Tuesday, July 3, 2012

Measuring Range to an ELT by Off-Tuning.

In the document CASARA Ottawa Electronic Search Guide (also published on Doc Stoc) an electronic search technique called Wing Blanking is proposed as an alternative to the Aural Null procedures. I will describe wing blanking in a later article, and point out some issues that arise when off-tuning is combined with wing blanking. In this article I want to examine their claims of determining range to the ELT using off-tuning.

Here is a quote from their document:
When the ELT is received with the radio off tuned by 100 KHz, perform another wing blanking bearing. This will give the aircraft a distance and bearing to the target. The distance will be <2 nautical miles which is the airborne off-tuned reception range (confirmed by experiment).
Confirmed by experiment they claim. But what does the math have to say about this? Plugging 2 nautical miles into the Friis equation we get 85.5 dB of attenuation for the propagation distance. We know that a properly functioning ELT is required to transmit 20 dBm (100 milliwats). A receiver should be able to pick up a signal which presents -107 dBm at the receiver input.

20 dBm - 85.5 dB = -65.5 dBm

So, for a receiver off tuned by 100 kHz to be able to detect a signal at a distance of less than 2 nautical miles, but not at a distance greater than 2 nautical miles there would have to be 41.5 dB of attenuation between the transmit antenna, receive antenna and off-tuning. Transmitter and receiver antenna installation and cabling might account for a few dB at each end, but the directivity of the antennas should add 4 dB. Let's call it a wash. So 41.5 dB of attenuation remains to be accounted for. So let's look at off-tuning.

Many people describe off-tuning as a way of reducing the sensitivity of the receiver. This is not a bad working hypothesis, but not really accurate. What is actually happening is the receiver, once tuned away from the designed transmitter frequency, is receiving an unwanted (or spurious) transmission that is on a near by frequency as a by-product of producing the intended frequency. Spurious transmissions are generally less powerful than the intended frequency, with the power falling of rapidly at first, but less rapidly the further they are from the intended frequency. This means that the first step of off tuning will make the greatest change in received signal strength, each step after will make less of a change. The FCC and Industry Canada (and no doubt other regulatory bodies) have regulations specifying how much power the spurious transmission may have. For ELTs Industry Canada says:

5.6 Unwanted Emissions

The power of unwanted emissions measured by an averaging meter of 300 Hz resolution bandwidth shall be attenuated below the level of the mean transmitter power (TP) by:
  1. at least 25 dB on any frequency removed from the centre of the authorized bandwidth by more than 50% up to and including 100% of the authorized bandwidth;
  2. at least 30 dB on any frequency removed from the centre of the authorized bandwidth by more than 100%.
The authorized bandwidth is 25 kHz.
100 kHz is four times the authorized bandwidth, so there must be at least 30 dB of attenuation, but will there be more? For airband transmitters that are not ELTs the specifications are:

5.2.2 Transmitter with A3E or A9W Emissions

For transmitters with A3E or A9W emissions, the mean power of any emissions shall be attenuated below the mean power of the transmitter, P as follows:
  1. When the frequency is removed from the equipment's channel centre frequency by more than 50% up to and including 100% of the channel bandwidth, the attenuation shall be at least 25 dB, measured with a bandwidth of 300 Hz;
  2. When the frequency is removed from the equipment's channel centre frequency by more than 100% up to and including 250% of the channel bandwidth, the attenuation shall be at least 35 dB, measured with a bandwidth of 300 Hz;
  3. When the frequency is removed from the equipment's channel centre frequency by more than 250% of the channel bandwidth, the attenuation for on-board aircraft transmitters shall be at least 40 dB; and the attenuation for ground transmitters shall be at least 43 + 10 log10 P (in watts) dB, measured with a bandwidth of 3 kHz.
The emission mask is shown in Figure 1.

So, even a regular airband transmitter is only required to have spurious signals at 100 kHz from the intended frequency attenuated by 35 dB. Should we expect that an ELT, with relaxed specifications would necessarily achieve the 41.5 dB attenuation required to provide the kind of range indication that CASARA Ottawa claims? How many different beacons vs airborne receiver combinations did they test to arrive at this conclusion? Were all the beacons tested by an approved avionics shop to ensure they conformed to ELT performance requirements? Were the beacon batteries providing the specified voltage and current? If any of these specifications or requirements were not met, then the ELT would not be producing the specified output power which could account for the attenuation. So, is the airborne reception range of an ELT when off-tuned 100 kHz less than two nautical miles? Or is this more unsubstantiated or imagined data similar to that used to support the Cardinal Pass? What harm can these claims do? Most of these questions can only be answered if CASARA Ottawa is willing to publish their experimental data. The last is quite easy to answer.

If we again use the Friis transmission equation we can work out the range necessary to provide the total attenuation needed to reduce the signal to an undetectable level at the 100 kHz offset. Using 30 dB at 100 kHz the result is about 7 nautical miles. The harm is quite apparent. Working from the assumption that the CASARA Ottawa document is correct, a search team could conclude they are less than two nautical miles from the ELT when they are in fact much further away. This could significantly delay the arrival of rescue personnel at the crash site as the search is concentrated several miles away.

Of course, different ELT beacons could have very different spurious attenuation at 100 kHz. All of the beacons available to CASARA Ottawa may have spurious emissions attenuated by 41.5 dB at 100 kHz. But does that tell us anything about the entire population of ELTs? On the other hand, if an actual search team is equipped with a more sensitive receiver, or the actual ELT has a good ground giving it more gain the situation could be even worse. And there are several other unknowable (in the case of a plane crash) parameters. All of this is also ignoring any propagation effects that may also be in play. But that should show up in the experimental data that CASARA Ottawa should be able to provide to justify their claims. I have requested this data from CASARA, let's see if they are willing and able to provide it. But really there are so many unknowable variables, no one should try to estimate range to an ELT based on the dubious indication of received signal strength provided by off-tuning.

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