Method for locating cable impairments
View Patent ↗Methods are disclosed for identifying and locating points of impairment in a cable plant, such as that used for cable television (CATV). The methods utilize both known characteristics of signals as well as propagation times in free space and within the cable in conjunction with accurate determination of locations at which measurements are taken. The methods can be applied to both RF cable leaks as well as points of ingress of interference.
1. A method for locating an impairment in a cable plant comprising:
a) transmitting a test signal having a known characteristic between a test node in the cable plant and a selected test point at a physical location outside the cable plant via the impairment;
b) measuring a total propagation time of the test signal between the test node and the test point; and
c) estimating from the total propagation time a maximum impairment distance in the cable plant between the impairment and the test node.
2. The method of claim 1 , wherein the total propagation time in step b) is measured using timing from an orbiting navigation satellite positioning system.
3. The method of claim 1 , wherein the physical location of the test point in step a) is measured using an orbiting navigation satellite positioning system.
4. The method of claim 1 , further comprising transmitting the physical location of the test point to the test node via the impairment.
5. The method of claim 1 , further comprising the step of using the known characteristic of the test signal to improve the total propagation time measurement accuracy.
6. The method of claim 5 , wherein the known characteristic of the test signal is selected from the group including of a test pattern, a single phase-locked TV carrier, multiple phase-locked TV carriers, wide bandwidth direct-sequence spread spectrum with long pseudo-noise spreading sequence, and unmodulated CW.
7. The method of claim 1 , further comprising the step of communicating the known signal characteristic from a transmission end to a receiving end over a separate communication path.
8. The method of claim 1 , further comprising repeating steps a) to c) at least two times for different selected test points to determine a change in proximity to the impairment.
9. The method of claim 1 , further comprising repeating steps a) to c) at least three times for different selected test points to directly estimate an impairment location.
10. The method of claim 9 , further comprising using a known layout of the cable plant to improve estimates of the impairment location.
11. The method of claim 9 , further comprising using a measured signal strength of a portion of the test signal received through the impairment to improve estimates of the impairment location.
12. The method of claim 9 , further comprising the step of displaying on a display unit measurement results comprising at least one of an estimate of the maximum impairment distance, a difference between two most recent estimates of the impairment location, and a graphical display of estimates of the maximum impairment distances as circles centered at each test point.
13. The method of claim 9 , wherein the impairment is a RF leak, and further comprising the step of measuring with a mobile receiver a time of arrival of the test signal from the test node in the cable plant at each test point.
14. The method of claim 9 , wherein the impairment is a point of ingress, and further comprising the step of measuring a time of arrival of the test signal from each test point at the test node in the cable plant.
15. The method of claim 14 , wherein a mobile transmitter is used for transmitting the test signal from each test point.
16. The method of claim 15 , further comprising transmitting the estimated impairment location to the test point for providing position feedback to a technician in the field.
17. The method of claim 1 , further comprising:
d) moving the test point along a test heading in relation to the cable plant;
e) measuring a Doppler frequency shift of at least one of the frequencies of the test signal;
f) estimating a heading error to the impairment from the Doppler frequency shift and a velocity of the test point;
g) selecting a new test heading from the estimated heading error; and
h) repeating steps d) to g).
18. The method of claim 17 , wherein the impairment is a RF leak, and further comprising the step of receiving the test signal at the test point from the test node in the cable plant.
19. The method of claim 17 , wherein the impairment is a point of ingress, and further comprising the step of receiving the test signal at the test node in the cable plant from the test point.