How did John Glenn know where he was when he splashed down in the Friendship 7?

Factual Questions
1

(This is from the recent movie “Hidden Figures”, but it’s not really about the movie itself. More wondering about reentry logistics…)

In one scene, an astronaut is about to splash down into the ocean after reentry, and he says “approaching 124.4434 N, 85.42233 E” (made-up coordinates). How was he able to know where he was landing? Was that just a dramatization, or was there actually a way to reliably position oneself to that degree of precision prior to GPS, during the daytime, and with nothing but open water all around you?

2

[ul]
[li]Glenn’s rescue ship was about 6 miles away when Friendship 7 splashed down, so the ship had the coordinates.[/li][li]Per https://en.wikipedia.org/wiki/Global_Positioning_System#Predecessors, in 1957 the US was using the Doppler effect to pinpoint the location of Sputnik 1.[/li][li]I don’t see John Glenn mentioning his (exact) location in http://mercury6.spacelog.org/page/00:04:32:39/.[/li][li]https://en.wikipedia.org/wiki/Mercury-Atlas_6#Splashdown lists his splashdown location at 21°20′N 68°40′W, about 120 miles northeast of the Dominican Republic and about 6 miles away from his rescue ship. [/li][li]Per http://www.satsig.net/lat_long.htm, your coordinates look to me like either about 2000 miles above the north pole or across the pole and in the middle of Siberia.[/li][/ul]

3

Pretty sure that was just the OP making up placeholder numbers (note the “(made-up coordinates)” comment) instead of going back and trying to transcribe from the movie. The actual scene may have had more accurate coordinates.

4

Yes, sorry, those were just made up numbers. I could go back and find the actual coordinates in the movie if you’re curious.

And actually, I rewatched that whole scene just now. Turns out I had misattributed the talking to Glenn. It was actually mission control telling him where he was, not him talking himself. There were just a few cuts back and forth and I didn’t realize the speaker had changed to someone in the control room (he kinda sounded like radio Glenn).

That probably explains it? They were able to use ship or ground based tracking to locate the capsule, and it wasn’t self triangulation its own position. Sorry for the confusion. And thanks for the history lesson regarding the Doppler tracking and other GPS predecessors.

5

Hmmm. If I were John Glenn and Mission Control is telling me my predicted splashdown position, I’d be thinking “Hey, don’t tell me - tell the guys who are supposed to come and get me.”

6

First it’s important to understand Mercury had no ability to change orbit, except a single retrofire to de-orbit. It was stuck in whatever orbit the booster provided. So there was no real need for on-board navigation, since the spacecraft was on a fixed path like a locomotive on fixed train tracks. In theory if you tracked the outbound spacecraft before it goes over the horizon, then perform calculations on that, it will later show up exactly where you expect.

This is how modern-day satellite tracking apps like GoSatWatch work. They can determine exactly when a given satellite will pass overhead because the orbit is fixed, unchanging and highly predictable. Mercury was like that: ‎GoSatWatch Satellite Tracking on the App Store

Gemini and Apollo had maneuvering ability to change orbits, so they had on-board inertial navigation and other systems to verify their 3D position and velocity in space.

By the time of Apollo, ground-based radar tracking was so accurate it could determine the location, direction and velocity of the spacecraft traveling to and from the moon, and even while orbiting the moon. I assume a less-developed version of that was available for Mercury, used to verify the pre-calculated orbit.

A ground-based or sea-based radar tracking station obviously knows its own location, and can determine by radar methods the distance to an orbiting target, the azimuth or compass direction to that target and the range/rate or closure rate of that target. Computer calculations from those determine the target position with respect to earth coordinates like lat/lon.

Even in the 1960s, radar technology was highly developed. Using only ground-based radar tracking (no on-board radar, no terminal homing guidance), the Nike Sprint ABM could be accurately guided while accelerating at 100 g to an incoming reentry vehicle moving at near-orbital velocity. This used only “command guidance” where the ground-based radar tracked both outbound missile and inbound target and essentially steered the two radar blips together.: https://www.youtube.com/watch?v=LsnkmpJhzlo

Even the old Hawk anti-aircraft missile had some ability to intercept other missiles. This was using mostly ground-based radar guidance, although the missile itself could sense reflected radar energy from the target. At 0:50 into this video, the frame-by-frame shows how accurate ground-based radar guidance could be: https://www.youtube.com/watch?v=SDvJGGiqIKE

7

Thats not quite true - well the retrofire isn’t precise and it isn’t random. They obviously had a little thruster system to turn and stabilise the module. Upon arrival in orbit, Glenn’s Mercury capsule turned its butt into the direction of travel, so that the best radio communications were established. The antenna were optimised for this position, because they are going to be used to control reentry…

So as you retrofire, the module would fine tune the attitude so that the retrofire was quite accurately retro-fire… but not necessarily exactly down the line of travel… accuracy of retrofire is the level of efficiency for orbit growth/shrink… if you are making large changes, you want efficency in them.

Anyway, with the de-orbit, descent, burn, the attitude and power of the thrust would correct the aim …
And then deploy the parachute over target …

8

Mercury had ZERO ability to change orbit. There is a difference between attitude control, which adjusts the pitch/roll/yaw of the spacecraft, vs translational control which changes orbital parameters. Mercury had attitude control only. Gemini and Apollo had translational control and could change orbital parameters.

Mercury missions could not change their orbit so the ground track for all future orbits of a given mission was exactly preordained from the moment of booster cutoff. This meant Mercury navigation was like being on a locomotive moving at a fixed velocity along railway tracks. To navigate all you need is an exact watch or timer, backed up by sightings of known landmarks. Mercury had a periscope which allowed the astronaut to verify where on the predetermined ground track he was, which in turn determined his exact latitude and longitude. You can see a typical Mercury ground track for several projected orbits here: https://upload.wikimedia.org/wikipedia/commons/3/3d/Mercury_Tracking_Network_2.png

9

Was he supposed to be able to go, “Oh, there goes Zanzibar. Oh, there’s California. Eta 32 minutes.”? He could identify landmasses accurately as he flew past them?

10

He could identify land masses, however it’s always possible widespread cloud cover could obscure them. Since the orbit ground track and spacecraft location was predetermined from the moment of booster cutoff, in theory a simple countdown clock could start at booster cutoff, calculated to fire the retro rockets at a specific time (hence geographic location).

In fact this was the method, however the retrofire clock was updated slightly just before retrofire, based on either ground sightings by the astronaut or by ground-based radar. So there was never any doubt where the spacecraft was in orbit, or what the corresponding lat/lon was – the spacecraft was in the exact place dictated by the orbit and the time since booster cutoff.

The updates to the retrofire timer were simply to increase the accuracy of the retro timing, hence the accuracy of the splashdown point.

To reiterate the answer to the OP question, Mercury had no need for navigation per se, since it was on a fixed, unchangeable path – similar to a train moving at a constant speed on railroad tracks.

The retrofire could be automatic, based on the retrofire count-down timer, or manual based on astronaut input, or remotely triggered based on ground control.

Once the retrofire happened and there was telemetric ground confirmation of this and the spacecraft attitude during retrofire, ground-based computers would calculate the projected lat/lon landing point.