GPS signal level is about -125dBm at surface, and good GPS receiver may acquire a signal down to maybe -150dBm or so. In effect you have a ~25dB margin and it is up to your receiver what is the maximum attenuation it can withstand in continuous operation. Go below Rx limit and you have no service. It is assumed that GPS signal is not affected by interferences, and due to the very low signal level it is obvious that it is easily jammed.

Cellphone signal is a bit different in a sense that you seldom have a situation with thermal noise as a limit. Due to frequency reuse the lower limit is set by interferences (I), information bandwidth and to some degree the technology. With rising bandwidth you have a rising thermal noise floor, and for simple voice call it will be about -125dBm. Because of interferences, the noise floor rises to, say, -110dBm, and you need extra power (carrier) for establishing indoor calls, say 20dB, so the cellphone signal level will be -90dBm and upwards. For broadband communications it is translated even more upwards. Only in sparsely populated areas with low interferences it may go lower down to the thermal noise limit (N), but in such cases you often have only intermittent service.

So you can't simply expect service only on the basis of signal strength. In GPS it is limited by S/N, while in cellular it is limited by C/I (carrier to interference).

Cellular frequency allocations are: 700 MHz 750 MHz 800 MHz 850 MHz 1700 MHz

2100 MHz 1900 MHz 2500 MHz

GPS frequencies are:  1575.42 MHz 1227.60 MHz 1381.05 MHz 1379.913 MHz 1176.45 MHz

I started to go into attenuation and interference but see that Mr. Virkes has covered it quite well.

Suffice it to say, the GPS signal is a very weak signal compared to almost all others.

JAC

If you keep your calculations in dBm units, you are free from frequency dependence. A milliwatt is a milliwatt at any frequency. As noise floor is strictly bandwidth dependent, link balance calculation is straightforward only in dBm units. Calculating with field in dBμV/m is perhaps looking kinda scientific, but it is a misleading complication and largely unnecessary.

To determine this, you would look at the skin depth equation, and would need to know the resistivity of the soil, which would depend on things like its salinity and water content.  This will tell you how the signal strength decays in the soil.  If you know the strength at the surface, then you can know what it is at any depth.  Cell phone signals are much stronger than GPS signals (in general), so given most soil make-ups, I would guess you could still receive cell phone signal but not GPS under the soil.

Thank You Sirs. Actually I was planning to use a GPS device with a PIG instrument that is run through the underground petroleum pipeline(1m below soil surface) for real time tracking. I did the experiment, burying cellular phone enclosed in a steel box up to 2 feet deep into the soil, & cellphone signal was still received in there.

So far as per your suggestions GPS tracking is ruled out underground. But you know about 'dead reckoning'. In my case, while the PIG is inserted into the pipeline, its exposed to one of the satellites. Now, inside the pipeline its speed is around 1 m/s, and we are having 'test lead points' at every 2 km (a wire welded to the external surface of the pipe & extending out the soil to the open air for taking voltage for cathodic protection. Could the 'test lead points' work as antennas?)

if the Test Lead Points (TLP) at every 2 km could act as antennas then at the first 'TLP', the PIG(that has steel extension touching the inner surface of pipeline) would be located (as the antenna(TLP) is exposed to sky). As the speed of the PIG is constant, the next 2 kms would be tracked by 'dead-reckoning' then again a TLP comes that will recorrect the position of the PIG and so on....  Possible?

As others have remarked, the GPS signal is much weaker than cell phone. There is no chance of reception in the buried metal pipe you describe.

The idea with the test leads is interesting, but it would not work, because the PIG is in a Faraday cage. And even if the signal would penetrate inside the tube, this would not give the position of the PIG, but the position of the test lead point. GPS positioning is based on the difference in time of arrival of the signals from different satellites. If this is measured at the antenna location and sent into the pipe towards the PIG, here will be a common delay for all those signals, depending on the position of the PIG, but the differences are fixed and correspond to the antenna position.

To solve your problem, you could use dead reckoning, e.g. using inertial sensors or a simple measuring wheel which rolls inside the pipe. If the pipe is empty you could even use a length of cable attached to the PIG. A transponder or echo device may also be a possibility.

An alternative approach is to equip the PIG with a radio transmitter. If this is powerful enough, it may penetrate through the pipe wall and may allow to positon the PIG from the surface. Not sure if this would work.

Gert Cuypers Sir, I was thinking that the signal is received & transmitted by the receiver/ transmitter fitted on the PIG.As the distance between the PIG & TLP will keep on changing, the delay will also change, so in that case the TLP would be 'virtually moving' with respect to the satellites at the PIG's speed. That may work in our favour. What you say?

Indeed, the delay will change, but the *differences* between the delays will not change. Imagine that the TLP is 10µs "closer" to one satellite than to another. From this difference, the position of the TLP can be uniquely determined (of course using more than two satellites in practice). All signals enter the pipe at the TLP, and from that point on they travel together at the same speed.

If the PIG moves away from the TLP, it will have to wait longer for the signals to arrive, say 1µs, but it will still see the 10µs difference between the two satellite timings. The PIG will conclude that its clock is running early (because it has to wait 1µs longer for the signals) and will adjust its clock to run 1µs behind the actual time. Then it will process the time differences (10µs) and conclude that it is at the position of the TLP, but at a later time than in reality.

Sir, If I use a cellphone(3G, 4G LTE), the satellite issue won't be there. I have tested & found that cellphone signal penetrates more that 2 feet depth of soil. What about using the cellphone network? In that case would TLPs work as antenna? Please guide

Also I was thinking what would be the locus of such a point whose distances from each of the 3 satellites is changing, but at any point of time the difference between every pair of the 3 satellites doesn't change?

I believe a better solution would be to acquire a pig with INS capabilities. It may also have a build-in GPS. The pig is told its initial position, or gets it from GPS. The INS is initialized and reports its positions along the way of the pipe. Its position at the end of the run is taken and compared with the INS for adjustment.

This is not new technology. It is used in the oil-patch and is quite robust.

JAC

Pigs are travelling through pipes, so the only meaningful measurement in space is a distance. And most pigable pipes are made of steel, which further complicates any meaningful radio communication with outside world.

For distance measurements you may also use inertial sensors. I did not play with those, but at a GNSS vulnerabilities conference those were presented as fairly useful.

Dear Mohd Sadaf: the problem of the time differences is not exclusively related to satellites, it also happens when using positioning using cell towers.

The cause is the following: most people think that the location is detemined from the distances to the satellites (or cell towers). In reality, this distance could only be known if the PIG (or GPS device) had a very accurate clock, with atomic clock precision. In that case the device could really calculate how long a signal has travelled, and how far away a satellite/cell tower is located.

In reality, the PIG/GPS device does not have such an accurate clock. However, it can accurately determine the time differences between the incoming signals from satallites/cell towers. (This is also the reason why at least four GPS satellites are needed to calculate a position instead of 3. A GPS device using an atomic clock would only need three satellites).

As mentioned before, the delay of the signals, going in the pipe from the TLP to the PIG will be interpreted as a clock fault in the PIG's internal clock. The differences will be fixed for the position of the TLP, and the calculated position will always be the TLP position.

If cell tower signals penetrate deep enough into the ground and in the pipe, this may be a solution. The TLPs will not help, however, because they have the same problem as described before.

To answer your question of the locus: the differences between 3 satellites are not enough to determine a point uniquely, because this only gives 2 independent differences. You need at least 4 satellites, as described before. There can only be one point with fixed differences from 4 satellites, and solving for this point will also give you a precise time.

thank you Sirs.

If the mobile phone or device is in the coverage area of a mobile base station operating on any frequency in use for cellular communications (450 MHz up to 2.5 GHz), the received signal strength of the mobile service will be significantly greater than the GPS signal strength, which will be down around the noise floor.

GPS signals use CDMA and the signal levels are lower than the noise floor.. the cell phone signals are much stronger than GPS .... generally the GPS signals are received at a level of -160dBW or -130dBm... and the received signal strength for each satellite varies with the position of the satellite from the receiver or the pseudorange of the satellite from the receiver as that determines the free space loss the signal will face. Along with that the elevation angle and the azimuth angle also determines the signal strength a GPS receiver is going to receive as the Antenna Gain Pattern of the GPS device changes with azimuth and elevation of the satellite signals (or in other words from which direction the signals are coming). Hope it helps.