Garmin's signal strength bars:  What they mean AND..
The meaning of WARM START,  COLD START,  AutoLocate, and "Search the Sky"

The meaning of the hollow and solid bars on the SV signal strength graph and the meaning of "warm start" and "cold start",  "AutoLocate"   and "Search the Sky" on Garmin's G-12XL et al seem to be misunderstood by many. Here is how they work.

First,  here is how Garmin defines their FOUR startup modes.

    Search the sky    -   Time, position, almanac, and ephemeris data all unknown.
    AutoLocate    -        Time, position, and ephemeris unknown, almanac known or partially known.
    Cold Start    -           Time and position known to within some limits, almanac known,  ephemeris unknown
    Warm start      -       Time and position known to within some limits, almanac known, at least 3 SVs Ephemeris are known from previous operation.

The satellites (SVs) broadcast two types of data, Almanac and Ephemeris. Almanac data is course orbital parameters for all SVs.   Each SV broadcasts Almanac data for ALL SVs.   This Almanac data is not very precise and is considered valid for up to several months.   Ephemeris data by comparison is very precise orbital and clock correction for each SV and is necessary for precise positioning.   EACH SV broadcasts ONLY its own Ephemeris data.  The validity of this data is dictated by the particular satellite and may be valid up to 4 to 6 hours.  Each set of ephemeris data gives a "fit" indication which tells how long the particular Ephemeris data is valid.  The  Ephemeris data is broadcast by each SV every 30 seconds so GPS receivers have frequent opportunities to receive and log this essential information.

When the GPS is initially turned on after being off for a period,  it "looks" for SVs based on where it "thinks" it is and  based on the almanac and current time. With this information,  appropriate SVs can be selected for initial search.  When the GPS receiver initially locks onto a SV, the Garmin satellite display initially shows "hollow" signal strength bars.   At this time,   the Ephemeris data has yet to be completely collected (or any existing is "stale") .  Once the ephemeris data is collected from EACH SV in turn, the associated signal strength bar will turn "solid" black and then the data from that SV is considered valid and available for navigation.

Multiple signal strength bars will turn solid at the same time and they go solid when ephemeris is successfully  collected in a parallel channel data correlator system.   All data frames from the SVs are broadcast in sync with
one another. The final ephemeral frame from each satellite would arrive at (approximately) the same time,
assuming that the particular SV began being tracked at the same time. Also the GPS does not need to get the three packet frames in order, or sequentially.  If a partial blockage disturbs one frame,   the GPS receiver will collect and store the others. Once we've received all 3, the signal bars go solid and lock can occur if enough SVs are "locked" and in suitable geometric arrangement.

The signal bars deserve some clarification for Garmin equipment.  On products in the GPS 12/12XL and II+ family, a hollow bar means that the satellite is not being used for navigation.  A solid bar means that the satellite is being used for navigation.  Being used for navigation implies that the ephemeris data is available.  However, not being used for navigation does not imply that the ephemeris data has not been collected.  On the other hand,  the mapping products (III, III+, NavTalk, 175/195...) show a gray bar when the particular satellite does not  have valid ephemeris data.  A solid bar means that valid ephemeris data has been collected.  However, a solid bar does not imply that the satellite is being used for navigation.

If power is cycled OFF on a GPS receiver,  then fairly immediately ON,    then the Ephemeris data is still "fresh",  and lock-on will be very quick since the GPS does not have to collect new Ephemeris data.  This is called a "warm" start.   As noted above,  "fresh" can mean up to several hours depending on information contained in the Ephemeris data packet for a particular SV.   On the other hand,  if the Ephemeris data is "stale" (as can be determined from data within the Ephemeris data packet) then the GPS will collect another Ephemeris data packet for any SV for which it has stale data.    (Actually,  the GPS is always trying to collect fresh Ephemeris data.)

If less than 3 SVs have "fresh" Ephemeris data when a unit is first turned ON,  lock cannot occur until at data from at least one additional SV must be acquired before the GPS can lock.  Note:  With SOME Garmin models,  when the GPS is powered OFF twice without getting a lock,  the GPS software "throws out" any Ephemeris data  it may have assuming it is stale.

The Ephemeris data comes in 3 packets.  The packets are linked together by an "issue of data" identification which, if the receiver properly checks the IOD, will ensure that the packets are all from the same data set.  In this way collection order does not affect the outcome.  This allows Garmin receivers (and others)  to be able to  "piece together"  these three data packets from two or three data transmissions in case part of a packet stream is received damaged.  This ability results in the fastest possible lock under adverse conditions.

If you are "motoring along"  lock can take MUCH longer than if you are "at rest" and out in the clear.  This is because the GPS must receive a COMPLETE Ephemeris data packet ERROR FREE before it will make the signal block "solid" indicating "lock" with a given SV.  Momentary tree blockage,  making a turn and momentarily losing signal from a given SV,  multipath from buildings,  mountains, etc. can all cause errors which will cause the
GPS to have to start over collecting one or more data messages which are essential to lock.

If the GPS has moved more than a few hundred miles or accurate time is lost, the Ephemeris  data will likely be invalid and if you are far enough off,  then none of the SVs that the Almanac thinks should  be overhead will be there. In such case, the GPS will have to  "search the sky" or be reinitialized so it can download new Ephemeris data (and perhaps a new almanac)  and start over.  In Garmin GPS receivers,  this function is called AUTOLOCATE.

The GPS units automatically re-collect Ephemeris data as often as they need to do so in the background. If you repeatedly  shade the antenna so that one or more SVs are blocked when it's trying to collect current ephemeris
on that SV, and it is unsuccessful in collecting and refreshing the data, when the signal returns, the bar will be hollow. Because of the time of applicability,  the units current time,  and depending on if a particular SVs ephemeris data needs refreshing,   this shading experiment will yield seemingly random results.

Maybe that clarifies it for everyone. For those who are not clear at this time.. Please read this and the previous discussion several times and try it with your Garmin GPS.   It really does work this way!

(Note: Yes! We know this is somewhat simplified information.  )

Joe Mehaffey
Revised:  12 July 1999