Why isn't it modulated...
It is modulated, according to the manufacturer (Dukane-SeaCom), the beacon 'pulse' is a once per second, 10 ms (10 thousandths of a second) gated (turns it on and then off) 37,500 cycle (37.5 KHz) ultrasonic 'tone'. That works out to be a short 2.66 cycle 'pulse' at 37.5KHz during the gate 'ON' time, once per second. The likely reason they chirp it, is to magnify the pulse-power while prolonging the battery life. In other words, doing so, allows the release of a significant tone blast (very short power 'on' time) without rapidly draining the battery. With a fresh battery, the chirp power is 157 dB.
...then it could even send info like how deep it is
However your point is 'why isn't the beacon transmitting a lot of telemetry data, info such as depth and GPS style position info?' To do the answer justice requires a fairly complicated answer, however, breifly, the DK120 (Dukane-SeaCom) beacon is designed to be survivable during a transient extremely violent event; be active for a relatively long period of time; and be fairly 'loud' over a long range in an incredibly, yet similar type signal, noisy environment, while remaining functional at extreme pressure depth, at low temperatures, and be battery operable inside a relatively small mass container. As one of the fellows here in the EEVblog succinctly said, 'KISS' or more verbosely, 'keep it simple Sue'. And because the beacon works in ocean water, and as I stated before, the environment is incredibly noisy and then the oceanic physics constrain the quantity and type of data that can be reliably recognized and that one point is a **BIGGIE** and then just simply, GPS does not work underwater. It would be very difficult if not impossible to make such a gadget as you ask about.
or the strength of the signal so the distance can be calculated.
Tough to do in ocean, as the media is so variable and without also transmitting internal health and likely precision time information, the distance info would only be relative at best. But the enviornment is so noisy and the bandwidth is so constrained, that the data transmission would need be long, in order to reliably sequence (and be recognizable) the intra-message symbols, but in of itself renders it unreliable as evidenced by the actual 'ping' durations being detected in the suspected MH370 search area.
However, there is always, seemingly, an alternative possible strategy available. A retired (US) NTSB investigator/engineer interviewed by US C
lueless N
etwork N
ews, early on, made an interesting suggestion which he emphatically re-embraced several times. His suggestion may in my opinion and obviously his, address several of the points raised. However, from an engineering perspective, some of the desired characteristics involve possibly significant practical engineering challenges in of themselves. The retired aircraft accident investigator (whose name I do not recall :[ ...), suggested that upon aircraft (assuming water) impact, that a satellite beacon would be released and deployed, that floats on the water, and being microwave enabled (exposed to atmosphere), it would be capable of wide-bandwidth transmission. Taking the idea a bit further, I speculate that such a device could be powered all the time, taking GPS location readings while the aircraft is in normal flight, but not transmitting, but just storing the data, so as not to add any more to the already congested RF spectrum. And then, God Forbid, when impact-trigger-released, the microwave-satellite-beacon would mechanically disconect, and also switch to internal, fully charged, battery power, and having stored a record of waypoints and at least the impact point if not post-impact position data. It would be able to uplink significant useful location data. The battery would not need to endure as long either, as the data would be immediately uplinked and likely received. However, this satcom-beacon would shock-burst the buffered data, periodically uplinking to conserve battery life, in any case. One of the challenges is how to deploy the beacon so that it doesn't become enmeshed in the wreckage tangle, possibly sinking uselessly to the bottom, or be so encumbered to lose line-of-sight with the sky overhead. Another challenge is how to prevent the beacon, floating on the surface, from being too far removed by surface currents from the initial impact point, but this may not actually be an issue, considering the several areas that were searched, each 'block' avg'g around 43,000
2 Km/(27,000
2 Mi),
because of the bad data given by the Malaysian 'authorities'.
In comparison ±200 hundred miles is like painting a target-icon on the impact point.
bench knob