoranges for all satellites. Since GPS uses the difference in the pseudoranges to each satellite to calculate the position, positioning isn't affected by cable delay.

The position calculated will be at the antenna, not at the receiver, which you can see by realizing that moving the antenna has a different effect on the pseudoranges to different satellites due to geometry, but moving the receiver has no effect at all (the cable length stays the same and so does the cable delay).

The time calculated by the GPS receiver will have an error equal to the cable delay, which is the length of the cable divided by the propagation velocity of the cable. The RG174 commonly used on "puck" antennas has a velocity of 0.66 c, which is about 5 nanoseconds per meter.

| improve this answer | |
\$\endgroup\$
  • 1
    \$\begingroup\$ I liked your answer but it seems to contradict @umar - any thoughts? \$\endgroup\$ – Andy aka Jul 1 '15 at 7:38
  • \$\begingroup\$ @Andyaka: where does it contradict? It seems both say its at the antenna and there is a time delay due to cable length. \$\endgroup\$ – PlasmaHH Jul 1 '15 at 8:39
  • 1
    \$\begingroup\$ The position is that of the antenna. The time is that of the signal arriving at the antenna plus a delay down the cable. \$\endgroup\$ – pjc50 Jul 1 '15 at 8:47
  • 1
    \$\begingroup\$ @PlasmaHH if the position is determined by the ratio of two (or more?) signals at the antenna then why should taking into consideration the cable's time delay matter. That's the contradiction as it appears to me. \$\endgroup\$ – Andy aka Jul 1 '15 at 9:39
  • 2
    \$\begingroup\$ @Andy GPS receiver gives both position and very accurate time. Time information gets affected by cable delay, but not the position. Hope i have understood your point. \$\endgroup\$ – User323693 Jul 1 '15 at 10:40
2
\$\begingroup\$

As already mentioned, the position is determined by differences in signals received by the antenna. So the cable and chip will be irrelevant for that.

However, when it comes to timing things get tricky.

As mentioned, you can calculate how long it takes for your signal to travel through the cable and correct for that, but when I did an experience a few years ago, we actually found the variability in timing to be in the order of microseconds.

So you can correct a few nanoseconds for your theoretical cable delay, but in practice the uncertainty in timing may be much larger.

| improve this answer | |
\$\endgroup\$
  • 2
    \$\begingroup\$ The uncertainty is mostly determined by whatever you use to calculate the time. The time information in the analog signal is clearly a lot more accurate than microseconds or the whole location thing would never work. Your receiver and whatever you use to check the times (actual computer running a non-real-time OS like Windows or Linux, maybe?) however may very well introduce uncertainties in that range. If that is what you use, correcting nanoseconds might really be in vain. If your timestamp consumer is something faster (FPGA, analog) it might not be. \$\endgroup\$ – DeVadder Jul 2 '15 at 13:50
-1
\$\begingroup\$

Although I agree that the phase center of the antenna is the calculated position. the real/actual GPS position is affected by the cable length.
A person holding the GPS can move anywhere within a radius of 50m from the center of the antenna and the GPS will not register any difference. Therefore the cable length does affect the accuracy of the GPS position, but because the cable length only causes a signal delay (about 300 ns), it does not affect the signal accuracy!

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.