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Could atomic clocks provide more accurate GPS positioning for GPS receivers? It’s my understanding that GPS works by measuring tiny differences in the time it t

by baddox 9y ago

Could atomic clocks provide more accurate GPS positioning for GPS receivers? It’s my understanding that GPS works by measuring tiny differences in the time it takes equivalent signals to reach the receiver from different satellites. It may be the case that other factors (like noise and refraction) dwarf any inaccuracies in the quartz clocks in our everyday GPS receivers.

azernik 9y ago
We don't need super accurate clocks in the GPS receivers; instead, the GPS satellites carry atomic clocks with them, and include timestamps in their broadcasts. So you get two signals at the same time, compare the time the satellite said it sent each signal, and you've got the difference in travel time.
- baddox 9y ago
  Oh, interesting. What’s the time granularity transmitted by the satellites? I assumed that the satellites broadcasted a course stream of times and the receiver had to finely measure its local time difference between receiving the same time from multiple satellites.
  - baddox 9y ago
    My assumption, for example: Receiver receives time “noon” from GPS satellite 1 at receiver-local time T=0. Receiver receives time “noon” from GPS satellite 2 at receiver-local time T=2 nanoseconds. Receiver receives a couple more “noon”s from other satellites, then uses its receiver-local time differences to trilateral its location.
  - azernik 9y ago
    So did some poking around; apparently, there are two timestamps in GPS messages, and the more precise one is packaged in a custom hash function and called the "ranging code". The timestep (called a "chip") is a bit under a microsecond. But yes, the receiver does have to measure small time differences between received signals; but that just requires a local clock with high resolution, not one with low drift or low absolute error. For example, if your local clock is a second off, but still measures time intervals to pretty high precision (say, 0.01% - losing a second every three years) that's probably not going to mess up your location fix very much. Especially when the problem is overconstrained as it usually is intentionally. Compare the theoretical minimum requirement of three visible satellites to make a measurement with the 4-12 satellites that the system guarantees will be in sight at any time.
campground 9y ago
From the article: > GPS receivers will benefit from onboard clocks. This is perhaps surprising, given that the GPS signal itself carries time information, but it happens because the signal from the satellite is weak—comparable to the power of a lightbulb transmitted across a continent. Landscape features, buildings, and interference make it harder to detect. To track this weak signal, the receiver must precisely lock onto the broadcast frequency. The more stable the local frequency reference, the faster and more reliable this tracking can be. > In hostile environments, such as battlefields, this becomes even more important. The GPS signal is vulnerable to jamming, and effective (but illegal) jammers are widely available and likely to be encountered in future wars. With precise timing information, GPS receivers could isolate the true signal above the noise of the jammer. The receivers could even allow navigation to survive partial destruction of the satellite network. > Present-day receivers must determine their positions by using signals from four or more satellites simultaneously, but a sufficiently accurate clock could instead use successive signals from a single satellite. Other defense applications include frequency-hopping communication, bistatic radar (in which an attacker stealthily acquires a radar signal from a target illuminated by a distant transmitter), and sensitive monitoring of enemy communications. For these reasons, portable clocks are of great interest to militaries in several countries.

by baddox 9y ago

azernik 9y ago

We don't need super accurate clocks in the GPS receivers; instead, the GPS satellites carry atomic clocks with them, and include timestamps in their broadcasts. So you get two signals at the same time, compare the time the satellite said it sent each signal, and you've got the difference in travel time.

baddox 9y ago
Oh, interesting. What’s the time granularity transmitted by the satellites? I assumed that the satellites broadcasted a course stream of times and the receiver had to finely measure its local time difference between receiving the same time from multiple satellites.
- baddox 9y ago
  My assumption, for example: Receiver receives time “noon” from GPS satellite 1 at receiver-local time T=0. Receiver receives time “noon” from GPS satellite 2 at receiver-local time T=2 nanoseconds. Receiver receives a couple more “noon”s from other satellites, then uses its receiver-local time differences to trilateral its location.
- azernik 9y ago
  So did some poking around; apparently, there are two timestamps in GPS messages, and the more precise one is packaged in a custom hash function and called the "ranging code". The timestep (called a "chip") is a bit under a microsecond. But yes, the receiver does have to measure small time differences between received signals; but that just requires a local clock with high resolution, not one with low drift or low absolute error. For example, if your local clock is a second off, but still measures time intervals to pretty high precision (say, 0.01% - losing a second every three years) that's probably not going to mess up your location fix very much. Especially when the problem is overconstrained as it usually is intentionally. Compare the theoretical minimum requirement of three visible satellites to make a measurement with the 4-12 satellites that the system guarantees will be in sight at any time.

campground 9y ago

From the article: > GPS receivers will benefit from onboard clocks. This is perhaps surprising, given that the GPS signal itself carries time information, but it happens because the signal from the satellite is weak—comparable to the power of a lightbulb transmitted across a continent. Landscape features, buildings, and interference make it harder to detect. To track this weak signal, the receiver must precisely lock onto the broadcast frequency. The more stable the local frequency reference, the faster and more reliable this tracking can be. > In hostile environments, such as battlefields, this becomes even more important. The GPS signal is vulnerable to jamming, and effective (but illegal) jammers are widely available and likely to be encountered in future wars. With precise timing information, GPS receivers could isolate the true signal above the noise of the jammer. The receivers could even allow navigation to survive partial destruction of the satellite network. > Present-day receivers must determine their positions by using signals from four or more satellites simultaneously, but a sufficiently accurate clock could instead use successive signals from a single satellite. Other defense applications include frequency-hopping communication, bistatic radar (in which an attacker stealthily acquires a radar signal from a target illuminated by a distant transmitter), and sensitive monitoring of enemy communications. For these reasons, portable clocks are of great interest to militaries in several countries.