How Does the OTPI Calculate the Exact Position of RF Emitters?

[nauman]

Hi all

I am interested to know how Aircraft born 'On Top Position Indicator (OTPI)' works for RF emitter (e.g. Sonobuoy) localisation? As per my understanding, the Direction Finding systems used in OTPI gives only bearing of incoming RF signals (no range measurement), how the exact position of RF emitter is being calculated?

Thanks

 

[anorlunda]

I can't find any aircraft On Top Position Indicator using Google. Can you provide a link?

Is your application marine? Are you trying to describe a NDB non-directional beacon for use in marine navigation?

 

[Baluncore]

There are several different types of OTPI that use different technology.

Early systems used a balanced orthogonal antennas mounted under the aircraft. By measuring the balance of the left-right pair and the fore-aft pair, a crude direction could be generated. The left-right pair made it possible to fly directly to the target, then as the fore-aft pair approached and reached balance, the aircraft was known to be over the position. Flying high was needed to initially acquire the signal, then it was necessary to fly lower to increase the accuracy of the position estimate.

GPS based systems receive the GPS position from the marker buoy and so can calculate direction, range and predict when the aircraft will be over that position. Those systems are not dependent on flying at low altitude for accuracy.

 

[anorlunda]

As usual, @Baluncore had better information than I did. Using the acronym OTPI, I found this article about OTPI and sonobouys.

http://jproc.ca/rrp/rrp3/seaking_otpi.html

 

[nauman]

There are several different types of OTPI that use different technology.

Early systems used a balanced orthogonal antennas mounted under the aircraft. By measuring the balance of the left-right pair and the fore-aft pair, a crude direction could be generated. The left-right pair made it possible to fly directly to the target, then as the fore-aft pair approached and reached balance, the aircraft was known to be over the position.

Thanks for help. Someone told me about a rotating (mechanically) antenna with cardioid beam pattern which tries to steer the null in the direction of RF received signal using a motor with feedback control system. Unfortunately, i could not get more detail about this technique. Do you have more information about this technique how to achieve 'OTPI' functionality using this approach?

Thanks

 

[Baluncore]

Someone told me about a rotating (mechanically) antenna with cardioid beam pattern which tries to steer the null in the direction of RF received signal using a motor with feedback control system.

I would be surprised if a servo drove a single antenna to a null since then there is no signal. It is much more likely that the servo drove a two antenna array to the point where the two antenna signals were of equal amplitude. The sum of the two antenna signals gives the received signal. The difference of the two antenna signals has a cardioid pattern, with the null when on target.
A synchronous detector, (the product of the sum and difference signals), would give the servo drive direction and velocity.

But that does not explain how an OTPI signal could be generated using that system.

A better description of the actual system, preferably a system name or part number could find the original training documents.

OTPI can be used to deliver a depth charge to a sonar buoy, a package to a beacon, or to locate an EPIRB.

 

[nauman]

It seams that OTPI is an old technique to find geolocation of RF emitter because aircraft have to reach on top of RF emitter to find its location as name suggest.
Are there any commercially available airborne RF Direction Finders (30MHz-400MHz) which can give direction as well as geolocation of RF emitters also?
I have searched some commercially available RF Direction Finders for airborne use, but they give only direction estimation of RF emitter.
Thanks

 

[Baluncore]

Are there any commercially available airborne RF Direction Finders (30MHz-400MHz) which can give direction as well as geolocation of RF emitters also?

Yes. Sidescan doppler. But you can't afford it. Also works from satellites.
If the beacon has GPS then the problem can be solved entirely by computation.
If the beacon has a transponder then both range and direction can be available.
See also; https://www.argos-system.org/using-argos/how-argos-works/
https://en.wikipedia.org/wiki/Argos_(satellite_system)

I have searched some commercially available RF Direction Finders for airborne use, but they give only direction estimation of RF emitter.

That is why they are called direction finders.

What are you really trying to do ?
What sort of beacon ? GPS ? Transponder ?
What type of aircraft ?

 

[pbuk]

It seams that OTPI is an old technique to find geolocation of RF emitter

No, all OTPI ever provided was a bearing (to one of a number of tactically deployed sonar-equipped buoys 'sonobuoys'). When a sonobuoy detected an enemy submarine the aircraft would fly along the bearing until it had a visual on the sonobuoy detected an overfly from the null (or had a visual) and could deploy its own dip sonar equipment for further investigation.

If the beacon has GPS then the problem can be solved entirely by computation.

And all modern beacons have GPS. OTPI was used in the 60's until the early 80's before GPS was available.
As @Baluncore asks, what are you really trying to do?

 

[nauman]

What are you really trying to do ?
What sort of beacon ? GPS ? Transponder ?
What type of aircraft ?

I am searching an airborne solution for helicopters (probably a RF Direction Finder with geolocation capability if such systems exist!) to find location of sonobuoys.
Sonobuoys have RF transmission in frequency range of 150MHz to 400MHz and no GPS is available on sonobuoys.

 

[pbuk]

I am searching an airborne solution for helicopters (probably a RF Direction Finder with geolocation capability if such systems exist!) to find location of sonobuoys.

What is the application?

Sonobuoys have RF transmission in frequency range of 150MHz to 400MHz and no GPS is available on sonobuoys.

If you can afford helicopters you can afford GPS on your sonobuoys - in fact you would probably have to pay to have it deleted.

 

[Baluncore]

I am searching an airborne solution for helicopters (probably a RF Direction Finder with geolocation capability if such systems exist!) to find location of sonobuoys.
Sonobuoys have RF transmission in frequency range of 150MHz to 400MHz and no GPS is available on sonobuoys.

A switched-antenna doppler-DF system will give an approximate bearing by measuring the phase of the received audio signal. Helicopter position is known by GPS, so software can plot virtual lines on a virtual map in the computer. Fly in the general direction of the target repeating as you go. Buoy drift, and early errors in bearing estimates can be eliminated as later estimates improve position. The doppler signal will fall to zero over the target.
https://en.wikipedia.org/wiki/Doppler_radio_direction_finding
https://hackaday.com/2018/01/23/shm...irection-finding-with-software-defined-radio/

 

[dlgoff]

A switched-antenna doppler-DF system will give an approximate bearing by measuring the phase of the received audio signal. Helicopter position is known by GPS, so software can plot virtual lines on a virtual map in the computer. Fly in the general direction of the target repeating as you go. Buoy drift, and early errors in bearing estimates can be eliminated as later estimates improve position. The doppler signal will fall to zero over the target.
https://en.wikipedia.org/wiki/Doppler_radio_direction_finding
https://hackaday.com/2018/01/23/shm...irection-finding-with-software-defined-radio/

Once I was flying with a friend to an Army reserve summer camp when we got lost over the Missouri Ozarks: https://en.wikipedia.org/wiki/Automatic_direction_finder

 

[Baluncore]

The Automatic Direction Finders for aviation operated on the original world wide web of long wavelength LF radio beacons. Those LF beacons have disappeared over the last ten years since GPS is now more reliable.

 

[DaveE]

Thanks for help. Someone told me about a rotating (mechanically) antenna with cardioid beam pattern which tries to steer the null in the direction of RF received signal using a motor with feedback control system. Unfortunately, i could not get more detail about this technique. Do you have more information about this technique how to achieve 'OTPI' functionality using this approach?

Thanks

This technique is fairly straightforward conceptually. It is just a way to steer an antenna in a circular pattern (think conical) without actually moving it. It's done mechanically in many forms for parabolic antennas (ref. nutator), but is also done electronically with phased array antennas. By looking for the point in the scan that the received signal is at its maximum, you will get a bearing. Coherent detection techniques (like lock-in amplifiers) can be very sensitive with accurate results. When the antenna is pointed directly at the source, the signal from the coherent detector is nulled (i.e. no preferred bearing).

This is typically how targeting radars track objects and how tracking antennas follow satellites. You will also find it in many other tracking or alignment applications. I designed exactly this type of system to align a laser resonator (a pair of parallel mirrors) to an accuracy on the order or 1mm at 1km, while moving the mirrors an imperceptible amount. You would need a spectrum analyzer to monitor the beam and I would have had to tell you to zoom into 30Hz to see it above the noise floor.

However, I'm not sure it's the best approach for your application. You may not need this level of sophistication and there are issues with target acquisition. This is typically better for tracking than searching, first you need to receive a signal before you can get a bearing.

 

[DaveE]

As per my understanding, the Direction Finding systems used in OTPI gives only bearing of incoming RF signals (no range measurement), how the exact position of RF emitter is being calculated?

A tracking system with a steerable beam (maybe a phased array antenna) could give you azimuth and elevation. Combined with your altitude data you can calculate range to the water surface.