Die GPS 12XL FAQ

V 1.0.2 vom 16.03.97 zurück zur GPS-Seite


Review of the Garmin 12XL GPS Receiver
by Joe Mehaffey and Jack Yeazel

Revision 7-Mar-97

(rev: Ext Ant, Current Draw, Dead Reckoning, Data Smoothing,++)

 I  (Joe) recently received a new G-12XL and Jack and I have  been 
 comparing it to my Magellan M-4000 and Jack Yeazel's Garmin  G-45 
 (not the 45XL).
 
 First,  a review of features and operation.  Most of the features 
 are  the  same as the Garmin 45XL unit according  to  news  group 
 correspondents. 
 
 As  others  have mentioned,  the G-12XL does lock on  to  the  SV 
 signals  quickly.   From a "cold" start,  after a 2000  mile  UPS 
 trip,  it locked on in 56 seconds.  After a 3 hour off time,  the 
 lock  up time was 20 seconds.  Specifications are "warm start"  ,  
 15  seconds,  "cold start" 45 seconds,  "autolocate"  5  minutes.  
 My Magellan M-4000 took 95 seconds from a "warm" start  situation 
 in the same environment.  Jack's G-45 is a bit faster than the M-
 4000.   A  new  feature  that  I have  not  seen  before  is  the 
 "Initialize by Country"  feature.  Apparently,  this  enhancement 
 was made on the Garmin 45XL version as well.  With this  feature,  
 you may select your country from a list and it will speed up  the 
 Autolocate from an unknown location.
 
 The  G-12XL's specification for position accuracy is  100  meters 
 with  SA,  15 meters exclusive of SA and 5 meters  with  suitable 
 DGPS  correction signals.  This is considerably better  than  the 
 DGPS  specs available with most low cost single and dual  channel 
 multiplexing systems.   The DGPS error  specification  for    the 
 G-45XL for instance is 10 meters.  This shows that reduced  error 
 capabilities  are  achievable  with even  low  cost  multichannel 
 receivers  as  compared with single and dual channel  units.   An 
 averaging  mode  is available in the G-12XL to  improve  waypoint 
 accuracy  in the presence of SA.  I have observed  error  estima-
 tions  in the range of 50ft with a few minutes of averaging at  a 
 spot.   We  hope  someone will test the G-12XL  in  a  controlled 
 environment  and let us hear how accurate the G-12XL's  indicated 
 error measurement readout really is.
 
 The G-12XL operates from 4 AA batteries or from external power in 
 the  range of 10 to 40 volts DC.  External current draw is  about 
 120ma (light off) and 150ma (light on).   Ten hours is  specified 
 as the AA battery life.  We have not tested this parameter.   The 
 data/power cable used is the same as the Garmin G-45 units.   The 
 external  antenna connector a miniature MCX coax jack is  located 
 on the rear of the unit just behind the "up arrow" button.  A six 
 inch  adapter lead MCX to BNC jack is available for  US$20.    We 
 measured  4.35  volts  on  the  connector  with  battery  voltage 
 measuring 5.05vdc on the G-12XL's test screen.  The Garmin  GA-26 
 amplified antenna and the Magellan M-4000 amplified antenna  both 
 work with the G-12XL (with adapter cable).  We believe most other 
 amplified GPS antennas will work as well.  The G-12XL  apparently 
 has  a diode switch to disable the internal antenna  (to  prevent 
 interaction) when the external antenna draws power (unlike the M-
 4000).
 
 The unit puts out a variety of NMEA protocols,  0180, 0182,  0183 
 ver  1.5,   and  0183 ver 2.0.  Other proprietary  sentences  are 
 output as well.  DGPS signals in the RTCM-104 format are  accept-
 ed.   A  setup screen permits selection of  the  I/O  combination 
 needed by a particular application.
 
 The G-12XL has a number of features and displays that are useful.  
 These include:
 a)   A  tape  style compass display screen showing  direction  of 
      travel,   track bearing,  speed,  trip  distance,  altitude,  
      lon/lat  position, and time of day.   On the 12XL, you  must 
      set in the time offset from UTC manually.  
 b)   A compass type display screen shows direction of travel  and 
      has  an  arrow pointing to the next waypoint.   This  screen 
      also  provides the name of the next waypoint,  the  bearing,  
      distance,   along  track speed,  speed,  cross  track  error 
      with  reference to the next waypoint.  This display  can  be 
      alternated  with two pushes of the "enter" button  with  the 
      highway display below.
 c)   A highway page type display screen shows off track  distance 
      similar  to a CDI display.  The CDI scale is  adjustable  in 
      units  of .25, 1.25, and 5.0 sm,  nm,  or km.   This  screen 
      includes  bearing,   distance,   track  over  ground  speed,  
      speed over ground,  estimated time of arrival,  and velocity 
      made  good,  all with reference to the next  waypoint.   The 
      name of the next waypoint is also displayed.  If you get too 
      far  off  course,  a message is presented  telling  you  the 
      proper course to steer to get back on course.
 d)   A  moving map display is provided to plot your  course  over 
      ground.   In  addition,   bearing,   distance,   track  over 
      ground,   and  speed over ground are provided on  this  same 
      display.   Any waypoints saved in the machine are  displayed 
      on the moving map page as they come within range of the  map 
      scale selected.  Note.  The "map" is a blank page until  you 
      put  in  your waypoints.  This screen has pan and  zoom  and 
      scales  from  .2 mile to 320 (miles or  other  units)  which 
      works  quite intuitively.  You can "point" at a waypoint  on 
      the  map and the distance and bearing to that waypoint  from 
      your  present  position  will be displayed.   You  can  also 
      "mark" and store a new waypoint by moving the cursor to  the 
      desired  position on the map and pressing the MARK key  plus 
      ENTER.   The map may be north up, or current track  up,   or 
      direction of current route segment up.  The map display  can 
      contain  position range rings,  plot straight lines  between 
      waypoints,  display "nearest" nine waypoints,  or put  names 
      beside waypoints.  The map screen also supports zoom and pan 
      features.
 e)   A  distance and sun screen is provided to compute  the  dis-
      tance  between any two waypoints and to display sunrise  and 
      sunset information for your locality.  
 f)   A simulator feature provides display of simulated motion and 
      simultaneously  outputs  simulated  tracking  data  to  your 
      computer  for  test  of mapping  software,   data  gathering 
      simulations and such.
 g)   A screen backlighting timer permits setting the backlight to 
      0,  15,  60, 120,  and 240 seconds when the unit is on  bat-
      tery power.  The screen will stay on continuously if  exter-
      nal  power is connected and the backlighting is  turned  on.  
      The backlighting is uniform and tinted blue and the  display 
      at night is easy to read.  The display is clear and  appears 
      to  be  the  same resolution as that  on  the  Garmin  G-45.  
      Screen contrast is also adjustable on the setup screen.
 h)   You can turn off the warning tone!  A screen permits beep on 
      warning messages,  keystrokes and warning messages, or NONE.
 i)   A message screen page allows you to view system warnings and 
      messages.   These  include such things as  approaching  way-
      point,  no DGPS position,  poor gps coverage, and battery is 
      low.   The total number of such advisory messages is  twenty 
      one.
 j)   A  satellite  status page comes up when you first  turn  the 
      unit  on.   It displays a "fuel"  or battery  gauge  showing 
      battery  remaining,  and a "compass"  display  of  satellite 
      numbers  (1...32) in view along with signal strength bars  for 
      each satellite potentially in view (up to 12).   If the unit 
      is  powered from an external source,  the "fuel"  gauge  bar 
      disappears.  The signal strength bars are hollow if the  GPS 
      has found the SV and is not yet locked and changes to  solid 
      black  when  lock is made to each satellite in  turn.   Each 
      signal strength bar is marked with the associated  satellite 
      number.   On the polar plot,  SV numbers are white on  black 
      when not locked black on white when locked. 
 k)   Waypoints may be named with a six character name.  
 l)   A track log is provided which can log from 0 to 999  points. 
      If 999 points are selected,  the track log actually  appears 
      to  record 1024 points and 1024 points are  also  referenced 
      during  download on the G-12XL's screen.  After the  maximum 
      selected number of track log points is recorded,  the G-12XL 
      discards  the oldest log points as new are added.  The  user 
      can  select  "automatic"  track  log  which  logs   whenever 
      direction or speed changes. Alternatively,  one can elect to 
      log  a track point at intervals from every second to  up  to 
      every 99 hours as needed by filling in a hhmmss entry in the 
      track log screen.  
 m)   A  track  back  feature provides the ability  to  steer  you 
      backwards  along  your initial outward course  without  your 
      having  to have entered waypoints manually during your  out-
      ward transit.
 n)   The G-12XL accepts the RTCM-104 version 2.0  DGPS correction 
      format version.  The GPS can control the frequency of  suit-
      ably equipped DGPS receivers when it is set to the RTCM/NONE 
      mode   (No  NMEA data output.).  DGPS beacon  frequency  and 
      signal strength will display on the G-12XL when DGPS activi-
      ty is present on the RTCM input.
 o)   External   I/O  signal  modes  available  are:    NMEA/NMEA,  
      RTCM/NMEA,  GRMN/GRMN,  and RTCM/NONE.  Alert messages  (and 
      optional beeps) alarm  DGPS signal failure when that mode is 
      enabled.  
 p)   A  waypoint proximity alarm can be programmed to  alert  the 
      user  with an audible "bong" when the selected  waypoint  is 
      being approached.  Up to 9 proximity waypoints may be used.
 q)   The  user  may select a Magnetic heading reference  or  true 
      north  as  required.  Other options are grid  reference  and 
      User selectable.
 r)   The  G12XL has 107 built in map datums plus  the  capability 
      for users to set in their own datum settings.  
 s)   The user may enter his own grid format if desired.
 t)   If the ENTER key is pressed and held when the unit is turned 
      ON,    an  undocumented  test  screen  is  activated   which 
      incidentally measures battery voltage.
 
 Subjective Observations of Performance
 
 Jack Yeazel and I  have been out testing the 12XL on the road and 
 in  the field.  The G-12XL has worked without a flaw that  either 
 of  us could find.  We uploaded and downloaded waypoints,  tracks 
 and  routes using Waypoint+(W95), and G7to..(DOS).  We  tried  it 
 out on SA4 and Delorme MapExpert,  and Vista.  No problems found.  
 We  compared  it with the G-45 (not an XL) and with  our  earlier 
 experiences  with  the G-45 tested against the  M-4000  for  lock 
 times and the ability to hold lock under various situations.   In 
 a  sentence,  the 12XL outperformed the other two in  every  test 
 for  lock  stability,  multipath performance,  re-lock  after  an 
 underpass,  and ability to suddenly change direction without loss 
 of lock.  We were very impressed.  We tried the G-12XL  (barefoot 
 [antenna wise]) on the dash with the G-45 with external amplified 
 antenna mounted to the windshield.  Still the G-12XL outperformed 
 the G-45 in every test.  We were very impressed at the speed that 
 the  speed  the  G-12XL  responded to  changes  in  direction  as 
 compared to the other two units.  At about 4mph,  the 12XL  would 
 complete a change of direction in about 15 feet,  the G-45  about 
 50ft,  and the M-4000,  about 60ft.  The G-12XL laid down  smooth 
 tracks  on our highway maps during all tests.  No  gaps,   jumps,  
 etc., (The G-12XL has data smoothing.)   The G-45 had a number of 
 minor  jumps of perhaps a few hundred feet at most  in  multipath 
 situations.   The M-4000 did the same with a couple of jumps  out 
 to  perhaps  800ft but returning within perhaps 30  seconds  once 
 lock  was re-established.   Note:  We were not able to  test  the 
 unit  under heavy tree cover as it is winter in Atlanta and  most 
 leaves  are off our trees.  However,  a number of  correspondents 
 have  told us that the G-12XL is approximately on a par with  the 
 excellent  Eagle  Explorer  receiver  performance  in  a   forest 
 situation.
 
 These are the major features We have observed in playing with the 
 G-12XL.   Our impressions are that the G-12XL is an  improved  G-
 45XL  with  a much faster acquisition of satellites  and  a  well 
 thought out user interface.  There are a few additional  features 
 that will appeal to many,  particularly the improved (12 channel) 
 receiver performance and quicker lock.  The more frequently  used 
 display  items  are easy to use,  but the  more  obscure  require 
 several  screens  and  menus and half a dozen  button  pushes  to 
 access.  All in all,  We like the unit much better than the older 
 units  we  have  tested.  (We did not have an EE  to  test,   but 
 likely it would track about as well as the G-12XL,  but as of now 
 no   data  cable  or  software  is  available  for   mapping   or 
 upload/download  with  the  EE  to the  best  of  our  knowledge.  
 Unfortunately,  it  is hard (for us) to dynamically  test  a  GPS 
 without the use of a moving map display system attached.  Without 
 a map display,  many GPS tracking errors can go unnoticed.)   
 
 We made these tests using two Toshiba 2150C laptop computers both 
 running  moving  map software.  The two gps units  were  operated 
 simultaneously  on the dash in front of the driver  (G-12XL)  and 
 with  amplified  external  antenna  on  the  lower  part  of  the 
 windshield  in front of the passenger (G-45).  The  laptops  were 
 positioned  so both displays could be watched simultaneously  and 
 then we drove over a course of perhaps 30 miles and evaluated the 
 relative  performance  of  the  GPS  equipment  by   observations 
 including  the  GPS "cookie trails' on the  moving  map  display.  
 Dropouts of the G-45 for instance would show a break or jump  off 
 the track of the "cookies" for that unit.
 
 More  Field  Tests (2/1/97):  I took the G-12XL  into  the  North 
 Georgia mountains where I have previously had problems with my M-
 4000.   The improvement in performance evident in the  G-12XL  is 
 substantial.  We drove for about 4 hours in twisty mountain roads 
 with  switchbacks  and steep mountains on the side of  the  road.  
 The  M-4000 would frequently lose lock and take up to minutes  to 
 recover.  The G-12XL never lost lock as long as it was laid  flat 
 on  the dash of the car.  The G-12XL produces position output  to 
 the computer much faster than the M-4000.  With the M-4000,   the 
 position  indicated  on the computer map usually  follows  behind 
 actual position by 200 to 300 feet making you unsure if the  next 
 road intersection is the one you want.  With the G-12XL,   points 
 plotted  on the display about every two seconds and seemed to  be 
 only  perhaps  50ft behind the actual location at the  moment  of 
 plot.   Several times,  just before we entered a  switchback,   I 
 removed  the  G-12XL  from the dash and put my  hand  across  the 
 antenna to block sigs.  Then after the switchback,  I put the  G-
 12XL  back on the dash.  Within less than 5 seconds,  the  G-12XL 
 recovered  lock.  In similar tests,  the M-4000 required up to  a 
 couple  of minutes to recover lock.  Multi-Channel  Parallel  GPS 
 receivers have MY vote!  A couple of EE owners have reported that 
 the  EE would work on the seat of the car beside them.   I  tried 
 this out with the G-12XL for a short time.  Perfect operation did 
 not  occur.  However,  it did provide a track almost all  of  the 
 time,  but sometimes, switchbacks would interrupt proper tracking 
 for 10 to 20 seconds.  There were 6 to 8 sats visible while I was 
 running  the  "on the seat" test,  still..  Not  Bad!   Actually,  
 this is superior performance.  Also,  multipath caused additional 
 position errors in the range of perhaps 200 feet SOME OF THE TIME 
 when  the  G-12XL was located on the seat instead  of  the  dash.  
 When  I  put  the  G-12XL back on  the  dash,   the  track  would 
 immediately  come back to normal error.  Therefore,  if  you  are 
 interested  in  accuracy,  the dash is the better  location.   An 
 outside antenna would be even better.  My hat is off to the EE in 
 this  regard  as  well since it is reported  to  provide  similar 
 performance.     With  the Garmin G-12XL,  I don't think  I  will 
 need  to  carry  my amplified external  antenna  around  anymore.  
 Operation  on  an  automobile  dashboard  appears  to  give  100% 
 satisfactory operation.
 
 We also noted that both the G-45XL and the G-12XL have a form  of 
 "dead  reckoning"  for moments when signal dropouts  occur.   For 
 instance,   if  the G-12XL is tracking along and  just  before  a 
 sharp turn you invert it and block its antenna,  it will continue 
 to track straight for about 30 seconds.  It also provides a  very 
 good data smoothing filter to throw out random fixes that are way 
 off  track.  This results in an exceptionally smooth track  on  a 
 moving  map  display even when multipath is present.   Even  with 
 this filter,  there was no overshoot apparent during quick stops,  
 sharp  turns,   and  similar  maneuvers  when  normal  continuous 
 tracking was taking place.
 
 If  anyone  has any additions,  questions,   suggestions,   error 
 corrections  other comments,  please feel free to Email  Jack  or 
 Joe.  
 
 Joe Mehaffey         and        Jack Yeazel
         w2jo@bellsouth.net          river5@peachnet.campus.mci.net
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