Understanding L2C ( L2 ) GPS
With the announcement of L2C GPS availability today readers have already emailed in dozens of questions. I’ll try to address as many of those as I can now. Understanding L2C requires a little bit of background into how GPS systems work. And I’ll admit I don’t have all of the answers right now.
GPS receivers work much like an FM radio. They are tuned to listen for signals being broadcast on certain stations, or frequencies. Radio stations broadcast from towers on the earth’s surface at about 100,000 watts. The high wattage allows the signal to travel beyond just “line of sight”. Radio stations broadcast on frequencies from 88 to 108 MHz.
GPS signals are broadcast from a couple dozen satellites which each revolve around the earth twice per day. However unlike a radio station of 100,000 watts the GPS satellites only have about 20-50 watts of power. This is why GPS reception is sometimes difficult near large buildings, under dense canopy, etc.
If your GPS receiver was a radio, you would tune it to 1575.42 MHz and listen to the response. Each satellite will identify itself and send a code used to calculate the time the signal arrived at the GPS receiver. The amount of time the signal took to travel from the GPS satellite to your GPS receiver is multiplied by the speed of light to calculate the distance you are from that satellite.
There is more than one frequency or “station” GPS satellites transmit on, however only one has been available for civilian use and it is called L1, sometimes called L1C for “L1 Civilian”.
Recently, a new GPS satellite was put into orbit. This satellite is capable of broadcasting not only on the L1 (L1C) frequency, but also a new L2 frequency. (L2C actually, the C denoting civilian). The L2 broadcasts are on 1227.5 MHz. (Note I’ve also seen references to it being on 1227.60).
So in order for your GPS receiver to be able to utilize information broadcast over L2 (L2C) your GPS receiver must be able to tune into that frequency. Keep in mind it takes at least three satellites to lock into your position. Currently there is only one satellite broadcasting on L2C however eight more are scheduled in the next two years. Some people have indicated to me that consumer level GPS receivers will not start hitting the market until there are 24 functional satellites broadcasting L2C information.
There are GPS applications (such as those used by professional surveyors) that currently can receive L2 GPS signals. However those are not L2C signals. There are other signals besides the Civil codes. There are also P codes, Y codes and M codes. We won’t get into that here.
“The activation of the L2C signal in January 2006 will be of great benefit to the civil community,” said Colonel Jester, Chief, Space Operations Branch, Air Force Space Command. “But they will be using this signal at their own risk until the command and control of L2C is realized in the Fiscal Year 2013 timeframe. Receivers able to fully utilize the L2C signal will be the responsibility of the civil community. The military will build receivers focused on the new M-code signal.”
So don’t expect to see any change in reception from your consumer GPS receiver today. L2C receivers appear to be a few years down the road…. And then L5 is coming down the line too….
So what the heck is the difference between L1 and L2 then? The L2 system’s signal is stronger. This allows for two improvements. The first is increased reception in troublesome areas such as near tall buildings, canyons, etc. It also will allow devices to reduce the amount of power they need to expend to receive a GPS signal. This will make putting accurate GPS receivers into mobile phones, watches, etc easier.