GPS Flashcards
of Satellites that Support GPS Technology
24, with 4 on reserve to be launched on need.
Trilateration
The method of determining the relative location of objects using the geometry of triangles.
How does a GPS unit measure distance?
The travel time of radio signals from at least four satellites. With three, we get two distance measurements, one of which is usually obviously wrong and can be rejected.
A satellite’s ideal location for collection is where?
High orbit
GPS information must be corrected for what?
Signal delays as it travels through the ionosphere and troposphere.
A GPS receiver must factor in the angle each signal is taking as it enters the atmosphere because that angle determines the length of the trip through the perturbing medium.
How can you measure distance?
Velocity x Time
Travel time for a satellite overhead
About 0.06 seconds
Pseudo-random Noise Code
A repeating radio signal broadcast by each GPS satellite and generated by each GPS receiver. In a given cycle, the satellite and the receiver start generating their codes at the same moment, and the receiver measures how much later the satellite’s broadcast reaches it. By multiplying that time by the speed of radio waves, the receiver can compute the distance between the satellite’s antenna and its own.
Where in space are satellites positioned?
About 11,000 miles up in space
How do we know where the satellites are exactly?
The spacings of the satellites are arranged so that a minimum of five satellites are in view from every point on the globe. Each has a precise recorded orbit.
On the ground all GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment.
Ephemeris Errors
These errors are caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites.
The DoD monitors these errors and sends corrections.
The satellite includes new corrected position information in the timing signals it’s broadcasting.
Dual Frequency Measurements
Physics says that as light moves through a given medium, low-frequency signals get “refracted” or slowed more than high-frequency signals.
By comparing the delays of the two different carrier frequencies of the GPS signal, L1 and L2, we can deduce what the medium (i.e. atmosphere) is, and we can correct for it. But only the military has access to the signals on the L2 carrier.
MultiPath Error
In the real world the signal will also bounce around on just about everything in the local environment and get to the receiver that way too.
The result is a barrage of signals arriving at the receiver: first the direct one, then a bunch of delayed reflected ones. This creates a messy signal.
Selective Availability
Basically the DoD introduced some “noise” into the satellite’s clock data which, in turn, added noise (or inaccuracy) into position calculations. The DoD may have also been sending slightly erroneous orbital data to the satellites which they transmitted back to receivers on the ground as part of a status message.
Together these factors made SA the biggest single source of inaccuracy in the system. Military receivers used a decryption key to remove the SA errors and so they’re much more accurate.
The idea is to make sure no hostile force uses GPS for weapons programs.
Differential GPS Accuracy
Differential GPS or “DGPS” can yield measurements good to a couple of meters in moving applications and sub-centimeter in stationary applications.
