Parameter Reference Manual

SCHOOL OF OCEANOGRAPHY
and
APPLIED PHYSICS LABORATORY
UNIVERSITY OF WASHINGTON
Version 66.12 May 2012

Chapter 1

Conventions and Introduction

1.1 Conventions

All parameters have a leading $ in their name. Parameters will be given in bold font, file names in italic. Example (not default) values follow each parameter's description in square brackets.

1.2 Introduction

This manual provides an alphabetical reference to the parameters that govern the operation of the Seaglider. All parameters described in this manual are changeable by the pilot via the command file, using the name,value (example: $SM_CC,475) convention. Clicking a parameter name jumps to that parameter's description. The "Back" button returns to the previous location in the document.

Values reported by the Seaglider in the log file associated with a dive include all the pilot-changeable parameters described in this document. The log file also contains other values that are given parameter-like names for consistency of parsing during post-dive data processing.

Chapter 2

Seaglider Command File (cmdfile) Directives

2.1 Introduction

Command file (cmdfile) directives control the fundamental state of autonomous Seaglider operations. The two fundamental states of autonomous Seaglider operation are diving and recovery. The diving state is the normal repeating sequence of canonical dives. The recovery state is when the Seaglider is held at the surface, calling to the basestation at pilot-specified intervals with an updated position and receiving instructions.

Command file (cmdfile) directives are given as the last (and perhaps only) line of the command file, as stored on the basestation and transferred to the Seaglider during its communication session.

2.2 Directives

$GO
This command will cause the glider to continue in its current mode of operation. If in an autonomous run, doing repeated dives, it will continue to dive according to its current set of parameters. If a $GO command is received while the Seaglider is in the recovery phase, the Seaglider will stay in the recovery phase.

$RESUME
This command will cause the Seaglider to resume diving from within the recovery phase, using its current set of parameters. It acts just like a $GO directive if the Seaglider is diving.

$QUIT
This command will cause the Seaglider to go immediately to the recovery phase. The Seaglider will hold at the surface, sleeping $T_RSLEEP minutes between the end of one communications session and the start of the next. There are about two minutes of communications overhead associated with each session, so that sessions are about ($T_RSLEEP + 2) minutes apart. Replacing the $QUIT directive with a $RESUME directive will cause the Seaglider to initiate a new dive with the existing set of parameters.

The state table below outlines the effect of each directive on the glider in each of the two autonomous run states: diving and recovery.

Chapter 3

Seaglider Parameters

Example (not default) values follow each parameter's description in square brackets.

$AD7714Ch0Gain
The gain assigned to the pressure sensor channel on the AD7714 analog-to-digital converter. The parameter takes two values: 128 for normal Seaglider operations with the installed Paine pressure sensor, and 1 for bench testing where a synthetic voltage is injected in place of the pressure sensor output to simulate diving. The advent of the parameters $SIM_W and $SIM_PITCH have obviated this parameter. [128]

$AH0_10V
The capacity of the 10V (low-voltage) primary battery pack (AmpHr). There is some safety factor in this number, but its accuracy has been verified in post-recovery depletion testing of Seaglider battery packs. The Seaglider goes into the recovery state if the total 10V battery pack amp-hours used on a mission equals or exceeds this value. [61.2]

$AH0_24V
The capacity of the 24V (high-voltage) battery pack (AmpHr). There is some safety factor in this number, but its accuracy has been verified in post-recovery depletion testing of Seaglider battery packs. The Seaglider goes into the recovery state if the total 24V battery pack amp-hours used on a mission equals or exceeds this value. [91.8]

$ALTIM_BOTTOM_PING_RANGE
The range (in meters) from the presumed apogee depth (the nominal depth at which the glider will begin its apogee maneuver) to ping for the bottom. With this parameter set the altimeter will only ping a maximum of once per dive. A value of 0 disables pinging. If the configured transponder device does not have a ping function, these parameters are ignored. See also $ALTIM_PING_DEPTH. [0]

$ALTIM_BOTTOM_TURN_MARGIN
The distance (in meters) from the altimeter-detected seafloor (or an obstacle) at which to initiate the apogee maneuver (bottom turn). A value of 0 disables use of the altimeter to determine the start of the apogee maneuver. [0]

$ALTIM_FREQUENCY
The frequency (kHz) to use for altimeter pings. Must be an integer between 10 and 25. [15]

$ALTIM_PING_DELTA
If the altimeter does not get a successful return and confirmation ping return at $ALTIM_PING_DEPTH, it will continue to issue pings at depth intervals of $ALTIM_PING_DELTA meters. See $ALTIM_PING_DEPTH.

$ALTIM_PING_DEPTH
The depth of the first altimeter ping (meters), if non-zero. If the altimeter gets a return, and a return to an immediate second confirmation ping, it will set the bottom depth equal to the current depth plus the altimeter range to the bottom. The apogee maneuver will be initiated at $ALTIM_BOTTOM_TURN_MARGIN meters above the bottom. If $ALTIM_PING_DEPTH is non-zero, the altimeter timeout will be set so that the maximum range is the larger of 0.75*$ALTIM_PING_DEPTH and 1.2*$ALTIM_TOP_PING_RANGE , if set. The first test is meant to exclude surface returns. Note that $ALTIM_PING_DEPTH and the $ALTIM_BOTTOM_PING_RANGE modes are mutually exclusive. If $ALTIM_BOTTOM_PING_RANGE is set, it will be honored to the exclusion of $ALTIM_PING_DEPTH.

$ALTIM_PULSE
Pulse width (ms) of altimeter pings. Must be an integer between 1 and 9. [5]

$ALTIM_SENSITIVITY
Sensitivity of the envelope detector on the altimeter. Must be an integer between 0 and 5. A value of 0 disables the envelope detector, causing the altimeter to trigger on any return at the right frequency. Values between 1 and 5 require that the return signal be above that voltage for the duration of a pulse width before triggering. [1]

$ALTIM_TOP_MIN_OBSTACLE
The minimum obstacle depth (m) to honor in initiating a subsurface finish. [2]

$ALTIM_TOP_PING_RANGE
The range (in meters) from the surface (this is simply depth) at which to ping the altimeter. A value of 0 disables a ping. If the configured transponder device does not have a ping function these parameters are ignored. [0]

$ALTIM_TOP_TURN_MARGIN
The distance (in meters) from an altimeter-detected obstacle at which to initiate the sub-surface finish. A value of 0 disables use of the altimeter to determine the start of the sub-surface finish. [0]

$APOGEE_PITCH
The intermediate pitch (position of the pitch mass) the glider will pitch to between the dive and climb phases, to prevent stalling. The glider assumes $APOGEE_PITCH when it observes a depth of greater than the apogee depth (whether by altimeter, $D_TGT or $D_GRID). The apogee maneuver also includes pumping the VBD to 0cc. After the apogee maneuver, the climb is initiated by changing both pitch and VBD to the opposite-signed values from the dive. [-5]

$C_PITCH
The center (neutral, or flat) position (A/D counts) for pitch. [2590]

$C_ROLL_CLIMB
The center (neutral, or straight flight) position (A/D counts) for roll during the climb (positive pitch control) phase. Note that the two roll centers will probably be different. This was implemented to correct for roll biases induced by physical asymmetries in the glider. [2150]

$C_ROLL_DIVE
The center (neutral, or straight flight) position (A/D counts) for roll during the dive (negative pitch control) phase. [2150]

$C_VBD
The center (neutrally buoyant at a specified density) position (A/D counts) for VBD. [2735]

$CALL_NDIVES
The number of profiles (dive/climb cycles) to perform before attempting communications. The Seaglider will surface normally following each dive, and GPS fixes 1 and 2 will be obtained at each surfacing, independent of the value of $CALL_NDIVES. [1]

$CALL_TRIES
The maximum number of phone calls to attempt during a surfacing between dives. [5]

$CALL_WAIT
The wait time (seconds) between call attempts during a communication session. This is time to allow the Iridium satellite geometry to change and perhaps improve the connection. [60]

$CAPMAXSIZE
The maximum size (in bytes, prior to compression) of the capture file to upload. If the capture file is to be uploaded (either due to critical output, $CAPUPLOAD set to 1, or a completed self-test) and the capture file size exceeds this value, the glider will create a new capture file that is of the size requested, per the following strategy. If there are no critical lines of output, then the first $CAPMAXSIZE bytes will be sent. If there are critical lines of output, then the new capture file will consist of the first 20 of the critical output lines, with a window of output lines surrounding each critical line.

$CAPUPLOAD
A Boolean value that determines if the capture file from the current dive should be uploaded or not: 0 means do not upload the capture file, 1 means upload the capture file. If critical output is captured during a dive, or a self-test has been completed, then the capture file is uploaded regardless of the value of $CAPUPLOAD. [0]

$CF8_MAXERRORS
The maximum number of CF8 errors allowed before the Seaglider goes into recovery state. A CF8 error is counted against the $CF8_MAXERRORS limit when a CF8 open or write call continues to fail (returns an error code) after three retries.

$COMM_SEQ
The specification of the sequence of file transfer to use. A value of zero indicates the standard communication file transfer sequence: command, targets, science, current dive log file, current dive data file, earlier un-transferred log and data files, pdoscmds.bat, sgdddd.pz.nnn (the results of the pdoscmds.bat commands), and any other files as commanded in pdoscmds.bat. A value of one indicates skipping the normal log and data file transmission step, and going directly to pdoscmds.bat: command, targets, science, pdoscmds.bat, sgdddd.pz.nnn and any other files as commanded in pdoscmds.bat. This was implemented as a way of getting to the pdoscmds.bat file in the event that communications problems or file corruptions prevented data transfers. It is a control mode to be used only when communications or other glider problems exist. [0]

$COMPASS_DEVICE
Configuration flags specifying the specific model and port for the compass and transponder/altimeter devices. This integer value is equal to port_number + 16*type_number. For example, for a TCM2-50 (type 0) on general purpose port 1, $COMPASS_DEVICE = 1 + 16*0 = 1. The array of available models is specific to each device. The compass device must be defined (parameter cannot be -1). For transponders not connected to a serial port (e.g., Benthos ENT-380) the null port (generally index 0) can be specified. See $DEVICE[1/2/3/4/ 5/6]. [1]

$COMPASS_USE
This parameter was used in development to assess compass problems. It allows for faking inputs and outputs in compass calibration and reporting. For normal operations, it should be set to 0, which yields current behavior of trusting inputs from the compass and using and reporting fully calibrated values.

• Bit 0 Heading source:
  • 0 = cal'd mag field w/PR corrections
  • 1 = direct from compass (effectively ignores PR bits)
• Bit 1 Autocal mode (experimental):
  • 0 = no autocalibration
  • 1 = leave compass on throughout flight and run autocalibration
• Bit 2 Offboard cal mode (experimental):
  • 0 = do not report mag values in data file
  • 1 = report mag values in data file for use in offboard cal
• Bits 3-4 pitch source for heading correction:
  • 0 = calibrated from sensor inputs per normal routine
  • 1 = use value direct from compass
  • 2 = calculate based on pitch mass position and gain
• Bits 5-6 roll source for heading correction
  • 0 = calibrated from sensor inputs per normal routine
  • 1 = use value direct from compass
  • 2 = calculate based on pitch mass position and gain
• Bits 7-8 pitch to report in data stream
  • 0 = calibrated from sensor inputs per normal routine
  • 1 = use value direct from compass
  • 2 = calculate based on pitch mass position and gain
• Bits 9-10 roll to report in data stream
  • 0 = calibrated from sensor inputs per normal routine
  • 1 = use value direct from compass
  • 2 = calculate based on pitch mass position and gain
• Bit 11 which compass device to use
  • 0 = COMPASS_DEVICE
  • 1 = COMPASS2_DEVICE

A value of 40 (0x28) will calculate a pitch and roll from control positions for use in the heading calibration but will report calibrated pitch and roll in the data stream. A value of 680 (40 + 640) will use the control position calculated attitudes for reporting as well. [0]

$COMPASS2_DEVICE
Same as $COMPASS_DEVICE for an optional second compass (used in glider development).

$COURSE_BIAS
A heading bias (degrees) to compensate for a glider's observed tendency to veer to one side. This value is subtracted from the desired heading to produce the target heading. [0]

$D_ABORT
The maximum depth (meters) for Seaglider operations. If this depth is reached, the dive is aborted and the glider immediately enters the recovery phase. [1090]

$D_BOOST
The depth (meters) above which only the boost pump will be run.[0]

$D_CALL
For non-zero values, require a depth shallower than this value before initiating the GPS and phone call portions of a surface maneuver. If this depth is not reached, a subsurface finish is executed.
[0]

$D_FINISH
Depth (meters) at which to consider a dive completed. Normally, this is 0, but may be a number greater than zero to specify the depth at which subsurface finish maneuvers should be started. Used only when an additional trigger to initiate a subsurface finish is present (see $N_NOSURFACE). [0]

$D_FLARE
The depth (meters) at which the glider flares to the computed desired pitch angle following the initial dive from the surface. The guidance and control (G&C) action at the start of the dive phase maintains full pitch forward as VBD bleeding takes place. A new G&C action is initiated as soon as a glider reaches $D_FLARE: pitch is adjusted first (the flare), then VBD is adjusted (continued bleed to the target VBD), then roll is actuated to turn the glider to the desired heading. [3]

$D_NO_BLEED
The depth (meters) below which the Seaglider will not bleed (move oil from the bladder into the internal reservoir, making the Seaglider less buoyant) on dives. This parameter also defines the depth at which the $T_NO_W parameter takes effect. [50]

$D_OFFGRID
The depth (meters) that the bathymetry map look-up routine returns in the event the Seaglider's position is outside an area for which the Seaglider carries a map. [1001]

$D_PITCH
If non-zero, depth (meters) that must be reached before surface pitch maneuver is executed. If a depth shallower than this value is not reached then execute a subsurface finish.
[0]

$D_SAFE
If non-zero, target depth to use when flying an escape route and limiting dive depth for VBD safety reasons. Used on escape routes when the escape was triggered by VBD max errors exceeded or uncommanded bleed.
[0]

$D_SURF
The depth (meters) at which the glider begins its approach to the surface. In order to collect data all the way to the surface, at $D_SURF the Seaglider computes how many more data samples to take, based on the observed vehicle vertical speed, depth, and the data sample interval. The number of additional points is limited to 50. The Seaglider then goes into passive G&C mode and collects that number of data points at the appropriate sample interval for the depth range. When complete, the Seaglider enters the surface phase. Note that this approach will occasionally result in the last few data samples being taken when the conductivity sensor is actually in air, giving unrealistic conductivity values. These samples can be removed in shore-side processing, if necessary.[3]

$D_TGT
The nominal depth (meters) at which the Seaglider begins the apogee phase, the transition from the negatively buoyant, pitch-down dive to positively buoyant, pitch-up climb. This depth is also used in conjunction with $T_DIVE to determine the specified vertical velocity for the dive and climb.

The actual depth of the starting point of the apogee maneuver may be determined by reading a digital bathymetric map ($D_GRID), or by using the altimeter. In either case, the vertical velocity specified by the combination of $D_TGT and $T_DIVE is retained by appropriate scaling of $T_DIVE. The apogee maneuver is not started until a depth greater than $D_TGT (or other depth trigger) is detected. In scenario runs, $D_TGT is also the depth at which the Seaglider enters the recovery phase. [990]

$DBDW
Overrides the hydro model calculated derivative of buoyancy with respect to w (vertical speed) used as a gain term in adjusting buoyancy to achieve the desired vertical speed. If parameter is zero then the onboard calculated value is used. Units are grams per m/s. Typical onboard calculated values are 2000--4000. [0]

$DEEPGLIDER
Indicates whether the glider is a standard Seaglider, or a Deepglider. 0 means that the glider is standard, and not a Deepglider. [0]

$DEEPGLIDERMB
Indicates whether the main board is intended for use in Deepglider. 0 means that the motherboard installed is intended for standard Seaglider, not for Deepglider. [0]

$DEVICE[1/2/3/4/5/6]
Configuration flags specifying device type and port for each of the six possible attached science sensors. Empty device slots are indicated with a parameter value of -1. Non-negative entries indicate that a device is attached. The encoding is specific to the version of the Seaglider software. These entries are set through the Seaglider menu system by the builder/assemblers.

$DIVE
The number of the next dive the Seaglider will start. [314]

$ESCAPE_HEADING
The base heading the Seaglider will steer in an escape recovery situation when either no position fix is available or no escape target was supplied in the targets file.

$ESCAPE_HEADING_DELTA
The actual heading steered by the Seaglider in an escape recovery situation will always be $ESCAPE_HEADING +/- $ESCAPE_HEADING_DELTA. The sign will switch (and thus the heading will toggle) when the bottom depth (as detected by altimetry or $T_NO_W) shallows by 5% relative to the depth at the last toggle.

$FERRY_MAX
The maximum correction (in degrees) to apply to the rhumb line to the active (next) waypoint when $NAV_MODE = 2. This is a safety limit to prevent spurious depth-averaged current calculations from giving the Seaglider a heading in the wrong direction. [45]

$FG_AHR_10V
Cumulative A-hr consumed from the 10V pack as tracked by the supervisor fuel gauge. This parameter is automatically updated whenever the fuel gauge is read and the supervisor's onboard accumulators are cleared. When reported in the log file it reflects the fuel gauge state at the start of the dive. This parameter is only relevant for RevC and later Seaglider motherboards. For RevB and earlier, this parameter is always zero.

$FG_AHR_24V
Cumulative A-hr consumed from the 24V pack as tracked by the supervisor fuel gauge. This parameter is automatically updated whenever the fuel gauge is read and the supervisor's onboard accumulators are cleared. When reported in the log file it reflects the fuel gauge state at the start of the dive. This parameter is only relevant for RevC and later Seaglider motherboards. For RevB and earlier, this parameter is always zero.

$FILEMGR
An integer parameter that specifies how aggressively to manage the onboard file system, as follows.

0: none
1: only store compressed files
2: delete splits on failed phone call

$FIX_MISSING_TIMEOUT
Time in days to tolerate lack of any valid navigation fix (GPS, RAFOS, Iridium geolocation) before triggering recovery.
[0]

$GLIDE_SLOPE
The absolute value of the maximum glide slope (degrees) allowed for the Seaglider. The desired glide slope is calculated on board the Seaglider to best achieve the goals of the next dive. The stall angle provides the lower limit; this parameter is the upper limit. [30]

$GPS_DEVICE
Configuration value specifying the model of the attached device. These devices have dedicated hardware ports on all motherboard revisions and as such a port specification is not necessary. A GPS device must be defined (cannot be -1). [32]

$HD_A
The hydrodynamic parameter representing the lift coefficient, determined empirically and used in the glider's on-board performance prediction and guidance calculations. [0.0029217]

$HD_B
The hydrodynamic parameter representing the drag coefficient, determined empirically and used in the glider's on-board performance prediction and guidance calculations.[0.010758]

$HD_C
The hydrodynamic parameter representing the induced drag coefficient, determined empirically and used in the glider's on-board performance prediction and guidance calculations. [2.8991001e-06]

$HEAD_ERRBAND
Deadband for heading (degrees). This value is used to determine if a correction to heading is required during an active G&C mode. If the absolute value of the difference between the actual heading and the desired heading is less than or equal to $HEAD_ERRBAND, no heading correction is made. If the difference is greater than $HEAD_ERRBAND, then a turn is performed until the desired heading is passed, or until the amount of time $T_TURN has elapsed. [10]

In a scenario run, $HEAD_ERRBAND is the deadband for roll angle. If the absolute value of the specified position of the mass shifter (in degrees) is less than or equal to $HEAD_ERRBAND, no roll is performed. [10]

$HEADING
A floating point value between -1.0 and 360.0 (true degrees, 0.0 and 360.0 are equivalent values) used in conjunction with certain navigation modes (controlled by the $NAV_MODE parameter) to determine the course steered by the Seaglider.

-If $NAV_MODE is 0, 1, or 2 and the value of $HEADING is between 0.0 and 360.0, the glider will use this value to synthesize a waypoint 20 km distant on the specified bearing from the current location.

-If $NAV_MODE is 3, $HEADING is added to the depth-averaged current calculated for the previous dive, to give a Seaglider heading that is the specified amount to the right of the current. [-1]

$HEAPDBG
A boolean value that toggles extended heap debugging. When set to 1, the glider maintained heap is checked for integrity before each memory allocation and free operation. 0 disables this checking. Note: due to the extensive output, it is not recommended that this parameter be used in conjunction with a DEBUG level of output on the SGLMALLOC service during field operations. [0]

$ICE_FREEZE_MARGIN
Temperature margin (degrees C) to apply to the freezing point calculation, weighted by ice condition, for surfacing decisions. For an in situ freezing point Tf and temperature T, the glider will surface if T > Tf + fm*(ic - 1)
where fm is the freeze margin and ic is the ice condition.

$ID
Seaglider identification (serial) number. This is a number between 1 and 999. Leading zeros are not required. This number is used in many ways, including creating the Seaglider's login on the basestation, in file naming conventions, and as a serial number for manufacturing purposes.

$INT_PRESSURE_SLOPE
The slope (psia per A/D count) calibration of the internal pressure sensor. The sensor has a span of 0 to 30 psia, with a 90mV output at full-scale at 12V excitation. The output is proportional to supply. The Seaglider excitation is 4.096V and the gain is 100, so 30 psia = 90*4.096/12*100*1. Full-scale = 90*4.096*gain*counts/mVolt, so the nominal slope is 0.009766 psia per A/D count. [0.0097660]

$INT_PRESSURE_YINT
The y-intercept of the linear calibration of the internal pressure sensor. [0]

$KALMAN_USE
The control parameter for the run state of the Kalman filter navigation program. The $NAV_MODE parameter controls whether the Kalman filter output heading is used to control the Seaglider. This separation of functions allows the Kalman filter to be run, but not used, while it "learns" the currents. Bits 0 and 1 together indicate the command mode of the filter and control its operation as described below.

0: Reset the Kalman filter. Set Kalman state vector and origin of local Kalman coordinate system to 0 and restart the filter.
1: Run the Kalman filter.
2: Do not run the Kalman filter.

-If the command mode of $KALMAN_USE are 0 or 1, and $NAV_MODE is 1, the Kalman filter results will be used to determine the Seaglider's heading.

-If $NAV_MODE is 1, but the command mode of $KALMAN_USE is 2 (not being run), the Seaglider acts as though $NAV_MODE is 0.

Bits 2 and 3 of $KALMAN_USE control whether to employ (0) or disable (1) the dirunal and semidiurnal components of the model, respectively. The mean component is always determined. For example, a $KALMAN_USE of 9 (= 8 + 1) runs the Kalman filter but disables the semidiurnal component of the model. A $KALMAN_USE value of 1 runs the Kalman filter with all components enabled.

$KERMIT
Controls the use of kermit (vs xmodem) for file transfers. This parameter was experimental, and has never been used operationally. The value controls what files kermit is used for: 0 =none, 1= data only, 2 =data only, batch mode transfer, 3= data and control, 4= data and control - data via batch mode.

$MASS
The mass of the glider in grams. Used in on-board buoyancy and current estimation calculations. [52000]

$MASS_COMP
The mass of any compressee in grams. Used in on-board buoyancy and current estimation calculations. [0]

$MAX_BUOY
The absolute value of the maximum negative buoyant force (grams) to which the glider is allowed to bleed during the dive phase. There is no restriction on positive buoyancy during the climb phase. [150]

$MINV_10V
The minimum allowable observed voltage on the 10 volt battery pack before the glider will stop diving and go into recovery. A value of 0 disables the check. [8]

$MINV_24V
The minimum allowable observed voltage on the 24 volt battery pack before the glider will stop diving and go into recovery. A value of 0 disables the check.[19]

$MISSION
The current Seaglider mission number. This value is intended to be unique for each of a particular Seaglider's various deployments. It has no effect on glider operations, but is reported back in data files for the purpose of data file distinction from other missions a glider may have performed. [0]

$MOTHERBOARD
An integer value indicating the motherboard revision carried by the glider and set by the builder or assembler. 1 = NOPP, 2 = A.1, 3 = AB, 4 = B.1, etc. [4]

$NAV_MODE
An integer value specifying the method used to choose a heading for the Seaglider to maintain on the next dive, according to the following table.

0: Steer constant heading ($HEADING)
1: Kalman filter (see $KALMAN_USE)
2: Ferry angle correction with respect to calculated depth-averaged current
3: Steer relative to depth-averaged current

Navigation flow proceeds as follows.

1. If we have calculated a valid depth-averaged-current (DAC) and $NAV_MODE = 3 (steer relative to DAC), synthesize a heading = currentdirection + $HEADING.

2. If a synthesized heading exists from (1) or $HEADING >= 0, synthesize a target 20 km away at a true bearing given by that heading. Otherwise,choose a target based on the usual selection process. Calculate range and bearing to the target.

3. Modify dive time and depth as needed to honor safeties and grids. Calculate speed limits - Seaglider minimum and maximum possible horizontalspeeds through the water. The speed limits and dive times fix the rangeof possibilities for the Seaglider's distance through the water.

4. Choose a glide slope that gets the Seaglider closest to the target (maximum speed if the target is distant, minimum speed if the target is too close).

5. At this point the Seaglider has a bearing and glide slope in hand. If $NAV_MODE = 0 or $NAV_MODE = 3, then these will be the heading and slope for this dive.

6. If $NAV_MODE = 2 (calculate set/ferry angle correction) and we have calculated a valid DAC, we apply $SPEED_FACTOR to the speed limits to account for the fact that the Seaglider actually won't achieve its ideal speed over the entire dive. We then iteratively calculate set corrections as a function of our horizontal speed through the water so that we optimize the Seaglider's travel toward the target (just as we chose our slope above). At each speed setting, the code uses a nonlinear solver to solve for ferry angle. First guess at speed is maximum. If the predicted distance over ground (DOG) with the set correction is less than the range to target (typical case) then the computation is complete. Otherwise we try the minimum speed. If the resulting DOG is greater than the range to target then the computation can't do any better and it is complete. If neither limit applies, we iterate via bisection to settle on the best speed. At convergence, the Seaglider has a ferry angle to steer and a horizontal speed to apply. The predicted horizontal speed is used to calculate a new value for glide slope.

$N_DIVES
The number of dives to complete before entering recovery. A value of zero (default) means dive until stopped by pilot command ($QUIT) or some other recovery condition occurs (battery exhaustion, error limit exceeded, etc.). [0]

$N_FILEKB
An integer value (-17 < $N_FILEKB < 17), which is the size (in kilobytes) and type (gzip-compressed or uncompressed) of file used for data uploading. Positive numbers direct the Seaglider to first use gzip to compress the data file, then split it into $N_FILEKB-sized pieces. Negative values for $N_FILEKB disable the gzip compression, but still cause the non-gzipped binary data file to be split into $N_FILEKB-sized pieces before transmission. A value of 0 means no splitting or compression is performed. The maximum allowed value of this parameter is currently set to 16. [4]

$N_GPS
The maximum number of seconds to wait for a GPS fix with HDOP = 2.0 or less. If no such fix is acquired, the last (most recent) GPS fix is used, which is probably, but not necessarily, the most accurate fix available in the specified time period. [20]

$N_NOCOMM
The number of dives that are allowed to occur without a complete and successful data communication session before actions are taken to improve communications, enter recovery, or navigate to a rescue position according to the value of $NOCOMM_ACTION. The default (and traditional) behavior with a value for $NOCOMM_ACTION of 0 is for the surface buoyancy parameter $SM_CC to be set to the maximum allowed by the software limits. This is a safety provision in the event $SM_CC is not sufficient to allow for a good antenna position. [1]

$N_NOSURFACE
An integer value that determines when the glider will finish the dive at $D_FINISH and when it will finish at the actual surface. For values greater than 1, the rule is that when the remainder of $DIVE divided by $N_NOSURFACE is zero, the glider will finish the dive at depth $D_FINISH. Other dives will finish at the surface. For negative values less than -1, this logic is reversed, and values of $DIVE divisible by $N_NOSURFACE will finish at the surface; all others will be subsurface finishes. Values of 1 and -1 are not allowed. A value of 0 disables this behavior. [0]

$NOCOMM_ACTION
Bitmask parameter controlling behavior after $N_NOCOMM dives have passed without successful communication with the basestation. The parameter is a logical or of the bits described below: