Thread: Questions, and I can't get answers

OK. I haven't kept up with much in GPS technical advancements since I retired. I knew the L2 was protected mil access at one time.

Originally Posted by Cane Pole

Lowrance L1/L2 : Same as Greg posted

Type L1, C/A code, 1.575 GHz Frequency/ L2 C/A code, 1.602 GHz Frequency

Not quite CP.
I stated that we did not use 1.602 GHz for the L1 C/A code. I read in a manual where it was stated that the L2 frequency was 1.602 GHz frequency. According to the GPS.gov web site; the L2 signal is (1227 MHz) 1.227 GHz (the L1 signal is 1575 MHz or 1.575 GHz, so that may be within the bandwidth of the L1 signal).

While I was looking, I found that there are some additional frequencies they are looking at employing that will give added accuracy for civilian GPS Receivers but they won’t happen anytime soon. The L2C (civilian use) signal should be available around 2018 and allow military (or better) accuracy. The L5 (for aviation safety services) signal around 2021 may allow sub-meter accuracies. The L1C signal (civilian use) will be available around 2026 and will use the same L1 1.575 GHz frequency as the current L1 signal. The same L1C signal is a “future standard” frequency being developed by other countries in their own GPS systems.


You have to watch those EPE (Estimated Position Error) numbers as these are generated by the company who produces the products and not all may be based on real-world data. The better measurement would be the HDOP (or PDOP) numbers as these are based on the locations of the GPS satellites used.

You also have to watch accuracy statements as these are usually expressed in either a 50% (often called CEP or Circular Error Probable) rate which means at least 50% of the time the position accuracy will be within the distance stated, or a 95% rate. Example: a GPS Receiver that has a CEP rating of 13 feet could also have a 95% rating of less than 5 feet – which would you use in your advertisement?

Ok. Like I said, I'm retired and don't keep up with it anymore. However, 2 frequencies must be better than one or the military wouldn't be using it.

Originally Posted by Cane Pole

Ok. Like I said, I'm retired and don't keep up with it anymore. However, 2 frequencies must be better than one or the military wouldn't be using it.

ok,I see were your going with this now

The P-code is much longer than the C/A code which gives you gain and accuracy improvements. For non-engineers, you get the extra gain because the receiver actually knows what pattern it is looking for and the longer pattern makes it easier for the processing software in the receiver to see the data. In a way it is like looking for a canoe or a supertanker in a dense fog.

When a GPS doesn't know where it is or doesn't have current ephemeris data, it can take a while to acquire a position fix. Because the P-code is much, much longer than the C/A code, it takes much longer to synchronize to the code and acquire position. Going back to the canoe, once you find a canoe it's much quicker to see if it is the right canoe than it is to verify that a supertanker is the one you are looking for.

The original concept was that receivers would synchronize to the C/A, find out what time it was, about where they were and get the ephemeris data. Once they had that they would be able to synchronize to the P-code relatively quickly. The C/A sequence is a kilobit long while the P-code's sequence is something like 3/4 of a terabit long. Military and high accuracy receivers use both the L1 and L2 to correct for atmospheric errors, which is a major advantage of being dual frequency.

Technology has changed since the original design that first went active almost 20 years ago. Hardware got faster, decoding techniques got much better and people figured out how to use the P-code without knowing the encryption code.

Changes are coming to GPS, with the addition of the civilian data on the L2 frequency and increases in sequence length on the C/A signal, civilian receivers will soon have two frequencies to use so they have the ability to correct for atmospheric errors internally and both codes will be substantially longer giving significant increases in decoding gain.

Probably more info than anybody wanted, but I find the system amazing. We have a high accuracy receiver at work. When it is mounted on a vehicle, you can actually see the wiggle caused by opening and closing doors.

I taught GPS before/during intiail WAAS. I took a course at Trimble Instruments and used there course ware (with permission) for my class course outline.

I can't remember much, and don't have enough interest anymore, so I ask questions. At one point im my life, I was very thorough. Don't know what happened.

it's called C.R.S. and it's a symptom of old age, so they tell me
(can't remember sh..er I mean stuff.)