Regarding New GPS Satellite

Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
hauptjm
Posts: 29
Joined: Mon Oct 19, 2015 3:26 pm

Regarding New GPS Satellite

Postby hauptjm » Tue Dec 18, 2018 10:34 am

Jim, what does this all mean to a layman?

The technical characteristics of GPS Block IIIA are:

1. L2C contains two distinct PRN sequences:
2. CM (for Civilian Moderate length code) is 10,230 bits in length, repeating every 20 milliseconds.
3. CL (for Civilian Long length code) is 767,250 bits, repeating every 1,500 milliseconds (i.e., every 1.5 s).
4. Each signal is transmitted at 511,500 bits per second (bit/s); however, they are multiplexed to form a 1,023,000 bit/s signal.
5. CM is modulated with a 25 bit/s navigation message with forward error correction, whereas CL contains no additional modulated data.
6. The long, non-data CL sequence provides for approximately 24 dB greater correlation protection (~250 times stronger) than L1 C/A.
7. L2C signal characteristics provide 2.7 dB greater data recovery and 0.7 dB greater carrier tracking than L1 C/A.
8. The L2C signals' transmission power is 2.3 dB weaker than the L1 C/A signal.
9. In a single frequency application, L2C has 65% more ionospheric error than L1.

jimh
Posts: 6905
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Regarding New GPS Satellite

Postby jimh » Tue Dec 18, 2018 10:53 am

The notation "L2C" refers to the frequency of the carrier signal being transmitted. "L" is for L-band. "2" is for the second signal in L-band. And "C" refers to civilian.

The signal in use now by most civilian GPS receivers is the L1 Coarse Acquisition (L1 C/A) signal.

Both L1 C/A and L2C are intended for use by civilian users.

L-band is the name given to the radio spectrum from 1,000 to 2,000-MHz or 1 to 2-GHz. There are a variety of microwave bands referred to with letter designators. For example C-band is 4 to 8-GHz. Ku-band is 12 to 18-Ghz. Most home satellite TV dishes are for Ku-band.

The L-band spectrum is generally allocated to and reserved for space to earth communication.

L1 C/A is transmitted at 1,575.42-MHz. This frequency is related to 1.023-MHz by being the 1540th multiple, that is, 1,575.42 = 1.023-MHz x 154.

L2C is transmitted at 1,227.60-MHZ. This frequency is related to 1.023-MHz by being the 1200th multiple, that is 1,227.60- = 1.023-MHz x 1200.

(The significance of 1.023 is due to that being the frequency rate of the modulating signal, discussed further below.)

Most existing GPS consumer-grade receivers are able to receive only the L1 C/A signal, and those receivers will not be able to receive the L2C signal. For just about every consumer-grade GPS receiver user, new hardware will be needed to receive the L2C signal.

The effect on consumers today with a consumer-grade GPS receiver of this new GPS III modernization will be nil, until the consumers upgrade to new GPS receiver hardware.

jimh
Posts: 6905
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Regarding New GPS Satellite

Postby jimh » Tue Dec 18, 2018 12:14 pm

The nature of GPS signals is all the signals in one family are all sent on the same carrier frequency. For example, all the satellites sending an L1 signal transmit that signal on 1,575.42-MHz. This may sound puzzling, as one would expect that the signals would mutually interfere with each other. That would be true, except that the signals are all modulated by a very particular and unique modulation signal, which creates what is known as Code Division Multiple Access (CDMA).

A normal receiver listening at 1,575.42-MHz would just hear a buzz of signals, but a GPS receiver is designed to be able to distinguish a multiple number of signals by demodulation techniques that use a matching signal to separate individual satellites. By the way, this same method, CDMA, is used in cellular telephone communication with terrestrial signals.

The modulating signal for GPS is an incredibly well-designed and ingenious digital signal that is sent at a rate of 1.023-Mbits/second for the L1 C/A signal. The digital "chip" is 1,023-bits long. Only certain combinations of bits are used, called Gold Codes (after the name of the inventor of them). These codes are also known as pseudo-random number or PRN codes. Each satellite uses its own reserved PRN.

The received signal is processed by a demodulator called a correlator. A correlator is an extremely efficient digital filter that can pick out just the parts of a wide-band digitally-modulated carrier that it needs, and in the process improve the signal-to-noise ratio. The correlator applies the same PRN code as the satellite signal it is searching for was modulated. This results in the desired signal popping out of the noise--literally. WIthout correlator demodulation, the signals from GPS satellites are so weak that they could not be received on Earth easily without huge antennas and special low-noise amplifiers. But the correlator demodulation method improves the signal-to-noise ratio by a factor of about 100-times.

The purpose of the modulating signal is to allow the receiver to align to the incoming signal as precisely as possible, because that precision is needed to accurately measure the time of arrival of the signal. After all, the finding of position from GPS signals depends on measuring the time of arrival with great precision.

Radio waves travel at close to the speed of light, or 300,000-kilometers-per-second. If the time of arrival is known to one-microsecond, then the distance resolution will be

(3 x 10^8-meters/1-second ) x (1 x 10^-6 second) = 3 x 10^2-meters or 300-meters.

If the receiver can precisely align and measure the phase of the arriving signal, say to 1-degree, then the resolution can improve to

300-meter x 1-degree/360-degrees = 0.8-meter--less than three feet.

jimh
Posts: 6905
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Regarding New GPS Satellite

Postby jimh » Tue Dec 18, 2018 12:25 pm

Re the length of the PRN codes: this is a complicated concept. The most basic relationship: the longer the code the more immune to being jammed or spoofed.

In the civilian codes, the codes themselves are short, well-known, and published. In the military codes, the codes are long, essentially unknowable, and kept secret.

jimh
Posts: 6905
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Regarding New GPS Satellite

Postby jimh » Tue Dec 18, 2018 1:47 pm

Re Nos 7, 8: I believe those comments refer to the actual transmitter power employed by the satellite to transmit the L2C signal is lower than the L1 C/A signal. This is an advantage because it reduces the power consumption of the satellite. The actual outcome at the receiver is the recovered L2C signal will have a better signal-to-noise ratio than the L1 C/A signal because of the increased gain the the correlator and other advantages to the modulating signal design. Again, this is rather complicated communication theory stuff.

jimh
Posts: 6905
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Regarding New GPS Satellite

Postby jimh » Tue Dec 18, 2018 1:58 pm

Re No. 9: the L2C is lower in frequency than the L1 C/A so there is more effect on the speed of propagation through the ionosphere on the L2C signal compared to the L1 C/A.

The mention of "single frequency application" is interesting. This is what just about every consumer GPS receiver is using now, just one signal that is on one frequency. The enhanced GPS receiver of the future will use two signals on two frequencies, both coming from the same satellite. That will mean there are two signals sent along the exact same path through the ionosphere to the earth receiver. This permits the delay in the ionosphere to be measured by comparing the time the two signals took to arrive at the receiver from the same satellite.

Here is an analogy:

Two submarines travel from Point A to Point B. They leave simultaneously, and they travel at the exact same speed through the water. The only difference is sub-1 travels at a depth of 100-meters and sub-2 travels at a depth of 200-meters. Assume that there is no current, except for a portion of their route, that affects them. If sub-1 arrives at Point B sooner than sub-2, then we can make an inference about the influence of any current in the water that affected their speed and work out a relationship of the current to depth.

In a similar but more complicated way--which I really do not understand exactly how the Physics work myself--when you get two radio signals from a remote point that travel the same path, you can work out the influence of any ionosphere propagation delay they experienced by some sort of algebra or other fancy math.

Why worry about this? Because random ionospheric propagation delay is one of the elements of the position fix error. By being able to reduce the ionospheric delay error the accuracy of the position fix is improved. We're talking about enough accuracy in the future to know which lane of a four-lane highway your car is in by GPS position. Of course, ultra-accurate highway maps must be made to achieve that sort of measurement, too.