Most Common Questions
How Far can I run cable from the receiving Antenna to a GPS Source Splitter?
What we are concerned with is total signal loss. For high gain antenna (35dB), most receivers can operate properly with 13dB +/- 2dB total loss or attenuation from the antenna to the receiver input. For lower gain antenna (26dB), most receivers function normal with 6dB +/- 2dB total loss or attenuation from the antenna to the receiver input.
Considerations include the cable loss between the antenna and the splitter, the splitter loss and the cable loss from the splitter to the receiving device. Those three things combined cannot exceed the 13dB +/- 2dB or the 6dB +/- 2dB above.
Remember the passive S12 adds approximately 4dB of loss and the passive S14 adds approximately 8dB of loss. The only thing left to do is calculate your cable loss.
Special Note: You can find low loss cable rated at GPS frequencies that are a plenum, semi-rigid or rigid type. Of course, you will pay extra for these types of cables. You can find the specifications of your cable on the internet. And as always, call or email us and we will be glad to help you out with any GPS project or installation.
Why are GPS Source splitters built with 200 OHM loads on DC blocked ports?
GPS source splitters can pass DC voltage or block DC voltage on every port. The 200 Ohm loads on DC blocked ports simulate the current draw of an attached antenna. Some GPS receivers will indicate an antenna fault if they do not detect this current draw.
Most GPS receivers supply antenna voltage through the input port. GPS Source splitters pass this DC voltage to the antenna via the Out1 port. Hence, the Out1 ports do not need a 200 Ohm load and the attached receiver will not indicate an antenna fault. For splitters with the Power Option where antenna voltage is supplied by the splitter, all Out ports are blocked and have a 200 Ohm load. Again, the attached receivers are satisfied with the 200 Ohm load a no antenna fault will be indicated.
Note: Remember, GPS Source products come standard with the customers choice of plug/jack (male/female) type N, SMA, TNC, or BNC connectors. Mix and match any connector configuration free of charge. We know that eliminating adapters removes excess loss and expense to every GPS distribution set up and configuration.
Do you accept credit card payments?
Yes, we accept all major credit cards and Electronic Bank Transactions as payment. There is not a method online so to place both domestic and international orders with credit cards, customers should call GPS Source to place orders with credit cards.
What are GPS Source's most common markets and applications?
We offer complete GPS solutions to almost all GPS markets worldwide. Our most common markets are military, cellular, timing, aviation, research, academics, aerospace, civil and commercial engineering and test.
What is the coverage of the GPS signal?
The Global Positioning System (GPS) provides accurate time information anywhere in the world. A GPS antenna requires a good view of the sky in order to receive information from the GPS satellites. Provided the antenna can view a large enough area of the sky, a signal lock can be achieved anywhere on the face of the planet.
What is the best place to locate the GPS antenna?
A GPS antenna needs to have a good view of the sky in order to obtain a signal lock. Ideally, a full 360 degree of the sky is required. However, in practice GPS antennas can obtain a signal lock with a partially obscured view. Often a good signal lock can be obtained by sitting the antenna on a window sill or ledge provided that the horizon is not too obscured.
As a general guide, the better the view of the sky provided the antenna, the better the chance of a good continuous signal lock.
Cable Delay Calculations
Calculating the propagation delay of coaxial cable
The delay of a cable or velocity factor is determined by the dielectric constant of the cable. The velocity factor is the speed at which an RF signal travels through a material compared to the speed the same signal travels through a vacuum. The velocity of propagation is inversely proportional to the dielectric constant. Lowering the constant increases the velocity. Generally, the higher the velocity factor, the lower the loss through a coaxial cable. Said another way, Velocity of Propagation (VP) or Velocity Factor (VF) is a parameter that characterizes the speed at which an electrical signal (e.g. radio) passes through a medium. Expressed as a number between 0 and 1 (or a percentage), it is the ratio of a signal's transmission speed to the speed of light in vacuum. Thus, transmission in a vacuum would have a VP of 1 (100%). VP equals the reciprocal of the square root of the dielectric constant of the material through which the signal passes.
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The maximum cable length that can be utilized by a GPS antenna is dependant on the gain of the antenna and the quality of the coax cable utilized.
The length of the cable and the type of cable, can affect the dynamic range of any GPS device. The dynamic range is essentially the range of signal levels over which it can operate. The low end of the range is governed by its sensitivity. While at the high end, it is governed by its overload or strong signal handling performance
Standard Cable Types
Cable type GPS Source Part Number LMR240 CA240 LMR400 CA400 RG8 C8
GPS Antenna Installation
A GPS device relies on the radio signals sent by the GNSS network situated in earth's orbit. But the GPS signal is very weak. The accuracy of GPS depends on the GPS signal strength that may degrade due to several reasons - one of them being the walls of buildings. Since the GPS signal is too weak to penetrate buildings, in order to use a GPS server or device indoors the signal has to be received outside and then amplified and rebroadcast inside.
The most common factor that interferes with a GPS signal is what is termed "Urban Canyon." This canyon refers to the high rise concrete buildings and skyscrapers that do not allow the signals to pass through them. They either totally block the radio signals sent by the satellites or alter their path so that they never reach the GPS servers or GPS device. Even if the signals are able to get through the buildings, they become so weak that it becomes weak for the GPS device's receiver to interpret the signal.
A GPS antenna is a device that helps focus and move the GPS signal to a GPS unit, whether it is a standalone unit or an embedded unit. A GPS antenna is used in a situation where the GPS unit itself is somehow removed from a line of sight to the sky, as in a building or a garage, to help the GPS “see” the sky.
If you are working in a GPS denied environment, a GPS antenna is the first step towards getting the GPS signal indoors. This list of questions can help you determine your antenna requirements, installation location and antenna type.
- Do you have access to the roof of your building? The best place for an antenna is a place on the roof with a direct line of sight to the GPS satellite system.
- Do you have permission or do you know the process to get permission and authorization to place an antenna on the roof of the building or garage? It is possible there is already a GPS antenna on your roof and you may be able to "split the signal" from the existing antenna.
- Do you have access to an unobstructed view of the horizon? You do not necessarily want to place the antenna at the highest point of the roof. You want to place it where it will get the most signal coverage.
- From the roof, can you reach the GPS unit with less than 300 feet of cable? The length of the cable can greatly affect the strength of the GPS signal. Three hundred feet is the approximate length at which you may have to start considering an amplifier to make the GPS signal stronger (so that you can use more cable).
- Is the distance of the antenna cable between 300 feet and 700 feet? Keeping the cable length under 700 feet limits the number of amplifiers you may need to push the GPS into the building.
- Do you prefer or do you know if your facility requires the use of a plenum rated cable? Plenum rated cable is more expensive, but it is flame resistant and has low smoke production. The idea being that if a building is involved in a fire, the plenum cable will not burn as easily as a standard PVC jacket and will product less smoke than a typical cable if it does catch fire. Plenum cable is generally 3 times the cost of regular cable, so take your time and make sure you actually need plenum cable. It may be reqquired because of fire codes.
- Will the antenna cable be installed in conduit? If so, is the conduit large enough to contain the "N" type connector on the GPS cable? If not, it may be necessary to have the connector installed on-site. This will require someone who is certified cable installer, because the connectors willl be attached via soldering.
- Is the installation area free of TV, FM, wireless and microwave radio transmitters? Any of this equipment will affect the GPS signal in a negative way. The GPS antenna may not work correctly around any kind of RF transmitter.
- Do you wish to have a surge suppressor installed to help protect the GPS equipment? GPS Source always recommends some kind of surge suppressor and a grounding kit when installing an antenna. Protect any communications and power lines at building entry points. If you use other antennas or aerials, such as a radio modem that distributes real-time correction messages, consider protecting those antennas as well. A correctly installed surge suppressor will route any energy from a lightning strike to the ground, before it enters the building.
- Do you need the antenna and cable to be shared with more than one GPS unit? This will require the purchase of a GPS splitter. They come in many sizes and types (1x2, 1x4, 1x8, 1x16, 1x32). Splitters can also be purchased with amplification, surge suppression or DC bias. See our large selection of GPS splitters for more information.
The GPS signal is different than standard radio signals (FM signal/AM signal, cellular, TV, two-way radio, etc.). It is different mainly by the amount of power it has when received by your roof/external antenna.
If you were standing next to the satelllite, you would be able to see a pattern, which is is represented (or known) as the normalized sinc function, similar to the picture below:
If that is the shape of the signal, why can’t you see it? The short answer is you are not standing next to the satellite. The long answer is the GPS satellites are 10,898 nautical miles in space and they don’t transmit with enough power for GPS signals to be as powerful as the terrestrial radio signals that you see with your spectrum analyzer.
Also, standard radio signals have a positive SNRs (signal-to-noise ratio). This means they are stronger than the noise in their band. The GPS signals have a negative SNR, i.e., they are weaker than the noise in their band. If GPS had a positive SNR, you would probably not need an antenna on your roof. It is possible you could receive it indoors without an outside antenna, just like your cell phone or AM/FM radio. But, the GPS signal has a negative SNR. By the time it makes it to the earth’s surface, it is about 26dB below the noise floor of the earth. That is why an external/roof antenna is needed. The GPS signal is available, but it is hidden under all the noise.
The GPS signal can be extracted from the noise. A typical GPS receiver has approximately 40dB of processing gain. When it looks at the “noise” it can see/detect something. It does this by performing what is called a correlation of the noise against the same codes that were used to create the GPS (CDMA) signal. Aclose analogy would be that of a comb with some of the teeth broken out that represents the pattern. If you comb the noise with a comb that is missing the exact same teeth as the comb that created the pattern…at exactly the right instant, you will get a match.
So what is the bump that you are seeing on your spectrum analyzer? What you are seeing is the response/shape of the antenna element (it acts like a filter) and the filter response of the antenna’s preamplifier. Their frequency response has a bell (Gaussian) curve shape, so that you see a bell-shaped bump on your analyzer. The noise is shaped by the filter, because noise is random. It has equal power across the band, so you wind up with noise I the shape of the filter response.
The GPS signal is there, but it is 26dB below the peak of the bump down in the noise or roughly 1,024 times weaker than the power of the noise. If the GPS signal is 26dB weaker than the signal when it arrives, is the GPS signal 26dB below the noise being measured? No, not necessarily. It is an ‘okay” rule of thumb, but keep in mind the exact signal level depends on the bandwidth of your system, the exact gains of all devices, their noise figures, temperature (remember noise figure changes with temperature), etc. Essentially, this “rule of thumb” will usually get you in the ballpark provided none of the gain stages are saturated.
“The GPS signal has a negative SNR…so what?”
Signal-to-noise ratio (or SNR) is a measure used to quantify how much a signal has been corrupted by noise. It is defined as the ratio of signal power to the noise power corrupting the signal. A ratio higher than 1:1 indicates more signal than noise. The GPS signal has a negative SNR, and is weaker than the noise in the band.
It is important when designing a GPS network with cascading gain stages that you keep the noise level in mind, because the noise is so much stronger than the GPS signal. It can drive cascaded gain stages into saturation. An experienced GPS network designer will strategically place gains and losses to avoid saturation issues. The other side of this coin is that you need to maintain link margins so that the signal does not dissipate before you place a following gain stage.
The way to think about it is like this, "if I have a GPS network that needs 60dB of gain, I shouldn’t place a 60dB amplifier at the output of the antenna and try to drive to the end of the network, because it will saturate and distort the signal. Likewise, I can’t place all the gain at the end of the network, because the signal will end before it reaches the amplifier at the end of the network". In other words, you will not have enough link margin. One needs to distribute the gains and losses so that you avoid saturation and maintain link margin across the GPS network. Call (719) 561-9520) if you need help with calculating link margins and designing your GPS network.
What Is M-Code
A major component of the US military modernization process is a new military signal. Called the Military Code, or M-Code, it was designed to further improve the anti-jamming and secure access of the military GPS signals.
The M-Code is designed to be autonomous, meaning that a user can calculate their position using only the M-code signal. From the P(Y)-code's original design, users had to first lock onto the C/A code and then transfer the lock to the P(Y)-code.
The M-Code is transmitted in the same L1 and L2 frequencies already in use by the previous military code, the P(Y)-code. The new signal is shaped to place most of its energy at the edges (away from the existing P(Y) and C/A carriers).
Section 913 of the Ike Skelton National Defense Authorization Act for Fiscal Year 2011, requires all military GPS user equipment purchased after FY 2017 to be M-Code capable, except in the case of cars or where waived by the Secretary of Defense.
GPS Source has several products that are M-Code capable, including the GLI-ECHO (smart splitter/amplifier), D3 (DAGR Distributed Device) and all Mil Spec Splitters.
Repeaters - Most Common
What is the range of a standard GPS repeater kit?
The standard GPS Source Repeater kit, which includes a GPSRK amplifier, 50 ft. of LMR240 coaxial cable and an L1A active receive antenna, has a range of 100ft line of sight. This can vary given steel structures such as ladders, equipment, etc. within this range; however, most applications can rely on this 100ft signal coverage.
When would I need a GPS repeater system?
Customers can test receivers and applications where a direct coaxial connection is not convenient or possible (e.g. emergency response vehicle or aircraft maintenance facility).
Customers can test receivers and/or applications with built-in integrated antennas enabling testing the entire system.
Customers can test multiple receivers at the same time.
Can I design a custom repeater system for my facility if the standard repeater kit doesn't fit my application?
Yes, custom repeater systems can be designed to accommodate virtually any facility or test scenario.
For example, custom repeater systems can be designed for multiple hangar or garage bays, multiple lab benches or installations requiring longer coaxial cable runs.
Do the L1/L2 repeater kits only transmit at these frequencies?
Antennas used in GPS Source repeater kits include filtering to prevent the re-transmission of frequencies outside of the GPS bands. If the filtering included in the GPS Source antennas is deemed insufficient, additional filtering can be specified in the GPSRK amplifier.
Should I order an L1 repeater kit or an L1/L2 repeater kit??
The L1 is standard for consumer or civilian applications. L1/L2 is used in survey and military applications.
What does the standard GPS kit come with?
The GPSRKL1, GPSRKL12, and GPSRKL12G repeater kits include a repeater amplifier with a built-in power supply and a gimbal mount, a passive retransmit antenna. GPS Source PNs METRO-RK and METROe-RK include all of the above plus 100-ft of LMR240 coaxial cable and an active receive antenna on a pole mount.
Note:* X = 1,2, or 3 and refers to power supply options: -1=110VAC, -2=230VAC, and -3=240VAC
Repeaters - FCC Licensing
Why do I need an FCC license to operate a GPS repeater?
The FCC requires commercial users (i.e. all non-Federal Government) within the U.S. to acquire and maintain a Part 5 experimental license to operate a GPS repeater kit. This is due to the potential for an improperly installed or improperly operated repeater kit to result in increased error in other GPS equipment in the vicinity.
Please refer to our FCC Licensing Instructions which provides detailed directions for how to apply and be granted an experimental FCC license.
Do I need an experimental license if I operate my repeater in a shielded room or if I use a repeater hood?
The FCC has provided GPS Source with the following statement regarding the requirements for licensing of GPS repeater systems in shielded environments:
"We generally don't require experimental licenses for operation inside true RF shielded rooms or anechoic chambers."
Federal Communications Commission
Experimental Licensing Branch
In the case of a GPS hat/hood system, the GPS hat/hood itself is a metal enclosure that is placed over the application antenna, thus constituting a closed Faraday cage, or 'shielded room' around the antenna. Furthermore, the GPS hat/hood system includes RF absorptive material inside of the hat, constituting an 'anechoic chamber'.
It is the understanding of GPS Source that the GPS hat/hood system meets the qualifications described by the FCC official above.
Measures have been taken to prevent RF energy from being radiated by the GPS hat/hood system (e.g. the hat has been designed to mount over the application antenna and has been coated with RF absorptive material), thus, any RF leakage from the system will be at extremely low levels and will be considered 'unintentional'. Unintentional radiation falls into a completely different regulatory category and signal level limits. The leakage levels from a GPS hat/hood system are far, far below those levels set by the FCC for unintentional radiation.
Please refer to our FCC Licensing Instructions which provides detailed directions for how to apply and be granted an experimental FCC license.
Splitters - Common Questions
How far can I run cable from the receiving antenna to a GPS Source splitter?
Can I get high isolation through a passive splitter?
No, you need to order custom gain to attain high isolation. This will then make the splitter active.
To determine what gain level you need for high isolation, refer to the high isolation chart.
Why are GPS Source splitters built with 200 OHM loads on DC blocked ports?
Do GPS Source splitters cover both L1/L2 frequencies?
Yes, GPS Source splitters are broadband; tuned and tested to work between 1GHz to 2GHz.
Can the GPS signal be split into 2 or more different receivers?
Yes. We have a standard S12 (splitter 2 outputs) that will split the GPS signal to two receivers. One output port will pass DC up to the antenna and the other output port is DC blocked with an RF load. You can choose the connector types, which can be the same or different depending upon your application. Also available are the S14, S18 and rack mount products.
Will the GPS Source splitter pass receiver DC to the antenna?
Yes. The standard configuration of GPS Source splitters will pass receiver DC voltage from output 1 to the antenna input. GPS Source splitters will use about 14ma of current.
If required, you can order the power option which will block all output ports and regulate DC voltage to the antenna input.
Do GPS Source splitters cover L1 & L2 GPS frequencies?
GPS Source splitters are broadband and tuned between 1GHz to 2 GHz. All splitters cover L1/L2, GLONASS, Galileo, and BeiDou frequencies standard.
Choosing a DC input voltage for a GPSS External Power Option
Input Voltage vs Output Voltage
Input voltage must be higher than the output voltage by the combined voltage drops of these circuit components (reverse polarity diode drop + min regulator drop + diode OR drop + output voltage)
Example for 5 volts output .7 + 1.4 + .7 +5 = 7.8. so if you need 5 volts out you should have at least an 8 volt power supply for the input voltage
Input Voltage vs Max Current
The maximum current that can be provided by the GPS external power option is determined by the maximum wattage the regulator has to dissipate.
The limit for most GPSS devices is 1 watt @ 25C higher temperatures will decrease this limit.
Remember to add the internal current of the device used to the amount of external current used for the total current load of the device.
Now consider a GPSS device using 48 ma total current (device current + antenna current) so the amount of power the device would be dissipating would be determined by the input voltage – reverse polarity diode drop (least case) – steering diode drop (least case) – output voltage so 8 - .3 - .3 – 5 = 2.4 volts.
2.4 volts across the regulator gives 2.4V * 48ma = 115mW of power dissipated by the regulator which is well below the 1 watt total dissipation limit.
Lets say instead of using an 8 volt power supply we chose instead to use a 24 volt power supply.
So the calculation would be 24 - .3 – 1.4 -.3 – 5 = 17 volts across the regulator so 17 * 48 = 816 mW of power dissipated by the regulator.
This is very close to the 1 watt limit of the linear regulator.
So when choosing a DC input level choose one high enough to over come the voltage drop of the circuit but not so high that it limits the amount of current the regulator can provide to below your required current.
If you only have a high voltage available then keep in mind it lowers the amount of total current the device can provide.