View Full Version : GPS Principles

12th Dec 2006, 03:32
I am studying for the Navigation ATPL and I am curious about some aspects of GPS principles.

The reader I have says that the GPS signal is weaker than the radio background noise. Knowing what to look for, the receiver can find the signal amongst the noise. This makes the antennae smaller than otherwise.

I am wondering;
why make the signal psuedo-random?
Is it for security?
So if you dont know what youre looking for, you wont find it?
Why is the same principle not applied to other satcomms (making all dishes smaller) ?

Thanks for any discussion

12th Dec 2006, 06:08
Satellites use an atomic clock (actually 4), for high precision, and continuously transmit their positions, plus a code number in a set code, at exactly the same time. The set code appears to be random, but isn’t, which is why it is known as Pseudo-Random Noise Code. It is the means by which a receiver recognises signals from a particular satellite.

Since the transmission time is known, the distance the signal has travelled can be calculated from its arrival time. The receiver matches each satellite's code with an identical copy in its database. By comparing any shift with its internal clock, it can calculate a pseudo-range, which is similar to comparing a broadcast copy of a song against one already playing on a CD player - if they both started at the same time, the received one would be slightly behind from the time delay. If several “songs” are received at the same time from multiple sources, the GPS receiver can correct for errors in its own clock and determine actual travel times.

The principle used is that, if three perfect measurements can locate a point in space, four imperfect ones can can eliminate clock offsets, or cancel out some timing errors. An error is therefore deliberately introduced, and algebra is used to compute where all possible points could intersect. The result given is your position. Because it is calculated, the word pseudo is used,and the measured distance between a satellite and a receiver is called a Pseudo-Range because the calculated range includes receiver clock error.


12th Dec 2006, 07:04
Why is the same principle not applied to other satcomms (making all dishes smaller) ?

GPS receivers don't ever transmit, so the antennas can be light duty. They are semi-omnidirectional, and the active element is a quarter wavelength of 1.6 GHz, or approx. 2.5 cm.

Voice satcomms, while in the same frequency spectrum, require high data rates and lots of power through a mechanically or electrically steered antenna continuously pointed to a geostationary satellite 25,000 miles up.

GPS, Simply spoken:
A bunch of satellites, orbiting thousands of miles above the center of the earth, transmitting a time signal, etc., requiring lots of software.
A receiver somewhere near the surface of the earth. Upon receiving three signals, it triangulates the distance from each satellite, with reference to the center of the earth. Did I say, Software?

Map? Database? Did I say, Software?

Compare that to an ILS, with the antennas anchored in concrete. 90 Hz modulation above and left; 150 Hz modulation below and right. Your position on the beam is determined by the difference in amplitude of 90 vs. 150. No atomic clocks, no software needed.

It takes mountains of data to prove the reliability of GPS and its software for CAT IIIb approaches and landings.


Paul Wilson
12th Dec 2006, 08:56
Also, if you want a stronger signal, you need more power to transmit it, which means a larger array of solar panels to provide the power. This makes the spacecraft bigger and heavier, and what that means is a much more expensive launch.

12th Dec 2006, 22:29
why make the signal psuedo-random?
Is it for security?

Anybody have an answer to this part of the question?

Self Loading Freight
12th Dec 2006, 23:11
A pseudorandom signal lets you overlay many signals on the same frequency.

If you've got the same sequence in your receiver as a pseudorandomly hopping transmitter, you know when and where to look for the signal. You get a statistically significant amount of information despite the channel being noisy. That noise also includes all the other pseudorandom signals, which you won't get a statistically significant amount of energy from - everyone on the same channel with a different pseudorandom sequence will think that, in effect, they're the only one there.

There'll be occasional collisions - and the more channels on the same frequency, the more collisions you'll get, so it's not a free lunch - but you can treat other signals as so many decibels of noise to include in your link budget. You can do all sorts of things (some of which GPS does) to increase your gain over noise too.

Why pseudorandom? If the signal you were listening for was truly random, you'd have no hope of finding it. If it wasn't random at all, then you'd get all sorts of effects when you and the other users on the channel were in sync (although you can do it that way - it doesn't work very well, mind).

There _are_ security uses for pseudorandom signals: with care, you can establish a useful channel which is well beneath the ambient noise in a channel and thus not visible to someone trying to detect, intercept or block you (up to a point: the more you know about the signal, the more you can do even without the sequence).

But in the open GPS service, the pseudorandom stuff is primarily there to share spectrum effectively. There's no secret stuff.

If you want to know more about this, look up CDMA (Code Division Multiple Access) - it goes alongside FDMA (Frequency - where you get your own channel) and TDMA (Time - where you get exclusive use of a channel for a slice of time).


12th Dec 2006, 23:22
Thanks to you all for your replies. Theres lots of useful info there


galaxy flyer
13th Dec 2006, 00:13

Go to Trimble's website www.trimble.com and they have a great tutorial on how GPS works. You might need a plug-in, but it's worth watching. I used it a lot when teaching GPS in the AF, but can't remember much. Very good, though!