1. Enabling Shallow Water Flight on Slocum Gliders
Chip Haldeman, David Aragon, Hugh Roarty, Scott Glenn, and Josh Kohut Rutgers University
Blue - < 9 m

2. Very broad…includes a wide variety of applications
Autonomous Platforms
Very broad…includes a wide variety of applications

3. In our field:
Autonomous Platforms
REMUS
Wave Glider

4. Autonomous Platforms – buoyancy driven
APEX Floats
Spray Glider
Autonomous Platforms – buoyancy driven
Seaglider
Slocum glider – only shallow water capable glider
Shallow water pump (4-30 m water depth)

5. Shallow Water Sampling Issues
UD_275 “OTIS”
“Bottom Sampling”
RU28
Pressure
Measured water depth
Pressure + raw altitude
“Dredging”

6. Shallow Water Sampling Issues
Why is this bad?
Data quality
– affects vertical and spatial resolution. Glider is still sampling, so there is plenty of data, but not where you want it.
Platform Risk
-- Sediment buildup in nosecone results in loss of buoyancy. Can’t float, can’t fly. Ejection weight?
-- Sediment in nosecone grinds between pump and diaphragm, causing leak and ending mission…or loss of the platform.
*Tip - Plug the nosecone for shallow coastal flights

7. OBTAINING HITS IN VERY SHALLOW WATER
This leaves a short window to obtain a hit during about 3 cycles or 12 seconds. Altimeter produces a hit every ~2 cycles.
OBTAINING HITS IN VERY SHALLOW WATER
REJECTED
ACCEPTED...
but 2 hits are required
Considered “on surface” or “too soon after inflection”
Too close to bottom (below minimum altimeter value)

8. ALTIMETRY: TAKING GROUND TRUTHING LITERALLY
Pitch angle depth correction
Altimeter filter: m_water_depth [n] = m_depth [n] + m_raw_altitude [n-1] Normally adds ~ 1 m to glider’s water depth calculation

9. ALTIMETRY: FINDING THE WRONG BOTTOM
Water depth = altitude + glider depth
Glider’s water depth = altitude (filtered) + glider depth
Actual water depth corrected for pitch
Actual depth is 21 m according to pitch corrected pressure sensor
Non-filtered pings show depth at about .3 m shallower, filtered pings show depth about 1.2 m deeper
In this example our altimeter, with the filter, is creating depths about 1.2 m deeper than actual.

10. SURFACE REFLECTIONS
As glider approaches bottom, hits below u_min_altimeter setting are rejected, but surface reflections are seen and accepted as “good”…so glider attempts to descend through ~10 m of sediment
10 m
REJECTED
“on surface/not diving” or hasn’t met “post-inflection time” requirements. Hits considered “good”, but rejected
ACCEPTED

11. Shallow Water Sampling Issues
How do we fix it?
Software solution, developed and tested at Rutgers, modifies default deepwater flight settings to enable shallow water flight. Confident of flight in 8 m of water…possibly as shallow as 6 m Obtaining valid altimetry in very shallow water - several default settings modified to allow more altimetry through
-- To avoid reflections, limit range of altimeter to less than water depth.

12. Pushing the envelope – very shallow water (6-7 m) flight
1-2 bottom solutions per dive; approx. 9 cycles or ~ 30 seconds
Flight in depths shallower than above will likely require a change in flight characteristics (pitch angle, pump throw, H stability) to slow flight and allow more valid altimetry. How do we address altimetry reflection issue?

13. Slocum gliders are capable of flight in shallow water, 4-30 m
Software fix modifies deepwater flight settings to enable shallow water flight
Bottom impacts and data degradation avoided

14. alt.mi ### alt.mi Shallow water (<15 m) altimetry coefficients
### David Aragon, Chip Haldeman, Rutgers University
### Account for shallow water; need enough good hits
sensor: u_reqd_depth_at_surface(m)
sensor: u_alt_min_post_inflection_time(sec)
sensor: u_alt_min_depth(m)
sensor: u_min_altimeter(m)
### Set this to less than actual water depth to avoid reflections
sensor: u_max_altimeter(m)
### Reduces filtered altimetry
sensor: u_alt_reqd_good_in_a_row(nodim) 1
sensor: u_alt_filter_enabled(bool)
sensor: u_alt_reduced_usage_mode(bool)
### Avoids bug in code; likely fixed but untested...
sensor: m_altitude_rate(m/s)
sensor: u_sound_speed(m/s)
### Keeps previous depth if new one not attained (2 yos)
sensor: u_max_water_depth_lifetime(yos) 2
### Attempts to limit max slope of bottom (use cautiously)
sensor: u_max_bottom_slope(m/m)
### Set this to get around false hits at depth, but use caution!!!
sensor: u_min_water_depth(m)
sensor: u_max_water_depth(m)